New results from Professor Aijun Ding and Wei Nie's team reveal the mechanism through which NO can enhance the formation of highly oxygenated biogenic molecules in the atmosphere


Highly oxygenated organic molecules (HOM) have recently been observed in various environments in the atmosphere. Owing to their high oxidation state and low volatility, HOM are a major source of secondary organic aerosol (SOA) and contribute significantly to new particle formation (NPF). Yet, given their extremely low concentrations, they have long been difficult to measure and identify effectively. Professor Aijun Ding and Professor Wei Nie's team have been collaborating with Academician Markku Kulmala's team from the University of Helsinki since 2014 to developmethods and gain insights for HOM, focusing particularly ontheinfluence of human activities onHOM formation and the impact of HOM on the air quality of urban atmosphere.

Fig. 1: The mechanism through which NO can enhance the formation of highly oxygenated organic molecules (HOM) from monoterpene oxidation at 278 K.

The team has conducted high-quality observations of HOM in several regions, including the Nordic forest region, the eastern mega-city cluster in China, and the Qinghai-Tibet Plateau (Yan et al., 2016; Liu et al., 2021; Xu et al., 2021; Nie et al., 2022; Liu et al., 2023), and confirmed that the interaction between nitrogen oxides (NOx) and organic peroxy radicals (RO2) greatly affected HOM formation; yet, the underlying mechanism remainslargely unclear. In this work, the team has performed dedicated experiments at the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber with molecular-level observations and simulations focusing on the HOM formation at low NO concentrations. A vital feature of these experiments is the precise control and monitoring of NO and monoterpenes at low concentrations. It shows that NO profoundly influences HOM formation even at extremely low concentrations (below 80 ppt). On the one hand, NOx leads to an enhancedformationof highly oxygenated organic nitrates (CHON-HOM) and inhibits the generation of highly oxygenated organic dimers (HOM-dimer) (Yan et al., 2020); on the other hand, low concentrations of NO can regulate the source-sink balance of highly oxygenatedperoxy radicals (HOM-RO2) and enhance the production of alkoxy radicals (RO)(Nie et al., 2023). The results revise the conventional perception that NO monotonicallyinhibits HOM formation and clarify the non-linear influence of NO on HOM formation. These insights suggest that HOM yields from typical boreal forest emissions can vary between 2%-6.5%, and HOM formation will not be completely inhibited even at high NO concentrations. The NO-induced RO autoxidation process ensures a significant amount of HOM formation at high NOx concentrations.

Fig. 2: Contour plot of the yields of highly oxygenated organic molecules (HOM) from monoterpene oxidation vs monoterpene and NOx concentration.

On June 8, 2023, this paper entitled "NO at low concentration can enhance the formation of highly oxygenated biogenic molecules in the atmosphere" was published in Nature Communications. Prof. Wei Nie and Associate Prof. Chao Yan are co-first authors, and Prof. Aijun Ding and Prof. Wei Nie are co-corresponding authors. The University of Helsinki, CERN, Carnegie Mellon University and Lund University, Sweden are collaborators in this work. This work was supported by the National Natural Science Foundation of China (NSFC) project (92044301, 42220104006, 42075101 and 41975154), the Jiangsu Provincial Collaborative Innovation Center of Climate Change, the Academy of Finland (grant nos. 317380, 320094 and 323255), the Fundamental Research Funds for the Centra Universities, the Swedish Research Council VR (project nr. 2019-05006), the Swedish Research Council FORMAS (project nr. 2018-01745), the strategic research area MERGE hosted by Lund University, US National Science Foundation (nos. AGS-1801574, AGS-1801897, and AGS-2132089), the H2020 European Research Council (CHAPAs (grant no. 850614).

Article links:

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