Characteristics Of Microbial Aerosols On Haze Days In Xi’an, China

In recent years, due to frequent happened in eastern China, haze draws the attention from all fields. Numerous studies have been conducted to investigate the physical and chemical properties, source apportionment and evolution processes of atmospheric PM during the haze events in Chinese cities. However, little is known until now about the biological properties of PM during haze episodes. To study the characteristics of microbial aerosols on haze days in Xi’an, China, air sampling were carried out on the campus of Chang’an University using six-stage cascade impactor and Omin particulate monitor from August 2014 to December 2015. Concentration and size distribution of viable bioaerosols and total microbe were analyzed. The results are as the follows.First, the haze weather of Xi’an mainly appeared in Autumn and Winter. During the haze days, the daily average concentration of viable bacteria and fungi is1297 CFU·m-3 and 1046 CFU·m-3, respectively, which were much higher than those detected during the non-haze days and exceeded the standard recommended by Chinese Academy of Sciences. The peak of the viable bacteria were in the size range of 3.3~4.7μm, whereas the peak of viable fungi were in the size range of 1.1~2.1μm,with the proportion of respirable fraction of 67.8% and 67.4%. And the number median diameter(NMD) were 2.01μm and 1.86μm, respectively, during haze days. Moreover, the concentration of bioaerosols has positive correlation with relative humidity, wind speed, AQI(PM2.5) and negative correlation with temperature.Second, during the haze days, the daily average concentration of total microbe range from 2.73×105±8.67×104 CFU·m-3 to 4.93×105±3.67×104 CFU·m-3. However, range from 2.01×105±3.20×104 CFU·m-3 to 5.50×105±7.24×104 CFU·m-3, during the non-haze days. And the concentration of total microbe was at a relatively high level in July and November. Moreover, during the haze days, the proportion of viable bioaerosols in the total microbe has increased, bacteria from 0.16% to 0.21% and fungi from 0.07% to 0.23%.Third, in order to characterize the transport pathway of the air parcels that passed through Xi’an, 72 h backward trajectory analysis was carried out by HYSPLIT-4 model. Simulation results show that air mass arriving at Xi’an came from two main pathways: the north and northeast direction, during the total microbe was in the high level. When the level of the total microbe was low, the air mass of Xi’an mainly came from the south and southwest direction.