If the stars at night have seemed big and bright lately, it might be because air pollution in Texas has plummeted during the coronavirus shutdown, according to satellite imagery.
Concentrations of nitrogen dioxide (NO2), a pollutant tied to fossil fuel burning, showed a significant decline across the state, according to imagery from the European Space Agency’s Sentinel 5P satellite.
Measurements from a NASA pollution monitor also confirm a dip in NO2 levels this spring compared with the past five years. The drop coincides with a decline in traffic beginning in mid-March, according to Texas Department of Transportation data, as businesses and schools closed in response to concerns about the virus’s spread.
“Satellite data from various agencies appear to show significant decreases in [NO2] across the state and generally agree with ground-based monitors located near roadways that show decreases in nitrogen oxides,” Brian McGovern, spokesman for the Texas Commission on Environmental Quality, said in a Friday email.
Launched in October 2017, the Sentinel 5P satellite is among those carrying instruments that measure a variety of trace gases in the atmosphere tied to pollution. In March and early April, satellite imagery began showing drops in NO2 in China, northern Italy, and the East Coast of the U.S.
A similar clearing of the air occurred over Texas in March and April. The Sentinel 5-\P imagery the Rivard Report obtained shows NO2 concentrations averaged over three 40-day periods: Dec. 29 through Feb. 4, Feb. 4 through March 15, and March 15 through April 23.
However, even with the overall lower levels of pollution, Texas cities still have still experienced some days with poor air quality. On Friday, San Antonio had its first high-ozone day in 2020, with levels reaching thresholds considered unhealthy for those with lung conditions and other sensitive groups.
As the number of coronavirus cases ratcheted up, Texas cities began shutting down certain businesses the week of March 15. Gov. Greg Abbott followed with a series of shutdowns starting that week and leading up to a statewide stay-home order on April 1.
That order expired Friday, replaced with a new one allowing businesses like restaurants, malls, and movie theaters to begin serving customers at 25 percent occupancy.
During the shutdown, Texans drove significantly less than they did in February, according to TxDOT data.
Statewide, traffic volume was down between 18 percent and 44 percent weekly from March 14 to April 24 compared to a baseline week of Feb. 22-28. In San Antonio, traffic volume was down 20 percent to 50 percent weekly.
Vehicles aren’t the only source of NO2 emissions. Coal plants, natural gas plants, and industrial sites that involve fossil fuel combustion also emit NO2.
During the pandemic, power plants also have been running at lower capacity, with many businesses and industries shut down or reducing their activity. Weekly demand for power has been down 4 percent to 5 percent since March 29, and 2 percent the week before that, according to the Electric Reliability Council of Texas, the state’s electrical grid operator.
However, pollution is not the only thing that influences the presence of NO2 in the atmosphere. Levels of the pollutant tend to vary with the season because sunlight drives chemical reactions that eventually convert NO2 into ozone.
“In addition to being controlled by emissions, NO2 is also controlled by the amount of sunlight that makes it to the surface,” said Daniel Anderson, a scientist at NASA’s Goddard Space Flight Center who has studied ozone formation over San Antonio.
“So, even for the same amount of emissions, you’ll have higher amounts of NO2 in the winter than in the summer because there is less sunlight,” Anderson said. “Some of the decrease you’re seeing in NO2 from one figure to the next is likely just due to this effect.”
A video Anderson shared from the Ozone Monitoring Instrument aboard NASA’s Aura research satellite shows NO2 levels fluctuating daily. The video compares NO2 emissions over San Antonio in 2020 to data from 2015 to 2019.
However, it’s clear that San Antonio’s NO2 levels in 2020 were much lower from mid-March through mid-April than they were on average over the last five years, according to a graph of the data. Similar trends can be seen for NO2 data in Houston and the Dallas-Fort Worth area.