Emerging investigator series: spatially and temporally resolved lightning NOx production rates obtained with space-based lightning detectors.

Abstract

Lightning is a dominant free tropospheric source of nitrogen oxides (NO_x), impacting climate by affecting global tropospheric ozone (O₃) and other key oxidants. We calculate temporally and spatially resolved lightning NO_x (LNO_x) production rates using lightning flash energies from space-based Lightning Imaging Sensors (LIS). Our production rates average ∼309 moles per flash (mol fl^-1) over oceans, peaking at midday, and ∼172 mol fl^-1 over land, peaking in the early morning. Replacing static values in the original GEOS-Chem model inventory with our spatially and hourly varying production rates yields global annual LNO_x emissions of 4.4 Tg nitrogen (N) for 2015-2019; almost 2 Tg N less than the original inventory. Annual terrestrial LNO_x emissions still exceed those over the ocean, due to more frequent lightning over land, but the relative proportion of maritime emissions increases. The largest decline in LNO_x emissions is north of 35°N in summer, as static 500 mol fl^-1 is replaced with 150-250 mol fl^-1. Global decline in LNO_x emissions decreases abundances of free tropospheric NO_x, O₃, the hydroxyl radical (OH) and peroxyacetyl nitrate (PAN). Hourly variability in LIS flash radiances is broadly consistent with high-frequency Geostationary Lightning Mapper (GLM) flash energies, though the comparison is limited to tropical America. Introducing hourly variability in LNO_x production rates leads to a greater than 10% localised decline in equatorial lower tropospheric O₃. Independent validation is critically needed, given our updated LNO_x emissions offer a viable solution to model overestimates in O₃ and OH and underestimates in the lifetime of the greenhouse gas methane.