Concentrations and size distributions of viable bioaerosols under various weather conditions in a typical semi-arid city of Northwest China

In recent years, atmospheric particulate matter (PM) has become one of the top pollutants affecting the air quality and human health in Xi'an, the largest city in semi-arid inland of China. Few studies have been carried out on the microbial fraction of PM (defined as bioaerosols) in this region, especially under specific weather conditions. In this study, airborne microbial samples in Xi'an city were collected from Aug. 2014 to Jul. 2015. The concentrations and size distributions of airborne viable bacteria and fungi were characterized under different weather conditions (e.g. sunny, cloudy, rainy and hazy days). The results showed that the concentrations of airborne viable microbes in Xi'an were lower than those in most cities worldwide due to the semi-arid climate feature. The concentrations of airborne viable microbes varied by weather conditions, with the highest value observed on hazy days and the lowest observed on rainy days. In particular, the mean concentrations of viable bacteria and fungi on the hazy days (1311 +/- 371 and 896 +/- 559 CFU/m(3)) exceeded the recommended permissible limit values in China. Moreover, the size distribution of airborne viable bacteria presented a similar unimodal pattern under four weather conditions, while no clear distribution pattern for airborne viable fungi was found in the non-haze weathers. Another important finding was that more than 60% viable bioaerosols were in respirable size range under all weather conditions. The present results can improve our understanding on the influence of viable bioaerosols on human health and air quality in semi-arid regions under various weather conditions.