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In the United States, industrial compounds are routinely transported by rail in pressurized vessels, often near urban areas. A rupture of a vessel, for example due to a derailment, can result in the rapid release of a liquid-aerosol-gas mixture. The health consequences of such a release, especially close to population centers, are not well understood. To address this question, a series of controlled experimental releases of pressurized chlorine (Cl 2) was conducted at the Dugway Proving Ground (Dugway, Utah). Each trial consisted of the sudden breach of a tank containing at least 4,500 kg (kg) of pressurized liquid Cl 2. In this paper, we report on measured Cl 2 concentrations in three test structures downwind of the release. Based on these data, we estimate the indoor-outdoor exchange, transport through a multi-room structure, and the first-order loss rate due to reaction or sorption. This loss rate is particularly important for consequence assessment. For example, in a mobile office with a ventilation rate of about three air changes per hour, the reaction loss rate was approximately 2.5 h−1. This accounts for a nearly 20 percent reduction in toxic load to indoor occupants. Finally, the paper discusses the modeling and analysis of a typical urban hazard assessment. • In 2015 and 2016, a series of controlled release experiments of pressurized chlorine (Cl 2) was conducted. • An experiment consisted of the sudden breaching of a tank containing greater than 4,500 kg (kg) of pressurized liquid Cl 2. • The measurement of the Cl 2 plume's transport through an array of downwind structures. • This paper reports the results of an experiment that is unlikely to be conducted often. • The reaction between the gas and indoor surfaces is important for hazard and consequence assessment. • The current study found that total Cl 2 gas loss was faster than the trailer's air change rate. [ABSTRACT FROM AUTHOR]

To predict atmospheric conditions in an urban environment, the land surface processes must be accurately described through the use of detailed land use (LU) and land cover (LC) data. Use of the U.S. Geological Survey (USGS) 25-category data, currently in the Fifth-generation Mesoscale Model (MM5), with the Noah land surface model (LSM) and MRF (medium-range forecast) planetary boundary layer (PBL) schemes resulted in the over-prediction of daytime temperatures in the Houston downtown area due to the inaccurate representation as a completely impervious surface. This bias could be corrected with the addition of canopy water in the urban areas from the evapotranspiration effects of urban vegetation. A more fundamental approach would be to utilize an LULC dataset that represents land surface features accurately. The Texas Forest Service (TFS) LULC dataset established with the LANDSAT satellite imagery correctly represents the Houston–Galveston–Brazoria (HGB) area as mixtures of urban, residential, grass, and forest LULC types. This paper describes how the Noah LSM and PBL schemes in the MM5 were modified to accommodate the TFS-LULC data. Comparisons with various meteorological measurements show that the MM5 simulation made with the high resolution LULC data improves the boundary layer mixing conditions and local wind patterns in the Houston Ship Channel, which is a critically important anthropogenic emission area affecting the HGB air pollution problems. In particular, when the synoptic flows are weak, the improved LULC data simulates the asymmetrically elongated Houston heat island convergence zone influencing the location of the afternoon Gulf of Mexico sea-breeze front and the Galveston Bay breeze flows. This paper is part I of a two-part study and focuses on the meteorological simulation. In part II, effects of using the different meteorological inputs on air quality simulations are discussed. [Copyright &y& Elsevier]

In February 2000, a detailed measurement campaign was conducted over a 9-day period within a mercury (Hg) cell chlor-alkali plant in the southeastern United States (US). The principal focus of this study was to determine the fugitive (non-ducted) airborne Hg emissions from the main production equipment located in the “cell building” during extended period operations, including maintenance events. Other Hg monitoring conducted in and around the plant also included surveys of process areas surrounding the cell building, measurement of Hg fluxes from soil and other exposed surfaces, and speciated Hg measurements of ambient air both “upwind” and “downwind” of the production area as well as the determination of Hg contained in output products and wastes. The study was a collaborative effort among scientists from the US Environmental Protection Agency, the US Department of Energy''s Oak Ridge National Laboratory, and the University of Michigan. This paper provides a description of the facility, the rationale and design of the study, and an introduction to companion papers containing the observational data. [Copyright &y& Elsevier]

Abstract: The purpose of this paper is to determine the impact of reduced atmospheric particulate resulting from the Clean Air Act of 1970 on changes in summer rainfall in the Atlanta, Georgia USA region. In order to determine if rainfall at nine candidate stations in the metropolitan area was influenced by changes in particulate concentrations within the 1948–2009 period, predicted rainfall characteristics were derived from rainfall frequencies at nine reference stations located more than 80 km from downtown Atlanta. Both parametric and non-parametric tests were used to test for significant differences between observed values and predicted values within 34 overlapping 30-year periods. For the country as a whole, emissions of PM10 (i.e. particulates with a diameter less than or equal to 10 μm) decreased by approximately 40% from 1970 to 1975. The reduction in emissions caused a rapid rebound in summer rainfall in the Atlanta region. There was suppression of rainfall over and downwind of the Atlanta urbanized area during 30-yr periods that comprise all or portions of the decades of the 1950s, 1960s, and 1970s. This suppression occurred even while urban-related factors that promote rainfall enhancement were present. During the 1948–1977 suppression period, there was a decrease in rainfall of at least 40 mm at affected locales, which is substantial given that the mean seasonal rainfall was approximately 300 mm. The rainfall suppression involved a decrease of heavy-rainfall days. Atlanta is most likely not a unique case; therefore, particulate-induced rainfall suppression might have occurred over and downwind of other U.S. urban areas prior to the late 1970s. [Copyright &y& Elsevier]

Abstract: Many cohort studies in environmental epidemiology require accurate modeling and prediction of fine scale spatial variation in ambient air quality across the U.S. This modeling requires the use of small spatial scale geographic or “land use” regression covariates and some degree of spatial smoothing. Furthermore, the details of the prediction of air quality by land use regression and the spatial variation in ambient air quality not explained by this regression should be allowed to vary across the continent due to the large scale heterogeneity in topography, climate, and sources of air pollution. This paper introduces a regionalized national universal kriging model for annual average fine particulate matter (PM2.5) monitoring data across the U.S. To take full advantage of an extensive database of land use covariates we chose to use the method of Partial Least Squares, rather than variable selection, for the regression component of the model (the “universal” in “universal kriging”) with regression coefficients and residual variogram models allowed to vary across three regions defined as West Coast, Mountain West, and East. We demonstrate a very high level of cross-validated accuracy of prediction with an overall R 2 of 0.88 and well-calibrated predictive intervals. In accord with the spatially varying characteristics of PM2.5 on a national scale and differing kriging smoothness parameters, the accuracy of the prediction varies by region with predictive intervals being notably wider in the West Coast and Mountain West in contrast to the East. [Copyright &y& Elsevier]

Abstract: The ESCAPE study (European Study of Cohorts for Air Pollution Effects) investigates relationships between long-term exposure to outdoor air pollution and health using cohort studies across Europe. This paper analyses the spatial variation of PM2.5, PM2.5 absorbance, PM10 and PMcoarse concentrations between and within 20 study areas across Europe. We measured NO2, NO x , PM2.5, PM2.5 absorbance and PM10 between October 2008 and April 2011 using standardized methods. PMcoarse was determined as the difference between PM10 and PM2.5. In each of the twenty study areas, we selected twenty PM monitoring sites to represent the variability in important air quality predictors, including population density, traffic intensity and altitude. Each site was monitored over three 14-day periods spread over a year, using Harvard impactors. Results for each site were averaged after correcting for temporal variation using data obtained from a reference site, which was operated year-round. Substantial concentration differences were observed between and within study areas. Concentrations for all components were higher in Southern Europe than in Western and Northern Europe, but the pattern differed per component with the highest average PM2.5 concentrations found in Turin and the highest PMcoarse in Heraklion. Street/urban background concentration ratios for PMcoarse (mean ratio 1.42) were as large as for PM2.5 absorbance (mean ratio 1.38) and higher than those for PM2.5 (1.14) and PM10 (1.23), documenting the importance of non-tailpipe emissions. Correlations between components varied between areas, but were generally high between NO2 and PM2.5 absorbance (average R 2 = 0.80). Correlations between PM2.5 and PMcoarse were lower (average R 2 = 0.39). Despite high correlations, concentration ratios between components varied, e.g. the NO2/PM2.5 ratio varied between 0.67 and 3.06. In conclusion, substantial variability was found in spatial patterns of PM2.5, PM2.5 absorbance, PM10 and PMcoarse. The highly standardized measurement of particle concentrations across Europe will contribute to a consistent assessment of health effects across Europe. [Copyright &y& Elsevier]

Abstract: Indoor air pollutants at high concentrations in poultry houses can potentially affect workers'' health, and animal welfare and productivity. This paper presents research results of a 2-year continuous monitoring of ammonia (NH3), carbon dioxide (CO2), hydrogen sulfide (H2S), and particulate matter (PM) concentrations from to date the most comprehensive study on a single farm in two 180,000-bird high-rise (HR) and two 200,000-bird manure-belt (MB) layer hen houses located in Indiana, USA. Air was sampled at ventilation fans of the mechanically-ventilated houses. Concentrations of NH3 and CO2 were measured with photoacoustic multi-gas monitors. Concentrations of H2S and PM10 were monitored with pulsed fluorescence analyzers and Tapered Element Oscillating Microbalances (TEOM), respectively. The 2-year mean ± standard deviation concentrations at ventilation fans of the four layer hen houses were 48.9 ± 39 and 51.9 ± 40.7 ppm in HR, and 13.3 ± 9.1 and 12.9 ± 10.5 ppm in MB for NH3; 26.4 ± 17.6 and 24.9 ± 19 ppb in HR, 40.0 ± 21.1 and 41.2 ± 31.5 ppb in MB for H2S; 1755 ± 848 and 1804 ± 887 ppm in HR, and 2295 ± 871 and 2285 ± 946 ppm in MB for CO2; and 540 ± 303 and 552 ± 338 μg m−3 in HR, and 415 ± 428 and 761 ± 661 μg m−3 in MB for PM10. Compared with the MB houses, concentrations of the HR houses were higher for NH3, and lower for CO2, H2S, and PM10 (P < 0.05). High concentrations of NH3 detected in winter represent potential challenges to workers'' health and animal welfare. Variations in pollutant concentrations at the exhaust fans were affected by outdoor temperature, ventilation, bird condition, and farm operation. A new weekly variation, characterized by significantly lower PM10 concentrations on Sundays, was identified and was related to the weekly schedule of house operational activities. [Copyright &y& Elsevier]

Abstract: Policy Relevant Background (PRB) ozone, as defined by the US Environmental Protection Agency (EPA), refers to ozone concentrations that would occur in the absence of all North American anthropogenic emissions. PRB enters into the calculation of health risk benefits, and as the US ozone standard approaches background levels, PRB is increasingly important in determining the feasibility and cost of compliance. As PRB is a hypothetical construct, modeling is a necessary tool. Since 2006 EPA has relied on global modeling to establish PRB for their regulatory analyses. Recent assessments with higher resolution global models exhibit improved agreement with remote observations and modest upward shifts in PRB estimates. This paper shifts the paradigm to a regional model (CAMx) run at 12km resolution, for which North American boundary conditions were provided by a low-resolution version of the GEOS-Chem global model. We conducted a comprehensive model inter-comparison, from which we elucidate differences in predictive performance against ozone observations and differences in temporal and spatial background variability over the US. In general, CAMx performed better in replicating observations at remote monitoring sites, and performance remained better at higher concentrations. While spring and summer mean PRB predicted by GEOS-Chem ranged 20–45ppb, CAMx predicted PRB ranged 25–50ppb and reached well over 60ppb in the west due to event-oriented phenomena such as stratospheric intrusion and wildfires. CAMx showed a higher correlation between modeled PRB and total observed ozone, which is significant for health risk assessments. A case study during April 2006 suggests that stratospheric exchange of ozone is underestimated in both models on an event basis. We conclude that wildfires, lightning NO x and stratospheric intrusions contribute a significant level of uncertainty in estimating PRB, and that PRB will require careful consideration in the ozone standard setting process. [Copyright &y& Elsevier]

Abstract: Increased use of ethanol in the United States fuel supply will impact emissions and ambient concentrations of greenhouse gases, “criteria” pollutants for which the U. S. EPA sets ambient air quality standards, and a variety of air toxic compounds. This paper focuses on impacts of increased ethanol use on ozone and air toxics under a potential implementation scenario resulting from mandates in the U. S. Energy Independence and Security Act (EISA) of 2007. The assessment of impacts was done for calendar year 2022, when 36 billion gallons of renewable fuels must be used. Impacts were assessed relative to a baseline which assumed ethanol volumes mandated by the first renewable fuels standard promulgated by U. S. EPA in early 2007. This assessment addresses both impacts of increased ethanol use on vehicle and other engine emissions, referred to as “downstream” emissions, and “upstream” impacts, i.e., those connected with fuel production and distribution. Air quality modeling was performed for the continental United States using the Community Multi-scale Air Quality Model (CMAQ), version 4.7. Pollutants included in the assessment were ozone, acetaldehyde, ethanol, formaldehyde, acrolein, benzene, and 1,3-butadiene. Results suggest that increased ethanol use due to EISA in 2022 will adversely increase ozone concentrations over much of the U.S., by as much as 1ppb. However, EISA is projected to improve ozone air quality in a few highly-populated areas that currently have poor air quality. Most of the ozone improvements are due to our assumption of increases in nitrogen oxides (NO x ) in volatile organic compound (VOC)-limited areas. While there are some localized impacts, the EISA renewable fuel standards have relatively little impact on national average ambient concentrations of most air toxics, although ethanol concentrations increase substantially. Significant uncertainties are associated with all results, due to limitations in available data. These uncertainties are discussed in detail. [Copyright &y& Elsevier]

Abstract: An ambient air monitoring campaign was performed in six counties (Sacramento, CA; Maricopa, AZ; Anoka, MN; Jefferson, KY; Harris, TX; and Pinellas, FL) between January 2008 and September 2009. The purpose of this paper is to compare the spatial and temporal variability of black carbon (BC) and particle-bound polycyclic aromatic hydrocarbons (p-PAHs), across these counties using continuous monitoring instruments – an Aethalometer and a Photoelectric Aerosol Sensor reporting in units of μg m−3 and fA, respectively. We explored temporal trends in these measurements to assess the potential impact of local combustion sources on air quality. Median BC concentrations ranged from 0.13 to 0.53 μg m−3; and median p-PAH values ranged from 0.31 to 4.18 fA. Hourly BC and p-PAH were elevated during morning rush hour and rapidly decreased later in the morning. Nighttime increases in BC and p-PAH were also observed in most counties. Diurnal patterns of BC and p-PAH were different on weekdays compared to weekends. Profiles of hourly ratios of p-PAH/BC in combination with meteorological data can provide insight into potential sources across the sites. Hourly ratios of p-PAH/BC which peaked during early morning and late afternoon hours suggest a dominating contribution of motor vehicle sources in four of the six counties. In two counties, hourly ratios remained elevated for several hours after rush hour and did not show a distinctive peak suggesting additional sources of BC and p-PAH. Such profiles were seen in both Jefferson KY and Harris TX, and may be attributed to coal combustion, petro-chemical industry and shipping activities, respectively. These results suggest that measurements of BC and p-PAH, combined with meteorological information and emission data are potentially useful to identify combustion sources impacting air quality. More research combining BC and p-PAH measurements with detailed source apportionment data is needed to more fully evaluate the utility of these real-time measures. [Copyright &y& Elsevier]

Abstract: In this paper, an analytical methodology is proposed for estimating diffuse dust emissions from bulk solids managing (storage, handling, and transport) activities in the ceramic industry. The methodology is based on analytical methods drawn from the AP-42, US Environmental Protection Agency reports and the Emission Estimation Technique (EET) Manual for Mining from Environment Australia. The analytical methods were evaluated by means of experimental campaigns in open and closed bulk solids storage and managing facilities in ceramic plants. Dust concentrations and meteorological variables were recorded in experimental field studies in order to implement the experimental models: a method based on Reverse Dispersion Modelling (RDM) was applied in open facilities, and the Roof Monitor Method (RMM) was used to estimate diffuse dust emissions in closed facilities. The proposed methodology was applied to 13 ceramic plants with different technological scenarios. The methodology enabled the diffuse PM10 emission factors associated with different dust abatement technologies to be determined. This methodology thus allows the Best Available Techniques (BATs) to be selected for reducing these emissions in ceramic and similar facilities. [Copyright &y& Elsevier]

Abstract: An emission pre-processing tool is generally needed to transform a bulk annual emission inventory into gridded, temporal (monthly, weekly, and hourly), and speciated emissions in order to use complex air quality models such as the Community Multi-scale Air Quality (CMAQ) model to assess control strategies, forecast air quality, and investigate pollution formation and transport processes. To support the regional air quality modeling in the Pearl River Delta (PRD), we developed an emission pre-processing system, Sparse Matrix Operator Kernel Emissions-PRD (SMOKE-PRD), based on the U.S. Environmental Protection Agency (USEPA)/SMOKE model. This paper introduces the methods and procedures for adapting the SMOKE model to the PRD regional bulk emissions. These include the compilation of the PRD local source-class clarification codes, the incorporation of the PRD local emission inventory, and the updating of spatial, temporal, and chemical speciation information. The SMOKE-PRD system was evaluated, and a case study on ozone simulation was conducted to demonstrate the applicability of the SMOKE-PRD system. Results show that the model can properly simulate temporal variations, spatial patterns, and peak values of O3 concentrations in the PRD region, suggesting the SMOKE-PRD system was successfully localized and can be used to provide model-ready emissions for regional air quality modeling in the PRD region. This work can be extended for adapting the SMOKE model to process emissions for modeling use in other regions of China. [Copyright &y& Elsevier]

Abstract: The rapid growth of energy consumption in conjunction with economic development during past decades in East Asia, especially China, caused severe air pollution problems at local and regional scales. Understanding of the meteorological conditions for air pollution is essential to the understanding of the formation mechanism of air pollutants and the development of effective emission control strategies to reduce air pollution. In this paper, the Fifth Generation National Center for Atmospheric Research (NCAR)/Pennsylvania State University (PSU) Mesoscale Model (MM5) modeling system is applied to simulate meteorological fields during selected six 1-month periods in 2007/2008 over a triple-nested modeling domain covering East Asia, the eastern China, and Shandong Province at horizontal grid resolutions of 36-, 12-, and 4-km, respectively. MM5 generally reproduces well the observations in the eastern China but performs worse in the western China and northeastern China. Largest biases occur in 2-m temperatures (T2) and wind speed and wind direction at 10-m in haze months (i.e., winter) and daily mean precipitation (Precip) in non-haze months (i.e., summer), due to limitations of the model in simulating snow cover and convective precipitation. Meteorological predictions agree more closely with observations at urban sites than those at the coastal and mountain sites where the model performance deteriorates because of complex terrains, influences of urban heat island effect and land/sea breezes, and higher elevations. Model results at 12-km in Shandong Province show an overall better performance than those at 4- or 36-km while the results at 4-km show worst performance due to inaccurate land use and the model’s incapability in simulating meteorological processes at a fine scale. [Copyright &y& Elsevier]

Abstract: In order to provide consistency for PM2.5 carbon data across urban and rural areas in the United States the Chemical Speciation Network (CSN) carbon sampling and analysis protocols were changed to resemble those of the Interagency Monitoring of Protected Visual Environments (IMPROVE). The “new” CSN carbon sampler, the URG-3000N, was introduced into the network in May 2007. In this paper collocated carbon measurements are used to (1) check for equivalency between the URG-3000N and the IMPROVE and (2) compare the URG-3000N and IMPROVE data with other collocated measurements. A sufficiently extensive dataset (over several years) enabled comparisons to be performed on a seasonal basis. The URG-3000N provided consistent measurements of PM2.5 carbon with the IMPROVE at both urban and rural locations. In contrast measurements by the CSN MetOne were higher by approximately 20–30% for total carbon at the urban location and 40–50% at the rural location. This information is important for assessing discontinuities in time series data due to changes in the method as occurred to the CSN carbon in 2007. [Copyright &y& Elsevier]

Abstract: The U.S. EPA periodically evaluates ambient concentrations, human exposures, and health risks for 180 hazardous air pollutants plus diesel particulate matter using modeled estimates from the National-Scale Air Toxics Assessment (NATA). NATA publishes estimates at the spatial resolution of U.S. Census tracts, which are subdivisions of a county. These local scale, model-predicted estimates from NATA are used extensively in community-based assessments; however, evaluation of NATA’s ambient concentrations and human exposure estimates against measurement data has been limited to date. This paper compares modeled annual average benzene results from the 2002 NATA with measured results from the 2004 to 2007 Detroit Exposure and Aerosol Research Study (DEARS) as a case study of the quality of NATA results. NATA model estimates support community-scale characterization and assessment. Benzene is particularly important as it was estimated by the 2002 NATA as the largest single air toxic pollutant in terms of cancer risk in the U.S. We found that the average ambient concentrations of benzene predicted by NATA were within 5 percent, on average, of the 24-h integrated average ambient concentrations measured in DEARS. The NATA human exposure estimates, which include only outdoor sources for benzene, were, on average, approximately half the measured breathing zone concentrations from DEARS. Our analyses support that the factors driving higher DEARS personal benzene concentrations relative to the NATA predicted exposure values are likely due, at least in part, to indoor sources. This work points to further community-scale modeling research to improve characterizations and assessments of human exposures. [Copyright &y& Elsevier]

Abstract: This paper compares spatial and seasonal variations and temporal trends in modeled and measured concentrations of sulfur and nitrogen compounds in wet and dry deposition over an 18-year period (1988–2005) over a portion of the northeastern United States. Substantial emissions reduction programs occurred over this time period, including Title IV of the Clean Air Act Amendments of 1990 which primarily resulted in large decreases in sulfur dioxide (SO2) emissions by 1995, and nitrogen oxide (NOx) trading programs which resulted in large decreases in warm season NOx emissions by 2004. Additionally, NOx emissions from mobile sources declined more gradually over this period. The results presented here illustrate the use of both operational and dynamic model evaluation and suggest that the modeling system largely captures the seasonal and long-term changes in sulfur compounds. The modeling system generally captures the long-term trends in nitrogen compounds, but does not reproduce the average seasonal variation or spatial patterns in nitrate. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines the impact of tripled anthropogenic emissions from China and India over the base level (gaseous species and carbonaceous aerosols for 2000) on air quality over the U.S. using the WRF/Chem (Weather Research and Forecasting – Chemistry) model at 1° resolution. WRF/Chem is a state-of-the-science, fully coupled chemistry and meteorology system suitable for simulating the transport and dispersion of pollutants and their impacts. The analyses in this work were focused on MAM (March, April and May). The simulations indicate an extensive area of elevated pollutant concentrations spanning from the Arabian Sea to the Northern Pacific and to the Northern Atlantic. MAM mean contributions from the tripled Asian emissions over the U.S. are found to be: 6–12 ppbv for CO, 1.0–2.5 ppbv for O3, and 0.6–1.6 μg m−3 for PM2.5 on a daily basis. [Copyright &y& Elsevier]

Abstract: The National Air Quality Forecast Capacity (NAQFC) system, which links NOAA’s North American Mesoscale (NAM) meteorological model with EPA’s Community Multiscale Air Quality (CMAQ) model, provided operational ozone (O3) and experimental fine particular matter (PM2.5) forecasts over the continental United States (CONUS) during 2008. This paper describes the implementation of a real-time Kalman Filter (KF) bias-adjustment technique to improve the accuracy of O3 and PM2.5 forecasts at discrete monitoring locations. The operational surface-level O3 and PM2.5 forecasts from the NAQFC system were post-processed by the KF bias-adjusted technique using near real-time hourly O3 and PM2.5 observations obtained from EPA’s AIRNow measurement network. The KF bias-adjusted forecasts were created daily, providing 24-h hourly bias-adjusted forecasts for O3 and PM2.5 at all AIRNow monitoring sites within the CONUS domain. The bias-adjustment post-processing implemented in this study requires minimal computational cost; requiring less than 10 min of CPU on a single processor Linux machine to generate 24-h hourly bias-adjusted forecasts over the entire CONUS domain. The results show that the real-time KF bias-adjusted forecasts for both O3 and PM2.5 have performed as well as or even better than the previous studies when the same technique was applied to the historical O3 and PM2.5 time series from archived AQF in earlier years. Compared to the raw forecasts, the KF forecasts displayed significant improvement in the daily maximum 8-h O3 and daily mean PM2.5 forecasts in terms of both discrete (i.e., reduced errors, increased correlation coefficients, and index of agreement) and categorical (increased hit rate and decreased false alarm ratio) evaluation metrics at almost all locations during the study period in 2008. [Copyright &y& Elsevier]

Abstract: This paper describes the further development and application of the Edinburgh–Lancaster Model for Ozone (ELMO). We replace straight-line back-trajectories with trajectories and associated meteorology supplied by the US National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) service to allow more realistic modelling of specific UK ozone episodes. We call this ELMO-2. Model performance is rigorously tested against observed ozone concentrations for two episodes recorded across 14 rural UK monitoring stations during the spring and summer of 1995. For both episodes, the afternoon concentrations (usually coinciding with the daily maxima) are captured well by the model and the diurnal ozone cycle is reproduced, although the amplitude in concentrations is generally smaller than the observed. The summer episode is investigated further through indicator species analysis and source attribution, and found to be mainly VOC-limited. European emissions account for the majority of ozone production. We demonstrate how improved modelling leads to better understanding of regional and local ozone production across the UK under episodic conditions. [Copyright &y& Elsevier]

Abstract: This paper provides a performance evaluation of the real-time, CONUS-scale National Air Quality Forecast Capability (NAQFC) that supported, in part, its transition into operational status. This evaluation focuses primarily on discrete forecasts for the maximum 8-h O3 concentrations covering the 4-month period, June through September, 2007, using measurements obtained from EPA''s AIRNow network. Results indicate that the 2007 NAQFC performed as well or better than previous configurations, despite the expansion of the forecast domain into the western half of the nation that is dominated by complex terrain. The mean, domain-wide, season-long correlation was 0.70. When examined over time, the domain-wide correlations exhibit a fairly consistent nature, with values exceeding 0.60 (0.70) over 90% (55%) of the days. The NAQFC systematically over-predicted the 8-h O3 concentrations, continuing a trend established by earlier NAQFC configurations, though to a lesser degree. The summer-long mean forecast value of 53.2ppb was 4.2ppb higher than the observed value, resulting in a domain-wide Normalized Mean Bias (NMB) of 8.7%. Most of the over-prediction is associated with observed concentrations less than 50ppb. In fact the model tends to under-predict when concentrations exceed 70ppb. As with the bias, the error associated with the latest configuration was also lower. The summer-long Root Mean Square Error of 13.0ppb (Normalized Mean Error (NME)=20.4%) represented marked improvements over earlier forecasts. Examination of the spatial distribution of both the NMB and NME reveals that the NAQFC was generally within 25% for the NME and 25% for the NMB over a majority of the domain. Several areas of poorer performance, where the NMB and NME often exceed 25% and in some cases 50%, were noted. These areas include southern California, where the NAQFC tended to under-predict concentrations (especially on weekends) and the southeast Atlantic and Gulf coasts regions, where the model over-predicted. Subsequent analysis revealed that the incorrect temporal allocation of precursor emissions was likely the source of the under-prediction in southern California, while inaccurate simulation of PBL heights likely contributed to the over-prediction in the coastal regions. [Copyright &y& Elsevier]

Windblown dust is known to impede visibility, deteriorate air quality and modify the radiation budget. Arid and semiarid areas with unpaved and unvegetated land cover are particularly prone to windblown dust, which is often attributed to high particulate matter (PM) pollution in such areas. Yet, windblown dust is poorly represented in existing regulatory air quality models. In a study by the authors on modeling episodic high PM events along the US/Mexico border using the state-of-the-art CMAQ/MM5/SMOKE air quality modeling system [Choi, Y.-J., Hyde, P., Fernando, H.J.S., 2006. Modeling of episodic particulate matter events using a 3D air quality model with fine grid: applications to a pair of cities in the US/Mexico border. Atmospheric Environment 40, 5181–5201], some of the observed PM10 NAAQS exceedances were inferred as due to windblown dust, but the modeling system was incapable of dealing with time-dependent episodic dust entrainment during high wind periods. In this paper, a time-dependent entrainment parameterization for windblown dust is implemented in the CMAQ/MM5/SMOKE modeling system with the hope of improving PM predictions. An approach for realizing windblown dust emission flux for each grid cell over the study domain on an hourly basis, which accounts for the influence of factors such as soil moisture content, atmospheric stability and wind speed, is presented in detail. Comparison of model predictions with observational data taken at a pair of US/Mexico border towns shows a clear improvement of model performance upon implementation of the dust emission flux parameterization. [Copyright &y& Elsevier]

The Interagency Monitoring of Protected Visual Environments (IMPROVE) program is a cooperative measurement effort in the United States designed to characterize current visibility and aerosol conditions in scenic areas (primarily National Parks and Forests) and to identify chemical species and emission sources responsible for existing man-made visibility impairment. In 2003 and 2004, the IMPROVE network began operating collocated samplers at several sites to evaluate the precision of its aerosol measurements. This paper presents the precisions calculated from the collocated data according to the United States Environmental Protection Agency''s guidelines . Revised requirements for designation of reference and equivalent methods for PM2.5 and ambient air quality surveillance for particulate matter: final rule, 1997. Code of Federal Regulations. Part IV: Environmental Protection Agency, vol. 40 CFR Parts 53 and 58, pp. 71–72. Available from 〈http://www.epa.gov/ttn/oarpg/t1/fr_notices/pm-mon.pdf〉]. These values range from 4% for sulfate to 115% for the third elemental carbon fraction. Collocated precision tends to improve with increasing detection rates, is typically better when the analysis is performed on the whole filter instead of just a fraction of the filter, and is better for species that are predominantly in the smaller size fractions. The collocated precisions are also used to evaluate the accuracy of the uncertainty estimates that are routinely reported with the concentrations. For most species, the collocated precisions are worse than the precisions predicted by the reported uncertainties. These discrepancies suggest that some sources of uncertainty are not accounted for or have been underestimated. [Copyright &y& Elsevier]

Upwind sources of NO x and SO2 play a crucial role in the amount of O3 and aerosols in the lower troposphere in the Mid-Atlantic US. This paper describes a novel method of clustering trace gas and aerosol profiles allowing for the quantification of the relationship between point sources and pollution levels. This improves our understanding of pollution origins and has the potential for prediction of episodes of poor air quality. A hierarchical clustering method was used to classify distinct chemical and meteorological events from over 200 aircraft vertical profiles in the lower troposphere. Profile measurements included O3, SO2, CO and particle scattering from June to August 1997–2003, in the Mid-Atlantic US (mostly in Maryland, Pennsylvania and Virginia). The clustering technique could discriminate distinct profile shapes including measurements made during the 2002 Canadian forest fires. Forty-eight-hour back trajectories were run for each profile and the integrated NO x and SO2 point source emissions encountered by each trajectory were calculated using data from the EPA Clean Air Market Division''s emissions database. There was a strong correlation between integrated NO x emissions and O3 profiles, indicating that O3 profiles are strongly influenced by and can be predicted with point source emissions. There is a prevalent concentration of SO2 over the eastern US with mixing ratios decreasing smoothly from about 3.5ppb near the surface to 0.2ppb at 2400m. [Copyright &y& Elsevier]

Outdoor air in several U.S. cities is now being continuously monitored for biological warfare agents. In the current implementation, all monitoring devices, which we refer to as “collectors,” are stationary. This paper discusses the trade-offs between stationary and moving collectors. A moving collector is more likely than a stationary collector to sample a clandestine release, but will experience a lower time-integrated concentration than an ideally placed stationary collector. Quantitative results are determined for some idealized cases, and suggest that moving collectors may offer substantial advantages over stationary collectors. [Copyright &y& Elsevier]

We estimate the contributions from biomass burning (summer wildfires, other fires, residential biofuel, and industrial biofuel) to seasonal and annual aerosol concentrations in the United States. Our approach is to use total carbonaceous (TC) and non-soil potassium (ns-K) aerosol mass concentrations for 2001–2004 from the nationwide IMPROVE network of surface sites, together with satellite fire data. We find that summer wildfires largely drive the observed interannual variability of TC aerosol concentrations in the United States. TC/ns-K mass enhancement ratios from fires range from 10 for grassland and shrub fires in the south to 130 for forest fires in the north. The resulting summer wildfire contributions to annual TC aerosol concentrations for 2001–2004 are 0.26μgCm−3 in the west and 0.14μgCm−3 in the east; Canadian fires are a major contributor in the east. Non-summer wildfires and prescribed burns contribute on an annual mean basis 0.27 and 0.31μgCm−3 in the west and the east, highest in the southeast because of prescribed burning. Residential biofuel is a large contributor in the northeast with annual mean concentration of up to 2.2μgCm−3 in Maine. Industrial biofuel (mainly paper and pulp mills) contributes up to 0.3μgCm−3 in the southeast. Total annual mean fine aerosol concentrations from biomass burning average 1.2 and 1.6μgm−3 in the west and east, respectively, contributing about 50% of observed annual mean TC concentrations in both regions and accounting for 30% (west) and 20% (east) of total observed fine aerosol concentrations. Our analysis supports bottom-up source estimates for the contiguous United States of 0.7–0.9TgCyr−1 from open fires (climatological) and 0.4TgCyr−1 from biofuel use. Biomass burning is thus an important contributor to US air quality degradation, which is likely to grow in the future. [Copyright &y& Elsevier]

The United States Environmental Protection Agency issues periodic reports that describe air quality trends in the US. For some pollutants, such as ozone, both observed and meteorologically adjusted trends are displayed. This paper describes an improved statistical methodology for meteorologically adjusting ozone trends as well as characterizes the relationships between individual meteorological parameters and ozone. A generalized linear model that accommodates the nonlinear effects of the meteorological variables was fit to data collected for 39 major eastern US urban areas. Overall, the model performs very well, yielding R 2 statistics as high as 0.80. The analysis confirms that ozone is generally increasing with increasing temperature and decreasing with increasing relative humidity. Examination of the spatial gradients of these responses show that the effect of temperature on ozone is most pronounced in the north while the opposite is true of relative humidity. By including HYSPLIT-derived transport wind direction and distance in the model, it is shown that the largest incremental impact of wind direction on ozone occurs along the periphery of the study domain, which encompasses major NO x emission sources. [Copyright &y& Elsevier]

As a part of the Relationships of Indoor, Outdoor, and Personal Air (RIOPA) study, 48h integrated residential indoor, outdoor, and personal exposure concentrations of 10 carbonyls were simultaneously measured in 234 homes selected from three US cities using the Passive Aldehydes and Ketones Samplers (PAKS). In this paper, we examine the feasibility of using residential indoor concentrations to predict personal exposures to carbonyls. Based on paired t-tests, the means of indoor concentrations were not different from those of personal exposure concentrations for eight out of the 10 measured carbonyls, indicating indoor carbonyls concentrations, in general, well predicted the central tendency of personal exposure concentrations. In a linear regression model, indoor concentrations explained 47%, 55%, and 65% of personal exposure variance for formaldehyde, acetaldehyde, and hexaldehyde, respectively. The predictability of indoor concentrations on cross-individual variability in personal exposure for the other carbonyls was poorer, explaining<20% of variance for acetone, acrolein, crotonaldehyde, and glyoxal. A factor analysis, coupled with multiple linear regression analyses, was also performed to examine the impact of human activities on personal exposure concentrations. It was found that activities related to driving a vehicle and performing yard work had significant impacts on personal exposures to a few carbonyls. [Copyright &y& Elsevier]

Improvements in measurement technology are permitting development of a more detailed scientific understanding of the cycling of mercury in the global atmospheric environment. Critical to advancing the state of knowledge is the acquisition of accurate measurement of speciated mercury (gaseous and particulate) at ground research stations in a variety of settings located around the globe. This paper describes one such research effort conducted at TVA''s Look Rock air quality monitoring site in Tennessee—a mountain top site (813m elevation) just west of the Great Smoky Mountains National Park. The Great Smoky Mountains National Park is the largest National Park in the eastern US and it receives environmental protection under a variety of US statutes. Gaseous and particle mercury species along with some additional trace gases were measured at Look Rock during two field studies totaling 84 days in the spring and summer of 2004. Average results for the entire sampling period are: gaseous elemental mercury Hg(0): 1. 65ngm−3, reactive gaseous mercury RGM: 0. 005ngm−3, particulate mercury Hg(p): 0. 007ngm−3. Literature review indicates that these levels are within the range found for other rural/remote sites in North America and worldwide. Reactive and particulate mercury comprised together less than 1%, on average, of total airborne mercury at Look Rock. When compared to the global background mercury literature, the Look Rock measurements demonstrate that the atmospheric mercury levels in the vicinity of the Great Smoky Mountains National Park are clearly dominated by the global atmospheric pool, not by local or regional sources. [Copyright &y& Elsevier]

Abstract: This paper examines the use of Moderate Resolution Imaging Spectroradiometer (MODIS) observed active fire data (pixel counts) to refine the National Emissions Inventory (NEI) fire emission estimates for major wildfire events. This study was motivated by the extremely limited information available for many years of the United States Environmental Protection Agency (US EPA) NEI about the specific location and timing of major fire events. The MODIS fire data provide twice-daily snapshots of the locations and breadth of fires, which can be helpful for identifying major wildfires that typically persist for a minimum of several days. A major wildfire in Mallory Swamp, FL, is used here as a case study to test a reallocation approach for temporally and spatially distributing the state-level fire emissions based on the MODIS fire data. Community Multiscale Air Quality (CMAQ) model simulations using these reallocated emissions are then compared with another simulation based on the original NEI fire emissions. We compare total carbon (TC) predictions from these CMAQ simulations against observations from the Inter-agency Monitoring of Protected Visual Environments (IMPROVE) surface network. Comparisons at three IMPROVE sites demonstrate substantial improvements in the temporal variability and overall correlation for TC predictions when the MODIS fire data is used to refine the fire emission estimates. These results suggest that if limited information is available about the spatial and temporal extent of a major wildfire fire, remotely sensed fire data can be a useful surrogate for developing the fire emissions estimates for air quality modeling purposes. [Copyright &y& Elsevier]

Abstract: Air pollutant emissions in the US generally peaked in 1970, the year that the Clean Air Act was passed, and have declined since, except for oxides of nitrogen (NO x ), which have remained steady or slowly increased. In 1995 Phase 1 of the Clean Air Act Amendments (CAAA) of 1990 went into effect resulting in a sharp drop in sulfur dioxide (SO2) emissions in most areas. Pollutant concentrations measured in precipitation and ambient air generally reflected the changes in emissions in most areas of the eastern US. Only the southern Appalachian Mountain region did not see an appreciable improvement in precipitation acidity over the last decade. Previous studies of cloud chemistry in the eastern US found no pattern of temporal trends, possibly because of large year-to-year variation in meteorology. In this paper, we present spatial and temporal trends in SO4 2− and NO3 − concentrations of cloud water samples collected in warm seasons only at two mountain sites (Whiteface Mountain, NY and Whitetop Mountain, VA). This analysis is based on a combined cloud chemistry data-set collected by EPA''s Mountain Cloud Chemistry Program (MCCP) (1986–1989) and Mountain Acid Deposition Program (MADPro) (1994–1999). Sample concentrations were (1) normalized by liquid water content (to reduce within-cloud variation) and (2) segregated into 90° arrival quadrants based on 36h back trajectory analysis (to diminish between-cloud or meteorological variation). For each quadrant at the two sites, annual (12 month) county emissions of SO2 and NO x were compiled out to 600, 1000, and1600km, and these values were compared with the annual means (warm season only) of normalized SO4 2− and NO3 − concentrations in hourly samples of cloud water (segregated by back-trajectory) collected at each site. For the period 1987–1999, Quadrant 3 (SW) for Whiteface Mt. and Quadrants 3 (SW) and 4 (NW) for Whitetop Mt. had the highest SO2 emissions and showed the largest decline in SO2 emissions after the CAAA was implemented. These same quadrants which had the largest decrease in emissions showed significant declines in cloud water SO4 2− over the time period. NO x emissions were highest in Quadrant 3 for Whiteface Mt. and in Quadrants 1 and 4 for Whitetop Mt. Only in Quadrant 1 at Whitetop Mt. did NO x emissions decrease during the study period (1987–1999). Cloud water NO3 − showed no consistent pattern at either mountain site with some quadrants having higher cloud water NO3 − values after Phase 1 of the CAAA and other quadrants having little change in cloud water NO3 − values. [Copyright &y& Elsevier]

Abstract: In this paper, we describe two advanced statistical techniques suited to address the following questions: which source categories of emissions affect given areas and where do these source categories come from? A source category is defined as a combination of volatile organic compounds (VOCs) associated with a specific industrial process. A discussion of the positive matrix factorization (PMF) multivariate receptor model is presented, and this PMF technique applied to hourly average concentrations of VOCs measured at five Photochemical Assessment Monitoring Stations (PAMS) located near the emission-rich Houston Ship Channel region in Texas. The observations were made between June and October 2003, and the PMF analysis was limited to nighttime measurements (21:00–06:00 CDT) to remove the complexity of photochemical processing and associated changes in the concentrations of primary and secondary VOCs. Six to eight VOCs source categories were identified for the five Ship Channel sites. Specific geographic areas associated with each source category were identified through the use of conditional probability functions that identify source regions when superimposed on maps of VOC emissions. [Copyright &y& Elsevier]

Abstract: Despite their relatively small population, heavy-duty diesel vehicles (HDDVs) are (in 2005) disproportionate contributors to the emissions inventory for oxides of nitrogen (NO x ) and particulate matter (PM) due to their high individual vehicle emissions rates, lack of engine aftertreatment, and high vehicle miles traveled. Beginning in the early 1990s, heavy-duty engine manufacturers began equipping their engines with electronic sensors and controls and on-board electronic computer modules (ECMs) to manage these systems. These ECMs can collect and store both periodic and lifetime engine operation data for a variety of engine and vehicle parameters including engine speed and load, time at idle, average vehicle speed, etc. The University of California, Riverside (UCR), under a contract with the California Air Resources Board (CARB), performed data analysis of 270 ECM data sets obtained from the CARB. The results from this analysis have provided insights into engine/vehicle operation that have not been obtained from previous on-board datalogger studies since those previous studies focused on vehicle operation and did not collect engine operating data. Results indicate that HDDVs spend a considerable amount of time at high-speed cruise and at idle and that a smaller percentage of time is spent under transient engine/vehicle operation. These results are consistent with other HDDV activity studies, and provide further proof of the validity of assumptions in CARB''s emission factor (EMFAC2002) model. An additional important contribution of this paper is that the evaluation of vehicle ECM data provides several advantages over traditional global positioning system (GPS) and datalogger studies: (1) ECM data is significantly cheaper than the traditional method ($50record–1 vs. ∼$2000record–1) and (2) ECM data covers vehicle operation over the entire life of the vehicle, whereas traditional surveys cover only short periods of surveillance (days, weeks, or months). It is worthwhile to note that this work was not intended to compare the various methods of data collection but to provide additional empirical support for the EMFAC2002 model and to explore the utility of this unique low-cost form of data collection and analysis. [Copyright &y& Elsevier]

Abstract: Exposure to carbonyls (aldehydes and ketones) can produce adverse health effects. It is known that various sources of carbonyls are often present inside residences but little is known about their indoor source strengths. In the present paper, we used a database established in the relationships of indoor, outdoor, and personal air (RIOPA) study to estimate indoor source strengths of 10 carbonyls and outdoor contributions to measured indoor concentrations of these carbonyls. We applied a mass balance model to analyze paired indoor and outdoor carbonyl concentrations simultaneously measured in 234 RIOPA homes. Among all the measured carbonyls, formaldehyde, and acetaldehyde had the strongest indoor source strengths with the estimated median values of 3.9 and 2.6mgh−1, respectively. Hexaldehyde also had large indoor source strengths with a median of 0.56mgh−1. Acetone had the largest variations in indoor source strengths ranging from undetected to 14mgh−1. The medians of the estimated indoor source strengths were 0.15mgh−1 for propionaldehyde, 0.18mgh−1 for glyoxal, 0.17mgh−1 for methylglyoxal, and 0.23mgh−1 for benzaldehyde. Acrolein and crotonaldehyde had the weakest indoor source strengths with no indoor sources detected in the majority of the RIOPA homes that were selected to have only nonsmoker residents. Consistent with the indoor source strength results, our estimated outdoor contributions to indoor concentrations were low for formaldehyde. In contrast, more than 90% of the indoor concentrations for acrolein and crotonaldehyde were from outdoor sources. The outdoor contributions to indoor concentrations for acetone, propionaldehyde, benzaldehyde, glyoxal, and methylglyoxal ranged from 10 to 90% across the RIOPA homes, suggesting that both indoor and outdoor sources had contributions to indoor concentrations of these compounds. [Copyright &y& Elsevier]

Abstract: Mercury concentrations in size fractions of airborne particulate matter obtained from urban and suburban sites in Beijing in 2003 and 2004 are presented in this paper. The average concentrations of total particulate mercury were 1.18±0.82ngm−3 with a range of 0.18–3.51ngm−3at the urban site and 0.68±0.62ngm−3 with a range of 0.13–2.40ngm−3 at the suburban site. This is much higher than what is found in Europe and Northern America. The results implie that significant anthropogenic sources in Beijing contribute to the concentration of particulate mercury in air and therefore, to the mercury deposition in this region. There are clear seasonal variations in concentration of particulate mercury with highest concentrations in winter at urban and suburban sites. This illustrates the important contribution from coal burning to particulate mercury during the heating season. The mercury concentration in each size fraction of airborne particulates was closely related to its size range. The highest mercury concentration was found in the size fraction less than 1.1μm with average concentrations of 0.59±0.50 and 0.29±0.26ngm−3 at the urban and suburban sites, respectively. The mercury in this size fraction accounts for 45.7±9.5% and 41.9±8.2% of the total particulate mercury, respectively. This implies that a large fraction of the particulate mercury in this area can be transported long range before it becomes deposited. Based on the concentrations of size-fractionated particulate mercury in ambient air, the dry deposition fluxes of mercury were estimated to 407μgm−2 a−1 at the urban site and 270μgm−2 a−1 at the suburban site. More than 90% of the deposition flux came from particles larger than 1.1μm despite a substantial fraction of the Hg found in particles less than 1.1μm. [Copyright &y& Elsevier]

Abstract: Cities in the Southwestern United States (Southwest) are often close to violating tropospheric ozone (ozone) and particulate matter (PM) federal air quality standards, and local climate and weather conditions play a large part in determining whether or not pollutant levels exceed the federally mandated limits and by what magnitude. The Kolmogorov–Zurbenko (KZ) filter method has been used in a number of studies in the Eastern United States to determine meteorological controls on ozone concentrations and to separate out those effects in order to examine underlying trends. The Southwest, however, experiences a different climate regime than other parts of the country, and atmospheric controls on air quality in the region have not been examined in this manner. This paper determines which meteorological variables most influence ozone and PM in the Southwest and examines patterns of underlying pollutant trends due to emissions. Ozone and PM data were analyzed over the time period 1990–2003 for the Southwest''s five major metropolitan areas: Albuquerque, NM; El Paso, TX; Las Vegas, NV; Phoenix, AZ; and Tucson, AZ. Results indicate that temperature and mixing height most strongly influence ozone conditions, while moisture levels (particularly relative humidity) are the strongest predictors of PM concentrations in all five cities examined. Meteorological variability typically accounts for 40–70% of ozone variability and 20–50% of PM variability. Long-term ozone trends are highly variable, but records from most stations indicate increasing concentrations over the last decade. Long-term trends in PM concentrations were relatively flat in all five cities analyzed but contained coincident extremes unrelated to meteorology. [Copyright &y& Elsevier]

Abstract: In the US, the majority of the on-road fleet and vehicle miles travelled are attributed to light-duty vehicles, which are fuelled almost entirely by gasoline. However, due to their significantly higher PM emission rates, emissions inventories have tended to attribute the majority of the mobile source PM to contributions from heavy-duty diesel vehicles and strategies to reduce mobile source PM have focused on the contribution from this source. A limited number of source attribution studies have implied that PM emission inventories over-estimate the diesel contribution and emissions from gasoline vehicles may be greater than previously believed. Other receptor-modelling studies have found diesel vehicles to be the dominant source of motor vehicle PM. The former conclusion is supported by recent on-road PM emission rate results obtained in a highway tunnel and a series of crossroad experiments. This paper describes the often-conflicting results obtained from receptor modelling studies and emission inventories and uses on-road emission factor results to estimate the relative contributions from the diesel and gasoline sectors of the fleet. [Copyright &y& Elsevier]

The city of Missoula, located in a high mountain valley in western Montana, contains one of the largest populations of residents in the entire Rocky Mountain region completely enclosed by mountains. Due to the surrounding topography and meteorological conditions, Missoula often experiences elevated concentrations of PM2.5. Recently, dry summers have exacerbated August particulate loads by immersing the valley in smoke from forest fires. Through a serendipitous set of circumstances, concentrations of PM2.5 (including chemical speciation) were measured throughout a yearlong program that captured both traditional urban particulates plus late summer forest fire particulates.This paper presents the results of the yearlong PM2.5 sampling program in which filter samples were collected over 24-h periods every 12 days at two locations. For the first time, concentrations of the fine fraction were simultaneously measured on the opposite end of the valley in Frenchtown (nearly 18 miles west of Missoula), thus providing information on the dispersive properties of PM2.5 in an enclosed airshed, and the spatial and seasonal variations within the Missoula Valley. Organic carbon (OC), elemental carbon (EC), trace elements, and ions were determined from the collected PM2.5 filter samples. The average mass of ambient PM2.5, as well as concentrations of OC, EC, elements, and ions measured over the entire year (22/3/00–17/3/01) were nearly identical on both sides of the valley. The annual average of PM2.5 was 15.4μgm-3 at both Missoula''s Boyd Park and at Frenchtown, with the highest concentrations of PM2.5 measured during the summer (26/6/00–18/9/00) and winter (23/12/00–17/3/01) seasons. The summer PM2.5 measurements were dominated by smoke from regional wildfires in Montana and Idaho, while thermal inversions in the valley contributed to the elevated winter concentrations. [Copyright &y& Elsevier]

This paper utilizes a rotated principal components approach first used by Eder, Davis, and Bloomfield (Atmos. Environ. 27a (16) (1993) 2645) to characterize non–urban daily 8-h maximum ozone (O3) concentrations within the eastern United States for the O3 seasons of 1993–2002. The analysis proceeds by selecting a spatially representative O3 database, imputing missing O3 concentrations using a spatial interpolation scheme, applying a rotated principal components analysis to delineate spatial regions of homogeneous concentrations, and investigating the temporal patterns exhibited by concentrations in each of the regions. Spatially, the analysis divides the eastern United States into five regions: a Northeast region, a Great Lakes Region, a Mid–Atlantic region, a Southwest region, and a Florida region. Concentrations are near the domain average for the Northeast, Great Lakes, and Southwest regions, are highest in the Mid–Atlantic region, and are lowest in the Florida region. Within each region the temporal patterns (e.g., seasonal trends, persistency, and annual trends) of O3 concentrations were quite different. The Northeast, Great Lakes, and Mid–Atlantic regions display a moderate amount of seasonal variability with peak concentrations occurring in late July, late June, and late July time periods, respectively. Conversely, the Southwest region exhibits a small amount of seasonal variability with peak concentrations occurring in late August, and the Florida region exhibits a relatively large amount of seasonal variability with peak concentrations occurring during the months of April and May. Ozone concentrations are most persistent in the Florida and Southwest regions (3–4 days of persistence), less persistent in the Mid–Atlantic region (2–3 days of persistence), and least persistent in the Northeast and Great Lakes regions (1 or 2 days persistence). These results demonstrate the spatial and temporal variability of O3 concentrations in the eastern United States and indicate that regional characterization (e.g., through aggregation of clusters of monitors) of O3 air quality may be a powerful yet simplifying air quality metric for determining seasonal trends, annual trends, and a variety of other O3 concentration characteristics. [Copyright &y& Elsevier]

The FAST-J algorithm was developed to quickly and accurately calculate photolysis rates under both clear and cloudy sky conditions. In this paper, photolysis rates of nitrogen dioxide were calculated using FAST-J and compared with measurements taken at two sites in the United States: Phoenix, Arizona, and Houston, Texas. The measurements were derived from either an actinic flux filter radiometer (Phoenix) or a spectroradiometer (Houston). A sun photometer sited nearby these radiometers provided irradiance measurements from which aerosol and cloud optical thicknesses were obtained. Aerosol single scattering albedo was not known, but was taken to be either 0.79 or 0.94, representative of either soot- or sulfate-like aerosols, respectively. These optical properties served as input to the FAST-J algorithm, which in turn was used to calculate photolysis rates. For both clear and cloudy sky cases, the modeled and measured photolysis rates agree within the uncertainties of the measurements for a single scattering albedo of 0.94. For a single scattering albedo of 0.79, the agreement is again within the uncertainty limits except for the cloudy sky case in Houston. The results suggest that the FAST-J code may be a practical algorithm for use in atmospheric chemical transport models that make repeated calls to photolysis rate subroutines. [Copyright &y& Elsevier]

Daily ambient aerosol samples were taken in Pittsburgh, Pennsylvania from the summer 2001 to the winter 2002 as part of the Pittsburgh Air Quality Study (PAQS). The study measured PM2.5 mass by the Federal Reference Method (FRM) and the PM2.5 chemical composition by a variety of filter-based and continuous instruments. This paper examines the mass balance between the FRM-measured mass and the sum of the aerosol chemical components. For the 7-month study period, the average FRM-measured mass is 11% greater than the sum of the mass of the aerosol chemical components. This mass balance discrepancy varies seasonally, with the average FRM-measured mass 17% greater than the sum of the chemical components for the summer months, with discrepancies as large as 30% during certain periods. Meanwhile, the FRM-measured mass was at or slightly below the sum of the chemical components for the winter months.The mass balance discrepancy and its seasonal shift cannot be explained by measurement uncertainty; instead the discrepancy is due to combination of retained aerosol water on the conditioned FRM filters and volatilization losses. The relative importance of these different effects varies with aerosol composition and causes the observed seasonal variation in the mass balance. The contribution of the aerosol water to the FRM-measured mass is estimated using continuous measurements of aerosol water at the site; volatilization losses are estimated from other filter-based instruments. Water contributes 16% of the FRM mass in the summer, and 8% of the FRM mass in the winter; it also appears responsible for episodes where the FRM-measured mass is significantly greater than the sum of components. Retention of water is greatest during acidic conditions, which commonly occur during the summer months. Volatilization losses are estimated at 5% of the FRM mass during the summer, and 9% for the winter. Volatilization losses appear to be most significant on days dominated by organic aerosol, or winter days with relatively high nitrate concentration. Accounting for the effects of water and volatilization losses closes the mass balance between the FRM and the sum of the chemical components, providing insight into the FRM measurements. [Copyright &y& Elsevier]

Building occupants, including cleaning personnel, are exposed to a wide variety of airborne chemicals when cleaning agents and air fresheners are used in buildings. Certain of these chemicals are listed by the state of California as toxic air contaminants (TACs) and a subset of these are regulated by the US federal government as hazardous air pollutants (HAPs). California''s Proposition 65 list of species recognized as carcinogens or reproductive toxicants also includes constituents of certain cleaning products and air fresheners. In addition, many cleaning agents and air fresheners contain chemicals that can react with other air contaminants to yield potentially harmful secondary products. For example, terpenes can react rapidly with ozone in indoor air generating many secondary pollutants, including TACs such as formaldehyde. Furthermore, ozone–terpene reactions produce the hydroxyl radical, which reacts rapidly with organics, leading to the formation of other potentially toxic air pollutants. Indoor reactive chemistry involving the nitrate radical and cleaning-product constituents is also of concern, since it produces organic nitrates as well as some of the same oxidation products generated by ozone and hydroxyl radicals.Few studies have directly addressed the indoor concentrations of TACs that might result from primary emissions or secondary pollutant formation following the use of cleaning agents and air fresheners. In this paper, we combine direct empirical evidence with the basic principles of indoor pollutant behavior and with information from relevant studies, to analyze and critically assess air pollutant exposures resulting from the use of cleaning products and air fresheners. Attention is focused on compounds that are listed as HAPs, TACs or Proposition 65 carcinogens/reproductive toxicants and compounds that can readily react to generate secondary pollutants. The toxicity of many of these secondary pollutants has yet to be evaluated. The inhalation intake of airborne organic compounds from cleaning product use is estimated to be of the order of 10 mg d−1 person−1 in California. More than two dozen research articles present evidence of adverse health effects from inhalation exposure associated with cleaning or cleaning products. Exposure to primary and secondary pollutants depends on the complex interplay of many sets of factors and processes, including cleaning product composition, usage, building occupancy, emission dynamics, transport and mixing, building ventilation, sorptive interactions with building surfaces, and reactive chemistry. Current understanding is sufficient to describe the influence of these variables qualitatively in most cases and quantitatively in a few. [Copyright &y& Elsevier]

To study the potential impacts of climate change on air quality and public health over the eastern United States, a coupled global/regional-scale modeling system consisting of the NASA-Goddard Institute for Space Studies Atmosphere–Ocean model, the MM5 mesoscale meteorological model and the Community Multiscale Air Quality (CMAQ) model for air quality has been developed. Evaluation results of the modeling system used to simulate climate and ozone air quality over the eastern United States during the five summers of 1993–1997 are presented in this paper. The results indicate that MM5 and CMAQ capture interannual and synoptic-scale variability present in surface temperature and ozone observations in the current climate, while the magnitude of fluctuations on shorter time scales is underestimated. A comparison of observed and predicted spatial patterns of daily maximum ozone concentrations shows best performance in predicting patterns for average and above-average ozone concentrations. The frequency distributions of the duration of extreme heat and ozone events show similar features for both model predictions and observations. Finally, application of a synoptic map-typing procedure reveals that the MM5/CMAQ system succeeded in simulating the average ozone concentrations associated with several frequent pressure patterns, indicating that the effects of synoptic-scale meteorology on ozone concentrations are captured by the modeling system. It is concluded that the GCM/MM5/CMAQ system is a suitable tool for the simulation of summertime surface temperature and ozone air quality conditions over the eastern United States in the present climate. [Copyright &y& Elsevier]

Fumigants are used to enhance the yield and quality of agricultural produce, which is critical to the maintenance of the production levels of carrots, potatoes, tomatoes, strawberries, melons, and many other crops grown in the US and throughout much of the world. With the worldwide phase-out of methyl bromide in progress, the continued availability of the remaining alternatives, such as metam-sodium, 1,3-dichloropropene, and chloropicrin, is becoming increasingly important. Metam-sodium has been used for over 40 years and is the second most widely used fumigant in the United States. Reduction in off-gassing rates of fumigants can promote health and safety benefits and an increased dose in the treatment zone, thereby increasing the potential efficacy of these products. On this basis, there is a need to evaluate off-gassing rates as a function of application and sealing methods. This paper summarizes recent research into the volatilization of the principal transformation product of metam-sodium, i.e., methyl isothiocyanate (MITC), into the atmosphere as a function of application and sealing methods. Seven field studies were conducted from 1999–2001 to evaluate the off-gassing rates of MITC from applications of metam-sodium by shank injection and chemigation using two different water sealing methods, i.e., standard water sealing and intermittent water sealing. MITC is slightly soluble in water. Irrigation of a field following an application helps to retain the compound in the soil, minimizing off-gassing while increasing the dose to the target pests. Intermittent water sealing involves applying water on an intermittent basis to minimize off-gassing rates during nighttime periods when relatively poor atmospheric dispersion conditions often occur. Research conducted by the Metam-Sodium TASK Force indicates that intermittent water sealing significantly reduces off-gassing rates both for shank injection and chemigation applications when compared with standard water sealing practices. [Copyright &y& Elsevier]

Precipitation scavenging can effectively remove particulates from the atmosphere. Interest in the phenomena waxed in the 1980s, but models developed at that time remain limited by the lack of both detailed, time-resolved wet deposition pattern measurements for model confirmation and real-time rain data for model execution. Recently, new rain products have become available that can revolutionize real-time use of precipitation scavenging models on the regional scale. We have utilized a 4-km, hourly resolution precipitation data set from the Arkansas Red-Basin River Forecast Center. A standard below-cloud aerosol scavenging model has been modified to incorporate the potentially larger scavenging in heavy rain events. This paper demonstrates the model on a sample rainfall data set. The simulations demonstrate the concentrating effect of rainfall, especially heavy rain, on deposition patterns. Wet deposition played an important role in the simulated fate and transport, removing as much as 70% of the released aerosol. [Copyright &y& Elsevier]

The East Bay Children''s Respiratory Health Study is examining associations between traffic-related pollutant exposures and respiratory health among children who reside and attend schools at varied proximity to northern California freeways. Chronic exposures are being inferred from outdoor pollutant concentrations at neighborhood schools. This paper reports primarily weeklong integrated NO2 and NOX concentrations measured with passive samplers placed outside at 10 elementary schools during 14 weeks in spring and 8 weeks in fall 2001. Measurements were also made outside selected student residences to examine spatial variability within three school neighborhoods. Regional concentrations of NO2 and NOX varied widely from week to week. School site data were normalized to measurements at a nearby regional monitoring station to facilitate analysis of relative pollutant exposures at the neighborhood schools. Normalized concentrations were consistent at each school throughout the study. Schools located upwind or far downwind of freeways were generally indistinguishable from one another and regional pollution levels. For school and neighborhood sites within 350 m downwind of a freeway, concentrations increased with decreasing downwind distance. The highest normalized concentrations occurred at a school located directly adjacent to a major freeway and a shopping center. In this case, normalized NO2 and NOX were ∼60% and ∼100% higher than regional background levels. At three schools within 130–230 m downwind of a freeway, normalized NO2 and NOX were ∼20–30% and ∼50–80% higher than regional levels. Validation testing of the passive samplers indicated precision of better than 5% for both NO2 and NOX when samplers were deployed outside for 1-week periods. Passive sampler results agreed with co-located chemiluminescence measurements to within 8% for NO2 and 3% for NOX. [Copyright &y& Elsevier]

Current modeling methods are inadequate for producing gridded mobile source inventories suitable for simulating fine-scale changes in emissions for transportation planning or regulatory control purposes. In this paper, we present a new model that more precisely simulates small-scale emission changes that can arise from, for example, implementation of traffic control measures in transportation networks. The new model, UCDrive, directly interfaces with most travel demand models and with California''s regulatory emission factor model EMFAC2001/2002. The model improves the sensitivity of emission estimates to variations in vehicle speeds and uses a theoretically consistent approach to spatially disaggregating mobile emissions. To demonstrate application of the model we use the Sacramento area network to perform a comparison study. We show that regional emissions with the new model replicate regulatory inventories, which is critical for using the model for regulatory purposes. We also show that the new model''s results better replicate regulatory totals than those derived with a frequently used gridded emissions inventory model. The new model should improve the ability to better simulate changes in mobile emissions from transportation projects and help to bridge the gap between gridded mobile emission inventories and regional mobile emission inventories. [Copyright &y& Elsevier]

This paper presents the first known values for emissions of polychlorinated dibenzodioxins and dibenzofurans (PCDDs/Fs) from combustion of agricultural field biomass. Wheat and rice straw stubble collected from two western US states were tested in a field burn simulation to determine emission factors. The resulting emission factor was approximately 0.5 ng toxic equivalency (TEQ)/kg burned for both sources. When coupled with published agricultural data on crop residue burning, about 1 g TEQ/year can be expected from wheat and rice straw residue burning, making this an apparently minor source of PCDDs/Fs in the United States. [Copyright &y& Elsevier]

This paper reviews sea level and elevated surface observations from Point Mugu and nearby Laguna Peak in southern Ventura County, rawin/ozonesonde data from Point Mugu, and aircraft measurements in the coastal zone and offshore obtained in support of the SCOS97-NARSTO effort. Streamline analyses of SCOS97-NARSTO wind data are also presented, which reveal synoptic and mesoscale influences on air flow over the southern California area. Complex diurnal, mesoscale and synoptic wind patterns redistribute pollutants across air basins. The data show that southeasterly flow, often present from morning into early afternoon, and coastal eddies and southerly surges convey ozone and precursors westward and northward where they may appear aloft as layers with several times the concentration of ozone at the surface. This flow can transport South Coast Air Basin (SCAB) pollutants via mountain passes to the desert where mixing with pollution from the San Joaquin Valley can occur. Climatological evidence shows this tendency for southerly flow in the earlier part of the day is a recurrent feature of southern California wind patterns, and a major mechanism for transport of ozone and precursors from the SCAB to coastal, inland and desert regions to the north. [Copyright &y& Elsevier]

Observed hydroxyl (OH) and hydroperoxy (HO2) radicals, collectively called HOx, were compared with OH and HO2 calculated by a box model that used the regional atmospheric chemistry mechanism and was constrained to the ancillary measurements during the PM2.5 Technology Assessment and Characterization Study-New York (PMTACS-NY) summer 2001 intensive in New York City. The measurements are described in the companion paper, Ren et al. (HOx concentrations and OH reactivity observations in New York City during PMTACS-NY2001, Atmospheric Environment, this issue). This comparison enables an investigation of HOx chemistry in this polluted urban atmosphere. For HO2, the observed concentrations and diurnal variation were usually well reproduced by the model calculations, with an observed-to-modeled ratio of 1.24, on average, for day and night. For OH, the model was generally able to match the measured concentrations during daytime with an observed-to-modeled ratio of about 1.10, but the calculations significantly underestimated OH during nighttime. The budgets of HOx show that its production was dominated by the photolysis of HONO, accounting for ∼56% of HOx production on average, during daytime due to relatively high HONO concentrations, while nighttime HOx production was mainly from the O3 reactions with alkenes. The OH reactivity measurements agree with the calculations to within 10% for both the composite diurnal variation and individual days. Calculations indicate that the reactions of OH with NO2, hydrocarbons, CO, NO, and carbonyls accounted for about 32%, 25%, 12%, 10% and 7% of total OH loss, respectively, in this urban area. Modeled instantaneous O3 production from HO2 and RO2 reactions with NO was 150±100 ppbv day−1. O3 production rates from measured HO2(P(O3)obsHO2) was greater than modeled HO2(P(O3)calcHO2) at higher values of NO. Average daily cumulative P(O3)obsHO2 was ∼140 ppbv day−1, a factor of 1.5, greater than average daily P(O3)calcHO2. [Copyright &y& Elsevier]

In community time series studies on the effect of particulate air pollution on mortality, particulate air pollution data are often available from multiple monitors. Published studies have combined the data from the multiple monitors using a simple or trimmed average. In this paper, I describe an alternative method for combining the particulate air pollution data available from multiple monitors. Using time series data from Cook County, Illinois the proposed method yields more meaningful biologically plausible particulate air pollution mortality effect estimates without loss of statistical accuracy and reduces the full set of monitors to a smaller set without loss of information. The proposed monitor combination may be more highly correlated with the average population exposure to particulate air pollution than a simple or trimmed average of the monitors. [Copyright &y& Elsevier]