Estimating Lightning NOx Production Using NO2 Columns from the TROPOMI Instrument and Flashes from the Geostationary Lightning Mappers

Nitric oxide (NO) is produced in lightning channels and quickly comes into equilibrium with nitrogen dioxide (NO₂) in the atmosphere.  The production of NO_x (NO + NO₂) leads to subsequent increases in the concentrations of ozone (O₃) and the hydroxyl radical (OH) and decreases in the concentration of methane (CH₄), thus impacting the climate system.  Global production of NO_x from lightning is uncertain by a factor of four.  NO_x production by lightning will be examined using NO₂ columns from the TROPOspheric Monitoring Instrument (TROPOMI) on board the Copernicus Sentinel-5 Precursor Satellite with an overpass time of approximately 1330 LT and flash rates from the Geostationary Lightning Mapper (GLM) on board the NOAA GOES-16 (75.2° W) and GOES-17 (137.2° W) satellites.  Where there is overlap in coverage of the two GLM instruments, the greater of the two flash counts is used.  Two approaches have been undertaken for this analysis:  a series of case studies of storm systems over the United States, and a gridded analysis over the entire contiguous United States, Central America, northern South America, and surrounding oceans.  A modified Copernicus Sentinel 5P TROPOMI NO₂ data set is used here for the case-study analysis to improve data coverage over deep convective clouds.  In both approaches, only TROPOMI pixels with cloud fraction > 0.95 and cloud pressure < 500 hPa are used.  The stratospheric column is removed from the total slant column, and the result is divided by air mass factors appropriate for deep convective clouds containing lightning NO_x (LNO_x).  Case studies have been selected from deep convective systems over and near the United States during the warm seasons of 2018 and 2019.  For each of these systems, NO_x production per flash is determined by multiplying a TROPOMI-based estimate of the mean tropospheric column of LNO_x over each system by the storm area and then dividing by a GLM-based estimate of the flashes that contribute to the column.  In the large temporal and spatial scale analysis, the TROPOMI data are aggregated on a 0.5 x 0.5 degree grid and converted to moles LNO_x*.  GLM flash counts during the one-hour period before TROPOMI overpass are similarly binned. A tropospheric background of LNO_x* is estimated from grid cells without lightning and subtracted from LNO_x* in cells with lightning to yield an estimate of freshly produced lightning NO_x, designated LNO_x.  Results of the two approaches are compared and discussed with respect to previous LNO_x per flash estimates.