Observing US Regional Variability in Lightning NO2 Production Rates

Lightning is a large and variable source of nitrogen oxides (NOx equivalent to NO + NO2) to the upper troposphere. Precise estimates of lightning NOx (LNOx) production rates are needed to constrain tropospheric oxidation chemistry; however, controls over LNOx variability are poorly understood. Here, we describe an observational analysis of variability in LNO2 with lightning type by exploiting U.S. regional differences in lightning characteristics in the Southeast, South Central, and North Central United States. We use satellite NO2 measurements from the Ozone Monitoring Instrument with Berkeley High Resolution vertical column densities, a combined lightning data set derived from the Earth Networks Total Lightning Network and National Lightning Detection NetworkTM measurements, and hourly winds from the European Centre for Medium-Range Weather Forecasts climate reanalysis data set (ERA5) over May-August 2014-2015. We find evidence that cloud-to-ground (CG) strokes produce a factor of 9-11 more NO2 than intracloud (IC) strokes for storms with stroke rates of at least 2,800 strokes center dot cell(-1)center dot hr(-1). We show that regional differences in LNO2 production rates are generally consistent with regional patterns CG and IC stroke frequency and stroke current density. A comparison of stroke-based and flash-based CG/IC LNO2 estimates suggests that CG LNO2 is potentially underestimated when derived with flash data due to the operational definition of CG lightning. We find that differences in peak current explain a large portion of CG/IC LNO2 variability, but that other factors must also be important, including minimum stroke rate. Because IC and CG strokes produce NOx in distinct areas of the atmosphere, we test the sensitivity of our results against the atmospheric NO2 vertical distribution assumed in the a priori profiles; we show that the relative CG to IC LNO2 was generally insensitive to the assumed NO2 vertical distribution.