Extinction coefficient of dry and deliquescent chemical components of PM_(2.) 5 in urban areas and their impacts on visibility deterioration

The chemical composition of PM2.5 and RH (relative humidity) significantly affected the atmospheric visibility. RH can lead to an increase in aerosol size and mass, enhancing the extinction coefficients (Bext), resulting in a rapid deterioration of visibility. This study conducted simultaneous observations of PM2.5 chemical compositions, meteorological factors, and Bext at commercial and industrial urban areas. A comparison was made between two locations in different seasons regarding the major water-soluble ions of PM2.5 and visibility. The study aimed to understand the reasons for differences in PM2.5 concentrations corresponding to a visibility threshold of 10 km (PTV10) for the two locations. Furthermore, it delved into the contribution of aerosol hygroscopicity to Bext under different pollution events. The results showed that winter had the poorest visibility in both locations. During this period, SO42− and NO3− constituted critical proportions of water-soluble ions in PM2.5 mass compositions for both commercial and industrial areas. The PM2.5 concentrations corresponding to PTV10 were 29.5 and 18.3 μg m−3 for the commercial and industrial areas, respectively. In periods of poor visibility, NH4NO3 was a significant component influencing Bext, accounting for up to 48.9% of the total Bext in industrial areas. Further analysis revealed that during pollution-retention periods, when RH > 75% compared to RH < 65%, the contribution of hygroscopicity to the Bext of (NH4)2SO4(hygro) increased by 1.7 times, and for NH4NO3(hygro), it increased by 5.1 times. Under conditions of high pollution and RH, NH4NO3(hygro) became the dominant factor in atmospheric Bext, leading to reduced visibility.