High-resolution geostationary satellite observations of free tropospheric NO2 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 (NOx ≡ 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 NOx 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 NOx 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 NOx parameterization in atmospheric models and the ability to use NO2 observations from space to quantify surface NOx emissions.