Exploring the driving factors of haze events in Beijing during Chinese New Year holidays in 2020 and 2021 under the influence of COVID-19 pandemic.

Unexpected outbreak of the 2019 novel coronavirus (COVID-19) has profoundly altered the way of human life and production activity, which posed visible impacts on PM and its chemical species. The abruptly emergency reduction in human activities provided an opportunity to explore the synergetic impacts of multi-factors on shaping PM pollution. Here, we conducted two comprehensive observation measurements of PM and its chemical species from 1 January to 16 February in Beijing 2020 and the same lunar date in 2021, to investigate temporal variations and reveal the driving factors of haze before and after Chinese New Year (CNY). Results show that mean PM concentrations during the whole observation were 63.83 and 66.86 μg/m in 2020 and 2021, respectively. Higher secondary inorganic species were observed after CNY, and K, Cl showed three prominent peaks which associated closely with fireworks burnings from suburb Beijing and surroundings, verifying that they could be used as two representative tracers of fireworks. Further, we explored the impacts of meteorological conditions, regional transportation as well as chemical reactions on PM. We found that unfavorable meteorological conditions accounted for 11.0 % and 16.9 % of PM during CNY holidays in 2020 and 2021, respectively. Regional transport from southwest and southeast (south) played an important role on PM during the two observation periods. Higher ratio of NO/SO were observed under high O and low RH conditions, suggesting the major pathway of NO and SO formation could be photochemical process and aqueous-phase reaction. Additionally, nocturnal chemistry facilitated the formation of secondary components of both inorganic and organic. This study promotes understandings of PM pollution in winter under the influence of COVID-19 pandemic and provides a well reference for haze and PM control in future.