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Transport is a key parameter in air quality research and plays a dominant role in the Colorado Northern Front Range Metropolitan Area (NFRMA), where terrain-induced flows and recirculation patterns can lead to vigorous mixing of different emission sources. To assess different transport processes and their connection to air quality in the NFRMA during the FRAPPÉ and DISCOVER-AQ campaigns in summer 2014, we use the Weather Research and Forecasting Model with inert tracers. Overall, the model represents well the measured winds, and the inert tracers are in good agreement with observations of comparable trace gas concentrations. The model tracers support the analysis of surface wind and ozone measurements and allow for the analysis of transport patterns and interactions of emissions. A main focus of this study is on characterizing pollution transport from the NFRMA to the mountains by mountain-valley flows and the potential for recirculating pollution back into the NFRMA. One such event on 12 August 2014 was well captured by the aircraft and is studied in more detail. The model represents the flow conditions and demonstrates that during upslope events, frequently, there is a separation of air masses that are heavily influenced by oil and gas emissions to the north and dominated by urban emissions to the south. This case study provides evidence that NFRMA pollution not only can impact the nearby foothills and mountain areas to the east of the Continental Divide but that pollution can 'spillover' into the valleys to the west of the Continental Divide. [ABSTRACT FROM AUTHOR]

We collected mercury observations as part of the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign over the southeastern US between 1 June and 15 July 2013. We use the GEOS-Chem chemical transport model to interpret these observations and place new constraints on bromine radical initiated mercury oxidation chemistry in the free troposphere. We find that the model reproduces the observed mean concentration of total atmospheric mercury (THg) (observations: 1:49±0:16 ngm-3, model: 1:51± 0:08 ngm-3), as well as the vertical profile of THg. The majority (65 %) of observations of oxidized mercury (Hg(II)) were below the instrument's detection limit (detection limit per flight: 58-228 pgm-3), consistent with model-calculated Hg(II) concentrations of 0-196 pgm-3. However, for observations above the detection limit we find that modeled Hg(II) concentrations are a factor of 3 too low (observations: 212±112 pgm-3, model: 67±44 pgm-3). The highest Hg(II) concentrations, 300-680 pgm-3, were observed in dry (RH<35 %) and clean air masses during two flights over Texas at 5-7 km altitude and off the North Carolina coast at 1-3 km. The GEOS-Chem model, back trajectories and observed chemical tracers for these air masses indicate subsidence and transport from the upper and middle troposphere of the subtropical anticyclones, where fast oxidation of elemental mercury (Hg(0)) to Hg(II) and lack of Hg(II) removal lead to efficient accumulation of Hg(II). We hypothesize that the most likely explanation for the model bias is a systematic underestimate of the Hg.(0)+Br reaction rate. We find that sensitivity simulations with tripled bromine radical concentrations or a faster oxidation rate constant for Hg.0/CBr, result in 1.5-2 times higher modeled Hg(II) concentrations and improved agreement with the observations. The modeled tropospheric lifetime of Hg(0) against oxidation to Hg(II) decreases from 5 months in the base simulation to 2.8-1.2 months in our sensitivity simulations. In order to maintain the modeled global burden of THg, we need to increase the in-cloud reduction of Hg(II), thus leading to faster chemical cycling between Hg(0) and Hg(II). Observations and model results for the NOMADSS campaign suggest that the subtropical anticyclones are significant global sources of Hg(II). [ABSTRACT FROM AUTHOR]

We collected mercury observations as part of the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign over the southeastern US between 1 June and 15 July 2013. We use the GEOS-Chem chemical transport model to interpret these observations and place new constraints on bromine radical initiated mercury oxidation chemistry in the free troposphere. We find that the model reproduces the observed mean concentration of total atmospheric mercury (THg) (observations: 1.49 ± 0.16 ng m-3, model: 1.51 ± 0.08 ng m-3), as well as the vertical profile of THg. The majority (65%) of observations of oxidized mercury (Hg(II)) are below the instrument's detection limit (detection limit per flight: 58-228 pg m-3), consistent with model-calculated Hg(II) concentrations of 0-196 ng m-3. However, for observations above the detection limit we find that modeled Hg(II) concentrations are a factor of 3 too low (observations: 212 ± 112 ng m-3, model: 67 ± 44 ng m-3). The highest Hg(II) concentrations, 300-680 pg m-3, were observed in dry (RH < 35%) and clean air masses during two flights over Texas at 5-7 km altitude and off the North Carolina coast at 1-3 km. The GEOS-Chem model, back trajectories and observed chemical tracers for these air masses indicate subsidence and transport from the upper and middle troposphere of the subtropical anticyclones, where fast oxidation of elemental mercury (Hg(0)) to Hg(II) and lack of Hg(II) removal lead to efficient accumulation of Hg(II). We hypothesize that the most likely explanation for the model bias is a systematic underestimate of the Hg(0)+Br reaction rate. We find that sensitivity simulations with tripled bromine radical concentrations or a faster oxidation rate constant for Hg(0)+Br, result in 1.5-2 times higher modeled Hg(II) concentrations and improved agreement with the observations. The modeled tropospheric lifetime of Hg(0) against oxidation to Hg(II) decreases from 5 months in the base simulation to 2.8-1.2 months in our sensitivity simulations. In order to maintain the modeled global burden of THg, we need to increase the in-cloud reduction of Hg(II), thus leading to faster chemical cycling between Hg(0) and Hg(II). Observations and model results for the NOMADSS campaign suggest that the subtropical anticyclones are significant global sources of Hg(II). [ABSTRACT FROM AUTHOR]

Arctic boundary layer nitrogen oxides (NOx DNO2CNO) are naturally produced in and released from the sunlit snowpack and range between 10 to 100 pptv in the remote background surface layer air. These nitrogen oxides have significant effects on the partitioning and cycling of reactive radicals such as halogens and HOx (OHCHO2/. However, little is known about the impacts of local anthropogenic NOx emission sources on gas-phase halogen chemistry in the Arctic, and this is important because these emissions can induce large variability in ambient NOx and thus local chemistry. In this study, a zero-dimensional photochemical kinetics model was used to investigate the influence of NOx on the unique springtime halogen and HOx chemistry in the Arctic. Trace gas measurements obtained during the 2009 OASIS (Ocean - Atmosphere - Sea Ice - Snowpack) field campaign at Barrow, AK were used to constrain many model inputs. We find that elevated NOx significantly impedes gas-phase halogen radical-based depletion of ozone, through the production of a variety of reservoir species, including HNO3, HO2NO2, peroxyacetyl nitrate (PAN), BrNO2, ClNO2 and reductions in BrO and HOBr. The effective removal of BrO by anthropogenic NOx was directly observed from measurements conducted near Prudhoe Bay, AK during the 2012 Bromine, Ozone, and Mercury Experiment (BROMEX). Thus, while changes in snow-covered sea ice attributable to climate change may alter the availability of molecular halogens for ozone and Hg depletion, predicting the impact of climate change on polar atmospheric chemistry is complex and must take into account the simultaneous impact of changes in the distribution and intensity of anthropogenic combustion sources. This is especially true for the Arctic, where NOx emissions are expected to increase because of increasing oil and gas extraction and shipping activities. [ABSTRACT FROM AUTHOR]

The springtime depletion of tropospheric ozone in the Arctic is known to be caused by active halogen photochemistry resulting from halogen atom precursors emitted from snow, ice, or aerosol surfaces. The role of bromine in driving ozone depletion events (ODEs) has been generally accepted, but much less is known about the role of chlorine radicals in ozone depletion chemistry. While the potential impact of iodine in the High Arctic is more uncertain, there have been indications of active iodine chemistry through observed enhancements in filterable iodide, probable detection of tropospheric IO, and recently, observation of snowpack photochemical production of I2. Despite decades of research, significant uncertainty remains regarding the chemical mechanisms associated with the bromine-catalyzed depletion of ozone, as well as the complex interactions that occur in the polar boundary layer due to halogen chemistry. To investigate this, we developed a zero-dimensional photochemical model, constrained with measurements from the 2009 OASIS field campaign in Barrow, Alaska. We simulated a 7-day period during late March that included a full ozone depletion event lasting 3 days and subsequent ozone recovery to study the interactions of halogen radicals under these different conditions. In addition, the effects of iodine added to our Base Model were investigated. While bromine atoms were primarily responsible for ODEs, chlorine and iodine were found to enhance the depletion rates and iodine was found to be more efficient per atom at depleting ozone than Br. The interaction between chlorine and bromine is complex, as the presence of chlorine can increase the recycling and production of Br atoms, while also increasing reactive bromine sinks under certain conditions. Chlorine chemistry was also found to have significant impacts on both HO2 and RO2, with organic compounds serving as the primary reaction partner for Cl atoms. The results of this work highlight the need for future studies on the production mechanisms of Br2 and Cl2, as well as on the potential impact of iodine in the High Arctic. [ABSTRACT FROM AUTHOR]

Organic peroxy (RO2) and hydroperoxy (HO2) radicals are key intermediates in the photochemical processes that generate ozone, secondary organic aerosol and reactive nitrogen reservoirs throughout the troposphere. In regions with ample biogenic hydrocarbons, the richness and complexity of peroxy radical chemistry presents a significant challenge to current-generation models, especially given the scarcity of measurements in such environments. We present peroxy radical observations acquired within a ponderosa pine forest during the summer 2010 Bio-hydroatmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen - Rocky Mountain Organic Carbon Study (BEACHON-ROCS). Total peroxy radical mixing ratios reach as high as 180 pptv (parts per trillion by volume) and are among the highest yet recorded. Using the comprehensive measurement suite to constrain a near-explicit 0-D box model, we investigate the sources, sinks and distribution of peroxy radicals below the forest canopy. The base chemical mechanism underestimates total peroxy radicals by as much as a factor of 3. Since primary reaction partners for peroxy radicals are either measured (NO) or underpredicted (HO2 and RO2, i.e., self-reaction), missing sources are the most likely explanation for this result. A close comparison of model output with observations reveals at least two distinct source signatures. The first missing source, characterized by a sharp midday maximum and a strong dependence on solar radiation, is consistent with photolytic production of HO2. The diel profile of the second missing source peaks in the afternoon and suggests a process that generates RO2 independently of sun-driven photochemistry, such as ozonolysis of reactive hydrocarbons. The maximum magnitudes of these missing sources (~120 and 50 pptv min-1, respectively) are consistent with previous observations alluding to unexpectedly intense oxidation within forests. We conclude that a similar mechanism may underlie many such observations. [ABSTRACT FROM AUTHOR]

Organic peroxy (RO2) and hydroperoxy (HO2) radicals are key intermediates in the photochemical processes that generate ozone, secondary organic aerosol and reactive nitrogen reservoirs throughout the troposphere. In regions with ample biogenic hydrocarbons, the richness and complexity of peroxy radical chemistry presents a significant challenge to current-generation models, especially given the scarcity of measurements in such environments. We present peroxy radical observations acquired within a Ponderosa pine forest during the summer 2010 Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen -- Rocky Mountain Organic Carbon Study (BEACHON-ROCS). Total peroxy radical mixing ratios reach as high as 180 pptv and are among the highest yet recorded. Using the comprehensive measurement suite to constrain a near-explicit 0-D box model, we investigate the sources, sinks and distribution of peroxy radicals below the forest canopy. The base chemical mechanism underestimates total peroxy radicals by as much as a factor of 3. Since primary reaction partners for peroxy radicals are either measured (NO) or under-predicted (HO2 and RO2, i.e. self-reaction), missing sources are the most likely explanation for this result. A close comparison of model output with observations reveals at least two distinct source signatures. The first missing source, characterized by a sharp midday maximum and a strong dependence on solar radiation, is consistent with photolytic production of HO2. The diel profile of the second missing source peaks in the afternoon and suggests a process that generates RO2 independently of sun-driven photochemistry, such as ozonolysis of reactive hydrocarbons. The maximum magnitudes of these missing sources (~ 120 and 50 pptv min-1, respectively) are consistent with previous observations alluding to unexpectedly intense oxidation within forests. We conclude that a similar mechanism may underlie many such observations. [ABSTRACT FROM AUTHOR]

Volatile organic compound (VOC) mixing ratios measured by five independent instruments are compared at a forested site dominated by ponderosa pine (Pinus Ponderosa) during the BEACHON-ROCS field study in summer 2010. The instruments included a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF-MS), a Proton Transfer Reaction Quadrupole Mass Spectrometer (PTR-MS), a Fast Online Gas-Chromatograph coupled to a Mass Spectrometer (GC/MS; TOGA), a Thermal Dissociation Chemical Ionization Mass Spectrometer (PAN-CIMS) and a Fiber Laser-Induced Fluorescence Instrument (FILIF). The species discussed in this comparison include the most important biogenic VOCs and a selected suite of oxygenated VOCs that are thought to dominate the VOC reactivity at this particular site as well as typical anthropogenic VOCs that showed low mixing ratios at this site. Good agreement was observed for methanol, sum of the oxygenated hemiterpene 2- methyl-3-buten-2-ol (MBO) and the hemiterpene isoprene, acetaldehyde, sum of acetone and propanal, benzene and the sum of methyl ethyl ketone (MEK) and butanal. Measurements of the above VOCs conducted by different instruments agree within 20 %. The ability to differentiate the presence of toluene and cymene by PTR-TOF-MS is tested based on a comparison with GC-MS measurements, suggesting a study-average relative contribution of 74% for toluene and 26% for cymene. Similarly, 2-hydroxy-2-methylpropanal (HMPR) is found to interfere with the sum of methyl vinyl ketone and methacrolein (MVK +MAC) using PTR-(TOF)- MS at this site. A study-average relative contribution of 85% for MVK+MAC and 15% for HMPR was determined. The sum of monoterpenes measured by PTR-MS and PTR-TOFMS was generally 20-25% higher than the sum of speciated monoterpenes measured by TOGA, which included - pinene, -pinene, camphene, carene, myrcene, limonene, cineole as well as other terpenes. However, this difference is consistent throughout the study, and likely points to an offset in calibration, rather than a difference in the ability to measure the sum of terpenes. The contribution of isoprene relative to MBO inferred from PTR-MS and PTR-TOF-MS was smaller than 12% while GC-MS data suggested an average of 21% of isoprene relative to MBO. This comparison demonstrates that the current capability of VOC measurements to account for OH reactivity associated with the measured VOCs is within 20 %. [ABSTRACT FROM AUTHOR]

Volatile organic compound (VOC) mixing ratios measured by five independent instruments are compared at a forested site dominated by ponderosa pine (Pinus Ponderosa) during the BEACHON-ROCS field study in summer 2010. The instruments included a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF-MS), a Proton Transfer Reaction Quadrupole Mass Spectrometer (PTR-MS), a Fast Online Gas-Chromatograph coupled to a Mass Spectrometer (GC/MS; TOGA), a Thermal Dissociation Chemical Ionization Mass Spectrometer (PAN-CIMS) and a Fiber Laser- Induced Fluorescence Instrument (FILIF). The species discussed in this comparison include the most important biogenic VOCs and a selected suite of oxygenated VOCs that are thought to dominate the VOC reactivity at this particular site as well as typical anthropogenic VOCs that showed low mixing ratios at this site. Good agreement was observed for methanol, the sum of the oxygenated hemiterpene 2-methyl-3-buten-2-ol (MBO) and the hemiterpene isoprene, acetaldehyde, the sum of acetone and propanal, benzene and the sum of methyl ethyl ketone (MEK) and butanal. Measurements of the above VOCs conducted by different instruments agree within 20%. The ability to differentiate the presence of toluene and cymene by PTR-TOF-MS is tested based on a comparison with GC-MS measurements, suggesting a study-average relative contribution of 74% for toluene and 26% for cymene. Similarly, 2-hydroxy-2-methylpropanal (HMPR) is found to interfere with the sum of methyl vinyl ketone and methacrolein (MVK+MAC) using PTR-(TOF)-MS at this site. A study-average relative contribution of 85% for MVK+MAC and 15% for HMPR was determined. The sum of monoterpenes measured by PTR-MS and PTR-TOF-MS was generally 20-25% higher than the sum of speciated monoterpenes measured by TOGA, which included α-pinene, β-pinene, camphene, carene, myrcene, limonene, cineole as well as other terpenes. However, this difference is consistent throughout the study, and likely points to an offset in calibration, rather than a difference in the ability to measure the sum of terpenes. The contribution of isoprene relative to MBO inferred from PTR-MS and PTR-TOF-MS was smaller than 12% while GC-MS data suggested an average of 21% of isoprene relative to MBO. This comparison demonstrates that the current capability of VOC measurements to account for OH reactivity associated with the measured VOCs is within 20%. [ABSTRACT FROM AUTHOR]

During the second Texas Air Quality Study 2006 (TexAQS II), a full range of pollutants was measured by aircraft in eastern Texas during successive transects of power plant plumes (PPPs). A regional photochemical model is applied to simulate the physical and chemical evolution of the plumes. The observations reveal that SO2 and NOy were rapidly removed from PPPs on a cloudy day but not on the cloud-free days, indicating efficient aqueous processing of these compounds in clouds. The model reasonably represents observed NOx oxidation and PAN formation in the plumes, but fails to capture the rapid loss of SO2 (0.37 h-1) and NOy (0.24 h-1) in some plumes on the cloudy day. Adjustments to the cloud liquid water content (QC) and the default metal concentrations in the cloud module could explain some of the SO2 loss. However, NOy in the model was insensitive to QC. These findings highlight cloud processing as a major challenge to atmospheric models. Model-based estimates of ozone production efficiency (OPE) in PPPs are 20-50% lower than observation-based estimates for the cloudy day. [ABSTRACT FROM AUTHOR]

We report the first observations of formaldehyde (HCHO) flux measured via eddy covariance, as well as HCHO concentrations and gradients, as observed by the Madison Fiber Laser-Induced Fluorescence Instrument during the BEACHON-ROCS 2010 campaign in a rural, Ponderosa Pine forest northwest of Colorado Springs, CO. A median noon upward flux of ~80 µg m-2 h-1 (~24 pptv m s-1) was observed with a noon range of 37 to 131 µg m-2 h-1. Enclosure experiments were performed to determine the HCHO branch (3.5 µg m-2 h-1) and soil (7.3 µg m-2 h-1) direct emission rates in the canopy. A zero-dimensional canopy box model, used to determine the apportionment of HCHO source and sink contributions to the flux, underpredicted the observed HCHO flux by a factor of 6. Simulated increases in concentrations of species similar to monoterpenes resulted in poor agreement with measurements, while simulated increases in direct HCHO emissions and/or concentrations of species similar to 2-methyl-3-buten-2-ol best improved model/measurement agreement. Given the typical diurnal variability of these BVOC emissions and direct HCHO emissions, this suggests that the source of the missing flux is a process with both a strong temperature and radiation dependence. [ABSTRACT FROM AUTHOR]

Transport and chemical transformation of well-defined New York City (NYC) urban plumes over the North Atlantic Ocean were studied using aircraft measurements collected on 20-21 July 2004 during the ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) field campaign and WRF-Chem (Weather Research and Forecasting-Chemistry) model simulations. The strong NYC urban plumes were characterized by carbon monoxide (CO) mixing ratios of 350-400 parts per billion by volume (ppbv) and ozone (O3) levels of about 100 ppbv near New York City on 20 July in the WP-3D in-situ and DC-3 lidar aircraft measurements. On 21 July, the two aircraft captured strong urban plumes with about 350 ppbv CO and over 150 ppbv O3 (~160 ppbv maximum) about 600 km downwind of NYC over the North Atlantic Ocean. The measured urban plumes extended vertically up to about 2 km near New York City, but shrank to 1-.5 km over the stable marine boundary layer (MBL) over the North Atlantic Ocean. The WRF-Chem model reproduced ozone formation processes, chemical characteristics, and meteorology of the measured urban plumes near New York City (20 July) and in the far downwind region over the North Atlantic Ocean (21 July). The quasi-Lagrangian analysis of transport and chemical transformation of the simulated NYC urban plumes using WRF-Chem results showed that the pollutants can be efficiently transported in (isentropic) layers in the lower atmosphere (<2-3 km) over the North Atlantic Ocean while maintaining a dynamic vertical decoupling by cessation of turbulence in the stable MBL. The O3 mixing ratio in the NYC urban plumes remained at 80-90 ppbv during nocturnal transport over the stable MBL, then grew to over 100 ppbv by daytime oxidation of nitrogen oxides (NOx =NO+NO2) with mixing ratios on the order of 1 ppbv. Efficient transport of reactive nitrogen species (NOy), specifically nitric acid (HNO3), was confirmed through the comparison of the CO/NOy ratio in photochemically fresh and aged NYC plumes, implying the possibility of long-range transport of O3 over the stable MBL over the North Atlantic Ocean in association with NOx regeneration mechanism. The impact of chemical initial and boundary conditions (IC/BCs) on modelled O3 urban plumes was investigated in terms of the background O3 level and the vertical structure of the urban plumes. Simulations with dynamic ("time-variant") chemical IC/BCs enhanced the O3 level by 2-12 ppbv on average in the atmospheric layer below 3 km, showing better agreement with the observed NYC plumes and biomass-burning plumes than the simulation with prescribed static IC/BCs. The simulation including MOZART-4 chemical IC/BCs and Alaskan/Canadian wildfire emissions compared better to the observed O3 profiles in the upper atmospheric layer (>~3 km) than models that only accounted for North American anthropogenic/biogenic and wildfire contributions to background ozone. The comparison between models and observations show that chemical IC/BCs must be properly specified to achieve accurate model results. [ABSTRACT FROM AUTHOR]

During the second Texas Air Quality Study 2006 (TexAQS II), a full range of pollutants was measured by aircraft in eastern Texas during successive transects of power plant plumes (PPPs). A regional photochemical model is applied to simulate the physical and chemical evolution of the plumes. The observations reveal that SO2 and NOy were rapidly removed from PPPs on a cloudy day but not on the cloud-free days, indicating efficient aqueous processing of these compounds in clouds. The model reasonably represents observed NOx oxidation and PAN formation in the plumes, but fails to capture the rapid loss of SO2 (0.37 h-1) and NOy (0.24 h-1) in some plumes on the cloudy day. Adjustments to the cloud liquid water content (QC) and the default metal concentrations in the cloud module could explain some of the SO2 loss. However, NOy in the model was insensitive to QC. These findings highlight cloud processing as a major challenge to atmospheric models. Model-based estimates of ozone production efficiency (OPE) in PPPs are 20-50% lower than observation-based estimates. Possible explanations for this discrepancy include the observed rapid NOy loss which biases 15 high some observation-based OPE estimates, and the model\s under-prediction of isoprene emissions. [ABSTRACT FROM AUTHOR]

This paper presents a detailed laboratory characterization of a thermal dissociation chemical ionization mass spectrometer (TD-CIMS) for the atmospheric measurement of Peroxyacetyl nitrate (PAN) and its homologues (PANs). PANs are efficiently dissociated in a heated inlet and the resulting peroxy acyl radicals are reacted with I- ions in a flow tube. The mass spectrometer detects the corresponding carboxylate ions. PAN, peroxypropionyl nitrate (PPN), peroxyisobutyryl nitrate (PiBN), peroxy-n-butyryl nitrate (PnBN), peroxyacryloyl nitrate (APAN), peroxycrotonyl nitrates (CPAN) and peroxymethacryloyl nitrate (MPAN) were cross-calibrated with both a dual channel GC/ECD and a total odd-nitrogen (NOy) instrument for the NCAR TD-CIMS' typical aircraft operation conditions. In addition, the instrument sensitivity to a number of more exotic PANs (peroxyhydroxyacetyl nitrate, methoxyformyl peroxynitrate, and peroxybenzoyl nitrate) was evaluated qualitatively by comparisons with a long-path FTIR instrument. The sensitivity for PPN is slightly higher than that of PAN. Larger aliphatic and olefinic PAN compounds generally showed lower sensitivities. We postulate that these differences are owing to secondary reactions in the thermal decomposition region, which either reduce the yield of peroxy acyl radicals or cause losses of these radicals through intramolecular decomposition. The relative importance of these secondary reactions varies considerably between different PAN species. Results also indicate that the reaction of the larger peroxy acyl radicals with the ion-water cluster, I- (H2O)n proceeds about an order of magnitude faster than with I- alone, as has been observed for peroxy acetyl radicals. Sensitivity variations among the individual PAN species at very low water vapor were observed. The results call for careful evaluation of each PAN species to be measured and for each desired operating condition of a TD-CIMS instrument. [ABSTRACT FROM AUTHOR]

The article provides an overview of 140 intercomparisons of collected data and record of measurement consistency presented during the Intercontinental Chemical Transport Experiment-B (INTEX-B) airborne field campaign in North America. It mentions its aim towards the investigation on pollution transport and transformation over the continent. It also notes its coordination with the Mega-city Initiative: Local and Global Research Observations (MILAGRO) program.

We interpret observations from the Intercontinental Chemical Transport Experiment, Phase B (INTEX-B) in spring 2006 using a global chemical transport model (GEOSChem) to evaluate sensitivities of the free troposphere above the North Pacific Ocean and North America to Asian anthropogenic emissions. We develop a method to use satellite observations of tropospheric NO2 columns to provide timely estimates of trends in NOx emissions. NOx emissions increased by 33% for China and 29% for East Asia from 2003 to 2006. We examine measurements from three aircraft platforms from the INTEX-B campaign, including a Canadian Cessna taking vertical profiles of ozone near Whistler Peak. The contribution to the mean simulated ozone profiles over Whistler below 5.5 km is at least 7.2 ppbv for Asian anthropogenic emissions and at least 3.5 ppbv for global lightning NOx emissions. Tropospheric ozone columns from OMI exhibit a broad Asian outflow plume across the Pacific, which is reproduced by simulation. Mean modelled sensitivities of Pacific (30° N-60°N) tropospheric ozone columns are at least 4.6 DU for Asian anthropogenic emissions and at least 3.3 DU for lightning, as determined by simulations excluding either source. Enhancements of ozone over Canada from Asian anthropogenic emissions reflect a combination of trans-Pacific transport of ozone produced over Asia, and ozone produced in the eastern Pacific through decomposition of peroxyacetyl nitrates (PANs). A sensitivity study decoupling PANs globally from the model's chemical mechanism establishes that PANs increase ozone production by removing NOx from regions of low ozone production efficiency (OPE) and injecting it into regions with higher OPE, resulting in a global increase in ozone production by 2% in spring 2006. PANs contribute up to 4 ppbv to surface springtime ozone concentrations in western Canada. Ozone production due to PAN transport is greatest in the eastern Pacific; commonly occurring transport patterns advect this ozone northeastward into Canada. Transport events observed by the aircraft confirm that polluted airmasses were advected in this way. [ABSTRACT FROM AUTHOR]

One of the major objectives of the Megacities Initiative: Local And Global Research Observations (MILAGRO-2006) campaign was to investigate the longrange transport of polluted Mexico City Metropolitan Area (MCMA) outflow and determine its downwind impacts on air quality and climate. Six research aircraft, including the National Center for Atmospheric Research (NCAR) C-130, made extensive chemical, aerosol, and radiation measurements above MCMA and more than 1000 km downwind in order to characterize the evolution of the outflow as it aged and dispersed over the Mesa Alta, Sierra Madre Oriental, Coastal Plain, and Gulf of Mexico. As part of this effort, free-floating Controlled-Meteorological (CMET) balloons, commanded to change altitude via satellite, made repeated profile measurements of winds and state variables within the advecting outflow. In this paper, we present an analysis of the data from two CMET balloons that were launched near Mexico City on the afternoon of 18 March 2006 and floated downwind with the MCMA pollution for nearly 30 h. The repeating profile measurements show the evolving structure of the outflow in considerable detail: its stability and stratification, interaction with other air masses, mixing episodes, and dispersion into the regional background. Air parcel trajectories, computed directly from the balloon wind profiles, show three transport pathways on 18-19 March: (a) high-altitude advection of the top of the MCMA mixed layer, (b) mid-level outflow over the Sierra Madre Oriental followed by decoupling and isolated transport over the Gulf of Mexico, and (c) low-level outflow with entrainment into a cleaner northwesterly jet above the Coastal Plain. The C-130 aircraft intercepted the balloon-based trajectories three times on 19 March, once along each of these pathways; in all three cases, peaks in urban tracer concentrations and LIDAR backscatter are consistent with MCMA pollution. In comparison with the transport models used in the campaign, the balloon-based trajectories appear to shear the outflow far more uniformly and decouple it from the surface, thus forming a thin but expansive polluted layer over the Gulf of Mexico that is well aligned with the aircraft observations. These results provide critical context for the extensive aircraft measurements made during the 18-19 March MCMA outflow event and may have broader implications for modelling and understanding long-range transport. [ABSTRACT FROM AUTHOR]

The article presents a study on the peroxyacetyl nitrate (PAN) measurement and detection via laser-induced fluorescence (LIF) technique, developed at the University of California, Berkeley. It says that PAN was formed in the atmosphere from the reaction of peroxy radicals and can be a temporary reservoirs for NOx (=NO+NO2). It also says that thermal dissociation-chemical ionization mass spectrometric (TD-CIMS) techniques are implemented for field measurements in the past few years.

The article offers information on the 140 intercomparisons of data collected during U.S. National Aeronautics and Space Administration's (NASA) Intercontinental Chemical Transport Experiment-B (INTEX-B) mission. It says that the INTEX-B, part of the NASA-led INTEX-NA mission, was the second major airborne field mission aiming to investigate the transport and transportation of pollution over the North American continent. It notes that it operates in coordination with MILAGRO and IMPEX missions.

We construct a global atmospheric budget for acetaldehyde using a 3-D model of atmospheric chemistry (GEOS-Chem), and use an ensemble of observations to evaluate present understanding of its sources and sinks. Hydrocarbon oxidation provides the largest acetaldehyde source in the model (128 Tg a-1, a factor of 4 greater than the previous estimate), with alkanes, alkenes, and ethanol the main precursors. There is also a minor source from isoprene oxidation. We use an updated chemical mechanism for GEOSChem, and photochemical acetaldehyde yields are consistent with the Master Chemical Mechanism. We present a new approach to quantifying the acetaldehyde air-sea flux based on the global distribution of light absorption due to colored dissolved organic matter (CDOM) derived from satellite ocean color observations. The resulting net ocean emission is 57 Tg a-1, the second largest global source of acetaldehyde. A key uncertainty is the acetaldehyde turnover time in the ocean mixed layer, with quantitative model evaluation over the ocean complicated by known measurement artifacts in clean air. Simulated concentrations in surface air over the ocean generally agree well with aircraft measurements, though the model tends to overestimate the vertical gradient. PAN:NOx ratios are well-simulated in the marine boundary layer, providing some support for the modeled ocean source. We introduce the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) for acetaldehyde and ethanol and use it to quantify their net flux from living terrestrial plants. Including emissions from decaying plants the total direct acetaldehyde source from the land biosphere is 23 Tg a-1. Other terrestrial acetaldehyde sources include biomass burning (3 Tg a-1) and anthropogenic emissions (2 Tg a-1). Simulated concentrations in the continental boundary layer are generally unbiased and capture the spatial gradients seen in observations over North America, Europe, and tropical South America. However, the model underestimates acetaldehyde levels in urban outflow, suggesting a missing source in polluted air. Ubiquitous high measured concentrations in the free troposphere are not captured by the model, and based on present understanding are not consistent with concurrent measurements of PAN and NOx: we find no compelling evidence for a widespread missing acetaldehyde source in the free troposphere. We estimate the current US source of ethanol and acetaldehyde (primary + secondary) at 1.3 Tg a-1 and 7.8 Tg a-1, approximately 60% and 480% of the corresponding increases expected for a national transition from gasoline to ethanol fuel. [ABSTRACT FROM AUTHOR]

The volatile organic compound (VOC) distribution in the Mexico City Metropolitan Area (MCMA) and its evolution as it is uplifted and transported out of the MCMA basin was studied during the 2006 MILAGRO/MIRAGEMex field campaign. The results show that in the morning hours in the city center, the VOC distribution is dominated by non-methane hydrocarbons (NMHCs) but with a substantial contribution from oxygenated volatile organic compounds (OVOCs), predominantly from primary emissions. Alkanes account for a large part of the NMHC distribution in terms of mixing ratios. In terms of reactivity, NMHCs also dominate overall, especially in the morning hours. However, in the afternoon, as the boundary layer lifts and air is mixed and aged within the basin, the distribution changes as secondary products are formed. The WRF-Chem (Weather Research and Forecasting with Chemistry) model and MOZART (Model for Ozone and Related chemical Tracers) were able to approximate the observed MCMA daytime patterns and absolute values of the VOC OH reactivity. The MOZART model is also in agreement with observations showing that NMHCs dominate the reactivity distribution except in the afternoon hours. The WRF-Chem and MOZART models showed higher reactivity than the experimental data during the nighttime cycle, perhaps indicating problems with the modeled nighttime boundary layer height. A northeast transport event was studied in which air originating in the MCMA was intercepted aloft with the Department of Energy (DOE) G1 on 18 March and downwind with the National Center for Atmospheric Research (NCAR) C130 one day later on 19 March. A number of identical species measured aboard each aircraft gave insight into the chemical evolution of the plume as it aged and was transported as far as 1000 km downwind; ozone was shown to be photochemically produced in the plume. The WRF-Chem and MOZART models were used to examine the spatial extent and temporal evolution of the plume and to help interpret the observed OH reactivity. The model results generally showed good agreement with experimental results for the total VOC OH reactivity downwind and gave insight into the distributions of VOC chemical classes. A box model with detailed gas phase chemistry (NCAR Master Mechanism), initialized with concentrations observed at one of the ground sites in the MCMA, was used to examine the expected evolution of specific VOCs over a 1-2 day period. The models clearly supported the experimental evidence for NMHC oxidation leading to the formation of OVOCs downwind, which then become the primary fuel for ozone production far away from the MCMA. [ABSTRACT FROM AUTHOR]

We construct a global atmospheric budget for acetaldehyde using a 3-D model of atmospheric chemistry (GEOS-Chem), and use an ensemble of observations to evaluate present understanding of its sources and sinks. Hydrocarbon oxidation provides the largest acetaldehyde source in the model (130 Tg a-1), with alkanes, alkenes, ethanol, and isoprene the main precursors. We use an updated chemical mechanism for GEOS-Chem, and photochemical acetaldehyde yields are consistent with the Master Chemical Mechanism. We apply SeaWiFS satellite observations to define the global distribution of light absorption due to marine dissolved organic matter (DOM), and estimate the corresponding sea-to-air acetaldehyde flux based on measured photoproduction rates from DOM. The resulting net ocean emission is 58 Tg a-1, the second largest global source of acetaldehyde. Quantitative model evaluation over the ocean is complicated by known measurement artifacts in clean air. Simulated concentrations in surface air over the ocean generally agree well with aircraft measurements, though the model tends to overestimate the vertical gradient. PAN:NOx ratios are well-simulated in the marine boundary layer, providing some support for the modeled ocean source. A key uncertainty is the acetaldehyde turnover time in the ocean mixed layer. We introduce the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) for acetaldehyde and ethanol and use it to quantify their net flux from living terrestrial plants. Including emissions from decaying plants the total direct acetaldehyde source from the land biosphere is 22 Tg a-1. Other terrestrial acetaldehyde sources include biomass burning (3 Tg a-1) and anthropogenic emissions (2 Tg a-1). Simulated concentrations in the continental boundary layer are generally unbiased and capture the spatial gradients seen in observations over North America, Europe, and tropical South America. However, the model underestimates acetaldehyde levels in urban outflow, suggesting a missing source in polluted air. Ubiquitous high measured concentrations in the free troposphere are not captured by the model, and based on present understanding are not consistent with concurrent measurements of PAN and NOx. We find no compelling evidence for a widespread missing acetaldehyde source in the free troposphere. We estimate the current US source of ethanol and acetaldehyde (primary+secondary) at 1.3 Tg a-1 and 7.0 Tg a-1, approximately 60% and 400% of the corresponding increases expected for a national transition from gasoline to ethanol fuel. [ABSTRACT FROM AUTHOR]

The volatile organic compound (VOC) distribution in the Mexico City Metropolitan Area (MCMA) and its evolution as it is uplifted and transported out of the MCMA basin was studied during the 2006 MILAGRO/MIRAGE-Mex field campaign. The results show that in the morning hours in the city center, the VOC distribution is dominated by nonmethane hydrocarbons (NMHCs) but with a substantial contribution from oxygenated volatile organic compounds (OVOCs), predominantly from primary emissions. Alkanes account for a large part of the NMHC distribution in terms of mixing ratios. In terms of reactivity, NMHCs also dominate overall, especially in the morning hours. However, in the afternoon, as the boundary layer lifts and air is mixed and aged within the basin, the distribution changes as secondary products are formed. The WRF-Chem (Weather Research and Forecasting with Chemistry) model and MOZART (Model for Ozone and Related chemical Tracers) were able to reproduce the general features of the daytime cycle of the VOC OH reactivity distribution showing that NMHCs dominate the distribution except in the afternoon hours and that the VOC OH reactivity peaks in the early morning due to high morning emissions from the city into a shallow boundary layer. The WRF-Chem and MOZART models showed higher reactivity than the experimental data during the nighttime cycle, perhaps indicating problems with the modeled nighttime boundary layer height. In addition, a plume was studied in which air was advected out of the MCMA and intercepted downwind with the DOE G1 on 18 March and the NCAR C130 one day later on 19 March. A number of identical species measured aboard each aircraft gave insight into the chemical evolution of the plume as it aged and was transported as far as 1000km downwind. Ozone and many OVOCs were photochemically produced in the plume. The WRF-Chem and MOZART models were used to examine the spatial and temporal extent of the 19 March plume and to help interpret the OH reactivity in the downwind plume. The model results generally showed good agreement with experimental results for the total VOC OH reactivity downwind and gave insight into the distributions of VOC chemical classes downwind. A box model with detailed gas phase chemistry (NCAR Master Mechanism), initialized with concentrations observed at one of the ground sites in the MCMA, was used to examine the expected evolution of specific VOCs over a 1-2 day period. The models clearly supported the experimental evidence for NMHC oxidation leading to the formation of OVOCs downwind, which then become the primary fuel for ozone production far away from the MCMA. [ABSTRACT FROM AUTHOR]

In March 2006 two instrumented aircraft made the first detailed field measurements of biomass burning (BB) emissions in the Northern Hemisphere tropics as part of the MILAGRO project. The aircraft were the National Center for Atmospheric Research C-130 and a University of Montana/ US Forest Service Twin Otter. The initial emissions of up to 49 trace gas or particle species were measured from 20 deforestation and crop residue fires on the Yucatan peninsula. This included two trace gases useful as indicators of BB (HCN and acetonitrile) and several rarely, or never before, measured species: OH, peroxyacetic acid, propanoic acid, hydrogen peroxide, methane sulfonic acid, and sulfuric acid. Crop residue fires emitted more organic acids and ammonia than deforestation fires, but the emissions from the main fire types were otherwise fairly similar. The Yucatan fires emitted unusually high amounts of SO2 and particle chloride, likely due to a strong marine influence on this peninsula. As smoke from one fire aged, the ratio ΔO3/ΔCO increased to ∼15% in <∼1 h similar to the fast net production of O3 in BB plumes observed earlier in Africa. The rapid change in O3 occurs at a finer spatial scale than is employed in global models and is also faster than predicted by microscale models. Fast increases in PAN, H2O2, and two organic acids were also observed. The amount of secondary organic acid is larger than the amount of known precursors. Rapid secondary formation of organic and inorganic aerosol was observed with the ratio ΔPM2.5/ΔCO more than doubling in ∼1.4±0.7 h. The OH measurements revealed high initial levels (>1×107 molecules/cm3) that were likely caused in part by high initial HONO (∼10% of NOy). Thus, more research is needed to understand critical post emission processes for the second-largest trace gas source on Earth. It is estimated that ∼44 Tg of biomass burned in the Yucatan in the spring of 2006. Mexican BB (including Yucatan BB) and urban emissions from the Mexico City area can both influence the March-May air quality in much of Mexico and the US. [ABSTRACT FROM AUTHOR]

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 NO[subx], CO, and O[sub3] concentrations (10—30% increase) are confined to distances <200 km, near surface. However, the extent of the influence is significantly larger at higher altitudes. BroaderMCMAimpacts (some 900 km Northeast of the city) are shown for specific outflow conditions in which enhanced ozone, NOy, 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 NO[subx] 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[NO[sub2]→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. [ABSTRACT FROM AUTHOR]

Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO3 in regionally polluted areas with large NOx levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NOx. Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO3 and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO3. A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction (>20%) of emitted isoprene. Organic nitrates produced from the NO3+isoprene reaction, though not directly measured, were estimated to account for 2-9% of total reactive nitrogen. The mass of isoprene oxidized by NO3 was comparable to and correlated with the organic aerosol loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1-17%, and isoprene SOA mass derived from NO3 was calculated to exceed that due to OH by approximately 50%. [ABSTRACT FROM AUTHOR]

Intercontinental Chemical Transport Experiment- B (INTEX-B) was a major NASA (Acronyms are provided in Appendix A.) led multi-partner atmospheric field campaign completed in the spring of 2006 (http://cloud1.arc.nasa.gov/intex-b/). Its major objectives aimed at (i) investigating the extent and persistence of the outflow of pollution from Mexico; (ii) understanding transport and evolution of Asian pollution and implications for air quality and climate across western North America; and (iii) validating space-borne observations of tropospheric composition. INTEX-B was performed in two phases. In its first phase (1-21 March), INTEX-B operated as part of the MILAGRO campaign with a focus on observations over Mexico and the Gulf of Mexico. In the second phase (17 April-15 May), the main INTEX-B focus was on trans-Pacific Asian pollution transport. Multiple airborne platforms carrying state of the art chemistry and radiation payloads were flown in concert with satellites and ground stations during the two phases of INTEX-B. Validation of Aura satellite instruments (TES, OMI, MLS, HIRDLS) was a key objective within INTEX-B. Satellite products along with meteorological and 3-D chemical transport model forecasts were integrated into the flight planning process to allow targeted sampling of air parcels. Inter-comparisons were performed among and between aircraft payloads to quantify the accuracy of data and to create a unified data set. Pollution plumes were sampled over the Gulf of Mexico and the Pacific several days after downwind transport from source regions. Signatures of Asian pollution were routinely detected by INTEX-B aircraft, providing a valuable data set on gas and aerosol composition to test models and evaluate pathways of pollution transport and their impact on air quality and climate. This overview provides details about campaign implementation and a context within which the present and future INTEX-B/MILAGRO publications can be understood. [ABSTRACT FROM AUTHOR]

The photochemical evolution of an anthropogenic plume from the New-York/Boston region during its transport at low altitudes over the North Atlantic to the European west coast has been studied using a Lagrangian framework. This plume, originally strongly polluted, was sampled by research aircraft just off the North American east coast on 3 successive days, and then 3 days downwind off the west coast of Ireland where another aircraft re-sampled a weakly polluted plume. Changes in trace gas concentrations during transport are reproduced using a photochemical trajectory model including deposition and mixing effects. Chemical and wet deposition processing dominated the evolution of all pollutants in the plume. The mean net photochemical O3 production is estimated to be -5 ppbv/day leading to low O3 by the time the plume reached Europe. Model runs with no wet deposition of HNO3 predicted much lower average net destruction of -1 ppbv/day O3, arising from increased levels of NOx via photolysis of HNO3. This indicates that wet deposition of HNO3 is indirectly responsible for 80% of the net destruction of ozone during plume transport. If the plume had not encountered precipitation, it would have reached Europe with O3 concentrations of up to 80 to 90 ppbv and CO between 120 and 140 ppbv. Photochemical destruction also played a more important role than mixing in the evolution of plume CO due to high levels of O3 and water vapour showing that CO cannot always be used as a tracer for polluted air masses, especially in plumes transported at low altitudes. The results also show that, in this case, an increase in O3/CO slopes can be attributed to photochemical destruction of CO and not to photochemical O3 production as is often assumed. [ABSTRACT FROM AUTHOR]

The NO-NO2 system was analyzed in different chemical regimes/air masses based on observations of reactive nitrogen species and peroxy radicals made during the intensive field campaign MIRAGE-Mex (4 to 29 March 2006). The air masses were categorized into 5 groups based on combinations of macroscopic observations, geographical location, meteorological parameters, models, and observations of trace gases: boundary layer (labeled as "BL"), biomass burning ("BB"), free troposphere (continental, "FTCO" and marine, "FTMA"), and Tula industrial complex ("TIC"). In general, NO2/NO ratios in different air masses are near photostationary state. Analysis of this ratio can be useful for testing current understanding of tropospheric chemistry. The ozone production efficiency (OPE) for the 5 air mass categories ranged from 4.5 (TIC) to 8.5 (FTMA), consistent with photochemical aging of air masses exiting the Mexico City Metropolitan Area. [ABSTRACT FROM AUTHOR]

Measurements of organic carbon compounds in both the gas and particle phases made upwind, over and downwind of North America are synthesized to examine the total observed organic carbon (TOOC) in the atmosphere over this region. These include measurements made aboard the NOAA WP-3 and BAe-146 aircraft, the NOAA research vessel Ronald H. Brown, and at the Thompson Farm and Chebogue Point surface sites during the summer 2004 ICARTT campaign. Both winter and summer 2002 measurements during the Pittsburgh Air Quality Study are also included. Lastly, the spring 2002 observations at Trinidad Head, CA, surface measurements made in March 2006 in Mexico City and coincidentally aboard the C-130 aircraft during the MILAGRO campaign and later during the IMPEX campaign off the northwestern United States are incorporated. Concentrations of TOOC in these datasets span more than two orders of magnitude. The daytime mean TOOC ranges from 4.0 to 456 µgC m-3 from the cleanest site (Trinidad Head) to the most polluted (Mexico City). Organic aerosol makes up 3-17% of this mean TOOC, with highest fractions reported over the northeastern United States, where organic aerosol can comprise up to 50% of TOOC. Carbon monoxide concentrations explain 46 to 86% of the variability in TOOC, with highest TOOC/CO slopes in regions with fresh anthropogenic influence, where we also expect the highest degree of mass closure for TOOC. Correlation with isoprene, formaldehyde, methyl vinyl ketone and methacrolein also indicates that biogenic activity contributes substantially to the variability of TOOC, yet these tracers of biogenic oxidation sources do not explain the variability in organic aerosol observed over North America. We highlight the critical need to develop measurement techniques to routinely detect total gas phase VOCs, and to deploy comprehensive suites of TOOC instruments in diverse environments to quantify the ambient evolution of organic carbon from source to sink. [ABSTRACT FROM AUTHOR]

Measurements of organic carbon compounds in both the gas and particle phases measured upwind, over and downwind of North America are synthesized to examine the total observed organic carbon (TOOC) over this region. These include measurements made aboard the NOAA WP-3 and BAe-146 aircraft, the NOAA research vessel Ronald H. Brown, and at the Thompson Farm and Chebogue Point surface sites during the summer 2004 ICARTT campaign. Both winter and summer 2002 measurements during the Pittsburgh Air Quality Study are also included. Lastly, the spring 2002 observations at Trinidad Head, CA, surface measurements made in March 2006 in Mexico City and coincidentally aboard the C-130 aircraft during the MILAGRO campaign and later during the IMPEX campaign off the northwestern United States are incorporated. Concentrations of TOOC in these datasets span more than two orders of magnitude. The daytime mean TOOC ranges from 4.0 to 456 μg Cm-3 from the cleanest site (Trinidad Head) to the most polluted (Mexico City). Organic aerosol makes up 3-17% of this mean TOOC, with highest fractions reported over the northeastern United States, where organic aerosol can comprise up to 50% of TOOC. Carbon monoxide concentrations explain 46 to 86% of the variability in TOOC, with highest TOOC/CO slopes in regions with fresh anthropogenic influence, where we also expect the highest degree of mass closure for TOOC. Correlation with isoprene, formaldehyde, methyl vinyl ketene and methacrolein also indicates that biogenic activity contributes substantially to the variability of TOOC, yet these tracers of biogenic oxidation sources do not explain the variability in organic aerosol observed over North America. We highlight the critical need to develop measurement techniques to routinely detect total gas phase VOCs, and to deploy comprehensive suites of TOOC instruments in diverse environments to quantify the ambient evolution of organic carbon from source to sink. [ABSTRACT FROM AUTHOR]

We present month-long observations of speciated acyl peroxy nitrates (APNs), including PAN, PPN, MPAN, APAN, and the sum of PiBN and PnBN, measured at the Mount Bachelor Observatory (MBO) as part of the INTEX-B collaborative field campaign during spring 2006. APN abundances, measured by thermal dissociation-chemical ionization mass spectrometry (TD-CIMS), are discussed in terms of differing contributions from the boundary layer (BL) and the free troposphere (FT) and in the context of previous APN measurements in the NE Pacific region. PAN mixing ratios range from 11 to 3955 pptv, with a mean value of 334 pptv for the full measurement period. PPN is linearly correlated with PAN (r²=0.96), with an average abundance of 6.5% relative to PAN; other APNs are generally <1% of PAN. Diurnal cycles and relationships of APNs with ozone reveal a gradient in hydrocarbon chemistry between the BL and the FT. On average, levels of APNs, ozone and PPN/PAN are substantially higher in FT air relative to BL air, suggesting that this site is strongly influenced by distant pollution sources. To estimate the impact of long-range transport of Asian pollution on atmospheric composition at MBO, we perform a detailed analysis utilizing HYSPLIT back trajectories. This analysis suggests that trans-Pacific transport of Asian pollution leads to substantial increases in APN and ozone mixing ratios at MBO, especially when transport occurs via the free troposphere. The ensemble of trajectories indicate that Asian-influenced free tropospheric air was sampled in ~16% of our data and contained a median PAN mixing ratio double that of the full dataset. [ABSTRACT FROM AUTHOR]