The fate of oxidized mercury (Hg) in clouds and fogs is affected by the complexation of oxidized Hg(II) with other chemical species present in cloud and fog water. Metal complexation often influences the types of reactions available to a metal in an aqueous system. The influence of pH, major inorganic ions, and organic acids on the complexation of Hg(II) and methylmercury (MeHg) was examined for a range of cloud and fog water compositions. Fog water was collected in the San Joaquin Valley, CA and rain water was collected at Devil's Lake State Park, WI to provide additional measurements of the chemical conditions of atmospheric media. A thermodynamic model was used to determine the speciation of Hg(II) and methylmercury (MeHg) over a range of atmospherically-relevant cloud and fog compositions. The speciation of Hg(II) in cloud and fog water was highly dependent on pH. For conditions found in most clouds and fogs, the chloride ion was the most important major ion controlling Hg(II) complexation, even under conditions of relatively low chloride content. However, Hg(OH)(2(aq)), and HgClOH(aq) were found to dominate over HgCl2 in locations with high pH due emissions of agricultural ammonia; i.e. San Joaquin and Sacramento Valleys, CA. At concentrations relevant to typical cloud and fog waters, carboxylic acids (e. g. formate and acetate) did not play a significant role in Hg(II) speciation. Methyl mercury was speciated as MeHgCl in most locations, except for the locations with high pH, where MeHgOH dominated. These results provide constraints over potential reaction pathways that may transform oxidized Hg(II) in clouds and fogs.

Northern cities in the Midwest experience frequent PM concentrations in excess of the Federal PM2.5 and PM10 National Ambient Air Quality Standards (NAAQS) during winter, which increasingly outnumber the summertime excesses the further north the cities are located. In contrast, cities located south of the Great Lakes tended to experience a greater number of excesses during summer. In order to ascertain the key components of PM contributing to these winter exceedances, we examined a regional elevated PM concentration episode which occurred between January 31, 2005 and February 6, 2005. The episode was caused by meteorological stagnation which affected 9 states in the Midwestern US (MN, WI, MI, IA, IL, IN, OH, MO, and KY). The temporal and spatial characteristics of PM2.5 and PM10-2.5 mass and bulk chemistry (nitrate, sulfate, ammonium, organic matter) were measured during this and other wintertime elevated PM episodes at Milwaukee, WI, and compared to Chemical Speciation Network (CSN) data. The observed elevated PM episodes demonstrated that nitrate was the driving anthropogenic component of the wintertime exceedances. This led to the conclusion that winter NOx controls on mobile and stationary sources, in combination with sufficient SO2, VOC and NH3 controls, should be further examined for the mitigation of wintertime PM2.5 episodes in the Midwest. The north-south gradient in exceedances of the PM2.5 NAAQS indicated that strategies for avoidance of violations of the 24-hour standard, and to a lesser degree the annual standard, would benefit from an emphasis on different seasons, and therefore different PM components, depending how far north or south the city is located in the Midwest.

Zinc concentrations in hobby rocket exhaust aerosols were measured by ICP-MS to be approximately 300 mg/g, revealing that hobby rocket motors emit particulate matter highly enriched in zinc. Zinc is a commonly used indicator species in receptor models for atmospheric particulate matter and is assumed to be emitted only by well-established point sources and mobile sources. The potential impact of the ephemeral particle-bound zinc from hobby rockets on atmospheric particulate zinc concentrations was estimated using a Gaussian puff model. The results from the model were compared to 24-hour averaged particulate zinc concentration data collected in three US urban centers. Potential impacts several kilometers downwind of the rocket launch site were found to be significant. Clearly the impact of ephemeral and unregistered emissions such as those from hobby rockets needs to be considered when using zinc in source apportionment models.

Experiments were conducted to empirically examine net changes in methylmercury concentration of atmospheric waters as function of irradiance. Methods were developed to allow experiments to be conducted at atmospherically relevant concentrations using trace metal clean techniques, over a range of aqueous matrices. Rain water was collected at Devil’s Lake State Park, WI, and simulated cloud water was created by water extraction of particulate matter collected at the same site. These waters were spiked with methyl mercury chloride and mercuric chloride and exposed to sunlight on the roof of a building. Experiments were conducted during typical summer conditions with respect to temperature, sunlight intensity and sunlight duration. For all cases, exposure to sunlight resulted in net loss of methylmercury: –0.022 ± 0.002 1/hr in rainwater at a total UVB flux of 8 kWhrs/m2; –0.008 ± 0.001 1/hr in simulated cloud water at a total UVB flux of 5.5 kWhrs/m2. For dark cases, no statistically significant formation in methylmercury from inorganic mercury was detected. Furthermore, laboratory experiments to form methylmercury from mercuric-acetate complexes did not give detectable yields. Given the results of this study, and the results of studies cited in this article, it is unlikely that homogeneous MeHg formation is fast enough to lead to the net formation of MeHg in atmospheric waters exposed to sunlight.

Six engineered nanomaterial (ENM) powders (nano-diamond, nano-silver, nano-titanium dioxide, single walled carbon nanotubes, multi-walled carbon nanotubes, and C60 fullerenes) were investigated to determine their aerodynamic and chemical characteristics. Materials were suspended in a controlled environmental chamber, collected on filters and cascading deposition impactors (MOUDI), and then underwent gravimetric and chemical analysis using standard atmospheric aerosol methodologies. The chemical analyses included examining elemental/organic carbon (EC/OC), soluble metals by ICP-MS, organics by TD-GCMS, and reactive oxygen species (ROS) macrophage assay. Chemical composition and toxicity were compared to urban ambient PM values to give context to the ENM results, allowing a relative assessment of aerosol characteristics and the risks associated with ENM emissions. The results show that ENM particle suspensions generally exist in the accumulation or coarse particle mode range, while large mass concentrations of Aitken-nuclei mode particles were not observed. Key findings include the following: the organic and elemental carbon analysis of the carbon structured ENM could not adequately reconstruct the mass of these carbon based materials, suggesting the carbon structure of these samples is too refractive or the carbonaceous material is not oxidized sufficiently to allow accurate quantification with standard thermal-optical EC/OC analysis; the materials exhibited very low quantities of PAHs and alkanes, with the majority of these constituents below detection limits; a select group of soluble metals were detected in concentrations similar to those observed in urban ambient samples on a mass per mass basis, and lastly, the biological activity of the ENM was found to be small (by in-vitro ROS macrophage assay), especially when compared to the activity of atmospheric PM2.5 in an urban location in the US.

The composition, concentration, and size of submicron particulate matter (PM1) were measured at five-minute resolution by an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) at a semi-rural location northwest of the Dallas-Fort Worth, TX, area during June 2011. Because of increased organic aerosol (OA) levels, focus here is placed on the period from June 17-30. The total measured PM1 mass concentration ranged between 1.1 and 16.5 mu g m(-3), with a mean of 4.4 +/- 2.6 (one s.d.) mu g m(-3). Significant variability is observed in the time series of total PM1 and of four individual HR-ToF-AMS species, particularly between June 21 and 25. The average PM1 mass composition was dominated by OA (55.0 +/- 14.8%) and sulfate (30.7 +/- 12.3%). Organic aerosol concentrations were correlated positively with carbon monoxide (CO) (R = 0.81). This study uses a variety of aging metrics and their relations to OA/Delta CO to characterize secondary organic aerosol. Photochemical age is estimated by using the toluene to benzene ratio. The average photochemical age was 26.7 +/- 5.3 h. Other metrics of age used in this work include the ratio of sulfate to total sulfur and the ratio of nitrogen oxides to total reactive nitrogen. The correlations between the OA/Delta CO and nitrogen aging metrics indicate consistent aging, and a weak relationship is observed between OA/Delta CO and sulfur aging. However, the relationship between photochemical age and OA/Delta CO does not show a statistically significant correlation. (C) 2015 Elsevier Ltd. All rights reserved.

Recent work has identified nitric acid (HNO3) as a potential precursor of nitrous acid (HONO), which is an important source of oxidants that regulate ozone and particulate pollution. Recent work in our laboratory has indicated that the reduction of HNO3 to HONO can occur homogeneously in the presence of surrogates for volatile organic compounds (VOCs) emitted by motor vehicles. This study focuses on the impact of environmental variables on the rate of formation of HONO in this process. The observed base case (25.0 degrees C and similar to 20.0% relative humidity (RH)) HONO formation rate was 0.54 +/- 0.09 ppb h(-1), values comparable to enhancements observed in HONO during morning rush hour in Houston, TX. The rate was enhanced at lower temperatures of similar to 20.0 degrees C, but the rate remained statistically similar (1 sigma) for experiments conducted at temperatures of 25 degrees C, 30 degrees C, and 35 degrees C. The assumption that multiple reactive components of the VOC mixture react with HNO3 is supported by this observation, and the relative importance of each reactive species in the reaction may vary with temperature. The enhanced rate at lower temperatures could make the proposed reaction mechanism more important at night. The formation rate of HONO does not change substantially when initial HNO3 concentration is varied between 400 and 4600 ppt, suggesting that the concentration of reactive VOCs was the limiting factor. The reduction of HNO3 to HONO appears not to occur heterogeneously on the aerosol surfaces tested. The presence of similar to 120 ppb of ammonia has no observable impact on the reaction. However, it is likely that UV irradiation (lambda = 350 nm) decreases the formation rate of HONO either by consuming the reactive VOCs involved or by directly interfering with the reaction. The "renoxification" of less reactive HNO3 to more reactive HONO has significant implications for daytime ozone and particulate pollution. Copyright (C) 2015 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.

We present field observations made in June 2011 downwind of Dallas-Fort Worth, TX, and evaluate the role of stabilized Criegee radicals (sCIs) in gaseous sulfuric acid (H2SO4) production. Zero-dimensional model calculations show that sCI from biogenic volatile organic compounds composed the majority of the sCIs. The main uncertainty associated with an evaluation of H2SO4 production from the sCI reaction channel is the lack of experimentally determined reaction rates for sCIs formed from isoprene ozonolysis with SO2 along with systematic discrepancies in experimentally derived reaction rates between other sCIs and SO2 and water vapor. In general, the maximum of H2SO4 production from the sCI channel is found in the late afternoon as ozone increases toward the late afternoon. The sCI channel, however, contributes minor H2SO4 production compared with the conventional OH channel in the mid-day. Finally, the production and the loss rates of H2SO4 are compared. The application of the recommended mass accommodation coefficient causes significant overestimation of H2SO4 loss rates compared with H2SO4 production rates. However, the application of a lower experimental value for the mass accommodation coefficient provides good agreement between the loss and production rates of H2SO4. The results suggest that the recommended coefficient for the H2O surface may not be suitable for this relatively dry environment.

The air quality in the outflow from Fort Worth, TX was studied in June 2011 at a location surrounded by oil and gas development in the Barnett Shale. The objectives of this study were to understand the major sources of volatile organic compounds (VOCs) and organic aerosols and explore the potential influence each VOC source had on ozone and secondary organic aerosol formation. Measurements of VOCs were apportioned between six factors using Positive Matrix Factorization (PMF): Natural Gas (25 +/- 2%; +/-99% CL); Fugitive Emissions (15 2%); Internal Combustion Engines (15 +/- 2%); Biogenic Emissions (7 +/- 1%); Industrial Emissions/Oxidation 1(8 +/- 1%); and Oxidation 2 (18 +/- 2%). Reactivity calculations suggest the Biogenic and Oxidation 2 factors were the most likely VOC sources to influence local ozone. However, enough OH reactivity was calculated for factors related to the oil and gas development that they could incrementally increase O-3. Three organic aerosol (OA) types were identified with PMF applied to high-resolution time-of-flight aerosol mass spectrometry measurements: hydrocarbon-like OA (HOA; 11% of mass) and two classes of oxidized OA (semi- and less-volatile OOA, SV and LV; 45% and 44%, respectively). HOA correlated with the Internal Combustion Engine VOC factor indicating that a large fraction of the HOA was emitted by gasoline and diesel motors. The SV-OOA correlated with the oxidized VOC factors during most of the study, whereas a correlation between LV-OOA and the oxidized VOC factors was only observed during part of the study. It is hypothesized that SV-OOA and the oxidized VOC factors correlated reasonably well because these factors likely were separated by at most only a few oxidation generations on the oxidation pathway of organic compounds. (C) 2015 Elsevier Ltd. All rights reserved.

Intensive air quality measurements made from June 22-25, 2011 in the outflow of the Dallas Fort Worth (DFW) metropolitan area are used to evaluate nitrous acid (HONO) sources and sinks. A two layer box model was developed to assess the ability of established and recently identified HONO sources and sinks to reproduce observations of HONO mixing ratios. A baseline model scenario includes sources and sinks established in the literature and is compared to scenarios including three recently identified sources: volatile organic compound-mediated conversion of nitric acid to HONO (S1), biotic emission from the ground (S2), and re-emission from a surface nitrite reservoir (S3). For all mechanisms, ranges of parametric values span lower- and upper-limit values. Model outcomes for 'likely' estimates of sources and sinks generally show under-prediction of HONO observations, implying the need to evaluate additional sources and variability in estimates of parameterizations, particularly during daylight hours. Monte Carlo simulation is applied to model scenarios constructed with sources S1-S3 added independently and in combination, generally showing improved model outcomes. Adding sources S2 and S3 (scenario S2/S3) appears to best replicate observed HONO, as determined by the model coefficient of determination and residual sum of squared errors (r(2) = 0.55 +/- 0.03, SSE = 4.6 x 10(6) +/- 7.6 x 10(5) ppt(2)). In scenario S2/S3, source S2 is shown to account for 25% and 6.7% of the nighttime and daytime budget, respectively, while source S3 accounts for 19% and 11% of the nighttime and daytime budget, respectively. However, despite improved model fit, there remains significant underestimation of daytime HONO; on average, a 0.15 ppt/s unknown daytime HONO source, or 67% of the total daytime source, is needed to bring scenario S2/S3 into agreement with observation. Estimates of 'best fit' parameterizations across lower to upper-limit values results in a moderate reduction of the unknown daytime source, from 0.15 to 0.10 ppt/s. (C) 2015 Elsevier Ltd. All rights reserved.

Nitric acid (HNO3) was reduced in a flow tube by volatile organic carbon compounds (VOCs) generated from engine oil vapor. The primary reaction product was believed to be HONO. The reaction was not enhanced when Teflon (R) Raschig rings were added to the flow tube to increase surface area, thereby showing the reaction to be homogeneous under the conditions studied. The HONO formation observed ranged between 0.1 and 0.6 ppb h(-1), with a mean of 0.3 +/- 0.1 ppb h(-1), for typical HNO3 concentrations of 4-5 ppb and estimated concentrations of the reactive components in the engine oil vapor between 200 and 300 ppt. The observations in this study compare well to a recently published field study conducted in Houston that observed average formation rates of 0.6 +/- 0.3 ppb h(-1). Water vapor was found to decrease the HONO formation rate by similar to 0.1 ppb h(-1) for every 1% increase in the water mixing ratio. (C) 2014 Elsevier Ltd. All rights reserved.

In this study, we examined the heterogeneous reduction of Hg(II) on the coal fly ash samples and synthetic aerosols under different light conditions in a controlled laboratory reactor. Three types of coal fly ashes were studied: a high carbon fly ash from a stoker boiler, a low carbon/low sulfate fly ash from a pulverized coal combustor burning low sulfur coal, and a high sulfate fly ash from a pulverized coal combustor burning high sulfur coal. The rate of Hg(II) reduction on the three diverse fly ash samples was found to be relatively fast with an average half-life of 1.6h under clear sky atmospheric conditions (under the irradiance of 1000W/m2). The reduction rate in the low sulfate/low carbon fly ash was approximately 1.5 times faster than with the other coal fly ash samples. Synthetic aerosols made of carbon black and levoglucosan produced Hg(II) reduction rates similar to coal fly ashes. However, aerosols composed of adipic acid resulted in reduction rates that were 3–5 times faster. The sensitivity of adipic acid reduction to light source wavelength was found to be greater than for the coal fly ash and other synthetic aerosols. Aerosols made from the water extracts of coal fly ash samples produced reduction rates equal to or slightly higher than with the native fly ash suggesting that the soluble components of fly ash play a significant role in the reduction mechanism. The measured reduction rates are likely important in the chemical processing of mercury in power plant plumes and potentially in the atmosphere and should be considered for incorporation in atmospheric transport models that are used to understand the fate of atmospheric mercury.

ADVERTISEMENT RETURN TO ISSUEPREVViewpointNEXTProgress on Understanding Atmospheric Mercury Hampered by Uncertain MeasurementsDaniel A. Jaffe*†‡, Seth Lyman§, Helen M. Amos∥, Mae S. Gustin⊥, Jiaoyan Huang⊥, Noelle E. Selin#, Leonard Levin∇, Arnout ter Schure○, Robert P. Mason◆, Robert Talbot¶, Andrew Rutter∞, Brandon Finley†, Lyatt Jaeglé‡, Viral Shah‡, Crystal McClure‡, Jesse Ambrose†, Lynne Gratz†, Steven Lindberg$, Peter Weiss-Penzias⊗, Guey-Rong Sheu∀, Dara Feddersen⧓, Milena Horvat◘, Ashu DastoorЯ, Anthony J. Hynes@, Huiting Mao∏, Jeroen E. Sonke★, Franz Slemr⧖, Jenny A. Fisher∫, Ralf Ebinghaus∮, Yanxu Zhang×, and Grant Edwards⪫View Author Information† School of Science, Technology, Engineering and Mathematics, University of Washington, Bothell, Washington 98011, United States‡ Department of Atmospheric Sciences, University of Washington, Seattle, Washington 98195, United States§ Bingham Entrepreneurship and Energy Research Center, Utah State University, Vernal, Utah 84078, United States∥ Department of Earth and Planetary Science, Harvard University, Cambridge, Massachusetts 02138, United States⊥ Department of Natural Resources and Environmental Sciences, University of Nevada, Reno, Nevada 89557, United States# Engineering Systems Division and Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 89557, United States∇ Palo Alto, California 94304, United States○ Electric Power Research Institute, Palo Alto, California 94304, United States◆ Department of Marine Sciences & Chemistry, University of Connecticut, Groton, Connecticut 06340, United States¶ Institute for Climate and Atmospheric Science, University of Houston, Houston, Texas 77004, United States∞ Department of Chemistry, Carroll University, Waukesha, Wisconsin 53186, United States$ Graeagle, California 96103, United States⊗ Department of Microbiology and Environmental Toxicology, University of California, Santa Cruz, California 95064, United States∀ Department of Atmospheric Sciences, National Central University, Jhongli, 320, Taiwan⧓ Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, United States◘ Department of Environmental Sciences, Jozef Stefan Institute, Ljubljana, SloveniaЯ Air Quality Research Division, Environment Canada, Dorval, Quebec M3H 5T4, Canada@ Division of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, United States∏ Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210, United States★ Observatoire Midi-Pyrénées, Université Paul Sabatier, 31062, Toulouse, France⧖ Atmospheric Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany∫ Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia∮ Department of Environmental Chemistry, Institute of Coastal Research, Geesthacht, Germany× School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States⪫ Faculty of Science, Macquarie University, Sydney, New South Wales 2109, Australia*E-mail: [email protected]Cite this: Environ. Sci. Technol. 2014, 48, 13, 7204–7206Publication Date (Web):June 18, 2014Publication History Received3 June 2014Published online18 June 2014Published inissue 1 July 2014https://pubs.acs.org/doi/10.1021/es5026432https://doi.org/10.1021/es5026432newsACS PublicationsCopyright © 2014 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views2098Altmetric-Citations81LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (644 KB) Get e-AlertscloseSUBJECTS:Calibration,Fluorescence,Mercury,Organic reactions,Quality management Get e-Alerts

Well-designed health studies and the development of effective regulatory policies need to rely on an understanding of the incremental differences in particulate matter concentrations and their sources. Although only a limited number of studies have been conducted to examine spatial differences in sources to particulate matter within an air shed, routine monitoring data can be used to better understand these differences. Measurements from the US EPA Chemical Speciation Network (CSN) collected between 2002–2008 were analyzed to demonstrate the utility of regulatory data across three sites located within 100 km of each other. Trends in concentrations, source contribution, and incremental excesses across three sites were investigated using the Positive Matrix Factorization model. Similar yearly trends in chemical composition were observed across all sites, however, excesses of organic matter and elemental carbon were observed in the urban center that originated from local emissions of mobile sources and biomass burning. Secondary sulfate and secondary nitrate constituted over half of the PM2.5 with no spatial differences observed across sites. For these components, the excess of emissions from industrial sources could be directly quantified. This study demonstrates that CSN data from multiple sites can be successfully used to derive consistent source profiles and source contributions for regional pollution, and that CSN data can be used to quantify incremental differences in source contributions of across these sites. The analysis strategy can be used in other regions of the world to take advantage of existing ambient particulate matter monitoring data to better the understanding of spatial differences in source contributions within a given air shed.

The contributions of anthropogenic and biogenic secondary organic carbon (SOC) to total PM2.5 mass are of interest to air quality management agencies required to demonstrate maintenance of the PM2.5 NAAQS. Reductions of SOC can be used in conjunction with the mitigation of other PM2.5 constituents to maintain PM2.5 concentrations below the regulatory limit. Currently, quantitative tools to understand the SOC source contributions to PM2.5 mass are not well developed, and the spatial variation of different types of SOC is not known.In this study concentrations of anthropogenic and biogenic SOC mass were determined using PM2.5 measurements made in Cleveland, OH and Mingo Junction, OH. Twenty-four hour averaged samples were collected on the EPA 1-in-6 day schedule over the course of one year between June 2007 and May of 2008. Organic molecular markers for anthropogenic and biogenic SOC were extracted from the PM2.5, silylated, and then analyzed by GC MS. Source apportionment calculations were conducted using the EPA CMB (v.8.2) software and organic molecular markers as source tracers.SOC concentrations calculated from SOC tracers measurements followed the expected seasonal patterns with maximum contributions during the summer and minimum contributions during the winter. Anthropogenic SOC constituted approximately 37% to the apportioned SOC and 6% to the measured OC, on average across both sites. Biogenic SOC contributed the 42% to the apportioned SOC, and 4% to the measured DC. Anthropogenic SOC contributed strongly to organic PM2.5 meaning that SOC may by partially controllable by reductions in VOC emissions from anthropogenic sources.Similarities in the month-to-month patterns in a-pinene markers were observed between Cleveland and Mingo Junction, suggesting a regional character to this type of SOC. However, such patterns were not readily apparent in the isoprene markers.Limitations were found in the current version of the model. Approximately half of the water soluble organic carbon unrelated to biomass burning (NB-WSOC) during spring, summer and early fall could not be apportioned by the CMB model with the SOC markers available during this study. This suggested that additional sources not included in the CMB model used in this study contributed to SOC, or that models using markers measured in chamber oxidations are not entirely representative of the study sites. The unapportioned OC did not correlate particularly well with any of the known OC sources. While performance of the model is limited due to uncertainties in the source profiles, the apportionments calculated still give a preliminary insight into the relative contributions to Soc from anthropogenic and biogenic emissions. (C) 2013 Elsevier Ltd. All rights reserved.

A laboratory reactor system was developed to examine the role of light and aerosol composition in the reduction of oxidized mercury (Hg(ii)) in laboratory-generated aerosols. Aerosolized sodium chloride, doped with mercury chloride, was exposed to light in a fixed-bed flow-through reactor. Three spectral ranges (UV, visible and a simulated solar spectrum) were examined, along with dark experiments, to investigate the role of light conditions in mercury reduction. In addition, the role of iron in the aerosol matrix was examined. The effluent from the reactor was analyzed for Hg(0) as evidence of reduction of Hg(ii) in the reactor. Significant reduction of Hg(ii) (1.5-9.9%) was observed for all three light sources and the rate of mercury reduction was proportional to the light irradiance. The presence of iron in the aerosol matrix inhibited the reduction rate and the degree of inhibition was dependent on the chemical form of the iron in the aerosol. The observed reduction reactions may be important chemical processes in the atmosphere and could be incorporated in atmospheric transport models that are used to understand the fate of atmospheric mercury.

Gaseous elemental mercury (GEM; Hg-(g)(0)) was oxidized by ozone and secondary hydroxyl radicals generated by the chemistry associated with the formation of secondary organic aerosols. The reaction was investigated in a 9-m(3) Teflon (R) batch reactor.The losses of GEM in ozone-only experiments compared well with numerical model predictions based on published reaction rates, and a second order rate analysis gave a reaction rate of (7.4 +/- 0.5) x 10(-19) cm(3) molecules(-1) s(-1), which was statistically indistinct from recent publications. Furthermore, the net oxidation of GEM observed in the SOA reaction system agreed well with a numerical model based on the GEM-ozone reaction rate determined in this study and a published GEM-OH oxidation rate.Recent modeling studies of mercury atmospheric cycling have found that use of laboratory-based GEM-ozone reaction rate coefficients caused overestimation of GEM oxidation, while theoretical studies cast doubt over the viability of the GEM-ozone oxidation reaction in the real atmosphere. The results presented here suggest that the reaction is viable in the atmosphere and that recent published reaction rates for GEM and ozone are pertinent for use in atmospheric models. An average of GEM-ozone rates determined during this and recent studies was 6.9 +/- 0.9 x 10(-19) cm(3) molecules(-1) s(-1). This value is recommended for use in future modeling studies. (C) 2012 Elsevier Ltd. All rights reserved.

This report contains an assessment by Sonoma Technology, Inc. of two near-road emission data studies conducted by others. In the first study, near-road Nitrogen dioxide (NO2) concentrations were measured in Las Vegas, NV, over one year between September 2007 and September 2008 at an elementary school located next to the US 95 freeway. The measurement site was 37 m from the edge of the road with a sound wall between the site and the road. The sample inlet was at the height of the top of the sound wall. US 95 is the road with a sound wall between the site and the road. The sample inlet was at the height of the top of the sound wall. Approximately 12% of the traffic on US 95 in 2007-2008 was heavy-duty diesel vehicles. In the second study, measurements next to interstate freeway I-710 near Los Angeles were made between February 2009 and March 2012, with winter and summer intensive measurement studies taking place in 2009. The majority of the analysis presented in this report focused on the intensive study periods. I-710 is a major trucking route to and from the Port of Long Beach, with annual average daily traffic ranging between 187,000 and 191,000 during 2009-2011 and with approximately 17-18% of the traffic comprising heavy-duty diesel vehicles. This study demonstrated that while both sites were below the 1-hr NO2 National Ambient Air Quality Standards (NAAQS), a reduction of the NAAQS to 80 ppb would likely cause the Los Angeles location to be above the standard. The data suggest that wind direction, wind speed, proximity of the monitor to the roadway, traffic patterns, total vehicle counts, fraction of heavy-duty diesel vehicles, urban background ozone concentrations, and urban background NO2 concentrations were the key factors influencing near-road NO2 concentrations.