A study of acidity on PM2.5 in Hong Kong using online ionic chemical composition measurements

Particle in-situ pH (pHIS), defined as pH of the aqueous phase on aerosols, is an important factor in influencing aerosol-phase chemistry and uptake of gaseous species by particles. In this study, a continuous system, Particle-into-Liquid System (PILS) coupled with two ion chromatographs, was used to obtain PM2.5 ionic chemical composition at a time resolution of 30 min at a suburban site in Hong Kong under three different synoptic conditions. The chemical composition data and meteorological parameters (e.g., temperature, relative humidity (RH)) are input into Aerosol Inorganic Model (AIM-III) for estimation of in-situ pH through calculation of H+ amount and aerosol liquid water content (LWC). The particle pHIS ranged from −1.87 to 3.12, with an average at −0.03, indicating the PM2.5 particles in Hong Kong are highly acidic. Unlike particle strong acidity, which was dominated by sulfate concentration, the amount of aerosol liquid water content could significantly influence in-situ particle acidity. Principal factor analysis has identified the equivalent concentration ratio between cations and anions (i.e., R+/−) and RH to be the two most important factors influencing the particle pHIS. pHIS under different synoptic conditions in this study could be well approximated by a single linear regression equation (slope: 0.95, R2: 0.93), i.e., pHIS = 4.94 R+/− + 3.11 RH − 5.70. Such an empirical equation provides a convenient mean in estimating particle in-situ acidity for assessing the role of acid-catalyzed aerosol reactions.

► Particle in-situ pH could be estimated by a single linear regression equation of two parameters.
► In-situ pH estimation indicates fine particles are highly acidic in Hong Kong.
► The diurnal variation in relative humidity could drive particle acidity to be higher during the day.