Impacts of Modifiable Factors on Ambient Air Pollution: A Case Study of COVID-19 Shutdowns.

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 PM 2.5 , CO, and NO 2 following the COVID-related closures at each site group were compared to measurements from "business-as-usual" periods. Overall, PM 2.5 concentrations decreased across the domain by ∼3 μg/m 3 . The morning rush-hour-induced CO and NO 2 concentrations at the high-traffic sites were both reduced by ∼50%, which is consistent with observed reductions in commuter traffic (∼50%). The morning rush-hour PM 2.5 enhancement from traffic emissions was reduced nearly 100%, from 1.4 to ∼0 μg/m 3 across all site groups. There was no significant change in the industry-related intraday variability of CO and PM 2.5 at the industrial sites following the COVID-related closures. If PM 2.5 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.