Surprising source of air pollution overtakes gas vehicles to become biggest toxin in US city

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Scientists have discovered a hidden source of pollution in Los Angeles that contributes about 60 percent to the toxins in city air during the summer months.

The new study led by the University of California, Berkley found that the over 10 million trees throughout LA are releasing harmful compounds that react to form 'smog,' which can cause allergic reactions or organ damage in high concentrations.

Worse, many of these emissions are released when vegetation endures hot temperatures, which triggers them to release the compounds as a way to cope — meaning the issue could magnify as climate change smashes heat wave records.  

Researchers are now urging California officials to rethink their climate change efforts, including which species of trees and shrubs to plant to green up the city.

Today, 60 percent of the air pollution in Los Angeles comes from a surprising source: plants

Even just trace exposures to smog has been linked to increased risks of heart attack, stroke, atrial fibrillation and pneumonia in recent studies.

The pollutant's lethal power in major US cities led to both the passage of the 1967 Air Quality Act and the 1970 Clean Air Act half a century ago. 

Traditionally, scientists have focused on the synthetic sources of smog, volatile organic compounds (VOCs) and the chemical compounds that react in air to form them — including ingredients in solvent-based paints, inks, and other products.

But the new study, published this June in the journal Science, broadened its focus to explore how naturally produced VOCs, called terpenoids, are stored and expelled by so-called 'biogenic' sources like trees, plants, and soil deposits.

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Researchers surveyed the city via sensors mounted atop an aircraft to collect their VOC data: both the variety of different of VOCs and the amount of them in the air.

The team discovered multiple unexpected biogenic sources including the region's citrus fruit-juicing and dairy-producing hubs as well as ethanol biofuel plants.

In the presence of sunlight, VOCs react with nitrogen oxides (NOx) to cause smog or 'ground-level ozone,' which is harmful to human health and other living things.

Both gasoline and diesel fuel release nitrogen during combustion, which latches onto oxygen in the air to create NOx: a now heavily regulated source of air pollution from automobiles, and the source of LA's infamous smog problem decades ago.

Study co-author Dr Ronald Cohen, a chemistry professor and director of UC Berkeley's Atmospheric Sciences Center, told DailyMail.com that those successful car regulations are why the city's VOC pollution is now only 40 percent man-made.

'On a hot day 15 years ago,' Dr Cohen wrote via email, 'transportation emissions were higher than biogenics [i.e. plants].' 

'Biogenics today are about the same on a hot day,' the Berkley chemist explained, 'but transportation emissions are now much lower.'

A two-engine UV-18A Twin Otter research aircraft (above) equipped with micro-meteorological sensors ('inlet' above) was used by the scientists to collect data on the shifting amount and chemical make-up of the air pollution in Los Angeles across weather conditions 

Above, flight path's of the researchers' twin engine Otter as it collected and measured the concentration of four air pollution chemical in the atmosphere above LA

'In the context of ozone,' he added, 'it's probably more important that transportation emissions have decreased, revealing the existing emissions from the biosphere.'

But despite this welcome news, the researchers' new study also found troubling spikes in the sheer volume of these naturally made 'biogenic terpenoid' VOCs in the air, specifically an increase in plant-based VOCs tied to high temperature days.

Both ozone and other chemical byproducts of VOCs in the atmosphere, known as organic aerosols, were higher in Los Angeles during the summer, the authors found.

Dr Cohen told DailyMail.com that this had as much to do with the increasing total number of heat waves per year as any particular record-breaking hot days. 

'It's probably better to think of the number of hot days increasing separately from the maximum temperature,' he advised. 'Both have occurred.'

Study co-author Dr Ronald Cohen, a chemistry professor and director of UC Berkeley's Atmospheric Sciences Center, told DailyMail.com that those successful car regulations are why the city's volatile organic compounds (VOCs) pollution is now only 40 percent man-made

The natural plant biology of blooming season and a few specific non-native plants introduced to LA have also contributed to these unwelcome summer spikes in VOCs.

Dr Cohen and his colleagues found that concentration rates for two plant-made VOCs — monoterpene and sesquiterpene — were higher in downtown Los Angeles than in the comparatively more lush San Bernardino Valley.  

While the researchers hold open the possibility that urban 'fragrance-related sources' may have added to those downtown numbers, they attributed part of the problem to VOC-emitting non-native trees like eucalyptus.

Sometimes called 'California's Most Hated Tree,' whole groves of this primarily Australian genus of tree were planted in the state beginning in the 1850s, primary for firewood and as windbreakers along the region's dry arid landscape.

'If you go back to California farm journals of the 1870s, '80s, '90s,' historian Jared Farmer told public radio station KQED, 'there's just report after report of disappointment, like "these trees are no good."'

'Flowering' seasons for eucalyptus and some other key species, the scientists found, are expected to increase VOC emissions leading to 'high ozone and particle pollution events.'

Within LA's San Joaquin Valley, the researchers also found spikes in VOC emissions linked to the state's famous orange groves, and its citrus processing facilities like juice factories and citrus packaging warehouses.

Wildfire smoke - like this massive plume from LA's Sepulveda Boulevard on October 28, 2019 - also contribute to the plant-based VOC mix in the regions air pollution

High ethanol emissions, another VOC, were found predictably from a 'green' ethanol biofuel plant in the valley as well. 

Both of these sources were identified with the aid of Google Maps data.

The team's so-called airborne flux measurements were pulled via multiple flights across weather conditions of a two-engine UV-18A Twin Otter research aircraft equipped with micro-meteorological sensors. 

The Twin Otter, which is flown out of LA's Burbank airport by the Naval Postgraduate School, is of a kind typically used by commercial skydiving companies. 

Dr Cohen told DailyMail.com that any future regulatory action should both aggressively curb the NOx auto exhaust that reacts with these plant-based VOCs but a plan should also be in place to avoid needlessly adding species that release VOCs.

'Policy advice would be (a) focus on reducing NOx emissions because the trees, shrubs and etc. are already planted,' Dr Cohen said, 'and (b) seek the lowest VOC-emitting trees for new planting.'