Insight from VOC flux measurements on managing air quality in cities

Managing ozone remains one of the most pressing and significant environmental challenges across megacities. Numerous studies suggest that biogenic emissions of volatile organic compounds (VOCs) play a key role in ozone formation in urban areas, yet direct evidence remains limited due to the complexity of the sources, sinks, and chemistry of biogenic VOCs. In the summer of 2021, we conducted VOC flux measurements in Beijing, a megacity in China, using the eddy covariance technique. We analyzed VOC flux data using positive matrix factorization allowing to identify prominent urban VOCs emission sources. Our findings highlight the decreasing importance of vehicle-related emissions for VOCs, while the demand for eliminating emissions from volatile chemical products has been increasing. Meanwhile, we discovered that more than half of the OH reactivity of the VOC flux originates from urban vegetation, underscoring the often-overlooked role of urban forests in air pollution. Surprisingly, the strong influence of biogenic emissions in Beijing largely governs the temperature dependence of ozone concentrations, aligning with the OH reactivity from VOC flux. This leads to ozone pollution episodes predominantly occurring on high-temperature days, when biogenic emissions, particularly isoprene, are substantially enhanced. Our study calls for a greater attention from air quality managers to the regulation of emissions from urban vegetation.