Advances in global NOx emission estimates using assimilation of TROPOMI NO2 data

Abstract:

Satellite observations of tropospheric NO₂ columns have proven useful for constraining surface NO_x emissions and its impact on air quality, atmospheric chemistry, and climate change as a precursor of tropospheric ozone and nitrate aerosols. The Tropospheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor satellite provides improved global pictures of global tropospheric NO₂ columns as compared with the Ozone Monitoring Instrument (OMI). This study demonstrates the advances in global NO_x emission estimates using the TROPOMI NO₂ retrievals (version 1.2) for the first time from a systematic comparison against data assimilation (DA) results using the QA4ECV OMI NO₂ retrievals (version 1.1) for April—May 2018, using the global chemical data assimilation system developed using an ensemble Kalan filter technique. The global RMSE reduction in tropospheric NO₂ column against the assimilated measurements was greater in TROPOMI DA (by 54%) than in OMI DA (by 38%), which can be attributed to 16% smaller super-observation errors of the TROPOMI retrievals than those of the OMI retrievals and 44% larger observation coverages of the TROPOMI retrievals than those of the OMI retrievals. TROPOMI DA also improved agreement in surface and tropospheric NO₂ concentration over the United States and Europe compared against surface in situ and aircraft (NASA ATom-4) measurements (by 12—84% in TROPOMI DA and by 2—70% in OMI DA). The global total NO_x emissions derived from TROPOMI DA were 15% smaller than those from OMI DA due to the systematic differences between the two retrievals, while providing larger analysis increments in TROPOMI DA by 42% in global average associated with the improved global coverage and reduced uncertainty. The global TROPOMI NO₂ DA can be expected to benefit studies on detailed spatial and temporal variations in ozone and nitrate aerosols and evaluation of bottom-up NO_x emission inventories.