Influence of urban outflow on water-soluble ions in PM2.5 and > PM2.5 particles at a suburban Ho Chi Minh City site, Vietnam

Abstract

In this study, a comprehensive dataset of daily water-soluble ions (WSIs) in PM2.5 and > PM2.5 particles was investigated at the suburban Ho Chi Minh City (HCMC) site, Vietnam. The study aimed to explore an impact of urban emission from the HCMC downtown on the air quality at the downwind region. The urban outflow was characterized by higher concentrations of WSIs in PM2.5 and their gaseous precursors even during the rainy months, and lower sea-salt fractions when compared to those observed under marine air masses. In PM2.5, SO42− was the most dominant species accounting for approximately 50% of the total ion content, followed by NO3−, K+, and NH4+. SO42− is mainly formed through gas-phase reactions especially under urban outflow when stronger oxidation capacity and higher levels of its precursors were observed. Based on Hysplit backward trajectories and concentration weighted trajectory analysis, it is found that the contribution of biomass burning to PM2.5 potassium was elevated consistent with the effect of air masses originated from the equatorial Southeast Asia countries. Hierarchical cluster analysis showed very strong correlation among three components K+–Cl−–NO3−, indicating that secondary NO3− formation through heterogeneous process was heavily dependent on the presence of biomass burning aerosols. The findings imply that the sampling site is impacted by urban emission and can be adopted as the mix receptor site for air quality management in the HCMC metropolitan area, an emerging megacity in developing country.