Smog chamber experiments were conducted to investigate the hygroscopicity of particles generated from photooxidation of α-pinene/NOx with different sulfate seed aerosols or oxidation conditions. Hygroscopicity of particles was measured by a tandem differential mobility analyzer （TDMA） in terms of hygroscopic growth factor （Gf）, with a relative humidity of 85%. With sulfate seed aerosols present, Gf of the aerosols decreased very fast before notable secondary organic aerosols （SOA） formation was observed, indicating a heterogeneous process between inorganic seeds and organic products might take place as soon as oxidation begins, rather than only happening after gas-aerosol partition of organic products starts. The final SOA-coated sulfate particles had similar or lower Gf than seed-free SOA. The hygroscopicity of the final particles was not dependent on the thickness but on the hygroscopicity properties of the SOA, which were influenced by the initial sulfate seed particles. In the two designed aging processes, Gf of the particles increased more significantly with introduction of OH radical than with ozone. However, the hygroscopicity of SOA was very low even after a long time of aging, implying that either SOA aging in the chamber was very slow or the Gf of SOA did not change significantly in aging. Using an aerosol composition speciation monitor （ACSM） and matrix factorization （PMF） method, two factors for the components of SOA were identified, but the correlation between SOA hygroscopicity and the proportion of the more highly oxidized factor could be either positive or negative depending on the speciation of seed aerosols present.
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,NH+ 4and 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]:[NH+4] ratio was usually higher than 1; however,it was less than 1 and the concentration of NH+4 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 NH+4 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 NH+ 4was 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 NH+4.It was found that the average relative humidity and temperature had great effects on the loss of NH+4.The loss of NH+4 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 NH+4,NO- 3and 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.
