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California's vehicle emissions have steadily declined over the years as environmental policies and advancing technology clean up traffic exhaust.
Yet since 2010, microscopic airborne particles and ground-level ozone have stubbornly refused to drop thanks to a rise in 'secondary sources' – many of which are the trees and shrubs greening up 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 of 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 recent airborne approach could directly measure airborne pollutants multiple times a second. This was achieved using an on-board mass spectrometer, which describes the spread of more than 400 types of emission in unprecedented detail.
Combining results with temperature patterns down to a resolution of 4 kilometers squared (about 2.5 miles squared), the team determined that botanical sources of VOCs, which include compounds like isoprene, monoterpenes, and sesquiterpenes, contributed to around 60 percent of the potential formation of secondary organic aerosols at the start of the LA summer.
Given that these botanical emissions increase with hot weather and drought, the problem could get worse as the summer continues. The researchers predict this is a problem we need to stay on top of as the world warms.
Ambient air pollution remains a significant health problem worldwide, in spite of efforts to reduce toxic emissions in transport and industry. Fine solid particles just micrometers in size increase the risk of heart disease and low birth weights, while ozone in the air we breathe has links with respiratory illness and increased mortality.
Key to the formation of both of these potentially toxic materials are VOCs – a wide variety of chemicals that both impact our health directly, and react in sunlight and the atmosphere to form particulates and gases like ozone.
Given an estimated 4.2 million premature deaths a year can be attributed to ambient airborne pollution, mostly in urban populations, health authorities are keen to find better ways of identifying sources of VOCs that can be mitigated in our biggest cities.
There's no end to potential producers of these pervasive compounds, with everything from pesticides to hair products to car upholstery to cleaning agents sweating out some kind of compound that is capable of generating something nasty in tiny amounts. So it's little surprise that volatile chemical products now contribute up to half of the fossil-fuel VOC emissions in industrialized cities.
What may be surprising is that the very green spaces that define clean living generate their own compounds in the form of terpenoids, which the analysis revealed contributed around 16 percent of the measured mass flux of VOCs.
Heavy debate has raged over the significance of biogenic versus industrial sources, especially when higher temperatures are taken into account.
"Monoterpene and sesquiterpene emissions typically increase exponentially with temperature, whereas isoprene emissions are known to increase with temperature and light and eventually decrease above a temperature threshold," the researchers note in their study.
Knowing the potential for a city's gardens to contribute to pollution isn't cause to reduce green spaces, which themselves keep temperatures cooler, and improve our health in other ways. Some can even remove certain species of VOC from the air.
To maximize their benefits, however, it would pay to better understand how factors like drought may increase large scale biogenic VOC emissions, and how the discarded blooms of plants like jacarandas – among the most abundant species in Los Angeles, although they are not native – contribute organic precursors of their own. Or even determining which kinds of plants might be lower emitters as global temperatures inevitably continue to rise.
This research was published in Science.
