Cars, trucks, and buses are among the top pollution sources in Philadelphia, given the vast network of normally traffic-choked roads within the city and surrounding areas.

The compounds emitted from those commuter tailpipes include particulate matter, nitrogen oxide, and carbon monoxide. In addition, the burning of fossil fuels produces volatile organic compounds, sulfur dioxide, and carbon dioxide, a greenhouse gas.

It’s been widely assumed that levels of those pollutants would drop significantly during the coronavirus shutdown, which has reduced rush-hour traffic to a fraction of its normal load.

Data from the Philadelphia Department of Public Health’s Air Management Services suggest levels of some pollutants have decreased in the city. The Inquirer published a story April 17 with data showing some of the decline. Then on Wednesday, Earth Day, city officials provided updated levels of two key pollutants before and after Gov. Tom Wolf’s orders to shut down businesses and require residents to stay home.

What the numbers say

The city monitors air quality through a network of stations that house instruments measuring gaseous, solid, and liquid aerosol pollutants.

Originally, at the request of The Inquirer, the health department analyzed averages from 10 monitors for nitrogen dioxide (NO2) and PM2.5 (atmospheric particulate matter under 2.5 micrometers), two pollutants that pose a threat to human health. It compared 16-day periods immediately before and immediately after Gov. Tom Wolf issued statewide mitigation orders on March 17. Both pollutants showed substantial drops.

The new city analysis includes a longer period used to examine pollutant levels. The new analysis compared values from Feb. 17 to March 17, before Wolf’s orders, and from March 17 through April 16, after the orders.

The orders closed schools and restaurants, halted non-essential travel, and traffic plunged. (A full stay-at-home order for Philadelphia went into effect March 23).

Nitrogen dioxide

The biggest drop came with NO2, an indicator for a group of gases known as nitrogen oxides (NOx). The monitors show a big decline in NO2, which is a good thing since it is harmful to human health.

NO2 is pumped into the air during the burning of fossil fuels used in motor vehicles and power plants. Breathing high concentrations of the gas can irritate airways, aggravate respiratory diseases, and lead to coughing, wheezing, and difficulty breathing.

Children and the elderly with asthma are generally at greater risk for the health effects of NO2, which may contribute to the development of asthma.

NO2 and other nitrogen compounds react with chemicals in the air to form particulate matter and ozone, both of which are harmful.

The average daily maximum one-hour NO2 concentration recorded in the month before the order was 37.5 parts per billion. After the order, the concentration had dropped to 29.2 parts per billion, reflecting a 22% drop.

Additional analysis showed NO2 levels are 22% lower this year than during the same period in 2019.

Data show decreases in nitrogen oxide from one month leading up to the coronavirus shutdown and one month after.
Philadelphia Department of Health
Data show decreases in nitrogen oxide from one month leading up to the coronavirus shutdown and one month after.

Particulates

The city also saw a decline in particulate matter, which is what you see in soot from vehicle exhaust. The tiny particles, smaller than one-10th the diameter of a human hair, are a health hazard and can penetrate deep into the lungs.

The monitors showed that PM2.5 dropped from a daily average of 7.4 micrograms per cubic meter of air to 6.1, a roughly 18% drop.

However, an analysis for roughly the same time period in 2019 showed no significant difference, which city officials attribute to the fact that PM2.5 is not just a local pollutant, and so could be influenced by sources elsewhere, as well as weather, which varies from year to year.

Data show decreases in PM2.5 (fine particulates) from one month leading up to the coronavirus shutdown and one month after.
Philadelphia Department of Health
Data show decreases in PM2.5 (fine particulates) from one month leading up to the coronavirus shutdown and one month after.

Carbon monoxide

Carbon monoxide, CO, is found in fumes produced by cars, trucks, small engines, and even stoves and grills. If it builds up indoors, it can poison people.

CO also dropped from 0.7 parts per million to 0.5 parts per million, about a 25% drop in concentrations.

Data show decreases in carbon monoxide from one month leading up to the coronavirus shutdown and one month after.
Philadelphia Department of Health
Data show decreases in carbon monoxide from one month leading up to the coronavirus shutdown and one month after.

Reducing pollution ‘pays dividends'

The health department said in a report that the data "shows that by restricting business and movement by the public, levels of certain air pollutants in Philadelphia will drop by significant amounts. While it is understood that maintaining this level of shutdown is inadvisable and dangerous, the Health Department maintains that there is something to be learned from it.”

The report continued: "Reducing the number of cars and trucks on the road and the amount of pollution from industrial point sources pays dividends in air quality. "

Joe Minott, executive director of the nonprofit Clean Air Council and a resident of Center City, said last week, “The difference in traffic has been breathtaking, for want of a better word. Even at the height of rush hour, it’s nothing.”

Minott said a drop in vehicle traffic and a commensurate drop in at least some pollutants “is exactly what you would expect. Mobile sources are the single largest source of air pollution. And the pollution is at ground level, so it has a very profound impact on nearby communities.”

Minott hopes that businesses will learn from the shutdown and allow more employees to work from home going forward — at least part of the time.

“I think we are already seeing these sorts of trends in general where younger people are less interested in owning cars and driving," he said. “And now, since we’ve been forced to implement work at home, will that become a new norm?”

Greenhouse gas

What about the chief greenhouse gas in the United States, carbon dioxide?

CO2 is widely dispersed in the atmosphere and can come from anywhere. Philadelphia does not monitor CO2 levels in its network.

However, the federal Energy Information Administration is forecasting that nationwide, “energy-related carbon dioxide (CO2) emissions will decrease by 7.5% in 2020 as the result of the slowing economy and restrictions on business and travel activity related to COVID-19.”

And even for 2021, the agency is forecasting that CO2 might only bounce back by 3.6%.

Anthony J. Broccoli, co-director of the Rutgers Climate Institute, said the only comparison to what we’re experiencing now is the 2007-09 recession, when there was a temporary reduction in greenhouse gas emissions.

But any lasting effect of the current economic slump on long-term changes in climate will be too small to notice.

"Much more drastic reductions in emissions that are sustained into the future would be required to stabilize the climate,” Broccoli said.

This story was updated April 22 to reflect newer pollutant numbers from its original publication date of April 17.