Search

During an airborne study in the Southeast United States, measured mixing ratios of biogenic hydrocarbons were systematically lower in air masses containing enhanced nitrogen oxides from power plants, which we attribute to increased concentrations of hydroxyl (OH) radicals within the power plant plumes. Plume transects at successively further downwind distances provide a decreasing gradient of nitrogen oxides (NO x) concentrations, which together with the implied loss rates of isoprene, constrains the OH dependence on NO x. We find that OH concentrations were highest at nitrogen dioxide concentrations near 1–2 ppbv and decreased at higher and at lower concentrations. These findings agree with the dependence of OH on NO x concentrations expected from known chemical reactions but are not consistent with some studies reporting direct OH measurements higher than expected in regions of the atmosphere with low NO x (NO < 0.08 and NO 2 < 0.46 ppbv) and high biogenic hydrocarbon emissions.

Volatile organic compounds (VOCs) emitted from residential wood and crop residue burning were measured in Colorado, U.S. When compared to the emissions from crop burning, residential wood burning exhibited markedly lower concentrations of acetonitrile, a commonly used biomass burning tracer. For both herbaceous and arboraceous fuels, the emissions of nitrogen-containing VOCs (NVOCs) strongly depend on the fuel nitrogen content; therefore, low NVOC emissions from residential wood burning result from the combustion of low-nitrogen fuel. Consequently, the emissions of compounds hazardous to human health, such as HNCO and HCN, and the formation of secondary pollutants, such as ozone generated by NOx, are likely to depend on fuel nitrogen. These results also demonstrate that acetonitrile may not be a suitable tracer for domestic burning in urban areas. Wood burning emissions may be best identified through analysis of the emissions profile rather than reliance on a single tracer species.

We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and MCM v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γglyx of 2 × 10−3, and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0–0.8 μg m−3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde (RGF = [GLYX]/[HCHO]), resulting from the suppression of δ-isoprene peroxy radicals (δ-ISOPO2). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of IEPOX peroxy radicals (IEPOXOO) with HO2. Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.

Tropospheric O3 has been decreasing across much of the eastern U.S. but has remained steady or even increased in some western regions. Recent increases in VOC and NOx emissions associated with the production of oil and natural gas (O&NG) may contribute to this trend in some areas. The Northern Front Range of Colorado has regularly exceeded O3 air quality standards during summertime in recent years. This region has VOC emissions from a rapidly developing O&NG basin and low concentrations of biogenic VOC in close proximity to urban-Denver NOx emissions. Here VOC OH reactivity (OHR), O3 production efficiency (OPE), and an observationally constrained box model are used to quantify the influence of O&NG emissions on regional summertime O3 production. Analyses are based on measurements acquired over two summers at a central location within the Northern Front Range that lies between major regional O&NG and urban emission sectors. Observational analyses suggest that mixing obscures any OPE differences in air primarily influenced by O&NG or urban emission sector. The box model confirms relatively modest OPE differences that are within the uncertainties of the field observations. Box model results also indicate that maximum O3 at the measurement location is sensitive to changes in NOx mixing ratio but also responsive to O&NG VOC reductions. Combined, these analyses show that O&NG alkanes contribute over 80% to the observed carbon mixing ratio, roughly 50% to the regional VOC OHR, and approximately 20% to regional photochemical O3 production.

Ethanol made from corn now constitutes approximately 10% of the fuel used in gasoline vehicles in the U.S. The ethanol is produced in over 200 fuel ethanol refineries across the nation. We report airborne measurements downwind from Decatur, Illinois, where the third largest fuel ethanol refinery in the U.S. is located. Estimated emissions are compared with the total point source emissions in Decatur according to the 2011 National Emissions Inventory (NEI-2011), in which the fuel ethanol refinery represents 68.0% of sulfur dioxide (SO2), 50.5% of nitrogen oxides (NOx = NO + NO2), 67.2% of volatile organic compounds (VOCs), and 95.9% of ethanol emissions. Emissions of SO2 and NOx from Decatur agreed with NEI-2011, but emissions of several VOCs were underestimated by factors of 5 (total VOCs) to 30 (ethanol). By combining the NEI-2011 with fuel ethanol production numbers from the Renewable Fuels Association, we calculate emission intensities, defined as the emissions per ethanol mass produced. Emission intensities of SO2 and NOx are higher for plants that use coal as an energy source, including the refinery in Decatur. By comparing with fuel-based emission factors, we find that fuel ethanol refineries have lower NOx, similar VOC, and higher SO2 emissions than from the use of this fuel in vehicles. The VOC emissions from refining could be higher than from vehicles, if the underestimated emissions in NEI-2011 downwind from Decatur extend to other fuel ethanol refineries. Finally, chemical transformations of the emissions from Decatur were observed, including formation of new particles, nitric acid, peroxyacyl nitrates, aldehydes, ozone, and sulfate aerosol.

We present measurements of methane (CH4) taken aboard a NOAA WP-3D research aircraft in 2013 over the Haynesville shale region in eastern Texas/northwestern Louisiana, the Fayetteville shale region in Arkansas, and the northeastern Pennsylvania portion of the Marcellus shale region, which accounted for the majority of Marcellus shale gas production that year. We calculate emission rates from the horizontal CH4 flux in the planetary boundary layer downwind of each region after subtracting the CH4 flux entering the region upwind. We find 1 day CH4 emissions of (8.0 ± 2.7) × 107 g/h from the Haynesville region, (3.9 ± 1.8) × 107 g/h from the Fayetteville region, and (1.5 ± 0.6) × 107 g/h from the Marcellus region in northeastern Pennsylvania. Finally, we compare the CH4 emissions to the total volume of natural gas extracted from each region to derive a loss rate from production operations of 1.0–2.1% from the Haynesville region, 1.0–2.8% from the Fayetteville region, and 0.18–0.41% from the Marcellus region in northeastern Pennsylvania. The climate impact of CH4 loss from shale gas production depends upon the total leakage from all production regions. The regions investigated in this work represented over half of the U.S. shale gas production in 2013, and we find generally lower loss rates than those reported in earlier studies of regions that made smaller contributions to total production. Hence, the national average CH4 loss rate from shale gas production may be lower than values extrapolated from the earlier studies.

We present OH observation results during the NACHTT-11 field campaign at the Boulder Atmospheric Observatory in Weld County, Colorado. The observed OH levels during the daytime (at noon) were ~ 2.7 × 106 molecules cm-3 at the ground level (2 m above ground level, AGL). HONO and ozone photolysis were the two dominant photochemical OH production pathways during the field campaign. However, alkene ozonolysis, found an important source for OH by two previous winter season OH observations, was a minor contribution to OH primary production (~5 %). To evaluate recycling sources of OH from HO2 and RO2, an observation constrained University of Washington Chemical Mechanism (UWCM) box model is employed to simulated ambient OH levels with different model scenarios. For the base run without constraining observed HONO, the model simulated OH significantly underestimates the observed OH level (20.8 times in the morning and 7.2 times in the daytime). This indicates that the known HONO sources incorporated in the UWCM model cannot explain the observed HONO level. Once HONO is constrained by the observation, the discrepancy between observation and model simulation improves (5.1 times in the morning and 2.1 times in the daytime) but still out of the measurement uncertainty rangemore » (35 %). We explore two possible reasons for the observed unexplainably high wintertime OH levels. First, potential roles of Cl atoms produce organic peroxy radicals from the reactions between Cl atmos and alkane compounds. However, the Cl levels during the observation period are estimated very low (~ 103 atoms cm-3) to explain the enhanced OH levels. Second, Impacts of higher HONO levels on the ground was evaluated. Strong HONO gradient towards ground was observed especially during the early morning (6 am to 8 am) was observed and the lowest level available for the HONO observation during the campaign is 5 m AGL. Once we assume the twice of the observed HONO levels averaged between 5 m to 15 m at 2 m AGL, model predicted OH levels agree well within the observation uncertainty range. Wintertime photochemistry has not been investigated as much as the summer season. The results of this study along with a limited number of winter OH observations clearly urge further investigation on tropospheric oxidation capacity in the winter season considering implications of tropospheric oxidation capacity to the short-lived climate forcers especially methane.« less

Ship-based measurements of gas phase hydrochloric acid (HCl), particulate chloride (pCl � ), and reactive nitrogen oxides (NOy) were made in the polluted marine boundary layer along the California coastline during spring 2010. These observations are used to assess both the rate of Cl atom production from HCl andthe role of direct HCl emissions andsubsequent partitioningas asource for pCl � . Observations of HCl made in coastal Southern California are broadly correlated with NOz (NOz ≡NOy - NOx), peaking at 11 A.M. The observed median HCl mixing ratio in Southern California is 1.3 ppb (interquartile range: 0.53-2.7 ppb), as compared to 0.19 ppb (interquartile range: 0.10-0.38 ppb) measured along the Sacramento River between San Francisco and Sacramento. Concurrent measurements of aerosol ion chemistry indicate that aerosol particles sampled in Northern California are heavily depleted in Cl � , corresponding to a mean pCldeficit of 0.05±0.03 (1σ) ppb for sub-10 μm aerosol particles. In comparison, aerosols measured in Southern California indicate that over 25% of particles showed an addition of Clto the particle population. Observations presented here suggest that primary sources of HCl, or gas phase chlorine precursors to HCl, are likely underestimated in the California Air Resource Board emissions inventory. These results highlight the need for future field observations designed to better constrain direct reactive halogen emissions.

We reanalyze a data set of hydrocarbons in ambient air obtained by gas chromatography-mass spectrometry at a surface site in Pasadena in the Los Angeles basin during the NOAA California Nexus study in 2010. The number of hydrocarbon compounds quantified from the chromatograms is expanded through the use of new peak-fitting data analysis software. We also reexamine hydrocarbon removal processes. For alkanes, small alkenes, and aromatics, the removal is determined by the reaction with hydroxyl (OH) radicals. For several highly reactive alkenes, the nighttime removal by ozone and nitrate (NO3) radicals is also significant. We discuss how this nighttime removal affects the determination of emission ratios versus carbon monoxide (CO) and show that previous estimates based on nighttime correlations with CO were too low. We analyze model output from the Weather Research and Forecasting-Chemistry model for hydrocarbons and radicals at the Pasadena location to evaluate our methods for determining emission ratios from the measurements. We find that our methods agree with the modeled emission ratios for the domain centered on Pasadena and that the modeled emission ratios vary by 23% across the wider South Coast basin. We compare the alkene emission ratios with published results from ambient measurements and from tunnel and dynamometer studies of motor vehicle emissions. We find that with few exceptions the composition of alkene emissions determined from the measurements in Pasadena closely resembles that of motor vehicle emissions.

[1] Measurements of carbonyl sulfide (OCS) have been proposed to provide a unique constraint on carbon assimilation by the biosphere that is independent of the influence of plant and soil respiration of CO2, but this constraint depends on a comprehensive understanding of the processes controlling OCS in the biosphere. We conducted a high-resolution temporal and spatial survey of OCS and CO2 mixing ratios during the California Nexus Experiment research cruise along the coast of California (U.S.) and into the Sacramento River Delta using a newly constructed compact quantum cascade laser spectrometer (precision for OCS of

[1] Several regions within California have significant air quality issues. Transport of pollutants emitted in one region to another region may add to the impact of local emissions. In this work, Lagrangian particle dispersion model simulations show the amounts of tracers that are transported within and among four regions, Southern California, the San Francisco Bay Area, the Central Valley, and the rest of the state. The simulations cover May and June of 2010, the California Research at the Nexus of Air Quality and Climate Change experiment period. Tracers of automobile emissions and one type of agricultural emission are used. Tracer mixing ratios are compared to airborne and ground-based measurements. The age of tracers in each location is also presented. Vertical profiles and diurnal cycles help to clarify the transport process. As is well known, Southern California emissions are transported to the east and affect the desert areas, and Bay Area automobile emissions are an important source of pollutants in the San Joaquin Valley. A novel result is that the Southern California Bight is filled with a mixture of well-aged carbon monoxide tracer from Southern California and the Bay Area. Air over the Bight is also affected by the agricultural emissions represented by the agricultural tracer, dominantly from the Central Valley where its sources are largest. There is no indication of transport from Southern California to the Central Valley. Emissions from the Central Valley do make their way to Southern California, as shown by the agricultural tracer, but automobile emissions from the Valley are insignificant in Southern California.

[1] Gaseous elemental mercury (GEM) in the atmosphere was measured during an oceanographic cruise in coastal waters between San Diego and San Francisco, California during the CalNex 2010 campaign. The goal of the measurements was to quantify GEM in the various environments that the ship encountered, from urban outflow, the Port of Long Beach and associated shipping lanes, coastal waters affected by upwelling, the San Francisco Bay, and the Sacramento ship channel. Mean GEM for the whole cruise was 1.41 ± 0.20 ng m–3, indicating that background concentrations were predominantly observed. The ship's position was most often in waters off the coast of Los Angeles (74% of time with latitude 0.05) higher than the whole cruise mean. South of 34.3°N, GEM was observed to vary diurnally and as a function of wind direction, displaying significantly higher concentrations at night and in the morning associated with general transport from the land to the sea. GEM and CO concentrations were positively correlated with a slope of 0.0011 ng m–3 ppbv–1 (1.23 × 10–7 mol mol–1) during periods identified as “Los Angeles urban outflow”, which given the inventoried CO emissions for the region, suggests a larger source of GEM than is accounted for by the inventory. The timing of the diel maximum in GEM (9:00 local time) was intermediate between the maxima of CO and NO2 (6:00) and that of NO and SO2 (10:00–12:00), suggesting that a mixture of urban and industrial sources were contributing to GEM. There was no observable postsunrise dip in GEM concentrations due to reaction with atomic chlorine in the polluted coastal atmosphere. On three occasions, significantly higher GEM concentrations were observed while in the Port of Long Beach (~ 7 ng m–3), and analyses of wind directions, ratios of GEM with other copollutants, and the composition of single particles, suggest that these plumes originated from the local waste incinerator in the Port area. A plume encounter from a large cargo ship allowed for the estimation of a mass-based emission factor for GEM (0.05 ± 0.01 mg kg–1 fuel burned). GEM enhancements observed in the Carquinez Straits, were lower than expected based on the observed NOx/SO2 ratios in the plumes and emissions inventories of the nearest oil refineries. In a region north of Monterey Bay known for upwelling, GEM in the air was positively correlated with dimethyl sulfide (DMS) in seawater and in the air. Using the observed GEM/DMS(g) relationship and the calculated mean DMS ocean-atmosphere flux for the cruise, an ocean-atmosphere flux of GEM of 0.017 ± 0.009 µmol m–2 d–1 was estimated. This flux was on the upper end of previously reported GEM ocean-atmosphere fluxes and should be verified with further measurements of Hg species in seawater and air.

[1] The sea breeze/land breeze diurnal circulation within the Los Angeles Basin and adjacent waters transports marine air into the basin during the day and urban air to Santa Monica Bay during the night. Nitryl chloride, ClNO2 is a nocturnal trace gas formed from the heterogeneous reaction of dinitrogen pentaoxide (N2O5) with chloride containing aerosol. Its photolysis after sunrise produces atomic chlorine radicals and regenerates NO2, both of which may increase ozone production. Mixing of the chloride source from marine sea salt with the urban NOx source in Los Angeles provides conditions ideal for the production of ClNO2. This paper presents an analysis using a wind profiler on the coast and measurements of ClNO2 and its precursors made from both ship and aircraft to assess the prevailing meteorological conditions important for ClNO2 production in this region, with a particular focus on the production over water within the land breeze phase of the circulation. A box model is used to calculate an upper limit to the amount of ClNO2 capable of being produced strictly over Santa Monica Bay during the land breeze. On three out of the four nights of ClNO2 measurements in Santa Monica Bay, the ClNO2 exceeds the upper limit calculated using the box model and shows that the majority of the ClNO2 is produced over the city and transported to Santa Monica Bay by the land breeze. This ClNO2 transport suggests the sea breeze more efficiently transports aerosol chloride inland than land breeze transports NOx offshore.

[1] The influence of convective mixing on surface ozone in Houston during TexAQS 2006 is examined. We use airborne lidar measurements of ozone and ship-based Doppler lidar measurements of winds, together with ship- and ground-based measurements of surface ozone to characterize horizontal and vertical mixing of ozone plumes from the Houston Ship Channel on two high-ozone days. We show that a stable capping layer trapped the plume in the boundary layer on 31 August, while shallow convection associated with active fair weather cumulus clouds mixed the plume with free tropospheric air on 17 August. Deep convection associated with an isolated air mass thunderstorm further decreased surface ozone near Galveston Bay in the late afternoon. High ozone thus affected a smaller area for a shorter period on 17 August, despite similar background concentrations and local production. We generalize these findings by comparing Houston ozone concentrations to National Weather Service (Lake Charles, LA) radiosondes. We show that for 1 June to 15 September 2006, stable conditions with high background ozone occurred 18% of the days leading to mean daily 8 h concentrations of 73 ± 11 ppbv. Shallow and deep convection associated with moderate to strongly unstable conditions lowered the mean ozone to 50 ± 11 ppbv (∼29% of days), while weaker convection associated with marginally unstable conditions reduced the mean concentrations to 63 ± 13 ppbv (∼11%). We use these observations to derive simple relationships between surface ozone and convective indicators that may prove useful for parameterization of convective venting in air quality models.

[1] Measurements made with a ship-based Doppler wind lidar during the summertime 2006 Texas Air Quality Study are used to study the relationship between lower-tropospheric vertical structure and winds and ozone (O3) concentrations in Houston, Texas, under two different flow regimes. We observed that strong southerly flow regimes, dominated by the subtropical anticyclone (Bermuda high) off the Atlantic coast of the United States, resulted in strong (i.e., high wind speed) onshore nocturnal low-level jets (LLJ) and low O3 and oxidant Ox (where Ox = O3 + NO2) concentrations at night and the following afternoon. In contrast, periods dominated by northerly or easterly flow resulted in relatively weak (low wind speed), but still onshore, nocturnal LLJs associated with higher concurrent and next-day concentrations of O3 and Ox. We present lidar data from 24 h example periods for each of these conditions and demonstrate how each type of flow regime is related to in situ ship-based ozone measurements. We expanded the study to include all days during the study when the ship was near Houston, to demonstrate how the strength of the meridional winds aloft show a better relationship to concurrent ship-measured Ox concentrations than the winds near the surface do. We found a strong relationship between a parameterization of the observed nocturnal jets, which reflect the synoptic conditions, to peak hourly O3 measured the next day at the ship and averaged throughout the Houston/Galveston/Brazoria continuous ambient monitoring stations monitoring network, indicating potential applications for planning air quality.

[1] We report measurements of NOx, SO2, CO, and HCHO mass-based emission factors from more than 200 commercial vessel encounters in the Gulf of Mexico and the Houston-Galveston region of Texas during August and September, 2006. For underway ships, bulk freight carriers have the highest average NOx emissions at ∼87 g NOx (kg fuel)−1, followed by tanker ships at ∼79 g NOx (kg fuel)−1, while container carriers, passenger ships, and tugs all emit an average of about ∼60 g NOx (kg fuel)−1. Emission of NOx from stationary vessels was lower, except for container ships and tugs, and likely reflects use of medium-speed diesel engines. Overall, our mean NOx emission factors are 10–15% lower than published data. Average emission of SO2 was lower for passenger ships and tugs and tows (6–7 g SO2 (kg fuel)−1) than for larger cargo vessels (20–30 g SO2 (kg fuel)−1). Our data for large cargo ships in this region indicate an average residual fuel sulfur content of ∼1.4% which is a factor of two lower than the global average of 2.7%. Emission of CO was low for all categories (7–16 g CO (kg fuel)−1), although our mean overall CO emission factor is about 10% higher than published data. Emission of HCHO was less than 5% that of CO. Despite considerable variability, no functional relationships, such as emissions changes with engine speed or load, could be discerned. Comparison of emission factors from ships to those from other sources suggests ship emissions in this region cannot be ignored.

[1] In situ measurements of aerosol optical and hygroscopic properties were made on board the National Oceanic and Atmospheric Administration R/V Ronald H. Brown during the Texas Air Quality Study–Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS). The aerosol light extinction coefficient (σep) was measured at 355, 532, and 1064 nm at 25%, 60%, and 85% relative humidity (RH) for both sub-1- and sub-10-μm-diameter particles with a cavity ring–down aerosol extinction spectrometer. The 532-nm σep was coupled with the 532-nm light absorption coefficient (σap) measured with a photoacoustic absorption spectrometer to calculate the aerosol single scattering albedo (ω) with absolute uncertainty 0.65 and >0.9, respectively). The optical data were used to estimate local, top of atmosphere aerosol-induced climate forcing (ΔFR). The ΔFR calculations were performed using both ω values measured at 25% RH and ω values converted to ambient RH. The calculated ambient ΔFR ranged from −7 to −40 W/m2 with absolute uncertainty between 0.7 and 2.5 W/m2. The results show that including aerosol hygroscopic properties in climate calculations is critical for improving estimates of aerosol forcing on climate.

[1] An extensive set of volatile organic compounds (VOCs) and other gas phase species were measured in situ aboard the NOAA R/V Ronald H. Brown as the ship sailed in the Gulf of Mexico and the Houston and Galveston Bay (HGB) area as part of the Texas Air Quality (TexAQS)/Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) conducted from July–September 2006. The magnitudes of the reactivities of CH4, CO, VOCs, and NO2 with the hydroxyl radical, OH, were determined in order to quantify the contributions of these compounds to potential ozone formation. The average total OH reactivity (ROH,TOTAL) increased from 1.01 s−1 in the central gulf to 10.1 s−1 in the HGB area as a result of the substantial increase in the contribution from VOCs and NO2. The increase in the measured concentrations of reactive VOCs in the HGB area compared to the central gulf was explained by the impact of industrial emissions, the regional distribution of VOCs, and the effects of local meteorology. By compensating for the effects of boundary layer mixing, the diurnal profiles of the OH reactivity were used to characterize the source signatures and relative magnitudes of biogenic, anthropogenic (urban + industrial), and oxygenated VOCs as a function of the time of day. The source of reactive oxygenated VOCs (e.g., formaldehyde) was determined to be almost entirely from secondary production. The secondary formation of oxygenated VOCs, in addition to the continued emissions of reactive anthropogenic VOCs, served to sustain elevated levels of OH reactivity throughout the time of peak ozone production.

[1] Mixing ratios of dimethyl sulfide (DMS) and its nighttime oxidant, the nitrate radical (N0 3 ), were measured in the summertime marine boundary layer (MBL) of the Gulf of Maine during the New England Air Quality Study-International Transport and Chemical Transformation campaign in 2004. DMS fluxes from the ocean were derived from simultaneous measurements of the wind speed and DMS in seawater. Day and night DMS oxidation rates were determined from modeled OH and measured NO 3 concentrations. The average DMS lifetime with respect to oxidation by OH at noon was 13.5 ± 3.4 (1σ) h, while at night, DMS lifetimes with respect to N0 3 oxidation varied by sampling region from 11 min to 28 h. Oxidation by photochemically generated halogen species likely also played a role during the day, although the nature and extent of the halogen species is more difficult to predict due to lack of halogen measurements. Closure of the DMS budget in the MBL required a vertical entrainment velocity of ∼0.4 cm s ―1 . This study suggests that entrainment of DMS out of the MBL competes with daytime oxidation and that the presence of pollution in the form of NO x and 0 3 in near-coastal regions at night results in nearly complete DMS oxidation within the MBL via reaction with N0 3 , with a much smaller contribution from entrainment. One potential implication of near-complete DMS oxidation within the MBL is a reduction of the amount of sulfur available for aerosol formation and growth at higher altitudes in the atmosphere.

provide chemical and physical characteristics including sulfate (SO4� ) mass, organic matter (OM) mass, black carbon (BC) mass, particulate matter (PM) mass, number concentrations (condensation nuclei (CN) > 5 nm), and cloud condensation nuclei (CCN). Optical characterization included multiple wavelength visible light absorption and extinction, extinction relative humidity dependence, and single scatter albedo (SSA). The global contribution of shipping PM was calculated to be 0.90 Tg a � 1 , in good agreement with previous inventories (0.91 and 1.13 Tg a � 1 from Eyring et al. (2005a) and Wang et al. [2008]). Observed PM composition was 46% SO4� , 39% OM, and 15% BC and differs from inventories that used 81%, 14%, and 5% and 31%, 63%, and 6% SO4� , OM, and BC, respectively. SO4� and OM mass were found to be dependent on fuel sulfur content as were SSA, hygroscopicity, and CCN concentrations. BC mass was dependent on engine type and combustion efficiency. A plume evolution study conducted on one vessel showed conservation of particle light absorption, decrease in CN > 5 nm, increase in particle hygroscopicity, and an increase in average particle size with distance from emission. These results suggest emission of small nucleation mode particles that subsequently coagulate/condense onto larger BC and OM. This work contributes to an improved understanding of the impacts of ship emissions on climate and air quality and will also assist in determining potential effects of altering fuel standards.

[1] The air quality and climate forcing impacts of atmospheric aerosols in a metropolitan region depend on the amount, composition, and size of the aerosol transported into the region; the input and removal of aerosols and aerosol precursors within the region; and the subsequent chemical processing in the atmosphere. These factors were studied in the Houston-Galveston-Gulf of Mexico region, aboard the NOAA R/V Ronald H. Brown during the Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS 2006). The aerosol measured in the Gulf of Mexico during onshore flow (low radon concentrations indicating no contact with land for several days) was highly impacted by Saharan dust and what appear to be ship emissions (acidic sulfate and nitrate). Mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol were 6.5 (4.6) μg m−3 and 17.2 (8.7) μg m−3, respectively. These mass loadings of “background” aerosol are much higher than typically observed in the marine atmosphere and thus have a substantial impact on the radiative energy balance over the Gulf of Mexico and particulate matter (PM) loadings (air quality) in the Houston-Galveston area. As this background aerosol moved onshore, local urban and industrial sources added an organic rich submicrometer component (66% particulate organic matter (POM), 20% sulfate, 14% elemental carbon) but no significant supermicrometer aerosol. The resulting aerosol had mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol of 10.0 (9.1) μg m−3 and 16.8 (11.2) μg m−3, respectively. These air masses, with minimal processing of urban emissions contained the highest SO2/(SO2 + SO4=) ratios and the highest hydrocarbon-like organic aerosol to total organic aerosol ratios (HOA/POM). In contrast, during periods of offshore flow, the aerosol was more processed and, therefore, much richer in oxygenated organic aerosol (OOA). Mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol were 20.8 (18.6) μg m−3 and 7.4 (5.0) μg m−3, respectively. Sorting air masses based on their trajectories and time over land provides a means to examine the effects of transport and subsequent chemical processing. Understanding and parameterizing these processes is critical for the chemical transport modeling that forms the basis for air quality forecasts and radiative forcing calculations.

[1] Ship and aircraft measurements of aerosol organic matter (OM) and water-soluble organic carbon (WSOC) were made in fresh and aged pollution plumes from major urban areas in the northeastern United States in the framework of the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. A large part of the variability in the data was quantitatively described by a simple parameterization from a previous study that uses measured mixing ratios of CO and either the transport age or the photochemical age of the sampled air masses. The results suggest that OM was mostly due to secondary formation from anthropogenic volatile organic compound (VOC) precursors in urban plumes. Approximately 37% of the secondary formation can be accounted for by the removal of aromatic precursors using newly published particulate mass yields for low-NOx conditions, which are significantly higher than previous results. Of the secondary formation, 63% remains unexplained and is possibly due to semivolatile precursors that are not measurable by standard gas chromatographic methods. The observed secondary OM in urban plumes may account for 35% of the total source of OM in the United States and 8.5% of the global OM source. OM is an important factor in climate and air quality issues, but its sources and formation mechanisms remain poorly quantified.

[1] The nocturnal nitrogen oxides, NO3 and N2O5, are important to the chemical transformation and transport of NOx, O3 and VOC. Their concentrations, sources and sinks are known to be vertically stratified in the nighttime atmosphere. In this paper, we report vertical profiles for NO3 and N2O5 measured from an aircraft (the NOAA P-3) as part of the New England Air Quality Study in July and August 2004. The aircraft data are compared to surface measurements made in situ from a ship and by long-path DOAS. Consistent with previous, vertically resolved studies of NO3 and N2O5, the aircraft measurements show that these species occur at larger concentrations and are longer lived aloft than they are at the surface. The array of in situ measurements available on the P-3 allows for investigation of the mechanisms that give rise to the observed vertical gradients. Selected vertical profiles from this campaign illustrate the role of biogenic VOC, particularly isoprene and dimethyl sulfide, both within and above the nocturnal and/or marine boundary layer. Gradients in relative humidity and aerosol surface may also create a vertical gradient in the rate of N2O5 hydrolysis. Low-altitude intercepts of power plant plumes showed strong vertical stratification, with plume depths of 80 m. The efficiency of N2O5 hydrolysis within these plumes was an important factor determining the low-level NOx and O3 transport or loss at night. Averages of nocturnal O3, NO2, NO3 and N2O5 binned according to altitude were consistent with the trends from individual profiles. While production rates of NO3 peaked near the surface, lifetimes of NO3 and N2O5 were maximum aloft, particularly in the free troposphere. Variability in NO3 and N2O5 was large and exceeded that of NO2 or O3 because of inhomogeneous distribution of NOx emissions and NO3 and N2O5 sinks.

[1] Measurements of peroxycarboxylic nitric anhydrides (PANs) were made during the New England Air Quality Study 2002 cruise of the NOAA RV Ronald H Brown. The four compounds observed, PAN, peroxypropionic nitric anhydride (PPN), peroxymethacrylic nitric anhydride (MPAN), and peroxyisobutyric nitric anhydride (PiBN) were compared with results from other continental and Gulf of Maine sites. Systematic changes in PPN/PAN ratio, due to differential thermal decomposition rates, were related quantitatively to air mass aging. At least one early morning period was observed when O3 seemed to have been lost probably due to NO3 and N2O5 chemistry. The highest O3 episode was observed in the combined plume of isoprene sources and anthropogenic volatile organic compounds (VOCs) and NOx sources from the greater Boston area. A simple linear combination model showed that the organic precursors leading to elevated O3 were roughly half from the biogenic and half from anthropogenic VOC regimes. An explicit chemical box model confirmed that the chemistry in the Boston plume is well represented by the simple linear combination model. This degree of biogenic hydrocarbon involvement in the production of photochemical ozone has significant implications for air quality control strategies in this region.

[1] We describe field measurements of nitrate radical photolysis rates, j(NO3), conducted during the International Consortium for Atmospheric Transport and Transformation (ICARTT) study in the summer of 2004 in the northeastern United States on board the NOAA research vessel Ronald H. Brown (RHB) and the NOAA aircraft WP-3. The photolysis rates of 17 other atmospherically important compounds were also measured. Direct measurements of spectral actinic fluxes using spectroradiometers were conducted on board the WP-3, which were then converted into photolysis rates. On board RHB, we used filter radiometers that specifically measured j(NO3) and were calibrated before and after the campaign by the spectroradiometers. NO3 photolysis rates ranged from below the detection limit of 10−5 s−1 at twilight to peak values of 0.5 s−1 over clouds at midday. The measurement uncertainties were 9% for the spectroradiometers and 14% for the filter radiometers. A field intercomparison between ship and aircraft instruments showed general agreement, indicating that aircraft data can be used to calculate the ship nadir radiation from the ocean surface. The measurements were used to evaluate the importance of photolysis of nitrate radicals in the troposphere. One result was that because of its spatial correlation with NO, NO3 daytime loss is dominated by reaction with NO in the free troposphere and the marine boundary layer. The tropospheric branching ratio between the two NO3 photolysis channels producing NO and NO2, was found to be (10.8 ± 1.2)% for NO in the lower troposphere.

[1] During the NEAQS-ITCT2k4 campaign in New England, anthropogenic VOCs and CO were measured downwind from New York City and Boston. The emission ratios of VOCs relative to CO and acetylene were calculated using a method in which the ratio of a VOC with acetylene is plotted versus the photochemical age. The intercept at the photochemical age of zero gives the emission ratio. The so determined emission ratios were compared to other measurement sets, including data from the same location in 2002, canister samples collected inside New York City and Boston, aircraft measurements from Los Angeles in 2002, and the average urban composition of 39 U.S. cities. All the measurements generally agree within a factor of two. The measured emission ratios also agree for most compounds within a factor of two with vehicle exhaust data indicating that a major source of VOCs in urban areas is automobiles. A comparison with an anthropogenic emission database shows less agreement. Especially large discrepancies were found for the C2-C4 alkanes and most oxygenated species. As an example, the database overestimated toluene by almost a factor of three, which caused an air quality forecast model (WRF-CHEM) using this database to overpredict the toluene mixing ratio by a factor of 2.5 as well. On the other hand, the overall reactivity of the measured species and the reactivity of the same compounds in the emission database were found to agree within 30%.

[1] Measurements were made on board the NOAA RV Ronald H. Brown during the second New England Air Quality Study (NEAQS 2004) to determine the source of the aerosol in the region and how sources and aging processes affect submicrometer aerosol chemical composition and optical properties. Using the Lagrangian particle dispersion model FLEXPART in combination with gas phase tracer compounds, local (urban), regional (NE U.S. urban corridor of Washington, D.C.; New York; and Boston), and distant (midwest industries and North American forest fires) sources were identified. Submicrometer aerosol measured near the source region (Boston Harbor) had a molar equivalence ratio near one with respect to NH4+, NO3−, and SO4=, had a large mass fraction of particulate organic matter (POM) relative to SO4=, and had relatively unoxidized POM. As distance from the source region increased, the submicrometer aerosol measured in the marine boundary layer became more acidic and had a lower POM mass fraction, and the POM became more oxidized. The relative humidity dependence of light extinction reflected the change in aerosol composition being lower for the near-source aerosol and higher for the more processed aerosol. A factor analysis performed on a combined data set of aerosol and gas phase parameters showed that the POM measured during the experiment was predominantly of secondary anthropogenic origin.

[1] The nitrate radical, NO3, and dinitrogen pentoxide, N2O5, are key reactive nocturnal nitrogen oxides in the troposphere. The daytime impact of NO3 and N2O5, however, is restricted by photochemical recycling of NO3 to NO2 and O3. In this paper, we report daytime measurements of N2O5 on board the NOAA research vessel Ronald H. Brown in the Gulf of Maine during the New England Air Quality Study–Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) campaign in the summer of 2004. Daytime N2O5 mixing ratios of up to 4 pptv were observed, consistent with predictions from a steady state analysis. Predicted and observed NO3 mixing ratios were below the instrumental detection limit of ∼1 pptv; the average calculated concentration was 0.09 pptv. Important impacts of daytime NO3 and N2O5 in the marine boundary layer included increased rates of VOC oxidation (in particular dimethyl sulfide) and enhanced NOx to HNO3 conversion, both of which scaled with the available NOx. Smaller effects of daytime NO3 and N2O5 included chemical destruction of O3 and a shift of the NO2:NO ratio. Because the rates of heterogeneous conversion of N2O5 and NO3 to HNO3 scale with the surface area available for uptake, the importance of daytime fog is discussed.

[1] Concentrations of NO3 and N2O5 were measured using an in situ instrument aboard the NOAA research vessel Ronald H. Brown in the marine boundary layer along the United States east coast as part of the New England Air Quality Study (NEAQS) in the summer of 2002. We analyze the results in terms of the loss partitioning and sink budgets for both of these compounds. Analysis of the data on nights with large N2O5 losses allowed for a determination of its heterogeneous uptake coefficient and gave γ(N2O5) = 0.03 ± 0.02. Reactions of NO3 with terrestrially emitted biogenic volatile organic compounds (isoprene and monoterpenes), advected into the marine boundary layer, and with DMS emitted from the ocean surface were also important. In general, loss of NO3 and N2O5 was rapid, and the partitioning between NO3 and N2O5 losses was roughly equal. Because rapid N2O5 loss consumes NOx at twice the rate of the reaction of NO2 with O3, whereas rapid NO3 loss leads to NOx removal at the same rate, the equal partitioning of losses indicates a nocturnal NOx loss rate of approximately 1.5 times the rate of NO2 + O3. Activation of halogens from the uptake of NO3 and N2O5 on sea salt was calculated to have produced substantial amounts of active Cl on some mornings through the nocturnal formation and sunrise photolysis of ClNO2 if the process proceeded at the rate determined by laboratory studies. However, there was no direct observational evidence to test the magnitude of the predicted source.

[1] Isoprene and its first and second generation photochemical products, methyl vinyl ketone (MVK), methacrolein (MACR), and peroxymethacrylic nitric anhydride (MPAN), were measured off the coast of New England during the 2002 New England Air Quality Study (NEAQS) on board the NOAA Research Vessel Ronald H. Brown. The results of these measurements were analyzed using a simple sequential reaction model that has been used previously to examine regional oxidant chemistry. The highest isoprene impact was observed in air masses that had passed over an area of high isoprene emission WSW of Boston. The relative concentrations of isoprene and its first generation products show that the photochemistry is consistently ‘‘older’’ than the isoprene photochemistry observed at continental sites. The sequential reaction model was also applied to the aldehyde-PANs (Peroxycarboxylic nitric anhydride) system, and the resulting PPN (peroxypropionic nitric anhydride)/propanal and PAN (peroxyacetic nitric anhydride)/ acetaldehyde relationships were consistent with additional sources of PAN in this environment, e.g., isoprene photochemistry. This isoprene source was estimated to result in approximately 1.6 to 4 times more PAN in this environment relative to that produced from anthropogenic VOCs (volatile organic compounds) alone.

[1] During the summer of 2004 large wildfires were burning in Alaska and Canada, and part of the emissions were transported toward the northeast United States, where they were measured during the NEAQS-ITCT 2k4 (New England Air Quality Study–Intercontinental Transport and Chemical Transformation) study on board the NOAA WP-3 aircraft and the NOAA research vessel Ronald H. Brown. Using acetonitrile and chloroform as tracers the biomass burning and the anthropogenic fraction of the carbon monoxide (CO) enhancement are determined. As much as 30% of the measured enhancement is attributed to the forest fires in Alaska and Canada transported into the region, and 70% is attributed to the urban emissions of mainly New York and Boston. On some days the forest fire emissions were mixed down to the surface and dominated the CO enhancement. The results compare well with the FLEXPART transport model, indicating that the total emissions during the measurement campaign for biomass burning might be about 22 Tg. The total U.S. anthropogenic CO sources used in FLEXPART are 25 Tg. FLEXPART model, using the U.S. EPA NEI-99 data, overpredicts the CO mixing ratio around Boston and New York in 2004 by about 50%.

[1] We have constructed a pulsed cavity ring-down spectrometer (CARDS) for simultaneous measurements of nitrogen dioxide (NO2), the nitrate radical (NO3), and dinitrogen pentoxide (N2O5) in the atmosphere. In this paper, we describe the development of the instrument to measure NO2 via its absorption at 532 nm. The NO2 detection channel was calibrated against a NIST traceable calibration standard as well as a photolysis-chemiluminescence (P-CL) NO2 detector. The absorption cross section of NO2 at 532 nm was determined to be (1.45 ± 0.06) × 10−19 cm2. The NO2 detection limit (1σ) for 1 s data is 40 pptv, and the instrument response is accurate within ±4% (1σ) under laboratory conditions. The linear dynamic range of the instrument has been verified from the detection limit to above 200 ppbv (r2 > 99.99%). For field measurements it is necessary to correct the CARDS NO2 signal for absorption by ozone. Under ambient conditions we report 1 s NO2 CARDS data with total uncertainty ±(4%, 60 pptv + 0.4 × (pptv/ppbv) × O3) (1σ). The instrument was deployed in the field during the New England Air Quality Study–International Transport and Chemical Transformation on board the NOAA research vessel Ronald H. Brown in the summer of 2004 and in Boulder, Colorado, in the winter of 2005. In both campaigns, CARDS and P-CL NO2 measurements were highly correlated (r2 > 98%), indicating the absence of interfering gas phase absorbers at 532 nm other than ozone and the suitability of CARDS to measure NO2 in the troposphere.

[1] A negative-ion proton-transfer chemical ionization mass spectrometer was deployed on a mobile tower-mounted platform during Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT) to measure nitrous acid (HONO) in the winter of 2011. High resolution vertical profiles revealed (i) HONO gradients in nocturnal boundary layers, (ii) ground surface dominates HONO production by heterogeneous uptake of NO2, (iii) significant quantities of HONO may be deposited to the ground surface at night, (iv) daytime gradients indicative of ground HONO production or emission, and (v) an estimated surface HONO reservoir comparable or larger than integrated daytime HONO surface production. Nocturnal integrated column observations of HONO and NO2 allowed direct evaluation of nocturnal ground surface uptake coefficients for these species (γNO2, surf = 2 × 10−6 to 1.6 × 10−5 and γHONO, surf = 2 × 10−5 to 2 × 10−4). A chemical model showed that the unknown source of HONO was highest in the morning, 4 × 106 molecules cm−3 s−1 (600 pptv h−1), declined throughout the day, and minimized near 1 × 106 molecules cm−3 s−1 (165 pptv h−1). The quantity of surface-deposited HONO was also modeled, showing that HONO deposited to the surface at night was at least 25%, and likely in excess of 100%, of the calculated unknown daytime HONO source. These results suggest that if nocturnally deposited HONO forms a conservative surface reservoir, which can be released the following day, a significant fraction of the daytime HONO source can be explained for the NACHTT observations.

[1] The nitrate radical, NO3, and dinitrogen pentoxide, N2O5, are two important components of nitrogen oxides that occur predominantly at night in the lower troposphere. Because a large fraction of NO2 reacts to form NO3 and N2O5 during the course of a night, their fate is an important determining factor to the overall fate of NOx (=NO and NO2). As a comprehensive test of nocturnal nitrogen oxide chemistry, concentrations of O3, NO, NO2, NO3, N2O5, HNO3 and a host of other relevant compounds, aerosol abundance and composition, and meteorological conditions were measured in the marine boundary layer from the NOAA research vessel Ronald H. Brown off the East Coast of the United States as part of the New England Air Quality Study (NEAQS) during the summer of 2002. The results confirm the prominent role of NO3 and N2O5 in converting NOx to HNO3 at night with an efficiency on par with daytime photochemical conversion. The findings demonstrate the large role of nighttime chemistry in determining the NOx budget and consequent production of ozone.

[1] Volatile organic compounds (VOCs) and some of their oxidants (O3, NO3) were measured on board the National Oceanic and Atmospheric Administration research ship R/V Ronald H. Brown along the coast of New England, downwind of New York, Boston, and Portsmouth and large forested areas in New Hampshire, Maine, and Massachusetts in July and August 2002. The diurnal variations of isoprene, monoterpenes, and aromatics were mainly dependent on their emissions and the abundance of the oxidants OH and NO3. Elevated mixing ratios of short-lived VOCs were only encountered at the ship, which was about 1–6 hours downwind of the sources, when the concentrations of the oxidants were low. For the biogenic compounds this was generally the case during morning and evening hours, when the lifetime of the biogenics was long because of low OH and NO3 concentrations. Most anthropogenic VOCs do not react with NO3, and therefore their mixing ratios remained elevated during the night. The products of isoprene oxidation, methyl vinyl ketone, methacrolein, and peroxymethacrylic nitric anhydride (MPAN) were, on average, more abundant than isoprene itself. Only during the transition periods from day to night, when oxidation rates were at a minimum, could isoprene exceed its products. The loss of the biogenic VOCs was dominated by reactions with NO3, whereas the loss of anthropogenics came mostly from reactions with OH. The oxygenated VOCs are the major contributor to the OH loss, except in close vicinity of emission sources. The total loss of biogenic compounds during the night was so effective that after one night of transport they were in most cases completely reacted away, whereas the mixing ratios of the anthropogenic compounds remained high during the night. The pool of reactive hydrocarbons at sunrise was thus typically dominated by anthropogenic VOCs.

[1] Gas phase nitric acid (HNO3) was measured at 5-min resolution on board the National Oceanographic and Atmospheric Administration (NOAA) research vessel Ronald H. Brown during the second leg (29 July to 10 August) of the New England Air Quality Study (NEAQS) 2002 cruise. A primary objective of the cruise was to improve understanding of the oxidation of NOx in, and removal of the oxidation products from, the polluted marine boundary layer east of northeastern North America. For the first 9 days of this leg the ship remained north of Cape Cod, and the cruise track did not extend much farther north than the New Hampshire-Maine border. During this period, HNO3 averaged 1.1 ppb and accounted for 19% of total reactive nitrogen oxides (measured NOy). On all days, peak HNO3 mixing ratios were observed in the early afternoon (average 2.3 ppb), at levels twofold to fourfold higher than the minima around sunrise and sunset. In these daytime peaks, HNO3/NOy averaged 28%. There were secondary nighttime peaks of HNO3 (0.9 ppb average), when HNO3 accounted for 16% of total reactive nitrogen oxides. This pronounced diurnal pattern confirms that production, and subsequent deposition, of HNO3 in the polluted marine boundary layer downwind of New England removes a significant fraction of the NOx exported to the atmosphere over the Gulf of Maine. Nitric acid was correlated with O3, particularly during the early afternoon interval when both molecules reached maximum mixing ratios (R2 = 0.66). The ozone production efficiency (OPE) inferred from the slope (10 ppb O3/ppb HNO3) was similar to the OPE of 9 estimated at the Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) Thompson Farm station in coastal New Hampshire during the study period.