Influence of Secondary Inorganic Aerosol on the Concentrations of PM_(2.5) and PM_(0.1) during Air Pollution Episodes in Hanoi, Vietnam

The high concentration of PM_(2.5) in Hanoi has been an issue of great concern. There were several periods during the winter when PM_(2.5) concentrations were higher than the Vietnamese ambient air quality standards (NAAQS) and WHO guidelines for daily PM_(2.5). In this study, the periods when daily PM_(2.5) concentrations exceeded the NAAQS of 50 µg m^(-3) for a minimum of two consecutive days were determined as episode periods. The study focuses on the impact of secondary inorganic aerosol (SIA) on PM_(2.5) episodes in the dry winter period in Hanoi. To calculate SIA, water-soluble ions of daily PM_(2.5) and PM_(0.1) samples which were collected on the rooftop of a three-storeyed building at an urban site in Hanoi, Vietnam (HUST site) from October 14 to December 31, 2020, were analyzed. Levels and SIA of PM_(2.5) and PM_(0.1) at a transportation site (CEM site) in an episode period from December 20 to December 28, 2020 were also measured. The contribution of SIA on PM_(2.5) and PM_(0.1) during those episodes, the effects of meteorological conditions, and long-range transport were investigated. The results showed that SIA contributed on average 29.0% and 14.1%, respectively, to PM_(2.5) and PM_(0.1) concentrations during air pollution episodes at HUST. Those were higher than the average contribution percentages of total SIA to PM concentrations in non-episode periods for PM_(2.5) (25.6%) and PM_(0.1) (10.6%) at HUST. Among meteorological factors, wind speed largely affected PM_(2.5) concentration and SIA of PM_(2.5). Relative humidity, pressure, temperature, and radiation had a good correlation with SIA of PM_(0.1) and a moderate correlation with PM_(0.1). Concentration-weighted trajectory analysis demonstrated that PM_(2.5) and SIA levels were also influenced by long-range transportation from the upper areas. This study highlighted the importance and served as pioneered research on SIA contribution to PM_(2.5) episodes in the country.