Rapid O ₃ assimilations – Part 2: Tropospheric O ₃ changes accompanied by declining NO _x emissions in the USA and Europe in 2005–2020

Tropospheric nitrogen dioxide (NOinline-formula₂) concentrations have declined dramatically over the United States (USA) and Europe in recent decades. Here we investigate the changes in surface and free-tropospheric Oinline-formula₃ accompanied by NOinline-formula₂ 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 Oinline-formula₃ observations. The assimilated Oinline-formula₃ concentrations demonstrate good agreement with Oinline-formula₃ observations. Surface Oinline-formula₃ 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 Oinline-formula₃ 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 Oinline-formula₃ concentrations over the USA and Europe, which resulted in a surface Oinline-formula₃ maximum in July–August in the simulations, which is in contrast to April in the observations. Furthermore, our analysis exhibits limited changes in surface Oinline-formula₃ concentrations; i.e., they decreased by inline-formula−6 % over the USA and increased by 1.5 % over Europe in 2005–2020. The surface observation-based assimilations suggest insignificant changes in tropospheric Oinline-formula₃ columns, namely inline-formula−3.0 % (USA) and 1.5 % (Europe) in 2005–2020. While the OMI-based assimilations exhibit larger decreases in tropospheric Oinline-formula₃ columns, with inline-formula−12.0 % (USA) and inline-formula−15.0 % (Europe) in 2005–2020, the decreases mainly occurred in 2010–2014, corresponding to the reported slower decline in free-tropospheric NOinline-formula₂ since 2010. Our analysis thus suggests that there are limited impacts of the decline in local emissions on tropospheric Oinline-formula₃ over the USA and Europe and advises more efforts to evaluate the possible contributions of natural sources and transport. The discrepancy in assimilated tropospheric Oinline-formula₃ columns further indicates the possible uncertainties in the derived tropospheric Oinline-formula₃ changes.