Recently, Jing He, a young researcher within Professor JieJiang’s research team at the School of Marine Science and Technology on Weihai campus, discovered that carbon dioxide (CO2) can react rapidly with atmospheric organic acids in atmospheric microdroplets. The result contributed to the key components of fine particulate matter (PM2.5) formation, offering new insights into the mechanism of atmospheric pollution formation. The findings, titled " Spontaneous Reaction between CO2 and Organic Acids in Water Microdroplets: Implications for the Formation of Secondary Organic Aerosols," were published in the “Journal of the American Chemical Society”.
Identifying the precise sources of Secondary Organic Aerosols (SOA) is critical for the effective control of PM2.5 levels. Current predictive models are built on established SOA formation pathways, but the model-predicted values are 1-2 orders of magnitude lower than the field observation measurements. This discrepancy indicated that the current understanding of the formation pathway for SOAs remains incomplete.
To address the above discrepancy, the research team focuses on the CO2, a ubiquitous atmospheric gas and long considered chemically inert in the context of aerosol formation. It is found that CO2could react rapidly with atmospheric organic acids within atmospheric water microdroplets, generating low-volatility compounds that serve as key building blocks for SOAs. To uncover this hidden mechanism, the team utilized a custom-built online mass spectrometry platform, coupling with radical quenching, isotope labeling, and high-resolution mass spectrometry, they successfully identified highly reactive carbocation intermediatesthat are typically too short-lived to be captured by traditional analytical methods. Furthermore, the team elucidated the complete reaction pathway involving the interaction of carbocations with the counterion HCO3-, derived from CO2.
The team further explored the environmental universality of the observed reaction. The CO2 rapidly reacted not only with dicarboxylic acids but also with prevalent benzene- and halogen-containing organic acids. The low-volatility compounds contribute to SOA formation and facilitate the growth of atmospheric particulate matter in size, exacerbating atmospheric PM2.5 pollution.The above results redefine CO2 from an "inert bystander" to an "active participant" in the atmosphere, establishing a direct link between CO2 and SOA formation for the first time. This work provides theoretical support for the precise identification of source apportionment and the prevention of urban atmospheric pollution.
CO₂ contributes to SOA formation through reaction with organic acids on atmospheric droplet surfaces: (a) Schematic diagram of the reaction mechanism; (b) Isotopic labeling of CO₂; (c) Identification of the highly reactive carbocation intermediate.
This research was conducted in collaboration with Dalian University of Technology and the University of Pennsylvania. Professor Jie Jiang served as the co-corresponding author, with Jing He as the co-first author. This research was supported by the National Natural Science Foundation of China and other finding resources.
Link to the Paper:https://pubs.acs.org/doi/10.1021/jacs.6c00050
