Comment on acp-2023-47

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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&ndash;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&ndash;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&ndash;2020. The surface observation-based assimilations suggest insignificant changes in tropospheric O₃ columns: -3.0 % (US) and 1.5 % (Europe) in 2005&ndash;2020. While the OMI-based assimilations exhibit larger decreases in tropospheric O₃ columns, -12.0 % (US) and -15.0 % (Europe) in 2005&ndash;2020, the decreases mainly occurred in 2010&ndash;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.