Applying the OMI NO2 Retrieval Algorithm to Estimate the Production Efficiency of Lightning NOx

Abstract

The Ozone Monitoring Instrument (OMI) on board the NASA Aura satellite was launched into sun-synchronous low-earth orbit in 2004. Its hyperspectral measurements have been an invaluable tool in determining trace-gas concentrations in the troposphere and stratosphere. Nitrogen dioxide (NO2) has a particularly prominent absorption signature in the violet and near-UV regions of the OMI spectrum. This signature can be exploited in retrievals of column amounts of NO2 attributable to both natural and anthropogenic sources. We outline the OMI NO2 retrieval algorithm and demonstrate its utility for inferring NOx (NO + NO2) amounts due to lightning. Lightning is the dominant source of NOx in the free troposphere, and most estimates of the concentration of lightning NOx (LNOx) require knowledge of the amount of this species produced per lightning flash. We present the largest spatial- and temporal-scale investigation of LNOx to date that combines satellite-based NOx estimates and lightning flash data. The study comprises five northern-hemisphere (NH) summers, including much of the mid-latitude regions in North America and Asia and adjacent waters. NO2 measurements are converted to LNOx and compared with flashes preceding OMI overpass by 2 hours. The flash counts are derived from ground-based World Wide Lightning Location Network (WWLLN) data that are adjusted for detection efficiency. We find reasonable correlation between the number of lightning flashes and the amount of LNOx produced and estimate mean efficiencies for the production of LNOx in various NH regions. Overall results indicate mole/flash values near the low end of those reported in previous LNOx studies, as well as a possible dependence of production efficiency on flash rate. These findings have potential implications in the chemistry of upper tropospheric trace gases and the global NOx budget.