Research shows Clean Air Act is likely responsible for dramatic decline in atmospheric organic aerosol

urban air
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The air we breathe contains particulate matter from a range of natural and human-related sources. Particulate matter is responsible for thousands of premature deaths in the United States each year, but legislation from the U.S. Environmental Protection Agency (EPA) is credited with significantly decreasing this number, as well as the amount of particulate matter in the atmosphere. However, the EPA may not be getting the full credit they deserve: new research from MIT's Department of Civil and Environmental Engineering (CEE) proposes that the EPA's legislation may have saved even more lives than initially reported.

"In the United States, the number of premature deaths associated with exposure to outdoor particulate exceeds the number of car accident fatalities every year. This highlights the vital role that the EPA plays in reducing the exposure of people living in the United States to harmful pollutants," says Colette Heald, associate professor of CEE and Earth, Atmospheric and Planetary Sciences.

The EPA's 1970 Clean Air Act and additional amendments enacted in 1990 address the health effects of particulate matter, specifically by regulating emissions of air pollutants and promoting research into cleaner alternatives. In 2011 the EPA announced that the legislation was responsible for a considerable decrease in particulate matter in the atmosphere, estimating over 100,000 lives saved every year from 2000 to 2010. However, the report did not consider organic aerosol, a major component of atmospheric particulate matter, to be a large contributor to the decline in particulate matter during this period. Organic aerosol is emitted directly from fossil fuel combustion (e.g. vehicles), residential burning, and wildfires but is also chemically produced in the atmosphere from the oxidation of both natural and anthropogenically-emitted hydrocarbons.

The CEE research team, including Heald; Jesse Kroll, an associate professor of CEE and of chemical engineering; David Ridley, a research scientist in CEE; and Kelsey Ridley SM '15, looked at surface measurements of organic aerosol from across the United States from 1990 to 2012, creating a comprehensive picture of organic aerosol in the United States.

"Widespread monitoring of air pollutant concentrations across the United States enables us to verify changes in air quality over time in response to regulations. Previous work has focused on the decline in particulate matter associated with efforts to reduce acid rain in the United States. But to date, no one had really explored the long term trend in organic aerosol," Heald says.

The MIT researchers found a more dramatic decline in organic aerosol across the U.S. than previously reported, which may account for more lives saved than the EPA anticipated. Their work showed that these changes are likely due to anthropogenic, or human, behaviors. The researchers' findings were published in a paper, "Causes and Consequences of decreasing atmospheric organic aerosol in the U.S." in the Proceedings of the National Academy of Sciences the week of December 25.

"The EPA report showed a very large impact from the decline in particulate matter, but we were surprised to see a very little change in the organic aerosol concentration in their estimates," explains David Ridley. "The observations suggest that the decrease in organic aerosol had been six times larger than estimated between 2000 and 2010 in the EPA report."

Using data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, the researchers found that organic aerosol decreased across the entire country in the winter and summer seasons. This decline in organic aerosol is surprising, especially when considering the increase in wildfires. But the researchers found that despite the wildfires, organic aerosols continue to decline.

The researchers also used information from the NASA Modern-Era Retrospective Analysis for Research and Applications to analyze the impact of other natural influences on organic aerosol, such as precipitation and temperature, and found that the decline would be occurring despite cloud cover, rain, and temperature changes.

The absence of a clear natural cause for the decline in organic aerosol suggests the decline was the result of anthropogenic causes. Further, the decline in organic aerosol was similar to the decrease in other measured atmospheric pollutants, such as nitrogen dioxide and carbon monoxide, which are likewise thought to be due to EPA regulations. Also, similarities in trends across both urban and rural areas suggest that the declines may also be the result of behavioral changes stemming from EPA regulations.

By leveraging the emissions data of organic aerosol and its precursors from both natural and anthropogenic sources, the researchers simulated organic aerosol concentrations from 1990 to 2012 in a model. The researchers found that more than half of the decline in organic aerosol is accounted for by changes in human emissions behaviors, including vehicle emissions and residential and commercial fuel burning.

"We see that the model captures much of the observed trend of organic aerosol across the U.S., and we can explain a lot of that purely through changes in anthropogenic emissions. The changes in organic aerosol emissions are likely to be indirectly driven by controls by the EPA on different species, like black carbon from fuel burning and nitrogen dioxide from vehicles," says Ridley. "This wasn't really something that the EPA was anticipating, so it's an added benefit of the Clean Air Act."

In considering mortality rates and the impact of organic aerosol over time, the researchers used a previously established method that relates exposure to particulate matter to increased risk of mortality through different diseases like cardiovascular disease or respiratory disease. The researchers could thus figure out the change in mortality rate based on the change in particulate matter. Since the researchers knew how much organic aerosol is in the particulate matter samples, they were able to determine how much changes in organic aerosol levels decreased mortality.

"There are costs and benefits to implementing regulations such as those in the Clean Air Act, but it seems that we are reaping even greater benefits from the reduced mortality associated with because of the change in organic aerosol," Ridley says. "There are health benefits to reducing organic further, especially in urban locations. As we do, natural sources will contribute a larger fraction, so we need to understand how they will vary into the future too."

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More information: D. A. Ridley el al., "Causes and consequences of decreasing atmospheric organic aerosol in the United States," PNAS (2017).
Citation: Research shows Clean Air Act is likely responsible for dramatic decline in atmospheric organic aerosol (2017, December 25) retrieved 17 October 2019 from
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User comments

Dec 25, 2017
It warms the heart to see bureaucrats looking out for each other.

Dec 26, 2017
The oft-repeated canard that particulates cause thousands of deaths each year in the United States is the worst kind of statistical malfeasance. It is a complete fabrication; a lie. Conversely, the idea that reductions in particulates by the EPA has saved thousands of lives is just as unsupportable.

No one can point to any death and attribute it to particulate pollution. It is based solely on the supposition that particulates exacerbate chronic lung conditions like asthma and emphysema and SHORTEN the life expectancy of sufferers. It's possible there is some merit to it, but so far no one has demonstrated that particulate pollution has "caused" a single death.

We should of course prevent or limit air pollution and particulate emissions based on actual evidence of harm, not on the statistical chicanery of the eternally unsatisfied science-denying environmental lobby.

Dec 26, 2017
I trust medical professionals over the opinion of an internet commenter.

Dec 27, 2017
gkam, if you had applied a little critical thinking, or even summoned a modicum of curiosity, you could have discovered for yourself that none of the authors of this paper are "medical professionals." Deep curiosity might have led you to question the last time you heard of someone dying of minuscule quantities of particulate pollution, then wonder who makes these claims and how they arrive at them, which leads to a fascinating look at epidemiological studies in general and the problems of identifying cause and effect as well as the disturbing revelation that people with preconceived biases torture data with statistical methods to reflect their foregone conclusion.

Incidentally, scientists and medical professionals are also human beings with biases. "Science" is not nearly as scientific as you think.

Dec 27, 2017
Ask, I studied the effects of particulates on human health in the master's course.

It is real. Shall I look up the real evidence for you?

Dec 27, 2017
Honestly don't waste your time. Trolls like askdad are just trying to get a response because they can't get people to talk to them in real life.

Dec 27, 2017
"Particulate matter is responsible for thousands of premature deaths in the United States each year," and not one death certificate shows that or that it was a significant contributing factor. All that is based on the Six Cities Study which has been debunked as bad science.

Dec 27, 2017
What about Raytheon's atmospheric heavy metal aerosols?

Dec 29, 2017
The earth gets wasted as trolls try and justify their pathetic destruction upon it.

Dec 29, 2017
Ask, I studied the effects of particulates on human health in the master's course.

It is real. Shall I look up the real evidence for you?

Askdaddy is as dumb as ever, stumbling upon his dumbfounded opinions over and over with no end in sight, because he has no regard whatsoever for the Earth's health, Only his dumbfounded opinions.

Dec 29, 2017
sorry gkam mistaken downvote.

Dec 29, 2017
It warms the heart to see bureaucrats looking out for each other.

Only to be exposed right here like the greedy corrupted idiots they are.

Dec 29, 2017
"Why most data scientists are frauds, according to a data scientist"

Who is this "data scientist", and why would you believe him? Is he a fraud?

Dec 30, 2017
The big fraud i see here is mackita, and gee golly, being an antigoracle sockpuppet, it comes as no surprise.

Jan 01, 2018
I trust medical professionals over the opinion of an internet commenter... Ask, I studied the effects of particulates on human health in the master's course
Aw georgie, fishing for detractors again? Attention is the only way psychopaths know they're alive. We know from what George kamburoff himself has told us, that the honorary MS he bought didnt require him to 'study' anything.

Anyone who uprates this sicko troll, or thinks he is a source of reliable info about anything, is an idiot.

Psychopaths lie for fun and profit. They cannot NOT lie.

Jan 01, 2018
Since aerosols and particulates reflect the sunlight and cool the planet it follows that the clean air act has contributed to global warming since the 70s and the not all of the so called temperature increase is due to increased CO2 levels.

Jan 01, 2018
"sorry gkam mistaken downvote."

Helo it is never a mistake to downvote gkam.

Jan 01, 2018
Makita was kind enough to share one prediction and here is another.


Remember James Hansen testified before congress about global warming in the very same year.

Jan 01, 2018


Ha, . hahahahahaha!

Jan 01, 2018
Right Gkam, only facts presented by the Ministry of Truth will be considered!

Jan 01, 2018
It is science.

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