Sensitivities of NOx transformation and the effects on surface ozone and nitrate

Abstract

As precursors to tropospheric ozone and nitrate, nitrogen oxide (NO_x ) 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 NO_x 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 NO_x and volatile organic compounds (VOC). Ozone production in industrial regions (the main NO_x emission source areas) increases in warmer conditions and slightly decreases following an increase in NO_x emissions due to NO_x 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 NO_x emission sources may be sensitive to NO_x transformation in remote source regions. The increase in NO₂ from NO_x titration over industrial regions results in an increase rate of total nitrate that remains higher than the increase rate of NO_x 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 (NO_y ) during the night. As a result, the transformation rate of NO_x to nitrate decreases. Examinations of the historical emissions and air quality records of a typical NO_x -limited area, such as Atlanta and a VOC-limited area, such as Los Angeles further confirm the conclusions drawn from the modeling experiments.

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