The Clean Air Status and Trends Network (CASTNET) is a long-term monitoring network designed to measure acidic pollutants and ambient ozone (O­₃) concentrations in rural areas across the United States. CASTNET is managed collaboratively by the U.S. Environmental Protection Agency – Clean Air Markets Division (EPA), the National Park Service – Air Resources Division (NPS), and the Bureau of Land Management – Wyoming State Office (BLM-WSO). In addition to EPA, NPS, and BLM-WSO, numerous other participants provide site operator support and grant land access including North American tribes, other federal agencies, States, private landowners, and universities.  

On March 13, 2020 the White House issued a national emergency concerning the COVID-19 outbreak and subsequently US state governors began issuing stay-at-home orders beginning with California on March 19, 2020 followed up by an additional 12 states issuing similar stay-at-home orders by March 23, 2020. By April 3, 2020, all but five US states have issued state-wide or partial-state stay-at-home orders.

We hypothesize that the stay-at-home orders likely reduced mobile, commercial, and industrial ozone-precursor emissions sufficiently to detect changes in regional air quality. To test this hypothesis, we compare hourly and daily maximum eight-hour average (DMA8) ozone values from 2020 against annual weekly averaged values from 2015-2019 from 75 CASTNET sites with more than 5 years of monitoring data.

Beginning the week of the declaration of the national emergency, March 9^th, through August 9^th, preliminary CASTNET ozone data show average relative percent differences of -7.4% in the 90^th percentile 1-hour ozone values.

We compare the CASTNET ozone data with meteorological datasets from CASTNET, NOAA, and the National Atmospheric Deposition Program (when present) to determine meteorological impacts on ozone formation. Lastly, we analyze mobility changes based on Google community mobility reports, inferring that changes in mobility patterns suggest reduced emissions from the transportation sector.