Trees Have Become an Unexpected Source of Air Pollution in Los Angeles

California has seen a steady decline in vehicle emissions over the years thanks to environmental policies and advancements in technology that have cleaned up traffic exhaust.

However, since 2010, microscopic airborne particles and ground-level ozone have remained stubbornly high due to an increase in ‘secondary sources’—many of which are the trees and shrubs greening up our city streets.

To map these emissions, a team of US researchers conducted nine aerial surveys over Los Angeles in June 2021 to directly measure fluctuating concentrations of volatile organic compounds (VOCs), which are precursors to particulate and ozone pollution and can originate from plants. Unlike previous maps, which estimated emissions based on known sources or modeled emission movements, this recent airborne approach directly measured airborne pollutants multiple times per second using an on-board mass spectrometer. This technique described the spread of more than 400 types of emissions in unprecedented detail.

Combining these results with temperature patterns at a resolution of 4 square kilometers (about 2.5 square miles), the team determined that botanical sources of VOCs, including 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 worsen as summer progresses. The researchers predict this issue will need ongoing attention as the world warms.

Ambient air pollution remains a significant health problem worldwide, despite efforts to reduce toxic emissions from 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 is linked to respiratory illness and increased mortality.

Key to the formation of both these potentially toxic materials are VOCs—a wide variety of chemicals that impact 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 are attributed to ambient airborne pollution, mostly in urban populations, health authorities are keen to identify sources of VOCs that can be mitigated in our biggest cities.

There is no shortage of potential producers of these pervasive compounds. Everything from pesticides to hair products to car upholstery to cleaning agents can release VOCs capable of generating something harmful in tiny amounts. Consequently, 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 associated with clean living generate their own compounds in the form of terpenoids. The analysis revealed that these contributed around 16 percent of the measured mass flux of VOCs.

There has been heavy debate over the significance of biogenic versus industrial sources, especially when higher temperatures are considered. “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 is not a reason to reduce green spaces, which themselves keep temperatures cooler and improve our health in other ways. Some plants can even remove certain species of VOCs from the air.

To maximize their benefits, it is essential to 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, though they are not native—contribute organic precursors of their own. Additionally, determining which types of plants might be lower emitters as global temperatures continue to rise is crucial.