Rapid assimilations of O₃ observations – Part 2: tropospheric O₃ changes in the United States and Europe in 2005–2020

Abstract

Abstract. Tropospheric nitrogen dioxide (NO2) concentrations have declined dramatically over the United States (US) and Europe in recent decades. Here we investigate the changes in surface and free tropospheric O3 associated with NO2 changes over the US and Europe in 2005–2020 by assimilating the Ozone Monitoring Instrument (OMI), and US 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 (US) and 35.3, 32.0 and 31.6 ppb (Europe); and tropospheric O3 columns are 35.5, 37.0 and 36.8 DU (US) 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 US and Europe, which resulted in a surface O3 maximum in July–August in simulations in contrast to April in observations. Furthermore, our analysis exhibits limited changes in surface O3 concentrations, i.e., decreased by -6 % over the US and increased by 1.5 % over Europe in 2005–2020. The surface observation-based assimilations suggest insignificant changes in tropospheric O3 columns: -3.0 % (US) and 1.5 % (Europe) in 2005–2020. While the OMI-based assimilations exhibit larger decreases in tropospheric O3 columns, -12.0 % (US) and -15.0 % (Europe) in 2005–2020, the decreases mainly occurred in 2010–2014, corresponding to the reported slowed declines in free tropospheric NO2. Our analysis thus suggests limited impacts of local emission declines on tropospheric O3 over the US and Europe and advises more efforts to evaluate the possible contributions of natural sources and transport.