[The TROPOspheric Monitoring Instrument (TROPOMI) is the satellite instrument on board the Copernicus Sentinel-5 Precursor satellite. The Sentinel-5 Precursor (S5P) is the first of the atmospheric composition Sentinels, launched on 13 October 2017, planned for a mission of seven years. From tropomi.eu] [From NASASport written by Dr. Aaron Naeger]    Preventative measures recently adopted to prevent further spread of COVID-19 in the U.S. have prompted an overall slowdown in economic activity and fewer vehicles on the roadways.  Since combustion engine powered vehicles can represent a major source of nitrogen dioxide (NO₂) emissions, less traffic on the roadways may lead to a significant reduction in NO₂ concentrations and, as a result, fine particulate matter (Particulate Matter less than 2.5 microns in diameter or PM2.5) as NO₂ emissions are a known precursor to PM formation. The Tropospheric Monitoring Instrument (TROPOMI), launched in 2017 as part of the polar-orbiting European Space Agency’s (ESA) Sentinel-5 precursor (Sentinel-5P) satellite, has advanced our capability to monitor fine-scale emission sources, including vehicular emissions along traffic corridors, with unprecedented spatial resolution of 5.6 x 3.5 km².  Daily midday scans of TROPOMI over the densely populated cities and heavily trafficked corridors in California during March 2020 show how the adoption of stricter COVID-19 measures have impacted air quality in the state.  To effectively examine the changes in air quality in California, we constructed weekday averaged NO₂ maps for March at 0.05° grid spacing from high-quality, cloud-free retrievals provided by TROPOMI level 2 data.  It is also important to note the role of natural weather variability on air pollutants during this seasonal transitional period, as warmer temperatures and higher mid-day solar angles lead to shorter NO₂ lifetimes and generally lower NO₂ column concentrations. [Figure 1. a) Gridded 0.05 x 0.05° NO₂ map from TROPOMI Offline (OFFL) L2 retrievals during 2-6 March 2020 (pre-shutdown) over California.  b-d) Same as a), except valid for b) 9-13 March during soft shutdown measures, c) 16-20 March when “shelter in place” orders were announced, d) full period of “shelter in place” orders during 23-27 March. Panel d) generated using near real-time (NRT) product due to 7-10 day lag of OFFL product. From NASASport] For the first weekday period of March (2-6 March) when COVID-19 measures were yet to be implemented, the largest tropospheric NO₂ concentrations were observed in Los Angeles and bordering counties with a less prominent peak in NO₂ around San Francisco (Fig. 1a).  The TROPOMI scans also resolved areas of enhanced NO₂ along the heavily trafficked corridor of State Route 99 (SR-99) in the Central Valley, particularly around the cities of Bakersfield and Fresno.  As initial, soft COVID-19 measures were adopted by businesses in California during the second weekday period in March (9-13 March), TROPOMI observed strong reductions in tropospheric column NO₂ around the large cities of Los Angeles and San Francisco along with noticeable decreases along SR-99 (Fig. 1b).  As California announced statewide “shelter in place” orders during the third weekday period of March (16-20 March), further decreases in NO₂ were apparent throughout all populated areas in the state and along SR-99 (Fig. 1c).  Noticeable decreases in NO₂ continued throughout much of the state during the fourth weekday period of March (23-27 March), especially around San Francisco (Fig. 1d).  Overall, these observed reductions in TROPOMI NO₂ throughout March are the result of decreased emissions on top of the seasonal changes in meteorological conditions.