California Air Pollution Drops In Every County During Pandemic

Californians have now been sheltering in place since March 20. While social distancing is slowing the spread of COVID-19, it’s also significantly reducing air pollutant and greenhouse gas levels across the state. Residents from San Diego to Sacramento are seeing clearer skies and breathing cleaner air, even in the center of cities that rarely experience a noticeable reduction in air pollution.

Statewide California Air Pollution Decreases Dramatically

From March 20 to April 9, all pollutants analyzed fell across the board — including ozone — when compared to averages in the same timeframe from 2017, 2018, and 2019.

County by County, Every Air Pollutant Drops

Fine particulate matter (PM₂.₅) levels dropped across the state, including counties in Central California, which often experience elevated levels of PM₂.₅ especially during the winter. Frequently elevated levels of PM₂.₅ and other pollutants in San Joaquin Valley (SJV) are related to the unique geography of the region, where episodic PM₂.₅ buildup events occur because of stagnant meteorological conditions in combination with excess emissions from urban, industrial and agricultural activities.

Fresno, Kings, Stanislaus, and Madera counties each had more than 70% reductions in PM₂.₅ when comparing average levels between March 20 — April 9, 2020 to average levels before the shelter-in-place order (Feb. 2 — March 2, 2020). Reduction in PM₂.₅ cannot be entirely attributed to shelter-in-place orders, however. Large decreases in PM₂.₅ in the SJV can occur seasonally, in the transition months between winter and spring. Additionally, rain events can cause a reduction in PM₂.₅ levels. More research is needed and underway to decouple the influence of seasonal changes, meteorology, and human intervention on PM₂.₅ concentrations observed.

Long-term exposure to high levels of fine particulate matter, which can either be emitted directly from a combustion source or formed in the atmosphere from complex chemical reactions, has been linked to cardiovascular and respiratory disease as well as cancer. Last week, Harvard released a study that shows that long-term exposure to even slightly elevated levels of fine particulate matter makes people more likely to die from COVID-19. Just a 1 μg/m3 increase in PM₂.₅ exposure over ten years increases the COVID-19 mortality rate by 15%.

Recent NO₂ approximations published in Nature from satellites show significant downward trends in NO₂ primarily for Los Angeles but not other large metropolitan areas in the United States. However, the ground-level measurements from regulatory sites presented here show how both NO₂ and NO (emissions associated with industrial and traffic activities, typically referred to as NOx) have been reduced across the state of California, including both rural and urban regions.

Unlike satellite approximations which give a birds-eye view of the entire column of air from space to the ground, ground-level measurements from regulatory sites and our own mobile sensor network represent the air humans breathe with high spatial and temporal resolution. Natural decreases in NO₂ concentrations occur in the Northern hemisphere relating to the angle of the sun from January to May. However, year-over-year comparisons of regulatory data indicate more than just seasonal causes for the significant decreases observed for NO₂ with the shelter-in-place order for California.

Carbon monoxide (CO) concentrations also dropped across the state with the shelter-in-place order. Processes that burn fossil fuel are a major source of CO in the ambient atmosphere. Kern County was the only county with observed increases in CO in this time period (29%).

While statewide average O₃ levels decreased by 5% compared to previous years, week-over-week changes show increasing O₃ for most of the state. O₃ is a secondary pollutant produced as a result of complex photochemistry and typically shows increased concentrations during springtime with more sunlight and increasing temperatures. As a result, interpreting the week-over-week changes and the impact of reduced traffic emissions is challenging. The year-over-year changes should take seasonality into account, but the relationship between emissions of precursors to O₃ (e.g. NOx and VOCs) and O₃ concentrations can vary significantly based on location (urban vs. rural), and other topographic and meteorological conditions. More research is needed to explain the changes observed in O₃ for most of the state.