High-resolution geostationary satellite observations of free tropospheric NO ₂ over North America and implications for lightning emissions

Free tropospheric (FT) nitrogen dioxide (NO₂) plays a critical role in atmospheric oxidant chemistry as a source of tropospheric ozone and of the hydroxyl radical (OH). It also contributes significantly to satellite-observed tropospheric NO₂ columns, which should be considered when using these columns to quantify surface emissions of nitrogen oxide radicals (NO_x ≡ NO + NO₂). But large uncertainties remain in the sources and chemistry of FT NO₂ because observations are sparse. Here, we construct a cloud-sliced FT NO₂ (700 to 300 hPa) product from the Tropospheric Emissions: Monitoring of Pollution (TEMPO) geostationary satellite instrument over North America. This product provides higher data density and quality than previous products from low Earth orbit instruments, including the first observations of the FT NO₂ diurnal cycle in different seasons. Combined with coincident observations from the Geostationary Lightning Mapper, the TEMPO data imply that lightning is the dominant source of FT NO_x in nonwinter seasons. Comparison of TEMPO FT NO₂ data with the Goddard Earth Observation System-Composition Forecasts (GEOS-CF) atmospheric chemistry model shows overall consistent magnitudes, seasonality, and diurnal variation, with a midday minimum in nonwinter seasons from photochemical loss. However, there are major discrepancies that we attribute to GEOS-CF's use of a standard cloud-top-height-based scheme for the lightning NO_x source. We find that this scheme underestimates offshore lighting flash density and misrepresents the diurnal cycle of lightning over land. Our FT NO₂ product provides a unique resource for improving the lightning NO_x parameterization in atmospheric models and the ability to use NO₂ observations from space to quantify surface NO_x emissions.