With the decline in aerosol air pollution from autos in LA, vegetation is rising as the principle supply – Watts Up With That?
[The ghost of Ronald Reagan makes an appearance~cr]
Plants that are high-emitting isoprenes can cause unhealthy aerosol levels during heat waves
UNIVERSITY OF CALIFORNIA, BERKELEY
Research news
IMAGE: In 2018, ORGANIC AEROSOLS accounted for around 23% of AEROSOL POLLUTANTS in LOS ANGELES (BLUE ON THE CAKE)
According to a new study by chemists at the University of California, Berkeley, California’s vehicle emissions restrictions have been so effective that in at least one urban area, Los Angeles, trees and other green plants may be the most vulnerable sources of dangerous aerosol pollution.
Aerosols – particles of hydrocarbons known as PM2.5 because they are less than 2.5 microns in diameter and easily lodge in the lungs – have been shown to cause cardiovascular and respiratory problems.
As a result of strict vehicle emissions laws, organic aerosol levels in the United States have been significantly reduced. However, in Los Angeles, which started at a higher level, the decline was particularly dramatic.
Based on pollution measurements over the past 20 years, UC Berkeley scientists found that PM2.5 concentrations in the Los Angeles Basin were half what they were in 1999 in 2012. As a result, there was almost no PM2 from 2016 to 2018 .5 violations in the area when the temperature was low, below 68 degrees Fahrenheit. However, aerosol concentrations increased at warmer temperatures – during the same period, 70% to 80% of days above 100 F exceeded the threshold of the National Ambient Air Quality Standard (NAAQS).
“The good news is that where we got the source and took action, those measures have been incredibly effective,” said Ronald Cohen, atmospheric chemist and professor of chemistry at UC Berkeley. “Twenty years ago, almost every day in LA violated a health-related standard. And now it’s only the hot days. “
As vehicle organic chemicals – compounds of carcinogens like benzene and toluene – dropped, air quality experts focused on other potential aerosol sources in cities with unhealthy concentrations. Many researchers believe personal care and household cleaning products – some seem as harmless as the citrus-scented limonene – could be the culprit. Given the temperature dependence of aerosol levels in Los Angeles, Cohen doubts this.
“There is a growing consensus that as the importance of automobiles grows, household chemicals will dominate the source of organics in the atmosphere, and therefore the source of aerosols,” he said. “I say I don’t understand how aerosols from these chemicals can be temperature dependent, so I think it’s probably something else. And trees are a good candidate. “
It is known that the higher the temperature, the more plants release organic chemicals. In many forest areas, trees are the source of organic chemicals that combine with man-made nitrogen oxides to form aerosols. President Ronald Reagan was partly right when he infamous 1981 declared: “Trees cause more pollution than cars.” It was at this time that scientists learned about the role of the forests around Atlanta in causing air pollution in that city.
Cohen and former Berkeley graduate student Clara Nussbaumer studied the organic chemical emissions of various plants known to grow or cultivated in the Los Angeles area and found that some, like the city’s iconic Mexican fan palms, produce many volatile organic compounds. Oak trees are also high emitters of organic chemicals.
They estimated that an average of 25% of the aerosols in the Los Angeles Basin come from vegetation that includes an estimated 18 million or more trees.
Vegetable aerosols are likely made up of the chemical isoprene – the building block of rubber, or plant chemicals like terpenes, which are made up of two or more isoprene building blocks that are combined to form a more complex molecule. According to Cohen, PM2.5 aerosols can be thought of as “tiny tiny pearls of candle wax” containing plant aerosols made up of many molecules of isoprene and terpenes found in pine resins.
“I’m not suggesting we get rid of plants, but I want people who think about large-scale planting to pick the right trees,” he said. “You should be picking low-emitting trees instead of high-emitting trees.”
The research was featured in the journal Environmental Science & Technology earlier this month.
How does global warming affect pollutants?
Cohen, who studied the temperature dependence of urban ozone levels to gain insight into the effects of climate change on pollutants, decided two years ago to study the temperature dependence of ozone and aerosol pollution in five counties in the Los Angeles Basin: Los Angeles , San Bernardino, Riverside, Orange and Ventura. He and Nussbaumer looked at data from 22 sites across the basin – eight in LA County, two in Orange County, five in Riverside County, four in San Bernardino County, and three in Ventura County – to study aerosols, and four locations – three in LA, one in San Bernardino – to study ozone.
The researchers found that at the beginning of the 21st century, the relationship between temperature and aerosol pollution was very different: as temperature rose, PM2.5 concentrations rose sometimes sharply, sometimes a little. Today the relationship is more linear: when temperature increases one degree, PM2.5 concentrations increase predictably by a set amount.
Cohen and Nussbaumer mainly focused on secondary organic aerosols (SOA), which form as particles when gaseous pollutants – mainly nitrogen oxides (NOx) and volatile organic compounds (VOCs) – react with sunlight. The same conditions create ozone.
Using a simple atmospheric model, they concluded that both regulated chemicals from vehicle exhaust and from cooking – primary organic aerosols such as benzene, toluene, ethylbenzene, and xylene – and isoprene from plants were precursors to the majority of the organic aerosols observed. Their model suggests that about a quarter of the SOA in the LA basin is made up of isoprene or other very similar compounds, and that these represent the majority of the temperature-dependent increase. While there is evidence that some temperature-dependent VOCs have been controlled over time, such as through evaporation of gasoline, isoprene is not one of them.
Cohen noted that as the use of electric cars increases, the importance of organic aerosols from vegetation becomes more important and mitigation measures are required to keep levels within legal limits during heat waves.
“Cars also contribute to ozone, and the LA basin has high ozone levels too, at high temperatures and for the same reason: there are more organic molecules that power chemistry when it’s hot,” Cohen said. “We want a strategy to think about which plants emit fewer hydrocarbons when exposed to heat or which other emissions we could control to prevent the formation of aerosols.”
Cohen hopes to use data from other urban areas, including the San Francisco Bay Area, to determine whether temperature-dependent aerosols are now dominating and whether vegetation is the culprit.
The study was funded in part by a grant (NA18OAR4310117) from the National Oceanic and Atmospheric Administration (NOAA). Cohen and Allen Goldstein, professors of environmental science, policy, and management at UC Berkeley, and civil and environmental engineering, also partnered with NOAA scientists and state and local air quality agencies in an experiment to measure emissions in Los Angeles at various temperatures observe. Cohen hopes that combining these different observation strategies in the LA basin will lead to better ideas for reducing high ozone and aerosol events in the basin, which can then serve as a guide to other major cities with poor air quality.
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Cohen also maintains a high-density network of 75 pollution monitoring stations in the Bay Area called BEACO2N. Cohen has just started working with William Berelson at the University of Southern California and the Los Angeles Unified School District to deploy BEACO2N nodes in that area of Southern California.
From EurekAlert!
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