Beijing's isoprene-emitting trees highlight ozone risks in greener, hotter cities

Recent studies in Beijing have highlighted the surprising role urban trees play in ozone formation, despite their generally positive environmental impact. Trees release biogenic volatile organic compounds (BVOCs) that, when combined with sunlight and nitrogen oxides, can contribute to ground-level ozone formation. While human activities also emit VOCs that react with NOx to create ozone, the researchers found that tree emissions can be particularly potent on hot days.

A study conducted at Jinan University in China monitored VOC emissions from the 102-meter Beijing Meteorological Tower and found that biogenic sources, primarily trees, made up only about one-tenth of all VOC emissions but contributed nearly half of the total reactivity. Isoprene, emitted by certain tree species, accounted for more than 90% of this chemical reactivity.

The researchers compared their findings with data from other cities and discovered that Beijing's isoprene emissions were the highest among observed cities, approaching levels seen in temperate forests. This was largely due to the types of tree species planted, with about 35% of Beijing's trees being isoprene-emitting species like weeping willow and Chinese white poplar. Cities with similar greenery levels but different tree compositions showed significantly lower isoprene emissions.

The study suggests that when selecting trees for urban greening projects, the potential for BVOC emissions should be considered. In Beijing, replacing high-emitting tree species with low-emitting ones in one-tenth of the urban tree population could reduce isoprene emissions by at least 29%. While conventional emission controls focusing on NOx and sunlight remain crucial, incorporating tree species composition could provide an additional strategy to mitigate hot-weather ozone risks.