Abstract
Abstract. Many recent studies have identified biological material as a major fraction of ambient aerosol loading. A small fraction of these bioaerosols consist of bacteria that have attracted a lot of attention due to their role in cloud formation and adverse health effects. Current atmospheric models consider bacteria as inert quantities and neglect cell growth and multiplication. We provide here a framework to estimate the production of secondary biological aerosol (SBA) mass in clouds by microbial cell growth and multiplication. The best estimate of SBA formation rates of 3.7 Tg yr−1 is comparable to previous model estimates of the primary emission of bacteria into the atmosphere, and thus this might represent a previously unrecognized source of biological aerosol material. We discuss in detail the large uncertainties associated with our estimates based on the rather sparse available data on bacteria abundance, growth conditions, and properties. Additionally, the loss of water-soluble organic carbon (WSOC) due to microbial processes in cloud droplets has been suggested to compete under some conditions with WSOC loss by chemical (OH) reactions. Our estimates suggest that microbial and chemical processes might lead to a global loss of WSOC of 8–11 and 8–20 Tg yr−1, respectively. While this estimate is very approximate, the analysis of the uncertainties and ranges of all parameters suggests that high concentrations of metabolically active bacteria in clouds might represent an efficient sink for organics. Our estimates also highlight the urgent need for more data concerning microbial concentrations, fluxes, and activity in the atmosphere to evaluate the role of bacterial processes as net aerosol sinks or sources on various spatial and temporal scales.
Highlights
Many recent atmospheric studies have been dedicated to the characterization and quantification of outdoor bioaerosols, since bioaerosols have been suggested to contribute to adverse health effects and cloud formation as ice-nucleating particles (Després et al, 2012)
Ccell denotes the ambient cell concentration (Table 1); Flive is the fraction of living cells, and it is assumed to be unity here; fractions of microbial processes in clouds (Fcloud) is the fraction of total time when bacteria are active in clouds (Table 2); and mcell is the average mass of a single cell, independent of the bacteria type
We have estimated the amount of biological mass that is formed in the atmosphere by growth and multiplication of bacterial cells (“secondary biological aerosol”, SBA)
Summary
Many recent atmospheric studies have been dedicated to the characterization and quantification of outdoor bioaerosols, since bioaerosols have been suggested to contribute to adverse health effects and cloud formation as ice-nucleating particles (Després et al, 2012). Biological material includes debris, pollen, bacteria, fungal spores, and viruses, and it is usually considered as being directly emitted to the atmosphere (primary biological aerosol, PBA; Jaenicke, 2005). In an urban and remote region in Germany, 24 % of all particles were found to include a biological fraction (Matthias-Maser and Jaenicke, 2000). Similar concentrations were observed at a remote high-altitude site with 16 %–64 % of the mass of particles with diameters of less than 10 μm being composed of biological mass (Wiedinmyer et al, 2009), whereas the PBA number fraction was much smaller (0.3 %–18 %) in Rome, Italy (Perrino and Marcovecchio, 2016). Bacteria only comprise a small fraction of the total biological aerosol mass, but they alone can contribute up to ∼ 20 % of the total number of particles with diameters greater than 0.5 μm (Bowers et al, 2012)
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