ABSTRACT
COVID-19-related closures offered a novel opportunity to observe and quantify the impact of activity levels of modifiable factors on ambient air pollution in real time. We use data from a network of low-cost Real-time Affordable Multi-Pollutant (RAMP) sensor packages deployed throughout Pittsburgh, Pennsylvania, along with data from Environmental Protection Agency regulatory monitors. The RAMP locations were divided into four site groups based on land use. Concentrations of PM2.5, CO, and NO2 following the COVID-related closures at each site group were compared to measurements from "business-as-usual" periods. Overall, PM,, concentrations decreased across the domain by similar to 3 mu g/m(3). The morning rush-hour-induced CO and NO2 concentrations at the high-traffic sites were both reduced by similar to 50%, which is consistent with observed reductions in commuter traffic (similar to 50%). The morning rush-hour PM2.5 enhancement from traffic emissions was reduced nearly 100%, from 1.4 to similar to 0 mu g/m(3) across all site groups. There was no significant change in the industry-related intraday variability of CO and PM2.5 at the industrial sites following the COVID-related closures. If PM2.5 National Ambient Air Quality Standards (NAAQS) are tightened, this natural experiment sheds light on the extent to which reductions in traffic-related emissions can aid in meeting more stringent regulations.