Figure caption: On March 19, California was one of the first states to set mandatory stay-at-home restrictions in an attempt to slow the spread of COVID-19. Arizona and Nevada followed suit around April 1. The Ozone Monitoring Instrument (OMI) on board NASA’s Aura satellite provide data that indicate that these restrictions have led to about a 31% decrease in NO₂ levels in the Los Angeles basin relative to previous years. NO₂, or nitrogen dioxide, is an air pollutant measured by OMI. The estimated reductions for other cities in the Southwest U.S. before and after the quarantine restrictions are 22% for the San Francisco Bay Area, 25% for San Diego and Tijuana, Mexico, 16% for Phoenix, and 10% for Las Vegas. The slider above shows satellite estimates of OMI NO₂ as an average of March 25 through April 25. The image on the left shows the mean of the 150-day period from 2015 through 2019, while the image on the right shows the mean of the 30-day period for 2020. The images are free and publicly-available and may be downloaded.