Gas-to-particle conversion of atmospheric ammonia and sampling artifacts of ammonium in spring of Beijing

Abstract

PM2.5 and gaseous pollutants (SO2, HNO2, HNO3, HCl, and NH3) were simultaneously collected by Partisol® Model 2300 Sequential Speciation Sampler with denuder-filter pack system in the spring of 2013 in Beijing. Water-soluble inorganic ions and gaseous pollutants were measured by Ion Chromatography. Results showed that the concentrations of NH3, NH4 + and PM2.5 had similar diurnal variation trends and their concentrations were higher at night than in daytime. The results of gas-to-particle conversion revealed that [NH3]:[NH4 +] ratio was usually higher than 1; however, it was less than 1 and the concentration of NH4 + increased significantly during the haze episode, indicating that NH3 played an important role in the formation of fine particle. Research on the sampling artifacts suggested that the volatilization loss of NH4 + was prevalent in the traditional single filter-based sampling. The excess loss of HNO3 and HCl resulted from ammonium-poor aerosols and semivolatile inorganic species had severe losses in the clean day, whereas the mass of NH4 + was usually overestimated during the single filter-based sampling due to the positive artifacts. Correlation analysis was used to evaluate the influence of meteorological conditions on the volatilization loss of NH4 +. It was found that the average relative humidity and temperature had great effects on the loss of NH4 +. The loss of NH4 + was significantly under high temperature and low humidity, and tended to increase with the increasing of absorption of gaseous pollutants by denuder. The total mass of volatile loss of NH4 +, NO3 − and Cl− could not be ignored and its maximum value was 12.17 μg m−3. Therefore it is important to compensate sampling artifacts for semivolatile inorganic species.