Molecular Characteristics of Ambient Organic Aerosols in Shanghai Winters Before and after the COVID-19 Outbreak

During the global pandemic of COVID-19, the world adopted different strategies to avoid the human and economic loss, and so does China. It was reported he reduction of human activities during this time period caused reduction in PM emissions. This study adopted a HPLC-Q-TOF-MS to compare the chemical compositions of ambient PM samples collected in Shanghai winter before (2018, 2019) and after (2021) the Covid-19 outbreak. The identified compositions were classified into subgroups of CHO, CHN, CHON, CHONS, CHOS and CHN compounds. Results showed that CHO compounds and CHON compounds were dominating the organic compounds. The average percentages of CHO- compounds were 57.97% in 2018, 58.98% in 2019, and 43.93% in 2021, respectively. The average percentages of CHON+ compounds were 52.74% in 2018, 50.34% in 2019, and 52.02% in 2021, respectively. The proportion of aliphatic compounds increased gradually during the three years, especially in 2021, indicating that CHO compounds were less affected by aromatic precursors after the Covid-19 outbreak. The contribution of anthropogenic emissions in Shanghai is weakened compared with the previous years. In addition, there was an enhanced emission source containing hydroxyl for CHOS compound formation in 2021. The variations of atmospheric oxidation degree among the three years were not significant.