Abstract
Abstract. As precursors to tropospheric ozone and nitrate, nitrogen oxide (NOx) in the present atmosphere and its transformation in response to emission and climate perturbations are studied by using the CAM-Chem model and air quality measurements from the National Emissions Inventory (NEI), Clean Air Status and Trends Network (CASTNET), and Environmental Protection Agency Air Quality System (EPA AQS). It is found that NOx transformations in present atmospheric conditions show different sensitivities over industrial and non-industrial regions. As a result, the surface ozone and nitrate formations can be divided into several regimes associated with the dominant emission types and relative levels of NOx and volatile organic compounds (VOC). Ozone production in industrial regions (the main NOx emission source areas) increases in warmer conditions and slightly decreases following an increase in NOx emissions due to NOx titration, which is opposite to the response in non-industrial regions. The ozone decrease following a temperature increase in non-industrial regions indicates that ozone production in regions that lack NOx emission sources may be sensitive to NOx transformation in remote source regions. The increase in NO2 from NOx titration over industrial regions results in an increase rate of total nitrate that remains higher than the increase rate of NOx emissions. The presented findings indicate that a change in the ozone concentration is more directly affected by changes in climate and precursor emissions, while a change in the nitrate concentration is affected by local ozone production types and their seasonal transfer. The sensitivity to temperature perturbations shows that a warmer climate accelerates the decomposition of odd nitrogen (NOy) during the night. As a result, the transformation rate of NOx to nitrate decreases. Examinations of the historical emissions and air quality records of a typical NOx-limited area, such as Atlanta and a VOC-limited area, such as Los Angeles further confirm the conclusions drawn from the modeling experiments.
Highlights
Surface ozone and particulate matter (PM) are two major pollutants that affect US air quality (Blanchard and Hidy, 2003; Jacob and Winner, 2008) and that have a deleterious effect on the human respiratory system and health in general
Since summer is the active period for ozone production and the availability of ozone measurements is the best in summer, we compare the summer daily maximum eight-hour (DM8H) surface ozone concentrations in order to evaluate the model performance
We find that the model is able to reproduce summer ozone concentrations over most of the contiguous US, with overall biases smaller than 20 ppb
Summary
Surface ozone and particulate matter (PM) are two major pollutants that affect US air quality (Blanchard and Hidy, 2003; Jacob and Winner, 2008) and that have a deleterious effect on the human respiratory system and health in general. US (Tao et al, 2007; Blanchard et al, 2012) Based on these measurements and inventories, we can assess the changes in the concentrations of surface ozone and total nitrate un- der present atmospheric conditions and further examine their Fig. 1F.iguSrceh1e. The measurements of chemical transformation of NOx includes daytime and night- surface ozone and total nitrate concentrations over typical time processes, which are both influenced by climate and megacities are analyzed to assess their temporal variability. This variability, which is associated with the variation of NOx and other precursor emissions, allows us to evaluate and better understand the conclusions drawn from numerical studies. After one year’s spin-up, the model result for the following three years is used in the analysis to perform inter-comparisons
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