Morphology, mixing state, and hygroscopicity of primary biological aerosol particles from a Chinese boreal forest

Abstract. Biological aerosols play an important role in atmospheric chemistry, clouds, climate, and public health. Here, we studied the morphology and composition of primary biological aerosol particles (PBAPs) collected in the Lesser Khingan Mountain boreal forest of China in summertime using transmission electron microscopy and scanning electron microscopy. Of all detected particles > 100 nm in diameter, 13 % by number were identified as PBAPs. In addition, 57 % of the PBAPs were identified as bacteria, followed by brochosomes (24 %) and fungi (19 %). The dominant size of bacteria was 1–4 μm, fungi was 2–4 μm, and brochosomes was 300–500 nm. The number size distribution of PBAPs coupled with the mass concentrations of PM_2.5 and PM₁₀ were used to estimate the total mass concentration of PBAPs, which is approximately 1.9 μg m⁻³ and accounts for 47 % of the in situ PM_2.5–10 mass. C, N, O, P, K, and Si are detected in all PBAP particles, and P represented a major marker to identify PBAPs. Moreover, there is a higher frequency and concentration of PBAPs at night compared with day. Bacterial and fungal particles displayed weak hygroscopicity with a growth factor of ~ 1.09 at RH = 94 %. Electron microscopy shows that approximately 20 % of the bacterial particles were internally mixed with metal, mineral dust, and inorganic salts in the boreal forest air. This work provides a database for both further understanding physicochemical state of individual PBAP particles from natural sources and expanding the scope of atmospheric implications.