To fully understand the physical and chemical influencing mechanisms of the frequent low visibility (VIS) events at urban Chongqing in recent years, this study explores the crucial causes for low VIS in Chongqing mainly from the perspectives of chemical light extinction combined with aerosol hygroscopic growth by using the simultaneous meteorological data and PM_2.5 chemical compositions from 2015 to 2021. Results show that the VIS<10 km cases were mainly observed in autumn and winter, with over half number of the days. Nitrate gradually dominated over sulfate and organics to be the predominant secondary inorganic component of PM_2.5 in both seasons, and nitrate possessed the largest contribution to the total light extinction. Along with the visibility deterioration and the increase of relative humidity (RH), the light extinction contribution due to nitrate and its corresponding hygroscopic growth enhanced significantly. Accompanied with the marked decrease in PM_2.5 concentrations since 2018, nitrate on average occupied a higher mass fraction and light extinction contribution at the same visibility level in comparison to previous years, specifically under high RH conditions. The above highlights that the continuous control of nitrate and relevant precursors will be beneficial for the effective improvement of low visibility in autumn and winter at urban Chongqing, given the precondition of generally low anthropogenic emission levels in the future.

为厘清近年来重庆主城区低能见度(VIS)事件仍然频发的理化影响机制, 利用2015—2021年的气象资料和同期的PM_2.5化学组分数据, 从PM_2.5化学消光和吸湿增长共同作用的角度探讨低VIS的原因。结果表明, 重庆主城区VIS<10 km的现象主要发生在秋冬季, 频率普遍高于50%。硝酸盐逐渐取代硫酸盐甚至有机物, 成为秋冬季PM_2.5中最主要的二次无机组分, 对消光的贡献最大。随着VIS恶化和相对湿度(RH)增大, 硝酸盐及其吸湿增长导致的消光贡献显著增强; 伴随2018年以来PM_2.5浓度的大幅度下降, 同等VIS水平下的硝酸盐具有更高的质量占比和消光贡献, 且在高RH下的吸湿贡献更为明显。在未来低人为排放的背景下, 控制硝酸盐及其前体物排放将有助于重庆主城区大气低VIS的有效改善。