Downtown LA’s Silent Killer: Study Finds Trees Drive Air Pollution

California’s vast trees could be hidden sources of pollution, with the Golden State’s 173 million urban canopy leading to a massive rise in “secondary source” emissions, just as climate change worsens.

This is according to new research led by Eva Pfannerstill from the University of California, Berkley. The study found that urban trees could cause persistently high levels of pollution, continuing to plague the state despite vehicle emissions declining rapidly over the past 15 years.

“Despite the great strides that have been made in reducing urban pollution in major North American and European cities, much remains unclear about the causes of such pollution,” according to the study – Temperature-dependent Aerosol emissions dominate and ozone formation in Los Angelespublished in Science.

To offset emissions and tackle the urban heat effect, California has been investing heavily in afforestation and tree planting to lift its urban tree canopy (which now stands at 15%) – as part of the world’s first and most ambitious climate deals.

However, according to Dr Pfannerstill, an atmospheric chemist, “about 60% of ozone and secondary organic aerosol formation potential in summertime Los Angeles is caused by biogenic terpenoid emissions, and this contribution strongly increases with temperature.”

Urban trees and plants have been found to release BVOC’s

Biogenic volatile organic compounds (or BVOCs) refer to volatile organic compounds released by plants (and trees), with researchers now arguing that trees and shrubs planted to green up city streets are keeping pollution levels persistently high across Los Angeles, as well as across other major cities including San Fransico, San Diego and Sacramento.

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 onboard 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 square kilometres, the team determined that botanical sources of VOCs, which include compounds like isoprene, monoterpenes, and sesquiterpenes, contributed to around 60% of the potential formation of secondary organic aerosols at the start of the LA summer.

Deadly urban hazes are a public health disaster

Ambient air pollution remains a significant health problem worldwide despite efforts to reduce toxic emissions in transport and industry. Fine solid particles, just micrometres in size, increase the risk of heart disease and low birth weights. At the same time, ozone in the air links respiratory illness and increased mortality.

VOCs—a wide variety of chemicals that directly impact our health and react in sunlight and the atmosphere to form particles and gases like ozone—are key to forming both of these potentially toxic materials.

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.

It may be surprising that the green spaces that define clean living generate their own compounds in terpenoids, which the analysis revealed contributed around 16% of the measured mass flux of VOCs.

Heavy debate has raged 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 that a city’s gardens have the potential to contribute to pollution is not a reason to reduce green spaces. They 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 understand better 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.

It would also pay to determine which kinds of plants might be lower emitters as global temperatures inevitably continue to rise.