A transition to nitrate-dominant in aerosol chemical composition have been observed in recent years. However, detailed knowledge on the relationships between the changed aerosol components and light extinction is less known. Here a second PMF (PMF²) method was introduced to explore light extinction apportionment. The result showed that nitrate ammonium (AN) was the dominant factors to light extinction in all seasons, and its contribution increased with aerosol mass loadings, highlighting the critical role of nitrate in both mass concentration and light extinction. Secondary organic aerosols (SOA) were also important factors, but showed higher contribution under relatively clean periods. We also explore the changes of light extinction after COVID-19 breakout. Extinction coefficient (b_ext) increased by 18-72%, which was mainly caused by AN and SOA. Considering enhanced contribution and high hygroscopicity of nitrate, our result highlight the significance of a strict emission control of nitrogen oxides (NOx) and ammonia (NH₃).

light extinction, nitrate ammonium, Beijing