Identification of coexistence of biological and non-biological aerosol particles with DAPI (4′,6-diamidino-2-phenylindole) stain

The fluorescent dye 4′,6-diamidino-2-phenylindole (DAPI) has been widely used to stain microorganisms in various environment media. We applied DAPI fluorescence enumeration to airborne microorganisms and found that non-biological particles, including organic compounds, minerals, and soot, were also visible upon exposure to UV excitation under fluorescence microscope. Using laboratory-prepared biological particles as the control, we investigated the feasibility of identifying both biological and non-biological particles in the same sample with DAPI staining. We prepared biological (bacterial, fungi, and plant detritus) and non-biological (biochar, soot, mineral, metal, fly ash, salt) particles in the laboratory and enumerated the particles and their mixture with DAPI. We found that mineral particles were transparent, and biochar, soot, metals and fly ash particles were black under a filter set at excitation 350/50 nm and emission 460/50 nm bandpass (DAPI-BP), while biological particles were blue, as expected. Particles of the water-soluble salts NaCl and (NH_(4))_(2)SO_(4) were yellow under a filter set at excitation 340–380 nm and emission 425 nm long pass (DAPI-LP). Case studies with samples of dustfall, atmospheric aerosols and surface soils could allow for the quantification of the relative number of different types of particles and particles with organic matter or salt coating as well. Fluorescence enumeration with DAPI stain is thus able to identify the co-existence of biological and non-biological particles in the air, at least to the extent of those examined in this study.

Light attenuation cross-section of black carbon in an urban atmosphere in northern China

Fine particulate matter （PM2.5） samples were collected over two years in Xi＇an, China to investigate the relationships between the aerosol composition and the light absorption efficiency of black carbon （BC）. Real-time light attenuation of BC at 880 nm was measured with an aethalometer. The mass concentrations and elemental carbon （EC） contents of PM2.5 were obtained, and light attenuation cross-sections （σATN） of PM2.5 BC were derived. The mass of EC contributed -5% to PM2.5 on average. BC σATN exhibited pronounced seasonal variability with values averaging 18.6, 24.2,16.4, and 26.0 m^2/g for the spring, summer, autumn, and winter, respectively, while averaging 23.0 m^2/g overall. σATN varied inversely with the ratios of EC/PM2.5, EC/[SO4^2- ], and EC/[NO3-]. This study of the variability in σATN illustrates the complexity of the interactions among the aerosol constituents in northern China and documents certain effects of the high EC, dust, sulfate and nitrate loadings on light attenuation.