Spatial variability of traffic-related air pollution reductions in the South Coast Air Basin during the COVID-19 stay-at-home period

Abstract:

As part of its COVID-19 response, California issued a mandatory stay-at-home order which went into effect on March 20, 2020. Concurrently, the concentration of traffic-related air pollutants such as black carbon (BC), ultrafine particles (UFP), carbon monoxide (CO), and nitrogen oxides (NO_x), showed appreciable reductions in the South Coast Air Basin (Basin). These reductions in pollutant concentrations began in early March before the stay-at-home order and coincided with heavy rainfall in the Basin. Subsequently, concentrations reached a minimum in late March/early April, then gradually increased back to more typical levels, in agreement with traffic flow data. We investigated the concentrations of the previously mentioned species at eight monitoring stations across the Basin (four near-road and four background sites) for pre-2020 and 2020 measurements during the stay-at-home period from March 20 to May 31. Compared to historical levels, reductions are nearly all statistically significant and range from 6 to 41%, dependent on the specific pollutant studied and location of the monitoring station. Generally, reductions are greater close to major highways and are less significant on the eastern side of the Basin where diesel trucks contribute to a greater proportion of vehicular traffic. Reductions in UFP are smaller than other species, likely due to primary emissions from sources other than traffic (e.g., cooking) or secondary formation. Despite the clear reductions in the studied pollutants, the direct effect of the stay-at-home order on the concentration of these species is convoluted by the effects of meteorology. Meteorologically normalized pollutant concentrations show that some fraction of these reductions is the result of weather, but that the reductions due to traffic in the Basin are still significant.