The role of seasonal characteristics in addressing scattering species for improved visibility in Shanghai, China.

Submicron particle extinction significantly contributed to reduced horizontal visibility during severe pollution episodes. However, seasonal variations in the scattering coefficients of chemical components, particularly with their size and mass concentration, are still not fully understood. This study investigated the scattering coefficients of the main chemical components of submicron particles (PM) in Shanghai using advanced instruments. We discovered that the main scattering species exhibited seasonal variations. Notably, organics (OA) and ammonium sulfate ((NH)SO) dominated in spring, summer, and autumn, whereas ammonium nitrate (NHNO) was the primary scattering species in winter. Analyzing the particle scattering coefficient size revealed that the scattering coefficients for OA in Shanghai exhibited bimodal distributions in spring, autumn, and winter, with peak sizes of 500-700 nm. The distribution was unimodal in summer, with a 400-500 nm peak size range. In addition, the peak particle size distributions of NHNO and (NH)SO demonstrated a pattern of winter > spring/autumn > summer, potentially owing to the different species sources and formation pathways throughout the four seasons. Seasonal variations in the mass scattering efficiency (MSE) showed that OA exhibited a higher MSE than NHNO or (NH)SO. Further, NHNO exhibited a higher MSE in winter, whereas (NH)SO exhibited the highest MSE in summer at 5.63. This study highlighted the importance of considering seasonal characteristics controlling scattered species and their precursors to enhance visibility. These findings can assist in formulating visibility control strategies in Shanghai.