This study presents the variation of chemical components in non-refractory submicron aerosols (NR-PM₁) measured by Q-ACSM in a coastal city of Southeast China in summer and winter during the years 2017, 2018, and 2020. As we know, long-term data on aerosol chemical composition were scarce in this region. The mass concentration of NR-PM1 decreased evidently, but the chemical composition of NR-PM1 showed a minor variation with OA increased and NO₃ decreased over the years. PMF/ME-2 models were performed to identify OA factors resolving POA, less oxidized OOA (LO-OOA), and more oxidized OOA (MO-OOA) in this study. Secondary components made dominant contributions of 85.1%–90.7% to NR-PM1, and OOA became more important in aerosols with time. The SOR, NOR, and MO-OOA/LO-OOA ratio as a function of LWC and Ox, combined with the diurnal variation of secondary aerosols (SAs), were performed to explore the main formation mechanism of NO₃, SO₄, and OOA. The results indicate that SO₄ and OOA factors in summer were mainly affected by photochemical reactions, while the characteristics and the main formation pathways of NO₃ and OOA factors in winter varied largely across the different years. The oxidized degree of OA had elevated in 2020 compared to 2017 and 2018 as the f44 (ratio of m/z 44 to total signal) of OA increased. Our study on the annual variation of aerosol chemical composition and the dominant formation pathways of secondary components will provide a scientific reference for air quality improvement.

Fig. 1. Monthly average of chemical composition (a) and its proportion (b) in NR-PM₁.