Inverse Modelling of Natural NOx Emissions and Implications for Ozone in the United States

Natural sources of nitrogen oxides (NO_x=NO+NO₂), including lightning and soil NO_x, are estimated to contribute more than 30 percent of total NO_x emissions in the United States. Lightning is the major source of NO_x emissions in the middle and upper troposphere. Due to the difficulties in understanding underlying processes and quantifying variant temporal and spatial distributions of natural NO_x, estimations of natural NO_x emissions remain significantly uncertain compared to anthropogenic sources. Satellite measurements of total NO₂ columns provide important top-down information to NO_x emission estimations. Previous estimations of natural NO_x relied mainly on ground-based measurements and satellite observations in natural NO_x dominant regions, the observations of which are limited. In this study, we inversely derive the natural NO_x emissions on a daily basis from a source-separated model, which could compute source-specific NO₂ columns for each natural and anthropogenic sources, using the 3-D Regional chEmical trAnsport Model (REAM) and the Ozone Monitoring Instrument (OMI) measurements during summer 2011. Results suggest a reduction in natural NO_x emissions of the model. We also analyze how ozone responses to projected natural NO_x emission reductions in the United States.