The characterization and quantification of viable and dead airborne biological particles using flow cytometry and double fluorescent staining

Primary biological aerosol particles (PBAPs) are important air pollutants because they are ubiquitous in the atmosphere; influence human health, air quality, atmospheric chemistry and physics; and play important roles in regulating atmospheric processes. In investigations of the effects of PBAPs in ambient environments, it is necessary to distinguish them from other particles. In this study, flow cytometry (FCM) was utilized in combination with permeant (SYBR Green dyes) and impermeant (Propidium Iodide, PI) nucleic acid fluorescent stains, to characterize and quantify the viable and dead airborne PBAPs in ambient aerosol in Beijing, China. The number concentrations of viable and dead PBAPs showed a consistent trend and correlated well with one another (R2 = 0.72, p < 0.01). However, no relationship was found between the number concentration of PBAPs and non-biological particles. The mean number concentrations of total particulate matter, dead PBAPs and viable PBAPs were 5.78 × 106 m−3, 1.11 × 106 m−3 and 7.09 × 105 m−3, respectively. The median PBAPs number concentrations (the sum of dead and viable) exhibited a seasonal cycle, i.e., highest in summer and autumn, lowest during winter and spring. Despite a lower median PBAPs number concentration during winter compared with other seasons, the viability peaked during winter, coinciding with the winter flu season. Stable synoptic conditions, polluted southwesterly air mass and long-range transported dust from the northwest desert region all contributed to the occurrence of explosive fluorescent PBAP episodes in Beijing, China.