Rapid O3 assimilations - Part 2: Tropospheric O3 changes accompanied by declining NOx emissions in the USA and Europe in 2005-2020

Tropospheric nitrogen dioxide (NO₂) concentrations have declined dramatically over the United States (USA) and Europe in recent decades. Here we investigate the changes in surface and free-tropospheric O₃ accompanied by NO₂ changes over the USA and Europe in 2005-2020 by assimilating the Ozone Monitoring Instrument (OMI) and U.S. Air Quality System (AQS) and European AirBase network O₃ observations. The assimilated O₃ concentrations demonstrate good agreement with O₃ observations. Surface O₃ concentrations are 41.4, 39.5, and 39.5 ppb (parts per billion; USA) and 35.3, 32.0, and 31.6 ppb (Europe) and tropospheric O₃ columns are 35.5, 37.0, and 36.8 DU (USA) and 32.8, 35.3, and 36.4 DU (Europe) in the simulations, assimilations, and observations, respectively. We find overestimated summertime surface O₃ concentrations over the USA and Europe, which resulted in a surface O₃ maximum in July-August in the simulations, which is in contrast to April in the observations. Furthermore, our analysis exhibits limited changes in surface O₃ concentrations; i.e., they decreased by −6 % over the USA and increased by 1.5 % over Europe in 2005-2020. The surface observation-based assimilations suggest insignificant changes in tropospheric O₃ columns, namely −3.0 % (USA) and 1.5 % (Europe) in 2005-2020. While the OMI-based assimilations exhibit larger decreases in tropospheric O₃ columns, with −12.0 % (USA) and −15.0 % (Europe) in 2005-2020, the decreases mainly occurred in 2010-2014, corresponding to the reported slower decline in free-tropospheric NO₂ since 2010. Our analysis thus suggests that there are limited impacts of the decline in local emissions on tropospheric O₃ over the USA and Europe and advises more efforts to evaluate the possible contributions of natural sources and transport. The discrepancy in assimilated tropospheric O₃ columns further indicates the possible uncertainties in the derived tropospheric O₃ changes.