Research confirms that clean air and emission control policies have substantially reduced both PM 2.5 aerosol and ozone concentrations in the Los Angeles (LA) Basin over the last two decades. However, concentrations of both have plateaued in recent years. and high-pollutant events still regularly exceed accepted thresholds on hot days. The plateau and the role of heat are due to a shift in pollution sources, according to University of California, Berkeley researchers Clara Nussbaumer and Ronald C. Cohen. As pollutants from vehicle emissions decline, the response of pollutants from other sources to temperature is becoming an important factor in high-aerosol and high-ozone events in the megacity region.
20 years of air quality data
Nussbaumer and Cohen, funded in part by the NOAA Climate Program Office’s Atmospheric Chemistry, Carbon Cycle, & Climate (AC4) program, analyzed air quality data from across the LA Basin for the past two decades (1999-2018). The bulk of their data focuses on organic aerosols, a specific category of particulate matter with a diameter smaller than 2.5 micrometers (PM 2.5) linked to a range of respiratory and cardiovascular illness. They used data from 22 measurement sites across the LA 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 sites (three in LA, one in San Bernardino) to look at ozone. Their research was recently detailed in the scientific journal Environmental Science & Technology.
Based on measurements, concentrations of PM 2.5 in the LA Basin essentially halved from 1999 to 2012. After 2012, however, Nussbaumer and Cohen found that the PM 2.5 concentration at each site remained more or less constant. This sharp decline followed by a plateau suggests that while emission controls were targeting the largest sources of pollutants in 1999, such as vehicle emissions, as those sources were reduced over time the most prominent sources now are those unaffected by current emissions policies.
While the average and max temperatures in the LA Basin have remained approximately constant over the last two decades, the relationship between temperature and aerosols has changed. Nussbaumer and Cohen’s analysis shows that when the temperature increases so too do PM 2.5 concentrations. Near the beginning of the 21st century, there was more variation in this relationship: if the temperature went up, sometimes PM 2.5 concentrations would increase a lot, sometimes a little. Now the relationship is more linear: if the temperature goes up, expect PM 2.5 concentrations to increase by a set amount for each degree rise in temperature.
As the relationship between aerosols and temperature becomes more clearly reflected in the observations it suggests that the decline in PM 2.5 is a result of a decrease in temperature-independent sources. Accordingly, from 2016-2018, there were nearly zero PM 2.5 violations at 68°F, while 70 to 80% of days over 100°F exceeded the National Ambient Air Quality Standard (NAAQS) threshold. “20 years ago, aerosol levels were unhealthy in LA no matter what the temperature,” Cohen explains, “but in the present it is only on the hotter days of the year.”
Indirect sources play the primary role
In 2018, organic aerosols comprised just under a quarter of PM 2.5 concentrations. Nussbaumer and Cohen found that organic aerosols were positively correlated with temperature and the largest contributor to high aerosol events.
Aerosols emitted directly into the atmosphere from combustion processes such as car exhaust, cooking, and wildfires are known as primary organic aerosol (POA). Secondary organic aerosol (SOA) form indirectly due to reactions of other emitted pollutants with sunlight. Nitrogen oxides (NOx) and volatile organic compounds (VOCs) are the usual precursors to any SOA; they also happen to be the same common prerequisites to ozone formation.
Scientists have long recognized that SOA in rural locations are extremely temperature dependent. Nussbaumer and Cohen’s recent work shows this is now true in cities too. Their research seeks to illuminate what emission sources are responsible for these temperature-dependent SOA and what exactly their presence means for air quality control in the LA Basin and other megacity regions like LA.
To determine SOA sources, Nussbaumer and Cohen first had to determine the role of various VOC precursors. They found that trends in VOC amounts in the air mirrored the trends in PM 2.5 concentrations. Early data records show multiple different sources of VOC molecules and that their emissions are less correlated with temperature. More recent records, however, show fewer sources with the majority of VOCs responding to temperature.
Anthropogenic VOC emissions associated with vehicles in the LA Basin have steadily declined since 1999. They are now so low that the contribution of biogenic, or natural, VOC concentrations and chemicals from other sources such as emissions from household and industrial chemicals (collectively referred to as volatile chemical products or VCP) are now important players in the chemistry of ozone and SOA. Anthropogenic VOCs from industrial and motor vehicle emissions that form aerosol are closely related to the BTEX (benzene, toluene, ethylbenzene, and xylene) family of chemicals. The most common biogenic source of VOCs for LA Basin is isoprene, naturally emitted by plants such as the Mexican fan palm trees the region is famous for.
Nussbaumer and Cohen created a back-of-the-envelope atmospheric model to test their conclusions about which chemical families are now running the aerosol show. They surmised that POA along with BTEX and isoprene together as SOA-precursors comprised the majority of the organic aerosols observed. Their model suggests that almost a quarter of the SOA in the LA Basin are formed by isoprene or other very similar compounds and these represent most of the temperature dependent increase. While there is evidence that some temperature-dependent VOCs have been controlled over time, such as those from evaporation of gasoline, isoprene is not one of them.
“These results point to the exceptional success of control measures aimed at passenger vehicles,” Cohen notes, “and uncover new sources for high aerosol that are from other sectors. Possibly lawns and trees, but also possibly chemicals, solvents, and home cleaning products that evaporate more intensely when it is hot.”
The future of urban air quality
Given the potential increase in temperature and heat wave intensity due to climate change, Nussbaumer and Cohen’s work indicates scientist and policy makers alike should pay attention to temperature-dependent aerosols. To keep an eye on California’s urban air quality, Cohen’s lab has maintained BEACO2N, a high-density network of measurements all over the Bay Area for years. All the collected data is publicly available for viewing and download. Many of the 75 locations with monitoring stations, are featured on the network’s website. Cohen has also just started a collaboration with William Berelson at University of Southern California (USC) and the LA Unified School District to deploy BEACO2N nodes in LA. Separately, along with his UC Berkeley collaborator, Allen Goldstein, Cohen is partnering with NOAA scientists and the state and local air quality agencies on an experiment to observe emissions in LA at different temperatures. Combining these different observing strategies in the LA Basin, Cohen hopes, “will lead to better ideas for reducing high ozone and aerosol events in the Basin, ones that can then be used as a guide in other major cities suffering from poor air quality.”
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