Long-term characterization of aerosol chemistry in cold season from 2013 to 2020 in Beijing, China

Abstract:

Severe haze episodes in cold season in Beijing have been mitigated greatly during the last decade. However, the changes in aerosol chemistry as responses to the large reductions in gaseous precursors during the two phases of the clean air action, i.e., phase Ⅰ (2013 – 2017) and phase Ⅱ (2018 – 2020) are less understood. Here we characterized such changes in cold season (January – March) by using five-year real-time aerosol particle composition measurements. Our results showed consistently large reductions for all chemical species from 2013 to 2020 with the largest decreases being chloride (95%) and organics (74%) followed by sulfate (69%), while the decreases in nitrate were comparatively small (44%). As a result, the nitrate contributions increased from 18% in 2013 to 30% in 2020, while those of sulfate were fairly stable by varying from 12% to 17% despite the decreased mass concentrations. Organic aerosol (OA) composition also changed significantly since 2018 which was characterized by large increases in the contributions of secondary OA and corresponding decreases in primary OA from fossil fuel combustion and cooking emissions. The changes in aerosol chemistry were closely related to the different reductions in gaseous precursors, e.g., SO₂ vs. NO₂, and the increases in photochemical processing, e.g., the increases in O₃, sulfur and nitrogen oxidation efficiency. Moreover, we found that the changes in aerosol chemistry in cold season during the phase Ⅱ of clean air action started to slow down. The changes in mass concentrations of PM_2.5 and secondary inorganic species were overall small during 2018 – 2020. Our results point towards a future challenge in mitigating air pollution in cold season which needs more stringent and scientific strategies to control secondary aerosol pollution in an environment with enhanced oxidation capacity and consistently high precursors.