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 [SO₄ ^2-]_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 [SO₄ ^2-]_measured ratio and pH under different regimes of relative humidity are discussed. © 2006 Elsevier Ltd. All rights reserved.