Discovery of the dominant roles of anthropogenic organic vapors on secondary organic aerosol formation over China’s megacities

Secondary organic aerosols (SOA) contribute a significant fraction to the particulate haze pollution in many urbanized regions, with profound impacts on climate and human health. The knowledge gaps in the sources and relevant chemical processes of SOA formation are the bottleneck for implementing effective measures to mitigate haze pollution. Low-volatility organic vapors are critical intermediates connecting the oxidation of volatile organic compounds (VOCs) to SOA, but the direct measurement of those highly reactive intermediate vapors poses a great challenge.

In the autaum 2018, the TRS project researchers conducted a comprehensive field study in Hong Kong, and during the same period, coordinated studies were concurrently carried out in three other Chinese megacities by mainland and international researchers in Beijing, Nanjing, and Shanghai. The results from the four cities revealed the significant roles of anthropogenic low-volatility organic vapors on the SOA formation in the urbanized environment, providing new insights for effectively mitigating the urban air pollution issues.

For the first time, the study characterized plenty of highly reactive oxygenated organic molecules (OOMs) in urban environments, and developed a novel classification framework to trace the measured OOMs and formed SOA to different precursors. The study showed that oxidation of anthropogenic VOCs dominates OOMs formation in the urban atmosphere, with approximately 40% contribution from aromatics and another 40% contribution from aliphatic hydrocarbons, a previously under-accounted class of VOCs. The irreversible condensation of these anthropogenic OOMs increases significantly in highly polluted conditions, accounting for a major fraction of SOA production. The study also unveiled that multi-step oxidation and auto-oxidation processes play key roles in OOMs formation, and nitrogen oxides (NOx) significantly affect the VOCs oxidation process, producing a considerable fraction of nitrogen-containing organic compounds. The results showed that the OOMs distribution and formation pathways present substantial homogeneity across China's three most urbanized regions, where more than 800 million people live and suffer from air pollution. The findings imply a possibility of solving air pollution issues with a uniform and effective mitigation strategy across these highly populated city clusters.

The study findings were published recently in Nature Geoscience. Dr. Zhe Wang, Co-PI of the TRS project, contributes as one of the co-first authors of the work. Other contributing authors include PC Prof. Tao Wang, Co-PIs Prof. Hai Guo and Prof. Shuncheng Lee, other Ph.D. students and researchers from the project.

Reference: Nie, W., Yan, C., Huang, D. D., Wang, Z., et al.: Secondary organic aerosol formed by condensing anthropogenic vapours over China’s megacities, Nature Geoscience, 15, 255–261 (2022). https://doi.org/10.1038/s41561-022-00922-5