Abstract
Abstract. Bacteria are constantly being transported through the atmosphere, which may have implications for human health, agriculture, cloud formation, and the dispersal of bacterial species. We simulate the global transport of bacteria, represented as 1 μm and 3 μm diameter spherical solid particle tracers in a general circulation model. We investigate factors influencing residence time and distribution of the particles, including emission region, cloud condensation nucleus activity and removal by ice-phase precipitation. The global distribution depends strongly on the assumptions made about uptake into cloud droplets and ice. The transport is also affected, to a lesser extent, by the emission region, particulate diameter, and season. We find that the seasonal variation in atmospheric residence time is insufficient to explain by itself the observed seasonal variation in concentrations of particulate airborne culturable bacteria, indicating that this variability is mainly driven by seasonal variations in culturability and/or emission strength. We examine the potential for exchange of bacteria between ecosystems and obtain rough estimates of the flux from each ecosystem by using a maximum likelihood estimation technique, together with a new compilation of available observations described in a companion paper. Globally, we estimate the total emissions of bacteria-containing particles to the atmosphere to be 7.6×1023–3.5×1024 a−1, originating mainly from grasslands, shrubs and crops. We estimate the mass of emitted bacteria- to be 40–1800 Gg a−1, depending on the mass fraction of bacterial cells in the particles. In order to improve understanding of this topic, more measurements of the bacterial content of the air and of the rate of surface-atmosphere exchange of bacteria will be necessary. Future observations in wetlands, hot deserts, tundra, remote glacial and coastal regions and over oceans will be of particular interest.
Highlights
The transport of microorganisms in the atmosphere could have important implications for several branches of science, including impacts on human health, agriculture, clouds, and microbial biogeography (Burrows et al, 2009)
Bauer et al (2003) found that bacteria in aerosol and cloud water samples were active as cloud condensation nuclei (CCN) at supersaturations between 0.07% and 0.11%, insoluble wettable particles of comparable size would not have been activated at such low supersaturations
Using a global chemistry-climate model, we investigated the transport of bacteria in the atmosphere and its sensitivity to scavenging and the source ecosystem
Summary
The transport of microorganisms in the atmosphere could have important implications for several branches of science, including impacts on human health, agriculture, clouds, and microbial biogeography (Burrows et al, 2009). Unraveling these effects has been difficult, partly because so little is known about the concentrations and sources of atmospheric microorganisms, or their transport pathways. By adjusting the simulation results to observed concentrations, we estimated the emissions of bacteria from ten lumped ecosystem classes as a first step towards a simple model of emissions of biological particles to the atmosphere Further details on the methodology can be found in the appendices, and supplementary tables and figures of simulation results are available in an online supplement: http://www.atmos-chem-phys.net/ 9/9281/2009/acp-9-9281-2009-supplement.pdf
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