Characteristics and plausible formation mechanisms of secondary inorganic and organic aerosols in four seasons and during haze episodes in Beijing

Abstract

Secondary inorganic aerosols (SIA) and secondary organic aerosols (SOA) are major components of atmospheric particulate matter, however, their exact formation pathways are variable during haze evolution and need to be further clarified. To fully elucidate the characteristics of sulfate, nitrate and secondary organic carbon (SOC) and compare their roles in haze formation, PM_2.5 samples were collected in downtown Beijing in four seasons, and PM₁, PM_2.5, and PM₁₀ samples were collected over the severe haze episodes in winter and spring of 2017. The sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR), and SOC/OC ratio were used to indicate the formation intensity of the respective secondary species. Based on the correlation analysis between secondary transformation ratios and their influencing factors, we found that relative humidity (RH) played a dominating role in NO₃⁻ formation and partitioning in the aerosol phase throughout the year, while SOR and SOC/OC were highly dependent on both O₃ and RH. Different from the monotonic increase of SOR and NOR with RH, the SOC/OC ratio first increased and then decreased with increasing RH. The severe haze episode in winter was characterized by simultaneously enhanced secondary production of SO₄²⁻, NO₃⁻ and SOC. Whereas, the haze event in spring was featured by dramatically enhanced NO₃⁻ concentration, signifying the uniquely high contribution of nitrate to haze formation in spring.