California vehicle emissions have fallen steadily over the years as environmental policies and advanced technology clean up traffic emissions.
However, since 2010, microscopic air particles and ground-level ozone have stubbornly refused to fall thanks to the rise of ‘secondary sources’ – many of which are the trees and shrubs that line our city streets.
To map the emissions, a team of US researchers took to the skies over Los Angeles nine times in June 2021 to directly measure fluctuating concentrations of volatile organic compounds (VOCs), which are precursors to the particulate and ozone pollution that can come from plants.
Unlike previous maps, which either estimated emissions based on known sources or modeled the movement of emissions, this latest airborne approach can directly measure air pollutants several times per second. This was achieved using an onboard mass spectrometer, which describes the spread of more than 400 types of emissions in unprecedented detail.
Combining the results with temperature models down to a resolution of 4 square kilometers (about 2.5 square miles), the team determined that botanical sources of VOCs, which include compounds such as isoprene, monoterpenes and sesquiterpenes, contributed about 60 percent of the possible formation of secondary organic aerosols in the early LA summer.
Given that these botanical emissions increase with hot weather and drought, the problem can get worse as the summer goes on. Researchers predict that this is a problem we must stay on top of as the world warms.
Ambient air pollution remains a significant health problem worldwide, despite efforts to reduce toxic emissions in transportation and industry. Fine particulate matter just micrometers in size increases the risk of heart disease and low birth weight, while ozone in the air we breathe is linked to respiratory disease and increased mortality.
Key to the formation of both of these potentially toxic materials are VOCs – a wide variety of chemicals that directly affect our health and react in sunlight and the atmosphere to form particles and gases such as ozone.
Given that an estimated 4.2 million premature deaths per year can be attributed to ambient air pollution, mostly in urban populations, health authorities are eager to find better ways to identify sources of VOCs that may soften in our larger cities.
There’s no end to the potential producers of these pervasive compounds, with everything from pesticides to hair products to car upholstery to cleaning agents sweating out a type of compound capable of generating something nasty in small amounts. . So it’s no surprise that volatile chemicals now contribute up to half of fossil fuel VOC emissions in industrialized cities.
What may be surprising is that the green spaces that define clean living generate their own compounds in the form of terpenoids, which the analysis found contributed to about 16 percent of the measured VOC mass flux.
A serious debate has raged over the importance of biogenic versus industrial sources, especially when higher temperatures are considered.
“Monoterpene and sesquiterpene emissions typically increase exponentially with temperature, while isoprene emissions are known to increase with temperature and light and eventually decrease above a temperature threshold,” the researchers note in their study.
Recognizing the potential for a city’s gardens to contribute to pollution is no reason to discount green spaces, which themselves keep temperatures cooler and improve our health in other ways. Some can even remove certain types of VOCs from the air.
To maximize their benefits, however, it would be beneficial to better understand how factors such as drought can increase large-scale biogenic VOC emissions and how the flowering of deciduous plants such as jacarandas—among the species most abundant in Los Angeles, although they are not native – they contribute their organic precursors. Or even determining which types of plants might have lower emissions as global temperatures inevitably continue to rise.
This research was published in science.