During 2020, the stay-at-home order mandated in response to the COVID-19 pandemic led to significant changes in traffic volumes in Cincinnati, OH. Air pollutant concentrations (PM2.5, black carbon (BC), carbon monoxide (CO), nitrogen dioxide (NO2), nitrogen dioxide (NOX), and ozone (O3)) monitored at two ground monitoring sites in the city of Cincinnati were analyzed intra-annually in 2020 to quantify if the stay-at-home order impacted air quality. Interannual analyses were also conducted to evaluate differences in 2020 data versus historical years (2016–2019). Traffic volume data were also analyzed, where it was observed that, compared to pre-pandemic 2020, total traffic counts decreased by up to 26.41% during Ohio’s stay-at-home order, while heavy-duty vehicle traffic increased by up to 26.95% during the latter half of 2020. Statistical analysis indicated nonuniform changes in air pollutant concentrations at both sites throughout 2020. During the lockdown period at the central monitoring site, PM2.5 increased by 9%, while NO2 decreased by 30% compared to pre-pandemic concentrations in 2020. For BC and CO, there were no significant changes.

In this study, ozone and particulate matter variations from four monitoring stations in the Southwest Ohio region were analyzed at different stages of the COVID-19 pandemic in 2020, and compared with those in 2019. These stations include a US EPA NCore site (Taft), an urbansuburban site (Sycamore), an industrial source site (Yankee) and a residential site near the source (Amanda). The air quality data were broken down to the lockdown period (March 23 to May 31) and the re-opening periods from June to December, 2020. Publicly available monitoring data on PM2.5, ozone and PM10 were used for analysis. PM2.5 reductions were non-uniform with strong seasonal variations. PM2.5 reductions were 4.04%, 15.6%, 11.63% at Sycamore, Taft and Yankee sites respectively during the lockdown, but increased 11.23% at Amanda. Reductions at Taft may be related to traffic restrictions while those at Yankee may be due to both reduced industrial A C C E P T E D M A N U S C R I P T2 production and source control measures. Ozone reductions were 7.94% and 6.50% at Sycamore 26 and Taft sites during the lockdown with Sycamore having higher ozone concentrations pre, 27 during and post lockdown. Ozone formation is NOx-limited in Southwest Ohio region and the 28 variations are uniform. Lower temperatures during the lockdown and fall of 2020 can also be a 29 contributing factor. The Air Quality Index (AQI) of combined ozone and PM2.5 improved during 30 the pandemic year. Consistent with a few other studies, COVID-19 restrictions did not bring 31 uniform pollutant reductions to the Southwest Ohio region.