Observations of Lightning NOx Production From Tropospheric Monitoring Instrument Case Studies Over the United States

Nitrogen oxides produced by lightning (LNOx) play an important role in determining mid- and upper-tropospheric concentrations of the hydroxyl radical (OH), methane (CH4), and ozone (O3). The moles of NOx produced per flash was examined using nitrogen dioxide (NO2) columns and cloud properties from the Tropospheric Monitoring Instrument (TROPOMI) and flash counts from the Geostationary Lightning Mapper (GLM) aboard the Geostationary Operational Environmental Satellite-16 (GOES-16) and Earth Networks Total Lightning Network (ENTLN) for 29 convective systems over the United States that occurred during 2018 and 2019. For each of the case studies, the LNOx production efficiency (PE) was estimated using TROPOMI pixels over deep convection. First, the NOx columns associated with the TROPOMI NO2 columns were estimated using a specially derived air mass factor (AMF). The tropospheric column due to recent lightning was then determined by subtracting from the median NOx column a background representative of the NOx column due to sources other than recent lightning. Then, the PE was calculated by multiplying the LNOx column by the storm area and dividing by the number of flashes contributing to the column. For a three-hour chemical lifetime, the mean PE was found to be 175 ± 100 mol per flash for optical flashes from GLM and 120 ± 65 mol per flash for radio-wave-detected flashes from ENTLN. The uncertainty associated with these values is mostly due to uncertainties in tropospheric background, AMF, and detection efficiency. GLM PE for individual systems was found to be positively correlated with optical energy.