The Regional Acid Deposition Model has been modified to create the Regional Particulate Model, a three‐dimensional Eulerian model that simulates the chemistry, transport, and dynamics of sulfuric acid aerosol resulting from primary emissions and the gas phase oxidation of sulfur dioxide. The new model uses a bimodal lognormal distribution to represent particles in the submicrometer size range. In addition to including the horizontal and vertical advection and vertical diffusion of the aerosol number concentration and sulfate mass concentration fields, the model now explicitly treats the response of the distribution parameters to particle coagulation within and between the modes, condensation of sulfate vapor onto existing particles, formation of new particles, evaporation and condensation of ambient water vapor in the presence of ammonia, and particle‐size‐dependent dry deposition. The model has been used to study how the degree of sulfuric acid neutralization by ambient ammonia affects the total aerosol concentrations and particle size distributions over eastern North America. Preliminary results for three representative locations, rural, near‐source, and nominal downwind of source, show that the effect is greatest for the rural and smallest for the near‐source regions, which corresponds with the largest and smallest values, respectively, of ammonium‐to‐sulfate molar ratios. The results indicate that the model could provide a tool for investigating the effects of various pollution control strategies, as well as new or alternative formulations of important aerosol processes.

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Chemical processing of sea-salt particles in coastal environments significantly impacts concentrations of particle components and gas-phase species and has implications for human exposure to particulate matter and nitrogen deposition to sensitive ecosystems. Emission of sea-salt particles from the coastal surf zone is known to be elevated compared to that from the open ocean. Despite the importance of sea-salt emissions and chemical processing, the US EPA's Community Multiscale Air Quality (CMAQ) model has traditionally treated coarse sea-salt particles as chemically inert and has not accounted for enhanced surf-zone emissions. In this article, updates to CMAQ are described that enhance sea-salt emissions from the coastal surf zone and allow dynamic transfer of HNO₃, H₂SO₄, HCl, and NH₃ between coarse particles and the gas phase. Predictions of updated CMAQ models and the previous release version, CMAQv4.6, are evaluated using observations from three coastal sites during the Bay Regional Atmospheric Chemistry Experiment (BRACE) in Tampa, FL in May 2002. Model updates improve predictions of NO₃^&minus;, SO₄^2&minus;, NH₄⁺, Na⁺, and Cl^&minus; concentrations at these sites with only a 8% increase in run time. In particular, the chemically interactive coarse particle mode dramatically improves predictions of nitrate concentration and size distributions as well as the fraction of total nitrate in the particle phase. Also, the surf-zone emission parameterization improves predictions of total sodium and chloride concentration. Results of a separate study indicate that the model updates reduce the mean absolute error of nitrate predictions at coastal CASTNET and SEARCH sites in the eastern US. Although the new model features improve performance relative to CMAQv4.6, some persistent differences exist between observations and predictions. Modeled sodium concentration is biased low and causes under-prediction of coarse particle nitrate. Also, CMAQ over-predicts geometric mean diameter and standard deviation of particle modes at the BRACE sites. These over-predictions may cause too rapid particle dry deposition and partially explain the low bias in sodium predictions. Despite these shortcomings, the updates to CMAQ enable more realistic simulations of chemical processes in environments where marine air mixes with urban pollution. The model updates described in this article are included in the public release of CMAQv4.7 (<a href="http://www.cmaq-model.org" target="_blank">http://www.cmaq-model.org</a>)

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Future changes in society and climate are expected to affect wildfire activity in the south-eastern United States. The objective of this research was to understand how changes in both climate and society may affect wildfire in the coming decades. We estimated a three-stage statistical model of wildfire area burned by ecoregion province for lightning and human causes (1992–2010) based on precipitation, temperature, potential evapotranspiration, forest land use, human population and personal income. Estimated parameters from the statistical models were used to project wildfire area burned from 2011 to 2060 under nine climate realisations, using a combination of three Intergovernmental Panel on Climate Change-based emissions scenarios (A1B, A2, B2) and three general circulation models. Monte Carlo simulation quantifies ranges in projected area burned by county by year, and in total for higher-level spatial aggregations. Projections indicated, overall in the Southeast, that median annual area burned by lightning-ignited wildfire increases by 34%, human-ignited wildfire decreases by 6%, and total wildfire increases by 4% by 2056–60 compared with 2016–20. Total wildfire changes vary widely by state (–47 to +30%) and ecoregion province (–73 to +79%). Our analyses could be used to generate projections of wildfire-generated air pollutant exposures, relevant to meeting the National Ambient Air Quality Standards.

Demand for air travel is projected to increase in the upcoming years, with a corresponding influence on emissions, air quality, and public health. The trajectory of health impacts would be influenced by not just emissions growth, but also changes in nonaviation ambient concentrations that influence secondary fine particulate matter (PM(2.5) ) formation, population growth and aging, and potential shifts in PM(2.5) concentration-response functions (CRFs). However, studies to date have not systematically evaluated the individual and joint contributions of these factors to health risk trajectories. In this study, we simulated emissions during landing and takeoff from aircraft at 99 airports across the United States for 2005 and for a 2025 flight activity projection scenario. We applied the Community Multiscale Air Quality (CMAQ) model with the Speciated Modeled Attainment Test (SMAT) to determine the contributions of these emissions to ambient concentrations, including scenarios with 2025 aircraft emissions and 2005 nonaviation air quality. We combined CMAQ outputs with PM(2.5) mortality CRFs and population projections, and evaluated the influence of changing emissions, nonaviation concentrations, and population factors. Given these scenarios, aviation-related health impacts would increase by a factor of 6.1 from 2005 to 2025, with a factor of 2.1 attributable to emissions, a factor of 1.3 attributable to population factors, and a factor of 2.3 attributable to changing nonaviation concentrations which enhance secondary PM(2.5) formation. Our study emphasizes that the public health burden of aviation emissions would be significantly influenced by the joint effects of flight activity increases, nonaviation concentration changes, and population growth and aging.

[1] The performance of the Multiscale Air Quality Simulation Platform (MAQSIP) in simulating the regional distributions of tropospheric ozone and particulate matter (PM) is evaluated through comparisons of model results from three-dimensional simulations against available surface and aircraft measurements. These applications indicate that the model captures the dynamic range of observations and the spatial trends represented in measurements. Some discrepancies also exist, however, and they are discussed in the context of model formulation, input data specification and assumptions, and variability and bias in measurements. The daily normalized bias (within ±20%) and normalized gross errors (<25%) for predicted surface level O 3 over an entire summer season are within the suggested performance criteria for management evaluation studies and are comparable to, if not smaller than, those reported previously for other regional O 3 models. Comparisons of modeled PM composition with speciated fine particle concentration measurements show that the model is able to capture the spatial variability in fine PM mass as well as in the inorganic component fractions. Both measurements and model results show that in the summertime in the eastern U.S., SO 4 2À is a relatively large component of fine PM mass; in contrast, NO 3 À is a significant fraction in the western U.S. in the wintertime case studied. The ability of the model to simulate the observed visibility indices (extinction coefficient and deciview) are evaluated through comparisons of model estimates using both a detailed Mie theory-based calculation (based on predicted aerosol size and number distributions) and an empirical mass reconstruction algorithm. Both modeled and observed data show that among the various aerosol components, in the eastern U.S. SO 4 2À contributes the largest fraction to the aerosol extinction (35-85%), while organic mass contributes up to 20-25%. In contrast, in the western U.S., SO 4 2À and NO 3 À have comparable contributions (20-50%) to the observed aerosol extinction. Comparisons with limited observational aircraft data, however, show moderate to poor correlation with measurements in the free troposphere. While these discrepancies can be attributed in part to model initialization and lateral boundary conditions specification, there is a need for further evaluation of the representation of boundary layer-free troposphere exchange mechanisms as well as the chemical mechanisms currently used in the model for representing chemistry in the free troposphere.