As a major precursor of ozone and PM2.5, emissions of nitrogen oxides (NOx=NO+NO2) have been subject to stringent control worldwide in the past decades. However, long-term observations have shown that the decrease of particulate nitrate (pNO3-) is much weaker than the NOx reductions. In extreme cases when NOx emissions have been suddenly reduced by large amounts, such as during the COVID-19 lockdown, pNO3-has even been seen to increase and severe haze remains to occur. This challenges the presumed efficacy of NOx reduction in mitigating particulatepollution.
Nanjing University, Beijing University of Chemical Technology, and the University of Helsinki collaborated in studying the sustained air pollution during COVID-19 lockdowns based on the comprehensive measurements in Beijing. We show that nocturnal nitrogen chemistry, usually negligible under polluted conditions, was amplified unintentionally and drastically, and this drove the formation of both pNO3- and secondary organic aerosol (SOA). First, the enhanced heterogeneous reactions of N2O5 on aerosol surfaces became the dominant source ofpNO3-during the lockdown, in contrast to a negligible contribution in the normal period; Second, ClNO2 and Cl2 formed in significant amounts provided a large pool of chlorine radicals, which rapidly oxidized various volatile organic compounds (VOCs), contributing to about half of the oxygenated organic molecules, which explains the enhanced formation of SOA.
Fig. 1.Relative contributions of PM2.5 componentsin different pollution levels (a), and concentrations of main nitrous species in different PM2.5 levels in the pre-lockdown (b and e) and lockdown periods (c and e).
A key to the amplification of nocturnal nitrogen chemistry is the relative abundance of NO and O3; when O3 is more abundant than NO, so-called “NO titration” is limited;however, when NO is in excess, “NO titration” suppresses nocturnal nitrogen chemistry over a wide area. As demonstrated by data collected in a dozen urban and suburban locations worldwide, nocturnal nitrogen chemistry is ubiquitously in a “suppression state” under polluted conditions. However, given that NOx has been continuously declining, we argue that the “NO titration” is diminishing, and nocturnal nitrogen chemistry is gaining increasing importance.From a pollution control point of view, our findings highlight that if O3 production cannot be controlled by simultaneously modulating VOC and NOx emissions, amplified nighttime nitrogen chemistry can cause sustained secondary aerosol pollution. Therefore, more attention should be given to nocturnal nitrogen chemistry during refinements to future multi-pollutant control strategies.
Fig.2. Observations of nighttime NOx (NO and NO2) and O3 levels in major cities around the globe.
This study, entitled “Increasing contribution of nighttime nitrogen chemistry to wintertime haze formation in Beijing observed during COVID-19 lockdowns”was published on 2023.10.12 in Nature Geoscience(https://www.nature.com/articles/s41561-023-01285-1). The first author, Dr. Chao Yan, was a postdoc researcherat the University of Helsinki and is currently working at Nanjing University as an associate professor. Dr. Yee Jun Tham from Sun Yat-sen University is the co-first author; Prof. Aijun Ding, Prof. Markku Kulmala, and Dr. Yee Jun Tham are co-corresponding authors of the paper. Prof. WeiNie from Nanjing University, Dr. Douglas Worsnopfrom Aerodyne Research (Inc), and Prof. Neil Donahue from Carnegie MellonUniversityhave important contributions to the paper. This study is funded by multiple projects, including the Natural Science Foundation of China under the Major Program of Integrated Projects, Key International Cooperation Projects, General Projects, and Jiangsu Collaborative Innovation Center for Climate Change.
Citation: Yan, C., Tham, Y.J., Nie, W. et al. Increasing contribution of nighttime nitrogen chemistry to wintertime haze formation in Beijing observed during COVID-19 lockdowns. Nat. Geosci. (2023). https://doi.org/10.1038/s41561-023-01285-1