Size dependence of in situ pH in submicron atmospheric particles in Hong Kong

Some studies have reported that small submicron atmospheric particles are more acidic than large submicron particles; other studies demonstrated a reversed trend. In this study, the size dependence of in situ pH in submicron particles in Hong Kong was investigated. The equivalent ratios of [NH4+]measured to [SO42−]measured in submicron particles were found to be generally less than unity and size dependent, suggesting the possibility of incomplete gas–aerosol equilibrium. The Aerosol Inorganic Model-II (AIM-II) model using measured ionic compositions with the gas–aerosol partitioning disabled was used to estimate the in situ pH in different sized particles. The estimated in situ pH of different sized submicron atmospheric particles was between −2.5 and 1.5 and it generally decreases with increasing submicron particle size. At such low in situ pH, the estimated HSO4− equivalent concentrations were 3.2 times (on average) of those of H+ in different sized particles. The trends of the size dependence of the [NH4+]measured to [SO42−]measured ratio and pH under different regimes of relative humidity are discussed.