Concentrations and size distributions of viable

Journal of Aerosol Science 106 (2017) 83–92

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Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci

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

MARK

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Yanpeng Lia,b, , Rui Lua, Wanxin Lia, Zhengsheng Xiea, Ying Songa a b

School of Environmental Science and Engineering, Chang’an University, Xi’an 710054, PR China Key Laboratory of Subsurface Hydrology and Ecology in Arid Areas, Ministry of Education, Xi’an 710054, PR China

AR TI CLE I NF O

AB S T R A CT

Keywords: Bioaerosols Concentration Size distribution Weather Semi-arid region

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/m3) 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.

1. Introduction With rapid urbanization and industrialization over the past 20 years, the increasing energy consumption and traffic vehicles have caused serious air pollution in Xi’an, the largest city in northwestern China. Atmospheric particulate matter (PM), especially fine fraction, has become one of the main pollutants affecting air quality in Xi’an (Niu et al., 2016). In addition to chemical components of PM, the microbial fraction of PM, generally named as bioaerosols, has substantial effects on atmospheric environment and human health. Bioaerosols are defined as airborne particles or large molecules carrying living organisms or released from living organisms (e.g., bacteria, fungi, viruses, pollen) (Ariyap and Amyot, 2004). It has been reported that bioaerosols may contribute as much as 25% to atmospheric aerosols (Jaenicke, 2005). Exposure to bioaerosols may lead to adverse health effects including infectious diseases, acute toxic effects, allergies and cancers (Douwes et al., 2003; Goldman and Huffnagle, 2009; Walser, Gerstnera, Brennera, Büngerb,

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Corresponding author at: School of Environmental Science and Engineering, Chang’an University, Xi’an 710054, PR China. E-mail address: [email protected] (Y. Li).

http://dx.doi.org/10.1016/j.jaerosci.2017.01.007 Received 19 August 2016; Received in revised form 20 January 2017; Accepted 23 January 2017 Available online 25 January 2017 0021-8502/ © 2017 Elsevier Ltd. All rights reserved.


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Eikmannc, Janssena and Herra, 2015). Therefore, bioaerosols have attracted more and more attention in recent years. Extensive studies have been conducted to quantify characteristics of airborne bacteria and fungi for indoor and outdoor environments in different regions of the world: residence or public buildings (Pastuszka et al., 2000; Kim and Chi, 2007; KarbowskaBerent et al., 2011; Frankel, Bekö, Timm, Gustavsen, Hansen and Madsen, 2012; Nasir and Colbeck, 2010; Li, Wang, Guo, Wang, Fu, Zhao and Wang, 2015a), agricultural or food processing settings (Zorman et al., 2008; Kim et al., 2009; Martin et al., 2010; Rajasekar and Balasubramanian, 2011), waste solid or waste water treatment factories (Huang et al., 2002; Fracchia et al., 2006; Grisoli, Rodolfi, Villani, Grignani, Cottica and Berri, 2009; Li, Qiu, Li, Ma, Niu, Wang and Feng, 2012; Li et al., 2013), urban traffic area or educational area (Wu, Chan, Rao, Lee, Hsu, Chiu and Chao, 2007; Fang et al., 2008; Gao et al., 2015; Li, Fu, Wang, Liu, Meng and Wang 2015b), and rural or coastal region (Li, Qi, Zhang, Huang, Li and Gao, 2011; Hurtado, Rodríguez, López, Castillo, Molina, Zavala and Quintana, 2014; Dong, Qi, Shao, Zhong, Gao, Cao and Chu, 2016). These studies have indicated that the concentrations and size distributions of bioaerosols have great regional and seasonal variations, depending on such biotic and abiotic factors as the type of microorganism species, environmental conditions, and human activities. For example, the concentration of total microorganisms ranged from 4800 CFU/m3 to 24,000 CFU/m3 in Beijing, China (Fang et al., 2008), and the concentration range of total airborne microbes was 8.49×104–2.11×106 Cells/m3 in Qingdao, China (Dong et al., 2016) while the total airborne fungi concentration was in the range of 184–16,979 spores/m3 in Cincinnati, Ohio, USA (Adhikari et al., 2006). Some studies have found that meteorological factors (e.g. relative humidity, temperature, wind direction and wind speed etc.) play major roles in bioaerosol concentrations and their transport (Jones and Harrison, 2004; Mouli et al., 2005; Burrows et al., 2009; Qi, Zhong, Shao, Gao, Wu, Huang and Ye, 2015; Zhong et al., 2016; Gao et al., 2016). High relative humidity can favour microbial growth, resulting in elevated bioaerosol concentrations. High temperature and intensive solar radiation may increase die-off rates and thus lead to the reduced microorganisms. However, most previous studies on bioaerosols in ambient air have been carried out on clean sunny days. More recently, several researchers begin to investigate the biological properties of PM during the haze episodes (Gao et al., 2015; Li et al., 2015b; Dong et al., 2016). It is worth noting that the haze day has been forecasted as a category of weathers (like sunny, cloudy and rainy) by China Meteorological Administration (CMA) in recent years. Different weather is well known to be characterized by different meteorological factors. However, available measurement data under other clean days and haze days are not yet sufficient although those existing studies have examined the correlation between airborne microbes and meteorological factors. In addition, in contrast to numerous studies in other geographic regions, the characteristics of bioaerosols in arid or semi-arid regions have been rarely investigated. Therefore, it is essential to determine the concentration and size distribution of bioaerosols in arid or semi-arid regions under various weather conditions, especially on hazy days, in order to provide a baseline understanding of their influence on human health when exposed to airborne microorganisms in various environments. The present study was carried out in a typical semi-arid city with the aim of addressing these gaps in our knowledge. Therefore, bioaerosol samples in Xi’an, China were collected and analyzed to characterize the concentrations and size distributions of bioaerosols on sunny, cloudy, rainy and hazy days from Aug. 2014 to Jul. 2015. The objectives of this study are to acquire the knowledge of bioaerosol characteristics under various weather conditions, and further to provide basic data for hazard evaluation of bioaerosols on human health and for future determination of Chinese official standard of outdoor air quality. 2. Materials and methods 2.1. Sampling sites Xi’an (34.22 °N, 109.18 °E, 424 m above sea level and 1100 km from the sea), located in the center of the Guanzhong Plain, has a population of over 8.468 million and an area of 39,064 km2. It is surrounded by the Loess Plateau and Qinling Mountain. As a typical semi-arid inland city, Xi’an has four distinct seasons with long summer and winter and short spring and autumn. In general, the climate feature in Xi’an is rainy and humid in summer and cold and dry in winter with an annual average temperature of 13.0–13.4 °C and annual precipitation of 558-750 mm. The prevailing wind direction is North-East i.e., NE 12% and East–North–East i.e., ENE 8%. The field sampling of ambient bioaerosols was carried out on the roof of School of Environmental Science and Engineering building of Chang’an University, which is located in the southern part of Xi’an city, as shown in Fig. 1. The building is approximately 20 m above the ground, which is situated between the 2nd and 3rd ring roads in Xi’an. The distance of the site from nearby major roads is about 400 m. The site is surrounded by the trees, greenbelts, residential and school buildings. There are no specific industrial pollution sources surrounding the site.

Fig. 1. Location of the sampling site at the south of downtown of Xi’an, China.

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2.2. Sampling and counting method A six-stage samplers (Westech, UK), installed at the height of about 1.0 m above the building roof surface, was employed to collect the viable bioaerosol samples with different size ranges. The range of aerodynamic diameter at each stage was: ≥7.0 μm (stage 1), 7.0–4.7 μm (stage 2), 4.7–3.3 μm (stage 3), 3.3–2.1 μm (stage 4), 2.1–1.1 μm (stage 5) and 1.1–0.65 μm (stage 6). Here fine particles and respirable particles were defined as particles with aerodynamic diameter less than 2.1 μm and 4.7 μm, respectively (Li et al., 2015b). The airborne viable bacterial and fungal samples were collected from Aug. 2014 to Jul. 2015. The collection campaigns started at 8:00 and 13:00 on each sampling day, respectively. The collections of bacterial and fungal samples were carried out successively at each single sampling time and each sample was collected for 10 min. Therefore, about 160 samples of the total 40 sampling days were used in this study. The sampler worked at a flow rate of 28.3 L/min and was sterilized by 75% ethanol between two samplings. Airborne fungi were collected on Sabouraud dextrose agar (Aoboxing Biotech, China) and then incubated at 28 °C for 72 h, while aerobic bacteria on nutrient agar (Aoboxing Biotech, China) and then incubated at 37 °C for 48 h. The colonies were counted as colony-forming units (CFU/m3) using positive-hole correction method (Andersen, 1958). During the sampling period, the concentration of fine particulate matter (PM2.5) was concurrently determined by the portable Haze-dust EPAM-5000 particulate monitor (SKC Inc., USA). Meteorological data, including ambient temperature, relative humidity (RH), wind speed and direction were simultaneously recorded by a portable automatic meteorological station (JLC-QGL, China). 2.3. Data analysis The concentration values reported below were represented as the mean values and standard deviation. LSD one-way analysis of variance (ANOVA) was carried out to examine significant differences in bioaerosol data during different seasons or under various weather conditions. A t-test was conducted to compare the bacterial and fungal concentrations under same weather condition, while non-parametric Mann Whitney-test was performed to compare differences between annual mean concentrations of airborne viable bacteria and fungi. Non-parametric Spearman's correlation analysis was applied to examine the relationships between bioaerosol concentrations and meteorological parameters as well as PM2.5. Given that the data were not normally distributed, non-parametric method was employed. A p-value of less than 0.05 was considered to be statistically significant difference at a confidence level of 95%. In addition, geometric median diameter (GMD) and geometric standard deviation (GSD) were also calculated to further describe the size distribution. IBM SPSS 19.0 was employed for all data analyses. 3. Results and discussion 3.1. Concentrations of viable bioaerosols in ambient air Annual mean concentration of viable bioaerosols in Xi’an from Aug. 2014 to Jul. 2015 was shown in Fig. 2. The concentrations of airborne viable bacteria and fungi varied from 97 to 1909 CFU/m3 and 67 to 1737 CFU/m3, with the mean values of 565 ± 464 and 399 ± 371 CFU/m3, respectively. The mean value of airborne viable bacteria was higher than that of airborne viable fungi (p < 0.01). Table 1 presented the comparison of concentrations of viable bioaerosols in different regions worldwide. It can be seen that the level of airborne bacteria detected in this study was lower than that in most cities in the world (Lee, Grinshpun, Martuzevicius, Adhikari, Crawford and Reponen, 2006; O’Gorman and Fuller, 2008; Haas et al., 2013; Gao et al., 2015). A main reason may be attributed to dissimilar meteorological and environmental conditions in different regions. As shown in Table 1, only Xi’an and

Fig. 2. Annual average concentrations of ambient bioaerosols in Xi’an from Aug. 2014 to Jul. 2015. Box frames represent the upper quartile and lower quartile, line represents the median, whiskers denote range, and “о” represents mean. “*” indicates a statistically significant difference between concentrations.

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Table 1 Comparison of concentrations of viable bioaerosols in different regions worldwide. Regions

Climate type

Concentration (CFU/m3)

Sampling period

Reference

viable bacteria

viable fungi

Xi’an, China

Semi-arid and continental climate

Aug.2014 – Jul.2015

Range: 97-1909 Mean: 565 ± 464

Range:67 −1737 Mean:399 ± 371

Current study

Beijing, China

Temperate monsoon climate

Jan.2013 – Jan.2014

Range: 80-5800 Mean: 1110 ± 976

Range:41 −7210 Mean: 948 ± 978

Gao et al. (2015)

Dunhuang, China

Arid and continental climate

Sep.2008 – Aug.2009

Range: 101-3800 Mean: 440 ± 77

–

Wang et al., (2010)

Cincinnati America

Temperate continental climate

Mar.2004 – Feb.2005

–

Range:0 −3882

Lee et al. (2006)

Dublin, Ireland

Maritime temperate climate

Jan.2005 -Dec.2005

–

Range: 30–6777 Mean: 915

O’Gorman et al. (2008)

Graz, Austria

Less windy illyric Climate

Jan.2008 –Jan.2009

Range: 0-2500

Range:30 −2300

Hass et al. (2013)

Tijuana, Mexico

Dry Mediterranean climate

Nov.2011-Feb.2013

Downtown:1700 ± 595 River valley: 40100 ± 21689

–

Hurtado et al. (2014)

Seoul, Korea

Temperate monsoon climate

July 1, 2013 – July 26, 2013

Range: 20-383 Mean: 107

Range: 60-930 Mean: 357

Heo et al. (2014)

Dunhuang, both located in the Northwestern China, are in semi-arid or arid region among those reported regions. The drying condition and strong solar radiation may provide an unfavorable condition for the growth and survival of airborne microorganism. Therefore, comparable concentrations of airborne microbes can be observed between Xi’an and Dunhuang city. Measurement data obtained in Duahuang, China (Wang, Ma, Ma, Wu, Ma and An, 2010) also supported the above explanation. The seasonal variation of viable bioaerosol concentrations was shown in Fig. 3. The concentrations of airborne viable bacteria were higher in autumn and winter, and lower in summer and spring (p < 0.05) with the highest concentrations observed in autumn. Similarly, Gao et al. (2016) reported the lowest mean bacterial concentrations in summer in Beijing. The possible reason for this low bacterial level is the high temperature and strong solar radiation. The average temperature here is 28.7 ± 3.9 °C in the summer was higher than 24.0 °C, above which the propagation and survival of airborne bacteria is universally decreased. However, the present result of higher bacterial concentrations in autumn and winter was contrary to several previous studies (Fang et al., 2008; Lee et al., 2016). According to Lee et al. (2016), bacteria concentrations should be low in winter due to disadvantageous condition for growth of most microorganisms in winter. However, higher bacterial concentrations detected in this study may be mainly attributed to frequent haze pollution occurred in Xi’an in late autumn and winter. The impact of weather and meteorological factors on bioaerosol concentrations will be discussed below. The seasonal variation of airborne viable fungi was a little different from that of airborne viable bacteria. The maximum amount of airborne viable fungi also occurred in autumn, while the minimum value occurred in winter. The seasonal variability of airborne viable fungi in this study was similar to those in previous reports (Fang et al., 2008; Li et al., 2011; Gao et al., 2016). As suggested by Fang et al. (2008), fungal spore was mainly influenced by nutrient levels, plant growth, and weather conditions. Due to the decreased vegetation as well as dry and cold weather towards winter, spore concentrations in ambient air also decreased, so that these

Fig. 3. Concentration variations of airborne viable bacteria and fungi under differnent seasons. “*” indicates a statistically significant difference between concentrations.

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Table 2 Meteorological parameters and PM2.5 concentrations under four weather conditions.

Sunny days Cloudy days Rainy days Hazy days

Temperature（°C）

Relative humanity（%）

Wind speed（m/s）

PM2.5（μg/m3）

22.4 ± 8.3 26.3 ± 6.7 21.0 ± 6.5 14.3 ± 6.7

31.9 ± 13.7 39.7 ± 23.2 73.9 ± 10.5 76.3 ± 9.7

1.7 ± 1.4 1.6 ± 0.8 1.2 ± 0.7 1.0 ± 0.8

46.2 ± 6.9a 48.0 ± 17.7a 35.0 ± 22.5a 143.0 ± 66.7b

a,b means that mean values within the column by the same letter are not significantly different (p > 0.05).

concentrations can be expected to be lower in the winter than in other seasons. The suitable temperature (19.4 ± 5.1 °C) and relative humidity (RH, 70.7 ± 12.5%) was helpful for the propagation and growth of fungi in autumn in Xi’an, resulting in higher concentration s of airborne fungi found in this study. 3.2. Concentration variations of airborne bacteria and fungi under different weather conditions According to CMA, haze episodes are generally referred to days with atmospheric visibility less than 10 km, RH below 90% and Air Quality Index (AQI) value of PM2.5 > 100 (the limit of Chinese air quality standards Grade II). Since the haze has recently been classified into one of the main weather conditions by CMA, 11, 14, 5 and 10 days of the total 40 sampling days were classified into four categories: sunny, cloudy, rainy and hazy days in this study, respectively. Note that the sunny, cloudy and rainy days refer to clean days or non-haze days in later discussion. Their mean meteorological parameters under four weather conditions were shown in Table 2. Based on the present observation, the hazy days in Xi’an mainly occurred in the fall and winter during the sampling period. The rainy days were primarily in the late summer or early autumn. These are typical weather features in Xi’an, China. Concentration variations of airborne viable bacteria, fungi and PM2.5 under various weather conditions during sampling period of one year were shown in Fig. 4. It can be observed that the concentration of PM2.5 on hazy and non-hazy days ranged from 81 to 279 mg/m3 and 22 to 80 mg/m3, respectively. On haze days, the daily mean concentrations of PM2.5 (143.0 ± 66.7 μg/m3) were significantly higher than Chinese daily average standard (75 μg/m3) and were much higher than those under other weather conditions. As observed in Table 2, relative humanity increased and wind speed were often less than 1 m/s on haze days, implying that the horizontal transport of aerosols was very weak. Therefore, it was favorable for the accumulation of air pollutants, resulting in higher concentrations of PM2.5 on the hazy days in Xi’an. It can be also seen that the concentrations of airborne viable bacteria and fungi varied daily, even in the same weather conditions. For example, the concentrations of airborne viable bacteria and fungi ranged from 127 to 944 CFU/m3 and from 165 to 780 CFU/m3 on cloudy days, respectively. A wider range of viable bioaerosol concentrations can be also found on hazy days: 602–1763 CFU/m3 for bacterial aerosols and 222–1736 CFU/m3 for fungal aerosols. To evaluate the present level of air contamination with microorganisms under various weathers in Xi’an, it is necessary to compare the present results with guideline values to regulate the permissible content of fungal and bacterial aerosols in outdoor air in China. To our knowledge, there are no official standards regarding bioaerosols in outdoor air in China, even in Europe and the US so far. China Scientific Ecology Centre, an unofficial organization, recommended that the guideline values should be less than 1000 CFU/m3 for total airborne bacteria and less than 500 CFU/m3 for airborne fungi in outdoor environments, respectively. It was

Fig. 4. Concentration variations of airborne viable bacteria, fungi and PM2.5 under different weather conditions.

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Fig. 5. Comparison of concentrations of viable bioaerosols under different weather conditions. Box frames represent the upper quartile and lower quartile, line represents the median, whiskers denote range, and “о” represents mean. “*” indicates a statistically significant difference between concentrations.

obvious that the concentrations of airborne viable bacteria on all non-haze days did not exceed this unofficial permissible limit value (1000 CFU/m3). Only on several non-haze days (i.e. May 9, 12 and July 15), the concentrations of airborne viable fungi exceeded the permissible limit value (500 CFU/m3). In contrast, the concentrations of airborne viable bacteria and fungi on most of the hazy days were greater than the reference values, suggesting that the high bioaerosol exposure levels on hazy days may have great potential to threaten the health of local citizens. To further characterize viable bioaerosols in different weathers, a comparison of concentrations of ambient bioaerosols under various weathers was shown in Fig. 5. The mean concentration of airborne viable bacteria and fungi varied by weathers with a trend of rainy days (251 ± 180 and 194 ± 85 CFU/m3) < sunny days (341 ± 158 and 254 ± 123 CFU/m3) < cloudy days (382 ± 216 and 312 ± 178 CFU/m3) < hazy days (1311 ± 371 and 896 ± 559 CFU/m3). Under each weather condition, the mean concentration of airborne viable bacteria was higher than that of airborne viable fungi although the differences were not statistically significant (p > 0.05). Moreover, the concentration differences were not statistically significant among rainy days, sunny days and cloudy days (p > 0.05), while the concentrations of viable bacterial and fungal aerosols on hazy days were significantly higher than those in other weathers (p < 0.05). Note that the non-simultaneous difference in monitoring bacterial and fungal concentrations may be small because both samples were collected at consecutive times within 30 min each sampling. Additionally, the current daily concentration data was the average of two measurements in the morning and afternoon on each sampling day. Therefore, one can compare the numbers of fungi vs. bacteria by using the present daily mean value. Although there were hardly previous studies on airborne microorganisms in cloudy or rainy weather, a correlation analysis between airborne microbes and meteorological factors might help to explain the reason of increased concentration of bioaerosols under such weather condition, as presented by Table 3. Positive correlations were found between the concentrations of viable microbes versus RH. This was consistent with the results reported by Li et al. (2015b). As indicated in Table 2, the RH was higher on the cloudy days than on the sunny days during the present sampling period. According to Jones and Harrison (2004), moisture in the air could alter integrity of cell walls or viral coats, and thus the increase of air humidity could facilitate the growth and survival of airborne microorganism on cloudy days. Solar radiation was known as an effective sterilization method for bioaerosols (Hwang et al., 2010), and thus reduced die-off rates from reduction in solar radiation on the cloudy days may lead to the accumulation of microorganisms. Rain is widely known to remove aerosol particles in ambient environments. The bioaerosol particles were captured and removed from the air by rain droplets and thus the concentration of bioaerosols decreased on rainy days. Nevertheless, Heo et al. (2014) recently found that concentrations of bacterial and fungal bioaerosols during rain events were approximately seven times those on non-rainy days in Seoul, Republic of Korea. They thought that humid environments produced by long-term rain events such as monsoon may be a sufficient condition for the growth of microorganisms and vibrations because of the splashing of droplets may facilitate the aerosolization of ground microorganisms. Difference between the two opposite results should be attributed mainly to different climate characteristics in different geographic region of observation locations. As a semi-arid city, there is no monsoon

Table 3 Pearson correlation coefficients between concentrations of viable bioaerosols vs. temperature, relative humidity and PM2.5.

Bacteria Fungi

Temperature (℃)

Relative humidity (%)

PM2.5 (μg/m3)

−0.403** −0.293

0.284 0.289

0.402* 0.230

** P < 0.01(2-tailed). * P < 0.05(2-tailed).

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season in Xi’an. The duration of rainy event was usually a few hours and rainfall was also small during the study period. Therefore, the humid environment produced by the rainy events in Xi’an was drastically different from that in Seoul. As a result, rain removal effect played a key role in the decreased concentration of bioaerosols on the rainy days in this study. Gao et al. (2015) recently presented the decreased airborne microbe level during urban haze events in Beijing. They suggested that high concentrations of toxic and hazardous substances on hazy days were probably attributed to adverse effects on the culturability of bacteria, leading to the inverse relationship between haze degree and concentration of viable airborne bacteria. Contrary to the observations in Beijing, the present study found a significant increase of fungal and bacterial aerosol concentrations on the haze days. Similar findings were obtained in two recent studies (Li et al., 2015b; Dong et al., 2016). Li et al. (2015b) collected airborne microbial samples in Xi’an from October 8th to 22th, 2014, and found that daily mean concentrations of airborne viable bacterial and fungal during the haze days were approximately 2–4 times and 4–7 times higher than those during the non-haze days, respectively. Dong et al. (2016) collected atmospheric bioaerosol particles from Oct. 2013 to Aug. 2014 in the coastal region of Qingdao, and also observed that the concentration of microbes increased greatly on hazy days compared with non-hazy days. The main reason, in our opinion, was that much more number of fine particles and relatively higher RH (shown in Table 2) during the hazy days provided a more favorable condition for microbial proliferation and growth, overwhelming the poisonous effects of toxic and hazardous substances in PM. Moreover, positive correlations between the concentrations of viable microbes versus PM2.5 (shown in Table 3) also indicated that the level of airborne viable microbes increased with increase of PM2.5 concentration. Another important reason might be due to that genera or species composition may change with the transformation from non-hazy days to hazy days. A further study on the airborne microbe community during the haze days should be helpful to test this hypothesis in the future. 3.3. Size distributions of airborne bacteria and fungi under different weather conditions Since bioaerosols with different aerodynamic diameters may be deposited in different positions of the respiratory system and result in different respiratory illnesses (Thomas, Webber, Sellors, Collinge, Frost and Stagg, 2008), effects of microbial aerosols on human health are not only related to their concentration, but also to their size distribution. Fig. 6 illustrated the size distributions of airborne viable bacteria and fungi under the four weather conditions. It can be seen that airborne viable bacteria presented a singpeak distribution pattern for every weathers. The highest proportions of airborne viable bacteria were detected on stage 3 (3.3– 4.7 µm) with fraction of 29.0%, stage 2 (4.7–7.0 µm) with 24.8%, stage 3 (3.3–4.7 µm) with 25.5%, and stage 3 (3.3–4.7 µm) with

Fig. 6. Size distributions of airborne viable bacteria and fungi under different weather conditions.

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25.9% on the sunny, cloudy, rainy and hazy days, respectively. The concentrations of airborne viable bacteria were not significantly different among the other stages under each weather condition. Similar sing-peak patterns have also been reported by previous studies in Beijing during non-haze days (Xu and Yao, 2013; Gao et al., 2015). This implied that the size distribution pattern was not affected by weather conditions and geographical regions. In contrast to airborne viable bacteria, the size distribution of airborne viable fungi under the non-haze weather conditions did not show clear sing-peak pattern. No statistically significant differences were detected on each stage under four weather conditions (p > 0.05). The size distribution was not consistent with previous results on non-haze days, i.e., a Gaussian distribution with peak in the size range of 2.1–3.3 µm in Beijing (Fang et al., 2008), 3.3–4.7 µm in Qingdao (Li et al., 2011), and 1.1–3.3 µm in Singapore (Zuraimi et al., 2009). The size distributions of airborne viable fungi are influenced by many factors such as microorganism species, spore age, sample culture medium, RH, and differences in aggregation rates of spores. At present, we cannot give further explanation on this phenomenon due to the lack of information on the species, age and nutrient source of fungi. Accordingly, a further study is necessary in the future. Under different weather conditions, the concentrations of airborne bacteria and fungi distributed between same stages were found to be different. For example, compared with the sunny days, the mean concentrations of both airborne bacteria and fungi were higher at most stages on the cloudy days, while lower at each stage on the rainy days. On the hazy days, there were significantly increased concentrations at every stages (p < 0.05). These variations in concentrations at each stage reflected effect of weathers on bioaerosol concentrations as discussed earlier. It is interesting that about 69.4%, 59.8%, 67.8% and 68.2% of airborne bacteria, and 70.7%, 65.9%, 63.1% and 65.5% of airborne fungi, were found in the size range of less than 4.7 μm, respectively, on the sunny, cloudy, rainy and hazy days, respectively. It indicated that more than 60% bioaerosols were in respirable size range under each weather condition. The respirable particles can deposit in lower respiratory tract. Therefore, exposure to ambient bioaerosols poses potential adverse impacts to human health under various weather conditions. Furthermore, the present results also suggested that the respirable fraction of bioaerosols did not significantly vary by weather condition. In addition, the proportion of respirable bioaerosols in ambient environment in the present study are comparable to those obtained in indoor environments (Pastuszka et al., 2000; Wang et al., 2010; Nasir and Colbeck, 2010), suggesting that the respirable fraction of bacteria and fungi may be independent on geographical and meteorological factors. To further quantify the size distribution of airborne bacteria and fungi, the geometric median diameter (GMD) and geometric standard deviation (GSD) can be also calculated, shown in Table 4. The GMD and GSD of airborne viable bacteria were 2.06 ± 1.36, 2.01 ± 1.26, 1.93 ± 1.28 and 2.00 ± 1.34 μm on the sunny, cloudy, rainy and hazy days, respectively, while values of airborne viable fungi were 1.72 ± 1.00, 1.85 ± 1.11, 1.90 ± 1.30, 1.86 ± 1.10 μm, respectively. Similar to the data obtained by Gao et al. (2015), the GMD of airborne bacteria was larger than that of fungi under each weather condition. It may be attributed to the fact that airborne bacteria exist primarily in clusters or attached to airborne particles. It can be found that the GMD of airborne viable bacteria on the sunny days was larger than that under other weather conditions, although no significant difference was found. This replied that the size distribution of airborne bacteria shifted to finer size range when the weather conditions changed from sunny days to other weathers. According to Li et al. (2015b), the relatively smaller GMD of airborne viable bacteria during the haze days may be due to the increased concentrations of fine particles. As discussed earlier, PM2.5 concentrations on the haze days were much higher than those on the sunny days. More fine particles became carriers for airborne microorganisms, which also provide nutrition for microorganism growth. As a result, the elevated percentage of bacterial particles present in the fine size range. As for rainy days, the bioaerosols could be captured and removed from the air by rain droplets. However, such removal effect on small particles was far less than on large particles. As a result, relatively more fine bioaerosols were still staying in the air, resulting in the relatively smaller GMD of airborne viable bacteria on the rainy days. In contrast, the GMD of viable fungal aerosols on the sunny days was lower than those under other weather conditions, although no significant difference was found. This difference in variation trend of bacterial and fungal GMD was most likely due to the differences in biological characteristics between two microorganism spcies. As suggested by Reponen (1995), the aerodynamic size of freshly released spores was larger than that of spores which had been airborne for a longer time. Further, environmental condition like RH of surrounding air had more significant effects on fungal growth than bacteria (Nasir and Colbeck, 2010).

4. Conclusions This study provided basic information on the concentrations and size distributions of ambient viable bioaerosols in Xi’an, a city in a semi-arid region under different weather conditions. The annual average concentrations of airborne viable bacteria and fungi in Table 4 Geometric median diameter (GMD) and geometric standard deviation (GSD) of viable bioaerosols under different weather conditions (µm). GMD ± GSD

Bacteria Fungi

Sunny days

Cloudy days

Rainy days

Hazy days

2.13 ± 1.36a 1.72 ± 1.00b

2.01 ± 1.26a 1.85 ± 1.01b

1.93 ± 1.28a 1.90 ± 1.30b

2.00 ± 1.34a 1.86 ± 1.10b

a and b means that mean values within the row by the same letter are not significantly different (p > 0.05).

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Xi’an were lower than most cities in other regions of the world due to the semi-arid climate feature. The concentration of airborne viable microbes also showed significant variations in different weathers, with the order of rainy days < sunny days < cloudy days < hazy days. The mean concentrations of viable bacteria and fungi on the hazy days were much higher than those on the non-haze days, and exceeded the health guideline suggested by Chinese non-governmental organization, suggesting that more attention should be paid to the potential health risk related to microorganisms in aerosols accompanied by the haze days. Another important finding was that more than 60% viable bioaerosols were in respirable size range under all weather conditions. In particular, the GMD of airborne viable bacteria were relatively higher on the sunny days than under the other three weather conditions. This finding replied that ambient microorganisms enriched in particles of more finer aerodynamic diameters when weather condition changes from sunny to others. As a primary research on bioaerosol characteristics under various environments, it should be noted that only a culture-based method was employed in the present study and the number of microorganisms in bioaerosols reported here is only for culturable ones on selective media, probably accounting for only ~10% of the total microorganisms. Although constrained to identification of a limited number of cultivatable species, it is still expected from the present results that the outdoor environments in Xi’an city may not be safe during the haze days in terms of human exposure to ambient bioaerosols. The present results can provide valuable information for prospective exposure assessment and establishment of Chinese official standard regarding bioaerosols in ambient air. 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