On-road vehicle emissions of carbon monoxide (CO), nitrogen oxides (NO x), and volatile organic compounds (VOCs) during 1995–2009 in the Atlanta Metropolitan Statistical Area were estimated using the Motor Vehicle Emission Simulator (MOVES) model and data from the National Emissions Inventories and the State of Georgia. Shahmatka v buhgalterii blank page. Statistically significant downward trends (computed using the nonparametric Theil-Sen method) in annual on-road CO, NO x, and VOC emissions of 6.1%, 3.3%, and 6.0% per year, respectively, are noted during the 1995–2009 period despite an increase in total vehicle distance traveled. The CO and NO x emission trends are correlated with statistically significant downward trends in ambient air concentrations of CO and NO x in Atlanta ranging from 8.0% to 11.8% per year and from 5.8% to 8.7% per year, respectively, during similar time periods. Weather-adjusted summertime ozone concentrations in Atlanta exhibited a statistically significant declining trend of 2.3% per year during 2001– 2009.

Although this trend coexists with the declining trends in on-road NO x, VOC, and CO emissions, identifying the cause of the downward trend in ozone is complicated by reductions in multiple precursors from different source sectors. Implications: Large reductions in on-road vehicle emissions of CO and NO x in Atlanta from the late 1990s to 2009, despite an increase in total vehicle distance traveled, contributed to a significant improvement in air quality through decreases in ambient air concentrations of CO and NO x during this time period. Emissions reductions in motor vehicles and other source sectors resulted in these improvements and the observed declining trend in ozone concentrations over the past decade. Although these historical trends cannot be extrapolated to the future because pollutant concentration contributions due to on-road vehicle emissions will likely become an increasingly smaller fraction of the atmospheric total, they provide an indication of the benefits of past control measures.

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Introduction On-road vehicle emissions contribute to deterioration in ambient air quality, particularly in urban areas. Oxides of nitrogen (NO x) and volatile organic compounds (VOCs) emitted by vehicles are precursors to the photochemical formation of ozone (O 3) and particulate matter (PM). NO x is also a contributor to acidic deposition. Additionally, NO x and carbon monoxide (CO) from on-road mobile sources can cause respiratory problems and have other harmful health effects. Large reductions in motor vehicle emissions have been achieved in the United States over the past four decades through regulations and voluntary measures taken by the automobile industry. For example, from 1970 to 2005, nationwide highway vehicle emissions of VOCs decreased by approximately 75% and emissions of PM and NO x decreased by over 50% though the total vehicles miles traveled (VMT) for highway vehicles increased more than 2-fold (). It is of interest to determine whether there have been statistically significant reductions in urban on-road emissions in the recent past (here, over an approximate 10–15-yr period) and whether such reductions, if present, are associated with statistically significant reductions in atmospheric pollutant concentrations.

We focus on the Atlanta Metropolitan Statistical Area (MSA) where on-road emissions constitute a large fraction of the total emissions inventory. Also, substantial improvements in air quality have been achieved in Atlanta, as indicated by the recent proposal by the U.S. Environmental Protection Agency (EPA) to approve the State of Georgia's request to redesignate 20 counties in metropolitan Atlanta as meeting the federal 8-hr O 3 standard of 75 ppb based on monitoring between 2008 and 2010; these counties had been in nonattainment since 1997. There is a wealth of scientific literature on temporal trends in emissions from mobile and other sources and trends in air quality in the United States (e.g.,;;;; ) and in the southeastern United States in particular (e.g.,,,,, ). All of these studies use mobile source emission data from the National Emissions Inventory (NEI) () for trends analyses. The NEI emission estimates are made by the EPA at 3-yr intervals, with frequent revision of the estimation procedures in each new version and additional revisions between versions, especially for the on-road inventory. Moreover, the NEI on-road sector emission data through 2005 were calculated using emission factors from the MOBILE6 (Mobile Source Emission Factor Model) vehicle emissions model, which has since been superseded by the Motor Vehicle Emission Simulator (MOVES) model, whereas the 2008 NEI on-road inventory was developed using MOVES.