Size distributions and temporal variations of biological aerosol particles in the Amazon rainforest characterized by microscopy and real-time UV-APS fluorescence techniques during AMAZE-08

摘要

As a part of the AMAZE-08 campaign during the wet season in the rainforestof central Amazonia, an ultraviolet aerodynamic particle sizer (UV-APS) wasoperated for continuous measurements of fluorescent biological aerosolparticles (FBAP). In the coarse particle size range(> 1 μm) the campaign median and quartiles of FBAP number and mass concentrationwere 7.3 × 104 m?3 (4.0–13.2 × 104 m?3) and0.72 μg m?3 (0.42–1.19 μg m?3), respectively,accounting for 24% (11–41%) of total particle number and 47%(25–65%) of total particle mass. During the five-week campaign inFebruary–March 2008 the concentration of coarse-mode Saharan dust particleswas highly variable. In contrast, FBAP concentrations remained fairlyconstant over the course of weeks and had a consistent daily pattern,peaking several hours before sunrise, suggesting observed FBAP was dominatedby nocturnal spore emission. This conclusion was supported by the consistentFBAP number size distribution peaking at 2.3 μm, also attributed tofungal spores and mixed biological particles by scanning electron microscopy(SEM), light microscopy and biochemical staining. A second primarybiological aerosol particle (PBAP) mode between 0.5 and 1.0 μm wasalso observed by SEM, but exhibited little fluorescence and no true fungalstaining. This mode may have consisted of single bacterial cells,brochosomes, various fragments of biological material, and smallChromalveolata (Chromista) spores. Particles liquid-coated with mixedorganic-inorganic material constituted a large fraction of observations, andthese coatings contained salts likely from primary biological origin. Weprovide key support for the suggestion that real-time laser-inducefluorescence (LIF) techniques using 355 nm excitation provide size-resolvedconcentrations of FBAP as a lower limit for the atmospheric abundance ofbiological particles in a pristine environment. We also show somelimitations of using the instrument for ambient monitoring of weaklyfluorescent particles < 2 μm. Our measurements confirm thatprimary biological particles, fungal spores in particular, are an importantfraction of supermicron aerosol in the Amazon and that may contributesignificantly to hydrological cycling, especially when coated by mixedinorganic material.