Concentration and size distribution of culturable airborne microorganisms in outdoor environments in Beijing, China

A one-year (from June 2003 to May 2004) study of airborne microbial concentration and size distribution was conducted systematically at three selected sampling sites in Beijing. Microbial samples were collected in triplicate for 3 min, 3 times per day, for 3 consecutive days of each month using FA-I sampler (imitated Andersen sampler, made by Applied Technical Institute of Liaoyang, China). Results showed that the concentration of total culturable microorganisms ranged from 4.8 X 10(2) colony forming units (CFU)/m3 to 2.4 X 10(4) CFU/m(3), with an arithmetic mean of 3.7 X 10(3) +/- 2.1 X 10(2) (standard deviation) CFU/m(3). As a whole, the percentage of airborne bacteria, accounting for 59.0% of the total culturable microorganisms, was significantly higher than those of airborne fungi (35.2%) and actinomycetes (5.8%). At RCEES (Research Center for Eco-Environmental Sciencesa culture and education area) and XZM (Xizhimena main traffic line), significantly higher percentages of airborne bacteria were found as compared to airborne fungi. No significant difference was observed between airborne bacteria and fungi at BBG (Beijing Botanical Gardena green garden area). The total microbial concentration was significantly higher at RCEES than at XZM and BBG (*P < 0.05), and no significant difference between XZM and BBG was observed (P 0.05). Total microbial concentrations were higher in summer and autumn, and lower in spring and winter at RCEES and XZM (*P < 0.05). At BBG, higher concentration was observed in summer during the sampling periods (***P < 0.001). The size distribution of airborne bacteria showed a skewed distribution at three sampling sites. The proportion of bacterial particles decreased gradually from Stage 1 (> 7.0 mu m) to Stage 6 (0.65 similar to 1.1 mu m), and there was an obvious downtrend on Stage 2 (4.7 similar to 7.0 mu m). Bacterial particles were mainly distributed at the first four Stages (> 2.1 mu m), accounting for 78.6% at RCEES, 84.0% at XZM, and 81.5% at BBG. The size distribution pattern of airborne fungi was found with normal logarithmic distribution at three sampling sites. Fungal particles were mainly distributed at Stage 3 (3.0 similar to 6.0 mu m), Stage 4 (2.0 similar to 3.5 mu m) and Stage 5 (1.0 2.0 m), composing 71.6% of the population at RCEES, 74.2% at XZM, and 68.3% at BBG. However, different distribution patterns were found in different dominant fungal genera. Cladosporium, Penicillium, and Aspergillus were present with normal logarithmic distribution, while Alternaria and sterile mycelia were found with skewed distribution. In opposition to the distribution pattern of fungal particles, the actinomycete particles at all sampling sites were primarily collected on Stage 1, Stage 5, and Stage 6, accounting for 61.1% at RCEES, 60.7% at XZM, and 64.8% at BBG.