California vehicle emissions have steadily declined over the years as environmental policies and advanced technology clean up exhaust fumes from traffic.
But since 2010, microscopic airborne particles and ground-level ozone have stubbornly refused to decline thanks to the rise of “secondary sources”—many of which are the trees and shrubs that green our city streets.
To map the emissions, a team of US scientists 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 particulate and ozone pollution that can come from plants.
Unlike previous maps that either estimated emissions based on known sources or modeled the movement of emissions, this recent airborne approach could directly measure pollutants in the air several times per second. This was achieved using the onboard mass spectrometer, which describes the spread of more than 400 types of emissions in unprecedented detail.
Combining the results with temperature patterns down to a resolution of 4 kilometers per square (about 2.5 miles per square), the team found that botanical sources of VOCs, which include compounds such as isoprene, monoterpenes, and sesquiterpenes, contributed about 60 percent of the potential formation of secondary organic aerosols on early LA summer.
As these botanical emissions increase with hot weather and drought, the problem could get worse as the summer continues. Scientists predict this is an issue we need to keep in mind as the world warms.
Air pollution remains a major health problem worldwide, despite efforts to reduce toxic emissions from transport and industry. Fine particulate matter measuring 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 both of these potentially toxic materials are VOCs – a wide range of chemicals that directly affect our health and react in sunlight and the atmosphere to form particles and gases such as ozone.
With an estimated 4.2 million premature deaths a year attributable to air pollution, mostly in urban populations, health authorities are scrambling to find better ways to identify sources of VOCs that can be mitigated in our largest cities.
The potential producers of these ubiquitous compounds are endless, from pesticides to hair products to car upholstery to cleaning products that sweat out some kind of compound capable of creating something nasty in tiny amounts. So it’s no surprise that volatile chemical products now contribute up to half of fossil fuel VOC emissions in industrial cities.
What may be surprising is that the very green areas that define clean living generate their own compounds in the form of terpenoids, which analysis revealed contributed approximately 16 percent of the VOC mass flux measured.
A heavy debate has raged over the importance of biogenic versus industrial sources, especially when higher temperatures are taken into account.
“Emissions of monoterpenes and sesquiterpenes typically increase exponentially with temperature, while isoprene emissions increase with temperature and light and eventually decrease above a temperature threshold,” the researchers note in their study.
Knowing the potential of urban gardens to contribute to pollution is no reason to reduce green spaces, which in themselves keep temperatures lower and improve our health in other ways. Some can even remove certain types of VOCs from the air.
However, to maximize their benefits, it would be worthwhile to better understand how factors such as drought can increase large-scale biogenic emissions of VOCs, and how discarded flowers of plants such as jacarandas—among the most abundant species in Los Angeles, although not native—contribute their own organic precursors. Or even determining which plant species might be lower emitters as global temperatures inevitably continue to rise.
This research was published in Science.