Physical and chemical characteristics of atmospheric aerosol under the different weather conditions in Hefei

To investigate the compositions and sources of atmospheric aerosols during haze episode in Hefei, samples of atmospheric aerosol were collected using the FA-3 Andersen cascade impactor in urban Hefei in the representative months of 2012 and 2013.The main water-soluble inorganic ions (WSII) (NH4+,Mg2+,Ca2+,Na+,K+,NO2-, NO3-,Cl-,SO42-) for each sample were measured. Based on the records of meteorological parameters during the observations, the samples were divided into four types: clear day, haze day, fog day and mist day. Then, the physical and chemical characteristics of aerosol under those four types of weather were analyzed and compared. The results showed that fog and haze in Hefei usually corresponded to light to heavy air pollution, and the clear day corresponded to good air quality. From clear day to fog or haze day, the concentration of fine particles (PM2.1 in this paper) increased evidently, with increasing percentages of WSII in PM2.1.The ratio of PM2.1to TSP was 43% for clear days, 52% for haze days, 65% for fog days and 64% for mist days. The ratio of WSII to PM2.1 was 46% (clear day), 67% (haze day), 61% (fog day), 80% (mist day).As for mass concentration, the top three ions in PM2.1 was SO42-,NO3- and NH4+ for fog (mist) and haze days, while the top three ions were SO42-, Ca2+ and NO3- on clear days. The top three ratios of WSII ions in PM2.1 on haze days, compared with the corresponding values on clear days, were 6.1 times larger for NO3-, 3.6 for NH4+ and 3.0 for SO42-, while the top three ratios of WSII ions in PM2.1 on fog (mist) days with the corresponding values on clear days were over 10.0 times for NO3-, over 5.0 for NH4+ and over 4.0 for Cl-. Under those four types of weather, the size spectra for those ions (NH4+,NO3-,Cl-,SO42-) related to anthropogenic activities were obviously different, showing double peaks, single peak and tri-peaks, while the size spectrum for natural emitted ions (e.g., Ca2+) was almost all double peaks under the four types of weather with slight changes. The balances between cations and anions were very good for aerosols below 3.3 μm and changed worse with the increasing size, and it was better on fog, mist and haze days than on clear days. The ratios and correlations among the major anions and between Cl- and Na+ show large differences between clear days and fog/haze days.