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Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol–radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibit an increasing trend with relative humidity (RH) in the North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth could play an important role in the PM2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the NCP were conducted using the WRF-Chem Model to comprehensively quantify contributions of the ALW effect to near-surface [PM2.5]. The WRF-Chem Model generally performs reasonably well in simulating the temporal variations in RH against measurements in the NCP. The factor separation approach (FSA) was used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM2.5]. The ALW not only augments particle sizes to enhance aerosol backward scattering but also increases the effective radius to favor aerosol forward scattering. The contribution of the ALW effect on the ARF and photochemistry to near-surface [PM2.5] is not significant, being generally within 1.0 µg m−3 on average in the NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases the effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM2.5] is 17.5 % on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80 %, the ALW progressively increases near-surface [PM2.5] but commences to decrease when the RH exceeds 80 % due to the high occurrence frequencies of precipitation.

Atmospheric aerosols scatter or absorb a fraction of the incoming solar radiation to cool or warm the atmosphere, decreasing surface temperature and altering atmospheric stability to further affect the dispersion of air pollutants in the planetary boundary layer (PBL). In the present study, simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the North China Plain (NCP) were performed using the Weather Research and Forecasting model with Chemistry (WRF-Chem) to comprehensively quantify contributions of aerosol shortwave radiative feedback (ARF) to near-surface (around 15 m above the ground surface) PM2.5 mass concentrations. The WRF-Chem model generally performs well in simulating the temporal variations and spatial distributions of air pollutants concentrations compared to observations at ambient monitoring sites in the NCP, and the simulated diurnal variations of aerosol species are also consistent with the measurements in Beijing. Additionally, the model simulates the aerosol radiative properties, the downward shortwave flux, and the PBL height against observations in the NCP well. During the episode, ARF deteriorates the haze pollution, increasing the near-surface PM2.5 concentrations in the NCP by 10.2 µg m−3 or with a contribution of 7.8 % on average. Sensitivity studies have revealed that high loadings of PM2.5 attenuate the incoming solar radiation reaching the surface to cool the low-level atmosphere, suppressing the development of the PBL, decreasing the surface wind speed, further hindering the PM2.5 dispersion, and consequently exacerbating the haze pollution in the NCP. Furthermore, when the near-surface PM2.5 mass concentration increases from around 50 to several hundred µg m−3, ARF contributes to the near-surface PM2.5 by more than 20 % during daytime in the NCP, substantially aggravating the heavy haze formation. However, when the near-surface PM2.5 concentration is less than around 50 µg m−3, ARF generally reduces the near-surface PM2.5 concentration due to the consequent perturbation of atmospheric dynamic fields.

In many parts of the developing world and economies in transition, small-scale traditional brick kilns are a notorious source of urban air pollution. Many are both energy inefficient and burn highly polluting fuels that emit significant levels of black carbon (BC), organic carbon (OC) and other atmospheric pollutants into local communities, resulting in severe health and environmental impacts. However, only a very limited number of studies are available on the emission characteristics of brick kilns; thus, there is a need to characterize their gaseous and particulate matter (PM) emission factors to better assess their overall contribution to emissions inventories and to quantify their ecological, human health, and climate impacts. In this study, the fuel-, energy-, and brick-based emissions factors and time-based emission ratios of BC, OC, inorganic PM components, CO, SO2, CH4, NOx, and selected volatile organic compounds (VOCs) from three artisanal brick kilns with different designs in Mexico were quantified using the tracer ratio sampling technique. Simultaneous measurements of PM components, CO, and CO2 were also obtained using a sampling probe technique. Additional measurements included the internal temperature of the brick kilns, mechanical resistance of bricks produced, and characteristics of fuels employed. Average fuel-based BC emission factors ranged from 0.15 to 0.58 g (kg fuel)−1, whereas BC∕OC mass ratios ranged from 0.9 to 5.2, depending on the kiln type. The results show that both techniques capture similar temporal profiles of the brick kiln emissions and produce comparable emission factors. A more integrated inter-comparison of the brick kilns' performances was obtained by simultaneously assessing emissions factors, energy efficiency, fuel consumption, and the quality of the bricks produced.

Diesel-powered vehicles are intensively used in urban areas for transporting goods and people but can substantially contribute to high emissions of black carbon (BC), organic carbon (OC), and other gaseous pollutants. Strategies aimed at controlling mobile emissions sources thus have the potential to improve air quality and help mitigate the impacts of air pollutants on climate, ecosystems, and human health. However, in developing countries there are limited data on the BC and OC emission characteristics of diesel-powered vehicles, and thus there are large uncertainties in the estimation of the emission contributions from these sources. We measured BC, OC, and other inorganic components of fine particulate matter (PM), as well as carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), ethane, acetylene, benzene, toluene, and C2-benzenes under real-world driving conditions for 20 diesel-powered vehicles encompassing multiple emission level technologies in Mexico City with the chasing technique using the Aerodyne mobile laboratory. Average BC emission factors ranged from 0.41–2.48 g kg−1 of fuel depending on vehicle type. The vehicles were also simultaneously measured using the cross-road remote sensing technique to obtain the emission factors of nitrogen oxide (NO), CO, total hydrocarbons, and fine PM, thus allowing for the intercomparison of the results from the two techniques. There is overall good agreement between the two techniques and both can identify high and low emitters, but substantial differences were found in some of the vehicles, probably due to the ability of the chasing technique to capture a larger diversity of driving conditions in comparison to the remote sensing technique. A comparison of the results with the US EPA MOVES2014b model showed that the model underestimates CO, OC, and selected VOC species, whereas there is better agreement for NOx and BC. Larger OC / BC ratios were found in comparison to ratios measured in California using the same technique, further demonstrating the need for using locally obtained diesel-powered vehicle emission factor database in developing countries in order to reduce the uncertainty in the emissions estimates and to improve the evaluation of the effectiveness of emissions reduction measures.

The aerosol–cloud interaction over the Tibetan Plateau has been investigated using a cloud-resolving weather research and forecasting model with a two-moment bulk microphysical scheme including aerosol effects on cloud condensation nuclei and ice nuclei. Two types of cumulus clouds with a similar convective available potential energy, occurring over the Tibetan Plateau (Cu-TP) and North China Plain (Cu-NCP) in August 2014, are simulated to explore the response of convective clouds to aerosols. A set of aerosol profiles is used in the simulations, with the surface aerosol number concentration varying from 20 to 9000 cm−3 and the sulfate mass concentration varying from 0.02 to 9.0 µg cm−3. Increasing aerosol concentrations generally enhances the cloud core updraft and maximum updraft, intensifying convections in Cu-TP and Cu-NCP. However, the core updraft is much stronger in Cu-TP than Cu-NCP, because of the early occurrence of the glaciation process in Cu-TP that is triggered at an elevation above 4000 m. The precipitation increases steadily with aerosol concentrations in Cu-NCP, caused by the suppression of the warm rain but occurrence of efficient mix-phased precipitation due to the reduced cloud droplet size. The precipitation in Cu-TP also increases with aerosol concentrations, but the precipitation enhancement is not substantial compared to that in Cu-NCP with high aerosol concentrations. The aerosol-induced intensification of convections in Cu-TP not only facilitates the precipitation but also transports more ice-phase hydrometeors into the upper troposphere to decrease the precipitation efficiency. Considering the very clean atmosphere over the Tibetan Plateau, elevated aerosol concentrations can remarkably enhance convections due to its specific topography, which not only warms the middle troposphere to influence the Asian summer monsoon but also delivers hydrometeors into the upper troposphere to allow more water vapor to travel into the lower stratosphere.

Rapid industrialization and urbanization have caused frequent occurrence of haze in China during wintertime in recent years. The sulfate aerosol is one of the most important components of fine particles (PM2. 5) in the atmosphere, contributing significantly to the haze formation. However, the heterogeneous formation mechanism of sulfate remains poorly characterized. The relationships of the observed sulfate with PM2. 5, iron, and relative humidity in Xi'an, China have been employed to evaluate the mechanism and to develop a parameterization of the sulfate heterogeneous formation involving aerosol water for incorporation into atmospheric chemical transport models. Model simulations with the proposed parameterization can successfully reproduce the observed sulfate rapid growth and diurnal variations in Xi'an and Beijing, China. Reasonable representation of sulfate heterogeneous formation in chemical transport models considerably improves the PM2. 5 simulations, providing the underlying basis for better understanding the haze formation and supporting the design and implementation of emission control strategies.

The organic aerosol (OA) concentration is simulated in the Guanzhong Basin, China from 23 to 25 April 2013 utilizing the WRF-CHEM model. Two approaches are used to predict OA concentrations: (1) a traditional secondary organic aerosol (SOA) module; (2) a non-traditional SOA module including the volatility basis-set modeling method in which primary organic aerosol (POA) is assumed to be semivolatile and photochemically reactive. Generally, the spatial patterns and temporal variations of the calculated hourly near-surface ozone and fine particle matters agree well with the observations in Xi'an and surrounding areas. The model also yields reasonable distributions of daily PM2.5 and elemental carbon (EC) compared to the filter measurements at 29 sites in the basin. Filter-measured organic carbon (OC) and EC are used to evaluate OA, POA, and SOA using the OC ∕ EC ratio approach. Compared with the traditional SOA module, the non-traditional module significantly improves SOA simulations and explains about 88 % of the observed SOA concentration. Oxidation and partitioning of POA treated as semivolatile constitute the most important pathway for the SOA formation, contributing more than 75 % of the SOA concentrations in the basin. Residential emissions are the dominant anthropogenic OA source, constituting about 50 % of OA concentrations in urban and rural areas and 30 % in the background area. The OA contribution from transportation emissions decreases from 25 % in urban areas to 20 % in the background area, and the industry emission OA contribution is less than 6 %.

Rapid industrialization and urbanization have caused severe air pollution in the Guanzhong basin, northwestern China, with heavy haze events occurring frequently in recent winters. Using the NCEP reanalysis data, the large-scale synoptic situations influencing the Guanzhong basin during wintertime of 2013 are categorized into six types to evaluate the contribution of synoptic situations to the air pollution, including “north-low”, “southwest-trough”, “southeast-high”, “transition”, “southeast-trough”, and “inland-high”. The FLEXPART model has been utilized to demonstrate the corresponding pollutant transport patterns for the typical synoptic situations in the basin. Except for “southwest-trough” and “southeast-high” (defined as favorable synoptic situations), the other four synoptic conditions (defined as unfavorable synoptic situations) generally facilitate the accumulation of air pollutants, causing heavy air pollution in the basin. In association with the measurement of PM2.5 (particulate matter with aerodynamic diameter less than 2.5 µm) in the basin, the unfavorable synoptic situations correspond to high PM2.5 mass concentrations or poor air quality and vice versa. The same analysis has also been applied to winters of 2008–2012, which shows that the basin was mainly influenced by the unfavorable synoptic situations during wintertime leading to poor air quality. The WRF-CHEM model has further been applied to simulate the selected 6 days representing the typical synoptic situations during the wintertime of 2013, and the results generally show a good agreement between the modeled distributions and variations of PM2.5 and the corresponding synoptic situations, demonstrating reasonable classification for the synoptic situations in the basin. Detailed meteorological conditions, such as temperature inversion, low-level horizontal wind speed, and planetary boundary layer, all contribute to heavy air pollution events in the basin under unfavorable synoptic conditions. Considering the proportion of occurrence of unfavorable synoptic situations during wintertime, reduction of emissions is the optimum approach to mitigate the air pollution in the Guanzhong basin.

In this study, the ozone (O3) formation in China's northwest city of Xi'an and surrounding areas is investigated using the Weather Research and Forecasting atmospheric chemistry (WRF-Chem) model during the period from 22 to 24 August 2013, corresponding to a heavy air pollution episode with high concentrations of O3 and PM2.5. The model generally performs well compared to measurements in simulating the surface temperature, relative humidity, and wind speed and direction, near-surface O3 and PM2.5 mass concentrations, and aerosol constituents. High aerosol concentrations in Xi'an and surrounding areas significantly decrease the photolysis frequencies and can reduce O3 concentrations by more than 50 µg m−3 (around 25 ppb) on average. Sensitivity studies show that the O3 production regime in Xi'an and surrounding areas is complicated, varying from NOx to VOC (volatile organic compound)-sensitive chemistry. The industrial emissions contribute the most to the O3 concentrations compared to biogenic and other anthropogenic sources, but neither individual anthropogenic emission nor biogenic emission plays a dominant role in the O3 formation. Under high O3 and PM2.5 concentrations, a 50 % reduction in all the anthropogenic emissions only decreases near-surface O3 concentrations by about 14 % during daytime. The complicated O3 production regime and high aerosol levels pose a challenge for O3 control strategies in Xi'an and surrounding areas. Further investigation regarding O3 control strategies will need to be performed, taking into consideration the rapid changes in anthropogenic emissions that are not reflected in the current emission inventories and the uncertainties in the meteorological field simulations.

The air quality of megacities can be influenced by external emission sources on both global and regional scales. At the same time their outflow emissions can exert an impact to the surrounding environment. The present study evaluates an SO2 peak observed on 24 March 2006 at the suburban supersite T1 and at ambient air quality monitoring stations located in the northern region of the Mexico City Metropolitan Area (MCMA) during the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. We found that this peak could be related to an important episodic emission event coming from Tizayuca region, northeast of the MCMA. Back-trajectory analyses suggest that the emission event started in the early morning at 04:00 LST and lasted for about 9 h. The estimated emission rate is about 2 kg s−1. To the best of our knowledge, sulfur dioxide emissions from the Tizayuca region have not been considered in previous studies. This finding suggests the possibility of "overlooked" emission sources in this region that could influence the air quality of the MCMA. This further motivated us to study the cement plants, including those in the state of Hidalgo and in the State of Mexico. It was found that they can contribute to the SO2 levels in the northeast (NE) region of the basin (about 42%), at the suburban supersite T1 (41%) and that at some monitoring stations their contribution can be even higher than the contribution from the Tula Industrial Complex (TIC). The contribution of the Tula Industrial Complex to regional ozone levels is estimated. The model suggests low contribution to the MCMA (1 to 4 ppb) and slightly higher contribution at the suburban T1 (6 ppb) and rural T2 (5 ppb) supersites. However, the contribution could be as high as 10 ppb in the upper northwest region of the basin and in the southwest and south-southeast regions of the state of Hidalgo. In addition, the results indicated that the ozone plume could also be transported to northwest Tlaxcala, eastern Hidalgo, and farther northeast of the State of Mexico, but with rather low values. A first estimate of the potential contribution from flaring activities to regional ozone levels is presented. Results suggest that up to 30% of the total regional ozone from TIC could be related to flaring activities. Finally, the influence on SO2 levels from technological changes in the existing refinery is briefly discussed. These changes are due to the upcoming construction of a new refinery in Tula. The combination of emission reductions in the power plant, the refinery and in local sources in the MCMA could result in higher reductions on the average SO2 concentration. Reductions in external sources tend to affect more the northern part of the basin (−16 to −46%), while reductions of urban sources in the megacity tend to diminish SO2 levels substantially in the central, southwest, and southeast regions (−31 to −50%).

The local and regional impacts of open fires and trash burning on ground-level ozone (O3) and fine carbonaceous aerosols in the Mexico City Metropolitan Area (MCMA) and surrounding region during two high fire periods in March 2006 have been evaluated using WRF-CHEM model. The model captured reasonably well the measurement-derived magnitude and temporal variation of the biomass burning organic aerosol (BBOA), and the simulated impacts of open fires on organic aerosol (OA) were consistent with many observation-based estimates. We did not detect significant effects of open fires and trash burning on surface O3 concentrations in the MCMA and surrounding region. In contrast, they had important influences on OA and elemental carbon (EC), increasing primary OA (POA) by ~60%, secondary OA (SOA) by ~22%, total OA (TOA = POA + SOA) by ~33%, and EC by ~22%, on both the local (urban) and regional scales. Although the emissions of trash burning are substantially lower than those from open fires, trash burning made slightly smaller but comparable contributions to OA as open fires did, and exerted an even higher influence on EC. Of the ~22% enhancement in SOA concentrations (equivalent to a ~15% increase in TOA) simulated, about two third was attributed to the open fires and one-third to the trash burning. On the annual basis and taking the biofuel use emissions into consideration, we estimated that open fires, trash burning and biofuel use together contributed about 60% to the loading of POA, 30% to SOA, and 25% to EC in both the MCMA and its surrounding region, of which the open fires and trash burning contributed about 35% to POA, 18% to SOA, and 15% to EC. The estimates of biomass burning impacts in this study may contain considerable uncertainties due to the uncertainties in their emission estimates in magnitude, temporal and spatial distribution, extrapolations and the nature of spot comparison. More observation and modeling studies are needed to accurately assess the impacts of biomass burning on tropospheric chemistry, regional and global air quality, and climate change.

The purpose of the present study is to investigate the uncertainties in simulating secondary organic aerosol (SOA) in Mexico City metropolitan area (MCMA) due to meteorological initial uncertainties using the WRF-CHEM model through ensemble simulations. The simulated periods (24 and 29 March 2006) represent two typical meteorological episodes ("Convection-South" and "Convection-North", respectively) in the Mexico City basin during the MILAGRO-2006 field campaign. The organic aerosols are simulated using a non-traditional SOA model including the volatility basis-set modeling method and the contributions from glyoxal and methylglyoxal. Model results demonstrate that uncertainties in meteorological initial conditions have significant impacts on SOA simulations, including the peak time concentrations, the horizontal distributions, and the temporal variations. The ensemble spread of the simulated peak SOA at T0 can reach up to 4.0 μg m−3 during the daytime, which is around 35% of the ensemble mean. Both the basin wide wind speed and the convergence area affect the magnitude and the location of the simulated SOA concentrations inside the Mexico City basin. The wind speed, especially during the previous midnight and the following early morning, influences the magnitude of the peak SOA concentration through ventilation. The surface horizontal convergence zone generally determines the area with high SOA concentrations. The magnitude of the ensemble spreads may vary with different meteorological episodes but the ratio of the ensemble spread to mean does not change significantly.

This work presents a simulation of the plume trajectory emitted by flaring activities of the Miguel Hidalgo Refinery in Mexico. The flame of a representative sour gas flare is modeled with a CFD combustion code in order to estimate emission rates of combustion by-products of interest for air quality: acetylene, ethylene, nitrogen oxides, carbon monoxide, soot and sulfur dioxide. The emission rates of NO2 and SO2 were compared with measurements obtained at Tula as part of MILAGRO field campaign. The rates of soot, VOCs and CO emissions were compared with estimates obtained by Instituto Mexicano del Petróleo (IMP). The emission rates of these species were further included in WRF-Chem model to simulate the chemical transport of the plume from 22 to 27 March of 2006. The model presents reliable performance of the resolved meteorology, with respect to the Mean Absolute Error (MAE), Root Mean Square Error (RMSE), mean bias (BIAS), vector RMSE and Index of Agreement (IOA). WRF-Chem outputs of SO2 and soot were compared with surface measurements obtained at the three supersites of MILAGRO campaign. The results suggest a contribution of Tula flaring activities to the total SO2 levels of 18% to 27% at the urban supersite (T0), and of 10% to 18% at the suburban supersite (T1). For soot, the model predicts low contribution at the three supersites, with less than 0.1% at three supersites. According to the model, the greatest contribution of both pollutants to the three supersites occurred on 23 March, which coincides with the third cold surge event reported during the campaign.

The contribution of garbage burning (GB) emissions to chloride and PM2.5 in the Mexico City Metropolitan Area (MCMA) has been investigated for the period of 24 to 29 March during the MILAGRO-2006 campaign using the WRF-CHEM model. When the MCMA 2006 official emission inventory without biomass burning is used in the simulations, the WRF-CHEM model significantly underestimates the observed particulate chloride in the urban and the suburban areas. The inclusion of GB emissions substantially improves the simulations of particulate chloride; GB contributes more than 60% of the observation, indicating that it is a major source of particulate chloride in Mexico City. GB yields up to 3 pbb HCl at the ground level in the city, which is mainly caused by the burning of polyvinyl chloride (PVC) in the garbage. GB is also an important source of PM2.5, contributing about 3–30% simulated PM2.5 mass on average. More modeling work is needed to evaluate the GB contribution to hazardous air toxics, such as dioxin, which is found to be released at high level from PVC burning in laboratory experiments.

As part of the MILAGRO 2006 field campaign, the exchange of atmospheric aerosols with the urban landscape was measured from a tall tower erected in a heavily populated neighborhood of Mexico City. Urban submicron aerosol fluxes were measured using an eddy covariance method with a quadrupole aerosol mass spectrometer during a two week period in March, 2006. Nitrate and ammonium aerosol concentrations were elevated at this location near the city center compared to measurements at other urban sites. Significant downward fluxes of nitrate aerosol, averaging −0.2 μg m−2 s−1, were measured during daytime. The urban surface was not a significant source of sulfate aerosols. The measurements also showed that primary organic aerosol fluxes, approximated by hydrocarbon-like organic aerosols (HOA), displayed diurnal patterns similar to CO2 fluxes and anthropogenic urban activities. Overall, 47% of submicron organic aerosol emissions were HOA, 35% were oxygenated (OOA) and 18% were associated with biomass burning (BBOA). Organic aerosol fluxes were bi-directional, but on average HOA fluxes were 0.1 μg m−2 s−1, OOA fluxes were −0.03 μg m−2 s−1, and BBOA fluxes were −0.03 μg m−2 s−1. After accounting for size differences (PM1 vs PM2.5) and using an estimate of the black carbon component, comparison of the flux measurements with the 2006 gridded emissions inventory of Mexico City, showed that the daily-averaged total PM emission rates were essentially identical for the emission inventory and the flux measurements. However, the emission inventory included dust and metal particulate contributions, which were not included in the flux measurements. As a result, it appears that the inventory underestimates overall PM emissions for this location.

One of the most challenging tasks for chemical transport models (CTMs) is the prediction of the formation and partitioning of the major semi-volatile inorganic aerosol components (nitrate, chloride, ammonium) between the gas and particulate phases. In this work the PMCAMx-2008 CTM, which includes the recently developed aerosol thermodynamic model ISORROPIA-II, is applied in the Mexico City Metropolitan Area in order to simulate the formation of the major inorganic aerosol components. The main sources of SO2 (such as the Miguel Hidalgo Refinery and the Francisco Perez Rios Power Plant) in the Mexico City Metropolitan Area (MCMA) are located in Tula, resulting in high predicted PM1 (particulate matter with diameter less than 1 μm) sulfate concentrations (over 25 μg m-3) in that area. The average predicted PM1 nitrate concentrations are up to 3 μg m−3 (with maxima up to 11 μg m−3) in and around the urban center, mostly produced from local photochemistry. The presence of calcium coming from the Tolteca area (7 μg m−3) as well as the rest of the mineral cations (1 μg m−3 potassium, 1 μg m−3 magnesium, 2 μg m−3 sodium, and 3 μg m−3 calcium) from the Texcoco Lake resulted in the formation of a significant amount of aerosol nitrate in the coarse mode with concentrations up to 3 μg m−3 over these areas. PM1−10 (particulate matter with diameter between 1 and 10 μm) chloride is also high and its concentration exceeds 2 μg m−3 in Texcoco Lake. PM1 ammonium concentrations peak at the center of Mexico City (2 μg m−3) and the Tula vicinity (2.5 μg m−3). The performance of the model for the major inorganic PM components (sulfate, ammonium, nitrate, chloride, sodium, calcium, and magnesium) is encouraging. At the T0 measurement site, located in the Mexico City urban center, the average measured values of PM1 sulfate, nitrate, ammonium, and chloride are 3.5 μg m−3, 3.5 μg m−3, 2.1 μg m−3, and 0.36 μg m−3, respectively. The corresponding predicted values are 3.7 μg m−3, 2.7 μg m−3, 1.7 μg m−3, and 0.25 μg m−3. High sulfate concentrations are associated with the transport of sulfate from the Tula vicinity, while in periods where southerly winds are dominant; the concentrations of sulfate are low. The underprediction of nitrate can be attributed to the underestimation of OH levels by the model during the early morning. Ammonium is sensitive to the predicted sulfate concentrations and the nitrate levels. The performance of the model is also evaluated against measurements taken from a suburban background site (T1) located north of Mexico City. The average predicted PM2.5 (particulate matter with diameter less than 2.5 μm) sulfate, nitrate, ammonium, chloride, sodium, calcium, and magnesium are 3.3, 3.2, 1.4, 0.5, 0.3, 1.2, and 0.15 μg m−3, respectively. The corresponding measured concentrations are 3.7, 2.9, 1.5, 0.3, 0.4, 0.6, and 0.15 μg m−3. The overprediction of calcium indicates a possible overestimation of its emissions and affects the partitioning of nitric acid to the aerosol phase resulting occasionally in an overprediction of nitrate. Additional improvements are possible by improving the performance of the model regarding the oxidant levels, and revising the emissions and the chemical composition of the fugitive dust. The hybrid approach in which the mass transfer to the fine aerosol is simulated using the bulk equilibrium assumption and to the remaining aerosol sections using a dynamic approach, is needed in order to accurately simulate the size distribution of the inorganic aerosols. The bulk equilibrium approach fails to reproduce the observed coarse nitrate and overpredicts the fine nitrate. Sensitivity tests indicate that sulfate concentration in Tula decreases by up to 0.5 μg m−3 after a 50% reduction of SO2 emissions while it can increase by up to 0.3 μg m−3 when NOx emissions are reduced by 50%. Nitrate concentration decreases by up to 1 μg m−3 after the 50% reduction of NOx or NH3 emissions. Ammonium concentration decreases by up to 1 μg m−3, 0.3 μg m−3, and 0.1 μg m−3 after the 50% reduction of NH3, NOx, and SO2 emissions, respectively.

In the present study, the impact of aerosols on the photochemistry in Mexico City is evaluated using the WRF-CHEM model for the period from 24 to 29 March during the MCMA-2006/MILAGRO campaign. An aerosol radiative module has been developed with detailed consideration of aerosol size, composition, and mixing. The module has been coupled into the WRF-CHEM model to calculate the aerosol optical properties, including optical depth, single scattering albedo, and asymmetry factor. Calculated aerosol optical properties are in good agreement with the surface observations and aircraft and satellite measurements during daytime. In general, the photolysis rates are reduced due to the absorption by carbonaceous aerosols, particularly in the early morning and late afternoon hours with a long aerosol optical path. However, with the growth of aerosol particles and the decrease of the solar zenith angle around noontime, aerosols can slightly enhance photolysis rates when ultraviolet (UV) radiation scattering dominates UV absorption by aerosols at the lower-most model layer. The changes in photolysis rates due to aerosols lead to about 2–17 % surface ozone reduction during daytime in the urban area in Mexico City with generally larger reductions during early morning hours near the city center, resulting in a decrease of OH level by about 9 %, as well as a decrease in the daytime concentrations of nitrate and secondary organic aerosols by 5–6 % on average. In addition, the rapid aging of black carbon aerosols and the enhanced absorption of UV radiation by organic aerosols contribute substantially to the reduction of photolysis rates.

Organic aerosol concentrations are simulated using the WRF-CHEM model in Mexico City during the period from 24 to 29 March in association with the MILAGRO-2006 campaign. Two approaches are employed to predict the variation and spatial distribution of the organic aerosol concentrations: (1) a traditional 2-product secondary organic aerosol (SOA) model with non-volatile primary organic aerosols (POA); (2) a non-traditional SOA model including the volatility basis-set modeling method in which primary organic components are assumed to be semi-volatile and photochemically reactive and are distributed in logarithmically spaced volatility bins. The MCMA (Mexico City Metropolitan Area) 2006 official emission inventory is used in simulations and the POA emissions are modified and distributed by volatility based on dilution experiments for the non-traditional SOA model. The model results are compared to the Aerosol Mass Spectrometry (AMS) observations analyzed using the Positive Matrix Factorization (PMF) technique at an urban background site (T0) and a suburban background site (T1) in Mexico City. The traditional SOA model frequently underestimates the observed POA concentrations during rush hours and overestimates the observations in the rest of the time in the city. The model also substantially underestimates the observed SOA concentrations, particularly during daytime, and only produces 21% and 25% of the observed SOA mass in the suburban and urban area, respectively. The non-traditional SOA model performs well in simulating the POA variation, but still overestimates during daytime in the urban area. The SOA simulations are significantly improved in the non-traditional SOA model compared to the traditional SOA model and the SOA production is increased by more than 100% in the city. However, the underestimation during daytime is still salient in the urban area and the non-traditional model also fails to reproduce the high level of SOA concentrations in the suburban area. In the non-traditional SOA model, the aging process of primary organic components considerably decreases the OH levels in simulations and further impacts the SOA formation. If the aging process in the non-traditional model does not have feedback on the OH in the gas-phase chemistry, the SOA production is enhanced by more than 10% compared to the simulations with the OH feedback during daytime, and the gap between the simulations and observations in the urban area is around 3 μg m−3 or 20% on average during late morning and early afternoon, within the uncertainty from the AMS measurements and PMF analysis. In addition, glyoxal and methylglyoxal can contribute up to approximately 10% of the observed SOA mass in the urban area and 4% in the suburban area. Including the non-OH feedback and the contribution of glyoxal and methylglyoxal, the non-traditional SOA model can explain up to 83% of the observed SOA in the urban area, and the underestimation during late morning and early afternoon is reduced to 0.9 μg m−3 or 6% on average. Considering the uncertainties from measurements, emissions, meteorological conditions, aging of semi-volatile and intermediate volatile organic compounds, and contributions from background transport, the non-traditional SOA model is capable of closing the gap in SOA mass between measurements and models.

The Mexico City Metropolitan Area (MCMA) experiences high loadings of atmospheric aerosols from anthropogenic sources, biomass burning and wind-blown dust. This paper uses a combination of measurements and numerical simulations to identify different plumes affecting the basin and to characterize transformation inside the plumes. The High Spectral Resolution Lidar on board the NASA LaRC B-200 King Air aircraft measured extinction coefficients and extinction to backscatter ratio at 532 nm, and backscatter coefficients and depolarization ratios at 532 and 1064 nm. These can be used to identify aerosol types. The measurement curtains are compared with particle trajectory simulations using WRF-Flexpart for different source groups. The good correspondence between measurements and simulations suggests that the aerosol transport is sufficiently well characterized by the models to estimate aerosol types and ages. Plumes in the basin undergo complex transport, and are frequently mixed together. Urban aerosols are readily identifiable by their low depolarization ratios and high lidar ratios, and dust by the opposite properties. Fresh biomass burning plumes have very low depolarization ratios which increase rapidly with age. This rapid transformation is consistent with the presence of atmospheric tar balls in the fresh plumes.

MILAGRO (Megacity Initiative: Local And Global Research Observations) is an international collaborative project to examine the behavior and the export of atmospheric emissions from a megacity. The Mexico City Metropolitan Area (MCMA) – one of the world's largest megacities and North America's most populous city – was selected as the case study to characterize the sources, concentrations, transport, and transformation processes of the gases and fine particles emitted to the MCMA atmosphere and to evaluate the regional and global impacts of these emissions. The findings of this study are relevant to the evolution and impacts of pollution from many other megacities. The measurement phase consisted of a month-long series of carefully coordinated observations of the chemistry and physics of the atmosphere in and near Mexico City during March 2006, using a wide range of instruments at ground sites, on aircraft and satellites, and enlisting over 450 scientists from 150 institutions in 30 countries. Three ground supersites were set up to examine the evolution of the primary emitted gases and fine particles. Additional platforms in or near Mexico City included mobile vans containing scientific laboratories and mobile and stationary upward-looking lidars. Seven instrumented research aircraft provided information about the atmosphere over a large region and at various altitudes. Satellite-based instruments peered down into the atmosphere, providing even larger geographical coverage. The overall campaign was complemented by meteorological forecasting and numerical simulations, satellite observations and surface networks. Together, these research observations have provided the most comprehensive characterization of the MCMA's urban and regional atmospheric composition and chemistry that will take years to analyze and evaluate fully. In this paper we review over 120 papers resulting from the MILAGRO/INTEX-B Campaign that have been published or submitted, as well as relevant papers from the earlier MCMA-2003 Campaign, with the aim of providing a road map for the scientific community interested in understanding the emissions from a megacity such as the MCMA and their impacts on air quality and climate. This paper describes the measurements performed during MILAGRO and the results obtained on MCMA's atmospheric meteorology and dynamics, emissions of gases and fine particles, sources and concentrations of volatile organic compounds, urban and regional photochemistry, ambient particulate matter, aerosol radiative properties, urban plume characterization, and health studies. A summary of key findings from the field study is presented.

Many recent models underpredict secondary organic aerosol (SOA) particulate matter (PM) concentrations in polluted regions, indicating serious deficiencies in the models' chemical mechanisms and/or missing SOA precursors. Since tropospheric photochemical ozone production is much better understood, we investigate the correlation of odd-oxygen ([Ox]≡[O3]+[NO2]) and the oxygenated component of organic aerosol (OOA), which is interpreted as a surrogate for SOA. OOA and Ox measured in Mexico City in 2006 and Houston in 2000 were well correlated in air masses where both species were formed on similar timescales (less than 8 h) and not well correlated when their formation timescales or location differed greatly. When correlated, the ratio of these two species ranged from 30 μg m−3/ppm (STP) in Houston during time periods affected by large petrochemical plant emissions to as high as 160 μg m−3/ppm in Mexico City, where typical values were near 120 μg m−3/ppm. On several days in Mexico City, the [OOA]/[Ox] ratio decreased by a factor of ~2 between 08:00 and 13:00 local time. This decrease is only partially attributable to evaporation of the least oxidized and most volatile components of OOA; differences in the diurnal emission trends and timescales for photochemical processing of SOA precursors compared to ozone precursors also likely contribute to the observed decrease. The extent of OOA oxidation increased with photochemical aging. Calculations of the ratio of the SOA formation rate to the Ox production rate using ambient VOC measurements and traditional laboratory SOA yields are lower than the observed [OOA]/[Ox] ratios by factors of 5 to 15, consistent with several other models' underestimates of SOA. Calculations of this ratio using emission factors for organic compounds from gasoline and diesel exhaust do not reproduce the observed ratio. Although not succesful in reproducing the atmospheric observations presented, modeling P(SOA)/P(Ox) can serve as a useful test of photochemical models using improved formulation mechanisms for SOA.

The contribution of HONO sources to the photochemistry in Mexico City is investigated during the MCMA-2006/MILAGO Campaign using the WRF-CHEM model. Besides the homogeneous reaction of NO with OH, four additional HONO sources are considered in the WRF-CHEM model: secondary HONO formation from NO2 heterogeneous reaction with semivolatile organics, NO2 reaction with freshly emitted soot, NO2 heterogeneous reaction on aerosol and ground surfaces. The WRF-CHEM model with the five HONO sources performs reasonably well in tracking the observed diurnal variation of HONO concentrations. The HONO sources included are found to significantly improve the HOx (OH+HO2) simulations during daytime and the partition of NO/NO2 in the morning. The HONO sources also accelerate the accumulation of O3 concentrations in the morning by about 2 h and subsequently result in a noticeable enhancement of O3 concentrations over the course of the day with a midday average of about 6 ppb. Furthermore, these HONO sources play a very important role in the formation of secondary aerosols in the morning. They substantially enhance the secondary organic aerosol concentrations by a factor of 2 on average in the morning, although they contribute less during the rest of the day. The simulated particle-phase nitrate and ammonium are also substantially enhanced in the morning when the four HONO sources are included, in good agreement with the measurements. The impact of the HONO sources on the sulfate aerosols is negligible because of the inefficient conversion of H2SO4 from SO2 reacting with OH.

The purpose of the present study is to investigate the sensitivity of ozone (O3) predictions in the Mexico City Metropolitan Area (MCMA) to meteorological initial uncertainties and planetary boundary layer (PBL) parameterization schemes using state-of-the-art meteorological and photochemical prediction models through ensemble forecasts. The simulated periods (3, 9, 15 and 29 March 2006) represent four typical meteorological episodes ("South-Venting", "O3-North", "O3-South" and "Convection-North", respectively) in the Mexico City basin during the MCMA-2006/MILAGRO campaign. Our results demonstrate that the uncertainties in meteorological initial conditions have significant impacts on O3 predictions, including peak time O3 concentrations ([O3]), horizontal and vertical O3 distributions, and temporal variations. The ensemble spread of the simulated peak [O3] averaged over the city's ambient monitoring sites can reach up to 10 ppb. The increasing uncertainties in meteorological fields during peak O3 period contribute to the largest unpredictability in O3 simulations, while the impacts of wind speeds and PBL height on [O3] are more straightforward and important. The magnitude of the ensemble spreads varies with different PBL schemes and meteorological episodes. The uncertainties in O3 predictions caused by PBL schemes mainly come from their ability to represent the mixing layer height; but overall, these uncertainties are smaller than those from the uncertainties in meteorological initial conditions.

A detailed analysis of OH, HO2 and RO2 radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify time-resolved ROx (sum of OH, HO2, RO2) radical production rates from day- and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O3), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH- and NO3 radicals, O3; and (4) NO, NO2, meteorological and other parameters. The ROx production rate was calculated directly from these observations; the MCM was used to estimate further ROx production from unconstrained sources, and express overall ROx production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO2 and HO2 radicals into OH radicals). Daytime radical production is found to be about 10–25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: Oxygenated VOC other than HCHO about 33%; HCHO and O3 photolysis each about 20%; O3/alkene reactions and HONO photolysis each about 12%, other sources 2* + water makes – if any – a very small contribution (~2%). The peak radical production of ~7.5 107 molec cm−3 s−1 is found already at 10:00 a.m., i.e., more than 2.5 h before solar noon. O3/alkene reactions are indirectly responsible for ~33% of these radicals. Our measurements and analysis comprise a database that enables testing of the representation of radical sources and radical chain reactions in photochemical models. Since the photochemical processing of pollutants in the MCMA is radical limited, our analysis identifies the drivers for ozone and SOA formation. We conclude that reductions in VOC emissions provide an efficient opportunity to reduce peak concentrations of these secondary pollutants, because (1) about 70% of radical production is linked to VOC precursors; (2) lowering the VOC/NOx ratio has the further benefit of reducing the radical re-cycling efficiency from radical chain reactions (chemical amplification of radical sources); (3) a positive feedback is identified: lowering the rate of radical production from organic precursors also reduces that from inorganic precursors, like ozone, as pollution export from the MCMA caps the amount of ozone that accumulates at a lower rate inside the MCMA. Continued VOC reductions will in the future result in decreasing peak concentrations of ozone and SOA in the MCMA.

A box model using measurements from the Mexico City Metropolitan Area study in the spring of 2003 (MCMA-2003) is presented to study oxidative capacity (our ability to predict OH radicals) and ROx (ROx=OH+HO2+RO2+RO) radical cycling in a polluted (i.e., very high NOx=NO+NO2) atmosphere. Model simulations were performed using the Master Chemical Mechanism (MCMv3.1) constrained with 10 min averaged measurements of major radical sources (i.e., HCHO, HONO, O3, CHOCHO, etc.), radical sink precursors (i.e., NO, NO2, SO2, CO, and 102 volatile organic compounds (VOC)), meteorological parameters (temperature, pressure, water vapor concentration, dilution), and photolysis frequencies. Modeled HOx (=OH+HO2) concentrations compare favorably with measured concentrations for most of the day; however, the model under-predicts the concentrations of radicals in the early morning. This "missing reactivity" is highest during peak photochemical activity, and is least visible in a direct comparison of HOx radical concentrations. We conclude that the most likely scenario to reconcile model predictions with observations is the existence of a currently unidentified additional source for RO2 radicals, in combination with an additional sink for HO2 radicals that does not form OH. The true uncertainty due to "missing reactivity" is apparent in parameters like chain length. We present a first attempt to calculate chain length rigorously i.e., we define two parameters that account for atmospheric complexity, and are based on (1) radical initiation, n(OH), and (2) radical termination, ω. We find very high values of n(OH) in the early morning are incompatible with our current understanding of ROx termination routes. We also observe missing reactivity in the rate of ozone production (P(O3)). For example, the integral amount of ozone produced could be under-predicted by a factor of two. We argue that this uncertainty is partly accounted for in lumped chemical codes that are optimized to predict ozone concentrations; however, these codes do not reflect the true uncertainty in oxidative capacity that is relevant to other aspects of air quality management, such as the formation of secondary organic aerosol (SOA). Our analysis highlights that apart from uncertainties in emissions, and meteorology, there is an additional major uncertainty in chemical mechanisms that affects our ability to predict ozone and SOA formation with confidence.

Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Aerosol Mass Spectrometer (AMS) and complementary instrumentation. Positive Matrix Factorization (PMF) of high resolution AMS spectra identified a biomass burning organic aerosol (BBOA) component, which includes several large plumes that appear to be from forest fires within the region. Here, we show that the AMS BBOA concentration at T0 correlates with fire counts in the vicinity of Mexico City and that most of the BBOA variability is captured when the FLEXPART model is used for the dispersion of fire emissions as estimated from satellite fire counts. The resulting FLEXPART fire impact factor (FIF) correlates well with the observed BBOA, acetonitrile (CH3CN), levoglucosan, and potassium, indicating that wildfires in the region surrounding Mexico City are the dominant source of BBOA at T0 during MILAGRO. The impact of distant BB sources such as the Yucatan is small during this period. All fire tracers are correlated, with BBOA and levoglucosan showing little background, acetonitrile having a well-known tropospheric background of ~100–150 pptv, and PM2.5 potassium having a background of ~160 ng m−3 (two-thirds of its average concentration), which does not appear to be related to BB sources. We define two high fire periods based on satellite fire counts and FLEXPART-predicted FIFs. We then compare these periods with a low fire period when the impact of regional fires is about a factor of 5 smaller. Fire tracers are very elevated in the high fire periods whereas tracers of urban pollution do not change between these periods. Dust is also elevated during the high BB period but this appears to be coincidental due to the drier conditions and not driven by direct dust emission from the fires. The AMS oxygenated organic aerosol (OA) factor (OOA, mostly secondary OA or SOA) does not show an increase during the fire periods or a correlation with fire counts, FLEXPART-predicted FIFs or fire tracers, indicating that it is dominated by urban and/or regional sources and not by the fires near the MCMA. A new 14C aerosol dataset is presented. Both this new and a previously published dataset of 14C analysis suggest a similar BBOA contribution as the AMS and chemical mass balance (CMB), resulting in 13% higher non-fossil carbon during the high vs. low regional fire periods. The new dataset has ~15% more fossil carbon on average than the previously published one, and possible reasons for this discrepancy are discussed. During the low regional fire period, 38% of organic carbon (OC) and 28% total carbon (TC) are from non-fossil sources, suggesting the importance of urban and regional non-fossil carbon sources other than the fires, such as food cooking and regional biogenic SOA. The ambient BBOA/ΔCH3CN ratio is much higher in the afternoon when the wildfires are most intense than during the rest of the day. Also, there are large differences in the contributions of the different OA components to the surface concentrations vs. the integrated column amounts. Both facts may explain some apparent disagreements between BB impacts estimated from afternoon aircraft flights vs. those from 24-h ground measurements. We show that by properly accounting for the non-BB sources of K, all of the BB PM estimates from MILAGRO can be reconciled. Overall, the fires from the region near the MCMA are estimated to contribute 15–23% of the OA and 7–9% of the fine PM at T0 during MILAGRO, and 2–3% of the fine PM as an annual average. The 2006 MCMA emissions inventory contains a substantially lower impact of the forest fire emissions, although a fraction of these emissions occur just outside of the MCMA inventory area.

We report the first measurements of particulate lead (Pb) from Aerodyne Aerosol Mass Spectrometers, which were deployed in and around Mexico City during the Megacity Initiative: Local and Global Research Observations (MILAGRO)/Mexico City Metropolitan Area 2006 (MCMA-2006) field campaigns. The high resolution mass spectrometer of one of the AMS instruments (HR-AMS) and the measured isotopic ratios unequivocally prove the detection of Pb in ambient particles. A substantial fraction of the lead evaporated slowly from the vaporizer of the instruments, which is indicative of species with low volatility at 600 °C. A model was developed in order to estimate the ambient particulate Pb entering the AMS from the signals in the "open" and the "closed" (or "background") mass spectrum modes of the AMS. The model suggests the presence of at least two lead fractions with ~25% of the Pb signal exhibiting rapid evaporation (1/e decay constant, τ3)2 particles, we estimated that the Pb ionization efficiency relative to nitrate (RIEPb) is 0.5. Comparison of time series of AMS Pb with other measurements carried out at the T0 supersite during MILAGRO (using Proton Induced X-ray Emission (PIXE), Inductively-Coupled Plasma Mass Spectrometry (ICP-MS) and single-particle counts from an Aerosol Time-of-Fight Mass Spectrometer (ATOFMS)) shows similar levels (for PIXE and ICP-MS) and substantial correlation. During part of the campaign, sampling at T0 was alternated every 10 min with an Aerosol Concentrator, which enabled the detection of signals for PbCl+ and PbS+ ions. PbS+ displays the signature of a slowly evaporating species, while PbCl+ appears to arise only from fast evaporation, which is likely due to the higher vapor pressure of the compounds generating PbCl+. This is consistent with the evaporation model results. Levels of particulate Pb measured at T0 were similar to previous studies in Mexico City. Pb shows a diurnal cycle with a maximum in the early morning, which is typical of primary urban pollutants. Pb shows correlation with Zn, consistent with previous studies, while the sources of Pb appear to be at least partially disjoint from those of particulate chloride. Back trajectory analysis of the T0 Pb data suggests the presence of sources inside the urban area SSW and N of T0, with different chemical forms of Pb being associated with different source locations. High signals due to particulate lead were also detected in the PEMEX site; again, no correlation between Pb and chloride plumes was observed, suggesting mostly different sources for both species.

The goal of this research is to quantify diesel- and gasoline-powered motor vehicle emissions within the Mexico City Metropolitan Area (MCMA) using on-road measurements captured by a mobile laboratory combined with positive matrix factorization (PMF) receptor modeling. During the MCMA-2006 ground-based component of the MILAGRO field campaign, the Aerodyne Mobile Laboratory (AML) measured many gaseous and particulate pollutants, including carbon dioxide, carbon monoxide (CO), nitrogen oxides (NOx), benzene, toluene, alkylated aromatics, formaldehyde, acetaldehyde, acetone, ammonia, particle number, fine particulate mass (PM2.5), and black carbon (BC). These serve as inputs to the receptor model, which is able to resolve three factors corresponding to gasoline engine exhaust, diesel engine exhaust, and the urban background. Using the source profiles, we calculate fuel-based emission factors for each type of exhaust. The MCMA's gasoline-powered vehicles are considerably dirtier, on average, than those in the US with respect to CO and aldehydes. Its diesel-powered vehicles have similar emission factors of NOx and higher emission factors of aldehydes, particle number, and BC. In the fleet sampled during AML driving, gasoline-powered vehicles are found to be responsible for 97% of total vehicular emissions of CO, 22% of NOx, 95–97% of each aromatic species, 72–85% of each carbonyl species, 74% of ammonia, negligible amounts of particle number, 26% of PM2.5, and 2% of BC; diesel-powered vehicles account for the balance. Because the mobile lab spent 17% of its time waiting at stoplights, the results may overemphasize idling conditions, possibly resulting in an underestimate of NOx and overestimate of CO emissions. On the other hand, estimates of the inventory that do not correctly account for emissions during idling are likely to produce bias in the opposite direction.The resulting fuel-based estimates of emissions are lower than in the official inventory for CO and NOx and higher for VOCs. For NOx, the fuel-based estimates are lower for gasoline-powered vehicles but higher for diesel-powered ones compared to the official inventory. While conclusions regarding the inventory should be interpreted with care because of the small sample size, 3.5 h of driving, the discrepancies with the official inventory agree with those reported in other studies.

The sensitivity of ozone production to precursor emissions was investigated under five different meteorological conditions in the Mexico City Metropolitan Area (MCMA) during the MCMA-2006/MILAGRO field campaign using the gridded photochemical model CAMx driven by observation-nudged WRF meteorology. Precursor emissions were constrained by the comprehensive data from the field campaign and the routine ambient air quality monitoring network. Simulated plume mixing and transport were examined by comparing with measurements from the G-1 aircraft during the campaign. The observed concentrations of ozone precursors and ozone were reasonably well reproduced by the model. The effects of reducing precursor emissions on urban ozone production were performed for three representative emission control scenarios. A 50% reduction in VOC emissions led to 7 to 22 ppb decrease in daily maximum ozone concentrations, while a 50% reduction in NOx emissions leads to 4 to 21 ppb increase, and 50% reductions in both NOx and VOC emission decrease the daily maximum ozone concentrations up to 10 ppb. These results along with a chemical indicator analysis using the chemical production ratios of H2O2 to HNO3 demonstrate that the MCMA urban core region is VOC-limited for all meteorological episodes, which is consistent with the results from MCMA-2003 field campaign; however the degree of the VOC-sensitivity is higher during MCMA-2006 due to lower VOCs, lower VOC reactivity and moderately higher NOx emissions. Ozone formation in the surrounding mountain/rural area is mostly NOx-limited, but can be VOC-limited, and the range of the NOx-limited or VOC-limited areas depends on meteorology.

A comparison of aromatic hydrocarbon measurements is reported for the CENICA supersite in the district of Iztapalapa during the Mexico City Metropolitan Area field experiment in April 2003 (MCMA 2003). Data from three different measurement methods were compared: a Proton Transfer Reaction Mass Spectrometer (PTR-MS), long path measurements using a UV Differential Optical Absorption Spectrometer (DOAS), and Gas Chromatography-Flame Ionization analysis (GC-FID) of canister samples. The principle focus was on the comparison between PTR-MS and DOAS data. Lab tests established that the PTR-MS and DOAS calibrations were consistent for a suite of aromatic compounds including benzene, toluene, p-xylene, ethylbenzene, 1,2,4-trimethylbenzene, phenol and styrene. The point sampling measurements by the PTR-MS and GC-FID showed good correlations (r=0.6), and were in reasonable agreement for toluene, C2-alkylbenzenes and C3-alkylbenzenes. The PTR-MS benzene data were consistently high, indicating interference from ethylbenzene fragmentation for the 145 Td drift field intensity used in the experiment. Correlations between the open-path data measured at 16-m height over a 860-m path length (retroreflector in 430 m distance), and the point measurements collected at 37-m sampling height were best for benzene (r=0.61), and reasonably good for toluene, C2-alkylbenzenes, naphthalene, styrene, cresols and phenol (r>0.5). There was good agreement between DOAS and PTR-MS measurements of benzene after correction for the PTR-MS ethylbenzene interference. Mixing ratios measured by DOAS were on average a factor of 1.7 times greater than the PTR-MS data for toluene, C2-alkylbenzenes, naphthalene and styrene. The level of agreement for the toluene data displayed a modest dependence on wind direction, establishing that spatial gradients – horizontal, vertical, or both – in toluene mixing ratios were significant, and up to a factor of 2 despite the fact that all measurements were conducted above roof level. Our analysis highlights a potential problem in defining a VOC sampling strategy that is meaningful for the comparison with photochemical transport models: meaningful measurements require a spatial fetch that is comparable to the grid cell size of models, which is typically a few 10 km2. Long-path DOAS measurements inherently average over a larger spatial scale than point measurements. The spatial representativeness can be further increased if observations are conducted outside the surface roughness sublayer, which might require measurements at altitudes as high as 10 s of metres above roof level.

In central Mexico during the spring of 2007 we measured the initial emissions of 12 gases and the aerosol speciation for elemental and organic carbon (EC, OC), anhydrosugars, Cl−, NO3−, and 20 metals from 10 cooking fires, four garbage fires, three brick making kilns, three charcoal making kilns, and two crop residue fires. Global biofuel use has been estimated at over 2600 Tg/y. With several simple case studies we show that cooking fires can be a major, or the major, source of several gases and fine particles in developing countries. Insulated cook stoves with chimneys were earlier shown to reduce indoor air pollution and the fuel use per cooking task. We confirm that they also reduce the emissions of VOC pollutants per mass of fuel burned by about half. We did not detect HCN emissions from cooking fires in Mexico or Africa. Thus, if regional source attribution is based on HCN emissions typical for other types of biomass burning (BB), then biofuel use and total BB will be underestimated in much of the developing world. This is also significant because cooking fires are not detected from space. We estimate that ~2000 Tg/y of garbage are generated globally and about half may be burned, making this a commonly overlooked major global source of emissions. We estimate a fine particle emission factor (EFPM2.5) for garbage burning of ~10.5±8.8 g/kg, which is in reasonable agreement with very limited previous work. We observe large HCl emission factors in the range 2–10 g/kg. Consideration of the Cl content of the global waste stream suggests that garbage burning may generate as much as 6–9 Tg/yr of HCl, which would make it a major source of this compound. HCl generated by garbage burning in dry environments may have a relatively greater atmospheric impact than HCl generated in humid areas. Garbage burning PM2.5 was found to contain levoglucosan and K in concentrations similar to those for biomass burning, so it could be a source of interference in some areas when using these tracers to estimate BB. Galactosan was the anhydrosugar most closely correlated with BB in this study. Fine particle antimony (Sb) shows initial promise as a garbage burning tracer and suggests that this source could contribute a significant amount of the PM2.5 in the Mexico City metropolitan area. The fuel consumption and emissions due to industrial biofuel use are difficult to characterize regionally. This is partly because of the diverse range of fuels used and the very small profit margins of typical micro-enterprises. Brick making kilns produced low total EFPM2.5 (~1.6 g/kg), but very high EC/OC ratios (6.72). Previous literature on brick kilns is scarce but does document some severe local impacts. Coupling data from Mexico, Brazil, and Zambia, we find that charcoal making kilns can exhibit an 8-fold increase in VOC/CO over their approximately one-week lifetime. Acetic acid emission factors for charcoal kilns were much higher in Mexico than elsewhere. Our dirt charcoal kiln EFPM2.5 emission factor was ~1.1 g/kg, which is lower than previous recommendations intended for all types of kilns. We speculate that some PM2.5 is scavenged in the walls of dirt kilns.

Large sulfur dioxide plumes were measured in the Mexico City Metropolitan Area (MCMA) during the MILAGRO field campaign. This paper seeks to identify the sources of these plumes and the meteorological processes that affect their dispersion in a complex mountain basin. Surface measurements of SO2 and winds are analysed in combination with radar wind profiler data to identify transport directions. Satellite retrievals of vertical SO2 columns from the Ozone Monitoring Instrument (OMI) reveal the dispersion from both the Tula industrial complex and the Popocatepetl volcano. Oversampling the OMI swath data to a fine grid (3 by 3 km) and averaging over the field campaign yielded a high resolution image of the average plume transport. Numerical simulations are used to identify possible transport scenarios. The analysis suggests that both Tula and Popocatepetl contribute to SO2 levels in the MCMA, sometimes on the same day due to strong vertical wind shear. During the field campaign, model estimates suggest that the volcano accounts for about one tenth of the SO2 in the MCMA, with a roughly equal split for the rest between urban sources and the Tula industrial complex. The evaluation of simulations with known sources and pollutants suggests that the combination of observations and meteorological models will be useful in identifying sources and transport processes of other plumes observed during MILAGRO.

Eddy covariance (EC) flux measurements of the atmosphere/surface exchange of gases over an urban area are a direct way to improve and evaluate emissions inventories, and, in turn, to better understand urban atmospheric chemistry and the role that cities play in regional and global chemical cycles. As part of the MCMA-2003 study, we demonstrated the feasibility of using eddy covariance techniques to measure fluxes of selected volatile organic compounds (VOCs) and CO2 from a residential district of Mexico City (Velasco et al., 2005a, b). During the MILAGRO/MCMA-2006 field campaign, a second flux measurement study was conducted in a different district of Mexico City to corroborate the 2003 flux measurements, to expand the number of species measured, and to obtain additional data for evaluation of the local emissions inventory. Fluxes of CO2 and olefins were measured by the conventional EC technique using an open path CO2 sensor and a Fast Isoprene Sensor calibrated with a propylene standard. In addition, fluxes of toluene, benzene, methanol and C2-benzenes were measured using a virtual disjunct EC method with a Proton Transfer Reaction Mass Spectrometer. The flux measurements were analyzed in terms of diurnal patterns and vehicular activity and were compared with the most recent gridded local emissions inventory. In both studies, the results showed that the urban surface of Mexico City is a net source of CO2 and VOCs with significant contributions from vehicular traffic. Evaporative emissions from commercial and other anthropogenic activities were significant sources of toluene and methanol. The results show that the emissions inventory is in reasonable agreement with measured olefin and CO2 fluxes, while C2-benzenes and toluene emissions from evaporative sources are overestimated in the inventory. It appears that methanol emissions from mobile sources occur, but are not reported in the mobile emissions inventory.

Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and complementary instrumentation. Mass concentrations, diurnal cycles, and size distributions of inorganic and organic species are similar to results from the CENICA supersite in April 2003 with organic aerosol (OA) comprising about half of the fine PM mass. Positive Matrix Factorization (PMF) analysis of the high resolution OA spectra identified three major components: chemically-reduced urban primary emissions (hydrocarbon-like OA, HOA), oxygenated OA (OOA, mostly secondary OA or SOA), and biomass burning OA (BBOA) that correlates with levoglucosan and acetonitrile. BBOA includes several very large plumes from regional fires and likely also some refuse burning. A fourth OA component is a small local nitrogen-containing reduced OA component (LOA) which accounts for 9% of the OA mass but one third of the organic nitrogen, likely as amines. OOA accounts for almost half of the OA on average, consistent with previous observations. OA apportionment results from PMF-AMS are compared to the PM2.5 chemical mass balance of organic molecular markers (CMB-OMM, from GC/MS analysis of filters). Results from both methods are overall consistent. Both assign the major components of OA to primary urban, biomass burning/woodsmoke, and secondary sources at similar magnitudes. The 2006 Mexico City emissions inventory underestimates the urban primary PM2.5 emissions by a factor of ~4, and it is ~16 times lower than afternoon concentrations when secondary species are included. Additionally, the forest fire contribution is at least an order-of-magnitude larger than in the inventory.

Mobile emissions represent a significant fraction of the total anthropogenic emissions burden in the Mexico City Metropolitan Area (MCMA) and, therefore, it is crucial to use top-down techniques informed by on-road exhaust measurements to evaluate and improve traditional bottom-up official emissions inventory (EI) for the city. We present the measurements of on-road fleet-average emission factors obtained using the Aerodyne mobile laboratory in the MCMA in March 2006 as part of the MILAGRO/MCMA-2006 field campaign. A comparison of our on-road emission measurements with those obtained in 2003 using essentially the same measurement techniques and analysis methods indicates that, in the three year span, NO emission factors remain within the measured variability ranges whereas emission factors of aldehydes and aromatics species were reduced for all sampled driving conditions. We use a top-down fuel-based approach to evaluate the mobile emissions from the gasoline fleet estimated in the bottom-up official 2006 MCMA mobile sources. Within the range of measurement uncertainties, we found probable slight overpredictions of mean EI estimates on the order of 20–28% for CO and 14–20% for NO. However, we identify a probable EI discrepancy of VOC mobile emissions between 1.4 and 1.9; although estimated benzene and toluene mobile emissions in the inventory seem to be well within the uncertainties of the corresponding emissions estimates. Aldehydes mobile emissions in the inventory, however, seem to be underpredicted by factors of 3 for HCHO and 2 for CH3CHO. Our on-road measurement-based estimate of annual emissions of organic mass from PM1 particles suggests a severe underprediction (larger than a factor of 4) of PM2.5 mobile emissions in the inventory. Analyses of ambient CO, NOx and CO/NOx concentration trends in the MCMA indicate that the early morning ambient CO/NOx ratio has decreased at a rate of about 1.9 ppm/ppm/year over the last two decades due to reductions in CO levels rather than by NOx. These trends, together with the analysis of fuel sales and fleet size, suggest that the relative contribution of diesel vehicles to overall NOx levels has increased over time in the city. Despite the impressive increase in the size of the vehicle fleet between 2000 and 2006, the early morning ambient concentrations of CO and NOx have not increased accordingly, probably due to the reported low removal rates of older vehicles, which do not have emissions control technologies, and partially due to the much lower emissions from newer gasoline vehicles. This indicates that an emission-based air quality improvement strategy targeting large reductions of emissions from mobile sources should be directed towards a significant increase of the removal rate of older, highly-polluting, vehicles.

Measurements of hydroxyl (OH) and hydroperoxy (HO2) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric ROx (OH + HO2 + RO2) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J-values and the concentrations of 97 chemical species. Several issues related to the ROx chemistry under polluted conditions are highlighted in this study: (i) Measured concentrations of both OH and HO2 were underpredicted during morning hours on a median campaign basis, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The model-predicted HO2/OH ratios underestimate the measurements for NO mixing ratios higher than 5 ppb, also consistent with previous urban field campaigns. This suggests that under high NOx conditions, the HO2 to OH propagation rate may be overestimated by the model or a process converting OH into HO2 may be missing from the chemical mechanism. On a daily basis (08:40 a.m.–06:40 p.m.), an analysis of the radical budget indicates that HONO photolysis, HCHO photolysis, O3-alkene reactions and dicarbonyls photolysis are the main radical sources. O3 photolysis contributes to less than 6% of the total radical production.

Accurate numerical simulations of the complex wind flows in the Mexico City Metropolitan Area (MCMA) can be an invaluable tool for interpreting the MILAGRO field campaign results. This paper uses three methods to evaluate numerical simulations of basin meteorology using the MM5 and WRF models: statistical comparisons with observations, "Concentration Field Analysis" (CFA) using measured air pollutant concentrations, and comparison of flow features using cluster analysis. CFA is shown to be a better indication of simulation quality than statistical metrics, and WRF simulations are shown to be an improvement on the MM5 ones. Comparisons with clusters identifies an under-representation of the drainage flows into the basin and an over-representation of wind shear in the boundary layer. Particle trajectories simulated with WRF-FLEXPART are then used to analyse the transport of the urban plume and show rapid venting and limited recirculation during MILAGRO. Lagrangian impacts were identified at the campaign supersites, and age spectra of the pollutants evaluated at those same sites. The evaluation presented in the paper show that mesoscale meteorological simulations are of sufficient accuracy to be useful for MILAGRO data analysis.

The impact of Mexico City (MCMA) emissions is examined by studying its effects on air quality, photochemistry, and on ozone production regimes by combining model products and aircraft observations from the MILAGRO experiment during March 2006. The modeled influence of MCMA emissions to enhancements in surface level NOx, CO, and O3 concentrations (10–30% increase) are confined to distances y, and MTBE mixing ratios over the Gulf of Mexico are linked to MCMA by source tagged tracers and sensitivity runs. This study shows that the "footprint" of MCMA on average is fairly local, with exception to reactive nitrogen, which can be transported long range in the form of PAN, acting as a reservoir and source of NOx with important regional ozone formation implications. The simulated effect of MCMA emissions of anthropogenic aerosol on photochemistry showed a maximum regional decrease of 40% in J[NO2→NO+O], and resulting in the reduction of ozone production by 5–10%. Observed ozone production efficiencies are evaluated as a function of distance from MCMA, and by modeled influence from MCMA. These tend to be much lower closer to MCMA, or in those points where modeled contribution from MCMA is large. This research shows that MCMA emissions do effect on regional air quality and photochemistry, both contributing large amounts of ozone and its precursors, but with caveat that aerosol concentrations hinder formation of ozone to its potential due to its reduction in photolysis rates.

Formaldehyde (HCHO) is a radical source that plays an important role in urban atmospheric chemistry and ozone formation. The Mexico City Metropolitan Area (MCMA) is characterized by high anthropogenic emissions of HCHO (primary HCHO), which together with photochemical production of HCHO from hydrocarbon oxidation (secondary HCHO), lead to high ambient HCHO levels. The CAMx chemical transport model was employed to evaluate the impact of primary HCHO on its ambient concentration, on the ROx radical budget, and on ozone (O3) formation in the MCMA. Important radical sources, including HCHO, HONO, and O3-olefin reactions, were constrained by measurements from routine observations of the local ambient air monitoring network and the MCMA-2003 field campaign. Primary HCHO was found not only to contribute significantly to the ambient HCHO concentration, but also to enhance the radical budget and O3 production in the urban atmosphere of the MCMA. Overall in the urban area, total daytime radical production is enhanced by up to 10% and peak O3 concentration by up to 8%; moreover primary HCHO tends to make O3 both production rates and ambient concentration peak half an hour earlier. While primary HCHO contributes predominantly to the ambient HCHO concentration between nighttime and morning rush hours, significant influence on the radical budget and O3 production starts early in the morning, peaks at mid-morning and is sustained until early afternoon.

Measurements of tropospheric hydroxyl (OH) and hydroperoxy (HO2) radicals were made during the MCMA (Mexico City Metropolitan Area) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These radicals were measured using a laser-induced fluorescence instrument developed at Indiana University. This new instrument takes advantage of the Fluorescence Assay by Gas Expansion technique (FAGE) together with direct excitation and detection of OH at 308 nm. HO2 is indirectly measured as OH by titration with NO inside the fluorescence cell. At this stage of development, IU-FAGE is capable of detecting 3.9×105 molecule/cm3 of both OH and HO2, with a signal to noise ratio of 1, an averaged laser power of 10-mW and an averaging time of 5-min. The calibration accuracies (1σ) are ±17% for OH and ±18% for HO2 using the water-vapor photolysis/O2 actinometry calibration technique. OH and HO2 concentrations were successfully measured at an urban site in Mexico City, with observed concentrations comparable to those measured in other polluted environments. Enhanced levels of OH and HO2 radicals were observed on several days between 09:30–11:00 a.m. and suggest an intense photochemistry during morning hours that may be due to elevated sources of HOx (OH+HO2) and a fast cycling between the radicals under the high NOx (NO+NO2) conditions of the MCMA.

In order to expand the currently limited understanding of atmospheric mercury source-receptor relationships in the Mexico City Metropolitan Area, real time measurements of atmospheric mercury were made at a downtown urban site, and a rural site on the outskirts of Mexico City, during March 2006. Numerous short-lived increases in particulate mercury (PHg) and reactive gaseous mercury (RGM) concentrations were observed at the urban site during the 17 day study, and less frequent increases in gaseous elemental mercury (GEM) concentrations were measured at both the urban and rural sites. The episodic increases observed were attributed to plume impacts from industrial point source emissions in and around Mexico City. Average concentrations and standard deviations measured during the study were as follows: i) urban site; PHg=187±300 pg m−3, RGM=62±64 pg m−3, GEM=7.2±4.8 ng m−3, and; ii) rural site; GEM=5.0±2.8 ng m−3. Several source regions of atmospheric mercury to the urban and rural sites were determined using Concentration Field Analysis, in which atmospheric mercury measurements were combined with back trajectory data to determine source regions. Only some source regions correlated to mercury emission sources listed in the Federal Pollutant Release and Transfer Register, leaving the rest unaccounted for. Contributions of anthropogenic mercury point sources in and around Mexico City to concentration averages measured at the urban site during the study were estimated to be: 93±3% of reactive mercury (PHg and RGM), and; 81±0.4% of GEM. Point source contributions to GEM measured at the rural site were 72±1%. GEM and reactive mercury (PHg+RGM) were not found to correlate with biomass burning at either of the measurement sites.

Mobile sources produce a significant fraction of the total anthropogenic emissions burden in large cities and have harmful effects on air quality at multiple spatial scales. Mobile emissions are intrinsically difficult to estimate due to the large number of parameters affecting the emissions variability within and across vehicles types. The MCMA-2003 Campaign in Mexico City has showed the utility of using a mobile laboratory to sample and characterize specific classes of motor vehicles to better quantify their emissions characteristics as a function of their driving cycles. The technique clearly identifies "high emitter" vehicles via individual exhaust plumes, and also provides fleet average emission rates. We have applied this technique to Mexicali during the Border Ozone Reduction and Air Quality Improvement Program (BORAQIP) for the Mexicali-Imperial Valley in 2005. We analyze the variability of measured emission ratios for emitted NOx, CO, specific VOCs, NH3, and some primary fine particle components and properties by deploying a mobile laboratory in roadside stationary sampling, chase and fleet average operational sampling modes. The measurements reflect various driving modes characteristic of the urban fleets. The observed variability for all measured gases and particle emission ratios is greater for the chase and roadside stationary sampling than for fleet average measurements. The fleet average sampling mode captured the effects of traffic conditions on the measured on-road emission ratios, allowing the use of fuel-based emission ratios to assess the validity of traditional "bottom-up" emissions inventories. Using the measured on-road emission ratios, we estimate CO and NOx mobile emissions of 175±62 and 10.4±1.3 metric tons/day, respectively, for the gasoline vehicle fleet in Mexicali. Comparisons with similar on-road emissions data from Mexico City indicated that fleet average NO emission ratios were around 20% higher in Mexicali than in Mexico City whereas HCHO and NH3 emission ratios were higher by a factor of 2 in Mexico City than in Mexicali. Acetaldehyde emission ratios did not differ significantly whereas selected aromatics VOCs emissions were similar or smaller in Mexicali. Nitrogen oxides emissions for on-road heavy-duty diesel truck (HDDT) were measured near Austin, Texas, as well as in both Mexican cities, with NOy emission ratios in Austin < Mexico City < Mexicali.

The emission characteristics of mobile sources in the Mexico City Metropolitan Area (MCMA) have changed significantly over the past few decades in response to emission control policies, advancements in vehicle technologies and improvements in fuel quality, among others. Along with these changes, concurrent non-linear changes in photochemical levels and criteria pollutants have been observed, providing a unique opportunity to understand the effects of perturbations of mobile emission levels on the photochemistry in the region using observational and modeling approaches. The observed historical trends of ozone (O3), carbon monoxide (CO) and nitrogen oxides (NOx) suggest that ozone production in the MCMA has changed from a low to a high VOC-sensitive regime over a period of 20 years. Comparison of the historical emission trends of CO, NOx and hydrocarbons derived from mobile-source emission studies in the MCMA from 1991 to 2006 with the trends of the concentrations of CO, NOx, and the CO/NOx ratio during peak traffic hours also indicates that fuel-based fleet average emission factors have significantly decreased for CO and VOCs during this period whereas NOx emission factors do not show any strong trend, effectively reducing the ambient VOC/NOx ratio. This study presents the results of model analyses on the sensitivity of the observed ozone levels to the estimated historical changes in its precursors. The model sensitivity analyses used a well-validated base case simulation of a high pollution episode in the MCMA with the mathematical Decoupled Direct Method (DDM) and the standard Brute Force Method (BFM) in the 3-D CAMx chemical transport model. The model reproduces adequately the observed historical trends and current photochemical levels. Comparison of the BFM and the DDM sensitivity techniques indicates that the model yields ozone values that increase linearly with NOx emission reductions and decrease linearly with VOC emission reductions only up to 30% from the base case. We further performed emissions perturbations from the gasoline fleet, diesel fleet, all mobile (gasoline plus diesel) and all emission sources (anthropogenic plus biogenic). The results suggest that although large ozone reductions obtained in the past were from changes in emissions from gasoline vehicles, currently significant benefits could be achieved with additional emission control policies directed to regulation of VOC emissions from diesel and area sources that are high emitters of alkenes, aromatics and aldehydes.

This study investigates the improvement of ozone (O3) simulations in the Mexico City basin using a three-dimensional variational (3DVAR) data assimilation system in meteorological simulations during the MCMA-2003 field measurement campaign. Meteorological simulations from the NCAR/Penn State mesoscale model (MM5) are used to drive photochemical simulations with the Comprehensive Air Quality Model with extensions (CAMx) during a four-day episode on 13–16 April 2003. The simulated wind circulation, temperature, and humidity fields in the basin with the data assimilation are found to be more consistent with the observations than those from the reference deterministic forecast. This leads to improved simulations of plume position, peak O3 timing, and peak O3 concentrations in the photochemical model. The improvement in O3 simulations is especially strong during the daytime. The results demonstrate the importance of applying data assimilation in meteorological simulations for air quality studies in the Mexico City basin.

Photochemistry in polluted atmospheres, particularly the formation of ozone (O3), depends not only on pollutant emissions, but also on meteorological conditions. In this study a 3-D chemical transport model CAMx was employed to investigate the O3 formation and its response to emissions reduction under three distinctively different meteorological conditions in the Mexico City Metropolitan Area during the MCMA-2003 field measurement campaign. The O3 formation characteristics and sensitivity to emissions change were found to be weakly dependent on the meteorological conditions. The evolution of O3 formation and its sensitivity to NOx and VOC levels were also examined along the photochemical plume transport pathway. The midday O3 production was found to undergo a rapid increase in a narrow range of chemical aging, while downwind plumes were characterized with low and constant O3 production, and plumes along their transport pathway were characterized by a combination of the two. The O3 formation was more VOC sensitive near the source area, but as the plume became chemically aged, O3 formation became progressively VOC insensitive and more NOx sensitive.

Continuous ambient measurements with aerosol time-of-flight mass spectrometry (ATOFMS) were made in an industrial/residential section in the northern part of Mexico City as part of the Mexico City Metropolitan Area-2006 campaign (MCMA-2006). Results are presented for the period of 15–27 March 2006. The submicron size mode contained both fresh and aged biomass burning, aged organic carbon (OC) mixed with nitrate and sulfate, elemental carbon (EC), nitrogen-organic carbon, industrial metal, and inorganic NaK inorganic particles. Overall, biomass burning and aged OC particle types comprised 40% and 31%, respectively, of the submicron mode. In contrast, the supermicron mode was dominated by inorganic NaK particle types (42%) which represented a mixture of dry lake bed dust and industrial NaK emissions mixed with soot. Additionally, aluminosilicate dust, transition metals, OC, and biomass burning contributed to the supermicron particles. Early morning periods (2–6 a.m.) showed high fractions of inorganic particles from industrial sources in the northeast, composed of internal mixtures of Pb, Zn, EC and Cl, representing up to 73% of the particles in the 0.2–3μm size range. A unique nitrogen-containing organic carbon (NOC) particle type, peaking in the early morning hours, was hypothesized to be amines from local industrial emissions based on the time series profile and back trajectory analysis. A strong dependence on wind speed and direction was observed in the single particle types that were present during different times of the day. The early morning (3:30–10 a.m.) showed the greatest contributions from industrial emissions. During mid to late mornings (7–11 a.m.), weak northerly winds were observed along with the most highly aged particles. Stronger winds from the south picked up in the late morning (after 11 a.m.), resulting in a decrease in the concentrations of the major aged particle types and an increase in the number fraction of fresh biomass particles. The highest wind speeds were correlated with the highest number fraction of fresh biomass particles (up to 76% of the submicron number fraction) when winds were coming directly from fires that were located south and southeast of the city based on MODIS fire count data. This study provides a unique clock of hourly changes in single particle mixing state and sources as a function of meteorology in Mexico City. These new findings indicate that biomass burning and industrial emissions can make significant contributions to primary particle loadings in Mexico City that are strongly coupled with local meteorology.

The evolution of ozone (O3) and 13 volatile organic compounds (VOCs) in the boundary layer of Mexico City was investigated during 2000–2004 to improve our understanding of the complex interactions between those trace gases and meteorological variables, and their influence on the air quality of a polluted megacity. A tethered balloon, fitted with electrochemical and meteorological sondes, was used to obtain detailed vertical profiles of O3 and meteorological parameters up to 1000 m above ground during part of the diurnal cycle (02:00–18:00 h). VOCs samples were collected up to 200 m by pumping air to canisters with a Teflon tube attached to the tether line. Overall, features of these profiles were found to be consistent with the formation of an upper residual layer during nighttime carrying over a fraction of the O3 from the previous day that contributes to the background concentration in surrounding regions. At the same time the release of heat stored in the urban surface forms a shallow unstable layer close to the ground, where the nocturnal emissions are trapped. After sunrise an O3 balance is determined by photochemical production, entrainment from the upper residual layer and destruction by titration with nitric oxide, delaying the ground-level O3 rise by 2 h. The subsequent evolution of the conductive boundary layer and vertical distribution of pollutants are discussed in terms of the energy balance, the presence of turbulence and the atmospheric stability.

As part of the Megacities Initiative: Local and Global Research Observations (MILAGRO) study in the Mexico City Metropolitan Area in March 2006, we measured particulate polycyclic aromatic hydrocarbons (PAHs) and other gaseous species and particulate properties, including light absorbing carbon or effective black carbon (BC), at six locations throughout the city. The measurements were intended to support the following objectives: to describe spatial and temporal patterns in PAH concentrations, to gain insight into sources and transformations of PAHs and BC, and to quantify the relationships between PAHs and other pollutants. Total particulate PAHs at the Instituto Mexicano del Petróleo (T0 supersite) located near downtown averaged 50 ng m−3, and aerosol active surface area averaged 80 mm2 m−3. PAHs were also measured on board the Aerodyne Mobile Laboratory, which visited six sites encompassing a mixture of different land uses and a range of ages of air parcels transported from the city core. A combination of analyses of time series, back trajectories, concentration fields, pollutant ratios, and correlation coefficients supports the concept of T0 as an urban source site, T1 as a receptor site with strong local sources, Pedregal and PEMEX as intermediate sites, Pico Tres Padres as a vertical receptor site, and Santa Ana as a downwind receptor site. Weak intersite correlations suggest that local sources are important and variable and that exposure to PAHs and BC cannot be represented by a single regional-scale value. The relationships between PAHs and other pollutants suggest that a variety of sources and ages of particles are present. Among carbon monoxide, nitrogen oxides (NOx), and carbon dioxide, particulate PAHs are most strongly correlated with NOx. Mexico City's PAH/BC mass ratio of 0.01 is similar to that found on a freeway loop in the Los Angeles area and approximately 8–30 times higher than that found in other cities. Evidence also suggests that primary combustion particles are rapidly coated by secondary aerosol in Mexico City. If so, their optical properties may change, and the lifetime of PAHs may be prolonged if the coating protects them against photodegradation or heterogeneous reactions.

The MILAGRO field campaign was a multi-agency international collaborative project to evaluate the regional impacts of the Mexico City air pollution plume as a means of understanding urban impacts on the global climate. Mexico City lies on an elevated plateau with mountains on three sides and has complex mountain and surface-driven wind flows. This paper asks what the wind transport was in the basin during the field campaign and how representative it was of the climatology. Surface meteorology and air quality data, radiosondes and radar wind profiler data were collected at sites in the basin and its vicinity. Cluster analysis was used to identify the dominant wind patterns both during the campaign and within the past 10 years of operational data from the warm dry season. Our analysis shows that March 2006 was representative of typical flow patterns experienced in the basin. Six episode types were identified for the basin-scale circulation providing a way of interpreting atmospheric chemistry and particulate data collected during the campaign. Decoupling between surface winds and those aloft had a strong influence in leading to convection and poor air quality episodes. Hourly characterisation of wind circulation during the MILAGRO, MCMA-2003 and IMADA field campaigns enables the comparisons of similar air pollution episodes and the evaluation of the impact of wind transport on measurements of the atmospheric chemistry taking place in the basin.

The surface reflectance ratio between the visible (VIS) and shortwave infrared (SWIR) radiation is an important quantity for the retrieval of the aerosol optical depth (τa) from the MODIS sensor data. Based on empirically determined VIS/SWIR ratios, MODIS τa retrieval uses the surface reflectance in the SWIR band (2.1 µm), where the interaction between solar radiation and the aerosol layer is small, to predict the visible reflectances in the blue (0.47 µm) and red (0.66 µm) bands. Therefore, accurate knowledge of the VIS/SWIR ratio is essential for achieving accurate retrieval of aerosol optical depth from MODIS. We analyzed the surface reflectance over some distinct surface covers in and around the Mexico City metropolitan area (MCMA) using MODIS radiances at 0.66 µm and 2.1 µm. The analysis was performed at 1.5 km×1.5 km spatial resolution. Also, ground-based AERONET sun-photometer data acquired in Mexico City from 2002 to 2005 were analyzed for aerosol depth and other aerosol optical properties. In addition, a network of hand-held sun-photometers deployed in Mexico City, as part of the MCMA-2006 Study during the MILAGRO Campaign, provided an unprecedented measurement of τa in 5 different sites well distributed in the city. We found that the average RED/SWIR ratio representative of the urbanized sites analyzed is 0.73±0.06 for scattering angles a averaged from sun-photometer measurements. The use of the new RED/SWIR ratio of 0.73 in the MODIS retrieval over Mexico City led to a significant improvement in the agreement between the MODIS and sun-photometer AOD results; with the slope, offset, and the correlation coefficient of the linear regression changing from (τaMODIS=0.91τa sun-photometer+0.33, R2=0.66) to (τaMODIS=0.96 τa sun-photometer−0.006, R2=0.87). Indeed, an underestimation of this ratio in urban areas lead to a significant overestimation of the AOD retrieved from satellite. Therefore, we strongly encourage similar analyses in other urban areas to enhance the development of a parameterization of the surface ratios accounting for urban heterogeneities.