COVID Quenching Effect on Mixed Air Quality

In April 2020, as Delaware and states across the country adopted social distancing measures to address the public health crisis caused by the coronavirus (COVID-19), University of Delaware professor Cristina Archer recalled that a group of people told her the sky looked bluer than usual.

This simple observation led Archer to investigate an important and complicated research question: the social distancing measures adopted in the United States and the consequent fewer people using various means of transport, have caused an improvement in air quality. all over the country?

Unfortunately, unlike those seen with clear blue skies, the answer is a bit murky.

“Just because people stayed home and just because they drove less didn’t necessarily mean the air quality was better,” said Archer, a professor in the College of Earth, Ocean and Environment’s School of Marine Science and Policy (CEOE ). “For some parameters the air quality was better, but for others it wasn’t. And it actually got worse in many places.”

The results of the study were recently published in Atmospheric Science and Technology Bulletin. The study was led by researchers from UD, Penn State and Columbia University.

From UD, co-authors include doctoral students Maryam Golbazi and Nicolas Al Fahel. Other co-authors of the study are Guido Cervone, a professor of geography and meteorology and atmospheric sciences at Penn State, and Carolynne Hultquist, a former PhD student in the Cervone lab and now a postdoctoral researcher at Columbia University’s Earth Institute.

To assess air quality, Archer focused on nitrogen dioxide and fine particulate matter, particularly PM2.5. The two pollutants are federally regulated and are both primary and secondary pollutants, meaning they can be emitted directly into the atmosphere or indirectly by chemical reactions.

Nitrogen dioxide is emitted during the combustion of fuel by all motor vehicles and airplanes, while particulate matter is emitted by airplanes and, among motor vehicles, mainly by diesel vehicles, such as heavy commercial diesel trucks. Both are also emitted by fossil fuel power plants, although particulate matter is mainly emitted by coal plants.

“Nitrogen dioxide is a good indicator of traffic,” Archer said. “We already had evidence from some preliminary documents in China that nitrogen dioxide had decreased significantly in areas of China where a blockade was in place.”

The researchers compared the measurements of these two pollutants in April 2020 with those of April in the previous five years, from 2015 to 2019.

They chose April because virtually every state had some sort of social distancing measure in place by April 1, which led to lifestyle changes.

“Even in states where there weren’t many infections, there was a shift in people’s mobility,” Archer said.

To quantify social distances, the researchers used a mobility index calculated and distributed by Descartes Labs, a predictive intelligence company that compiles large data sets from around the world. Their algorithms took into account the maximum distance people traveled in a day by tracking the user’s location multiple times a day while using selected apps on their smartphones.

“One of the big uncertainties in trying to predict future air quality is how the atmosphere will respond to lower emissions of certain pollutants,” Cervone said. “COVID-19 has given us some insight into the effects of lower emissions rates on the environment. We had this unique situation that showed us what happens if people stop driving.”

In addition, they used 240 soil monitoring sites to measure nitrogen dioxide and 480 for particulate matter, as well as satellite data from the National Aeronautics and Space Administration (NASA) Ozone Monitoring Tool (OMI) to measure the tropospheric column. total, the lowest layer of the earth’s atmosphere – nitrogen dioxide.

Overall, they found that there were statistically significant large decreases in nitrogen dioxide in 65% of monitoring sites, with data from NASA’s OMI satellites showing an average drop in 2020 of 13% over the whole country compared to the average. of the previous five years.

Particulate concentrations, however, were not significantly lower in 2020 than in the past five years. It was also more likely to be higher than lower in April 2020 than the previous five years.

“Unsurprisingly, we found that air quality improved in terms of nitrogen dioxide at all stations. So as soon as people started staying at home, traffic was reduced and air quality was reduced. was better for nitrogen dioxide, “Archer said. “But when we looked at the particulates, there was almost no difference. There was no improvement on average over the particulate matter and at 24% of the sites, April 2020 was worse than the previous five years for particulate concentrations.” .

Golbazi echoed these sentiments by saying that the researchers “expected that reducing transport would reduce nitrogen dioxide concentrations. However, our findings on PM2.5 were surprising because we learned that this scale of human mobility reduction. it didn’t really reduce PM2. 5 concentrations. “

This is significant because while nitrogen dioxide is a precursor to other pollutants, such as the formation of ground level ozone and the formation of nitric acid in acid rain, which will ultimately lead to negative impacts on human health, particulate matter is dangerous in itself. .

Fine particulate matter is an air pollutant made up of microscopic particles that pose a great risk to human health because they can penetrate directly into human lungs, bloodstream and even the heart.

Archer said they are working on a hypothesis as to why particulate matter could have increased while nitrogen dioxide decreased.

One possible explanation is that particulate matter is emitted from diesel vehicles, which handle most of the nation’s deliveries. While traffic for gasoline passenger vehicles decreased in April, diesel trucks and normal freight traffic did not change significantly.

Additionally, more particulate matter is emitted when people use heat to heat their personal homes than office spaces.

“In the office it is very unlikely that you have a stove or a fireplace, but at home you are,” Archer said. “And in April 2020, although in many states it is already hot season, we had an incredibly cold April in the Northeast. So particulate matter is probably responding to those changes in residential heating and to normal or even higher than normal. freight transport. road and diesel vehicles. “

Archer said the next steps in the research are to run more studies to confirm their hypothesis. Additionally, the work will be presented at the upcoming fall meeting of the virtual American Geophysical Union.

As for those bright, sunny days observed in Delaware at the start of closing in April, Archer said it was probably just a matter of perception.

“People used to tell me, ‘I think the skies are bluer,’ but they weren’t,” he said. “It was probably a coincidence that there were sunny, clear days. In Delaware, our skies were actually more polluted in April 2020 due to above-average particulate concentrations.”