Observations of fluorescent and biological aerosol at a high-altitude site in Central France

Abstract

Total bacteria, fungal spore and yeast counts were compared with UV Light-Induced Fluorescence (UV-LIF) measurements of ambient aerosol at the summit of the Puy de Dˆome (pdD) mountain in Central France (1465 ma.s.l), which represents a background elevated site. Bacteria, fungal spores and yeast were enumerated by epifluorescence microscopy (EFM) and found to number 2.2 to 23 L−1 and 0.8 to 2 L−1, respectively. Bacteria counts on two successive nights were an order of magnitude larger than in the intervening day. A Wide Issue Bioaerosol Spectrometer, version 3 (WIBS-3) was used to perform UV-LIF measurements on ambient aerosol sized 0.8 to 20 µm. Mean total number concentration was 270 L−1 (σ = 66 L−1) found predominantly in a size mode at 2 µm for most of the campaign. Total concentration (fluorescent + non-fluorescent aerosol) peaked at 500 L−1 with a size mode at 1 µm because of a change in air mass origin lasting around 48 h. The WIBS-3 features two excitation and fluorescence detection wavelengths corresponding to di ff erent biological molecules. The mean fluorescent particle concentration after short-wave (280 nm; Tryptophan) excitation was 12 L−1 (σ = 6 L−1), and did not vary much through the campaign. In contrast the mean concentration of particles fluorescent after long-wave (370 nm; NADH) excitation was 95 L−1(σ = 25 L−1), and a nightly rise and subsequent fall of up to 100 L−1 formed a strong diurnal cycle in the latter. The fluorescent populations exhibited size modes at 3 µm and 2 to 3 µm, respectively. A hierarchical agglomerative cluster analysis algorithm was applied to the data and used to extract di ff erent particle factors. A cluster concentration time series representative of bacteria was identified. This was found to exhibit a diurnal cycle with a maximum peak appearing during the day. Analysis of organic mass spectra recorded using an Aerosol Mass Spectrometer 25 (AMS; Aerodyne Inc.) suggests that aerosol reaching the site at night was more aged than that during the day, indicative of sampling the residual layer at night. Supplementary meteorological data and previous work also show that pdD lies in the residual layer/free troposphere at night, and this is thought to cause the observed diurnal cycles in organic-type and fluorescent aerosol particles. Based on the observed disparity between bacteria and fluorescent particle concentrations, fluorescent non-PBA is likely to be important in the WIBS-3 data and the surprisingly high fluorescent concentration in the residual layer/free troposphere raises questions about a ubiquitous background in continental air during the summer.