Photochemical ozone formation oriented VOC source apportionment and health economic burdens in Pearl River Delta

Why Ozone in the Air Matters to Everyday Life

Ground-level ozone is not the protective layer high in the sky; it is a harmful gas that irritates lungs, worsens heart and breathing problems, and shortens lives. In China’s Pearl River Delta (PRD)—a densely populated, fast-growing region that includes Guangzhou and Shenzhen—ozone levels have kept rising even as some pollutants have been controlled. This study asks a simple but vital question: which invisible gases in city air are really driving ozone formation, and what is the human and economic cost?

Invisible Ingredients in City Air

The researchers focused on volatile organic compounds (VOCs), a broad family of easily evaporated chemicals emitted by vehicles, industry, fuel use, and even trees. Over summer and winter of 2024–2025, they measured 96 different VOCs in four PRD cities. Overall VOC levels averaged about 43 parts per billion, but varied strongly from place to place: industrial Dongguan had up to twice the concentrations seen in more service-oriented Guangzhou and Shenzhen. The most abundant gases were alkanes, many linked to natural gas use and petrochemical activities, along with halogenated compounds and aromatic solvents from factories and manufacturing.

Sunlight, Reactive Gases, and Ozone Production

Not all VOCs are equally important for ozone. What matters most is how quickly they react in sunlight with highly reactive hydroxyl radicals, setting off chain reactions that turn nitrogen oxides from traffic and industry into ozone. Using a chemical aging method, the team separated what was simply present in the air from what had already been “burned up” by sunlight—its photochemical loss. They found that relatively rare but highly reactive gases, especially alkenes such as isoprene and butadiene and aromatic solvents like toluene and styrene, were consumed rapidly and played an outsized role in ozone formation. Summer conditions of strong sun and high temperature greatly accelerated these reactions, while high humidity and high air pressure tended to slow them or trap pollutants near the ground.

Who Really Fuels Ozone: Cars, Factories, or Trees?

To link specific activities to ozone, the authors built a new “photochemical ozone formation potential” (PL-OFP) source apportionment method. They combined a statistical source model with the measured chemical reactivity of each VOC. At first glance, natural gas and biomass burning dominated VOC concentrations, and vehicle exhaust and certain industries appeared to have the highest potential to make ozone. But once they accounted for how much of each source’s gases were actually consumed in sunlight, the picture flipped. More than 70% of biogenic VOCs—mainly isoprene from vegetation—were destroyed by photochemical reactions, making biological emissions the largest real contributor to ozone in summer, with natural gas and biomass burning becoming more important in winter. Vehicle emissions still had a high “potential” to create ozone, but after reactions, their true contribution shrank markedly.

Health and Money Lost to Polluted Air

The team then translated ozone levels into human impacts using established epidemiological risk models. Across the four cities, they estimated 8,522 premature deaths per year linked to short-term ozone exposure, mostly in older and more densely populated Guangzhou. By assigning an economic value to statistical life based on local income levels, they calculated that ozone pollution costs the PRD region about 4.9 billion US dollars annually, or roughly four thousandths of its total economic output. Of this, around 1.2 billion dollars and more than 2,000 deaths were specifically tied to ozone formed from the photochemical loss of VOCs, with reactive gases from vegetation and natural gas or biomass burning responsible for the majority of that burden.

What This Means for Cleaner Air and Safer Lives

The study shows that simply targeting the biggest or most obvious VOC sources is not enough to tame ozone. Instead, air-quality policies must pay close attention to how reactive each gas is and how strongly it feeds ozone chemistry under real weather conditions. In the PRD, that means giving priority to controlling low-concentration but highly reactive VOCs from vehicle exhaust, specific industrial processes, and combustion, while also considering how human activities interact with natural emissions from vegetation. By focusing on these “spark plug” chemicals rather than just total emissions, cities can design more effective strategies that protect public health and reduce the substantial economic losses associated with ozone pollution.

Citation: Deng, W., Wang, L., Huang, J. et al. Photochemical ozone formation oriented VOC source apportionment and health economic burdens in Pearl River Delta. npj Clean Air 2, 16 (2026). https://doi.org/10.1038/s44407-026-00055-8

Keywords: ground-level ozone, volatile organic compounds, Pearl River Delta, air pollution health impacts, photochemical smog