Inter-particle and gas-particle interactions in sampling artifacts of PM in filter-based samplers

Abstract

Despite the recent rapid development of real-time measurements of chemical compositions in atmospheric aerosols, filter-based aerosol sampling for an extended period of time with post-sampling chemical analysis is still very common. The collected particles interact with both the in-coming gas and each other. These interactions lead to sampling artifacts. This paper investigates the role of aerosol ionic composition, inter-particle and gas-particle interactions in the sampling artifacts of PM 2.5 at high relative humidity. This is achieved by comparing PM 2.5 samples collected with and without a denuder/filter pack system at urban and suburban sites in Hong Kong. The samples collected with a denuder/filter pack were used to estimate the artifact-free ambient gas and particulate concentrations. Because of the prevailing high relative humidity (>70%RH) in HK, the collected particles were expected to have deliquesced and form an internal mixture, according to thermodynamic predictions. It was found that an ambient molar particulate [NH 4 + ]/[SO 4 2− ] ratio of 1.5 is a critical condition to the sampling artifact characteristics of PM 2.5 . The urban samples were all ammonium rich (AR; [NH 4 + ]/[SO 4 2− ] greater than 1.5) and were characterized by a high nitrate concentration and low strong-acidity. They had a significant strong acidity loss of 73%, roughly half of which was due to the neutralization of acidity by ammonia. The suburban samples were all ammonium poor (AP; [NH 4 + ]/[SO 4 2− ] less than or equal to 1.5) and were characterized by a low nitrate concentration and high strong-acidity. Sampling artifacts of the acidity, ammonium and sulfate were not important in the AP samples. In both the AR and AP samples, the individual evaporation of HNO 3 and HCl were the primary reactions for the artifacts of nitrate and chloride while the concomitant evaporation of HNO 3 and HCl with NH 3 was insignificant. In general, the extents of the sampling artifacts of nitrate and chloride increased with increasing gas-to-particulate concentration ratio of these two species. A methodology to estimate the artifact-free ambient concentrations of nitrate and chloride from measurements of filter pack samplers (without any denuders) for deliquesced particles is proposed.