Aerosol acidity and its impact on sulfate and nitrate of PM2.5 in southern city of Beijing-Tianjin-Hebei region

• Published in: Atmospheric pollution research 2024-02, Vol.15 (2), p.101997, Article 101997
• Main Authors: Wei, Zhe, Tahrin, Norhaslinda Mohamed
• Format: Article
• Language: English
• Subjects: Activity coefficient; Beijing-Tianjin-Hebei; PM2.5; SO42
• ISSN: 1309-1042; 1309-1042
• DOI: 10.1016/j.apr.2023.101997

Aerosol acidity, similar to the pH of a solution, influences the water-phase reactions within aerosols, thereby impacting the formation and composition of particle components. This study investigated the chemical composition of PM2.5, pH of PM2.5 and its impact on formation of SO42− and NO3− in Handan city from March 2015 to February 2016. The results showed that NO3− was the dominant inorganic secondary ion in PM2.5, accounting for 16.4% of PM2.5, slightly higher than SO42− at 14.2%. The average pH of PM2.5 in Handan was 4.3, indicating moderate acidity, with higher acidity observed in spring and winter. Nitrogen oxidation ratio (NOR) was synergistically influenced by pH and aerosol water content (AWC), while sulfur oxidation ratio (SOR) was associated with high aerosol acidity or AWC. The activity coefficient of NO3− exhibited a negative correlation with activity coefficient of [H+], with an R2 value of −0.20 (“-” means negative correlation). Conversely, a positive correlation was observed between activity coefficient of SO42− and [H+], with an R2 value of 0.19. The partition coefficient (ε(NO3−)) of nitrate, representing its assignment to the solid phase, is given by the function y = −0.5x2+4.2x-7.6 (R2 = 0.60), where x corresponds to pH and y corresponds to ε(NO3−). It suggested that NO3− is most likely to enter the solid phase in significant quantities under moderately acidity condition (at around pH = 4.2). A significant correlation observed between pH, surface tension, and particle radius, indicating that pH influences the aging process of aerosols. However, the impact of pH varies depending on the pollution episode. •The aerosols in Handan have a moderate acidity level, with a pH value of 4.3.•The partition coefficient of ε(NO3−) will decrease as pH increases•The pH exhibits a significant correlation with both surface tension and particle radius.