Abstract
Abstract. Primary biological aerosol particles (PBAPs) may play an important role in aerosol–climate interactions, in particular by affecting ice formation in mixed phase clouds. However, the role of PBAPs is poorly understood because the sources and distribution of PBAPs in the atmosphere are not well quantified. Here we include emissions of fungal spores and bacteria in a global aerosol microphysics model and explore their contribution to concentrations of supermicron particle number, cloud condensation nuclei (CCN) and immersion freezing rates. Simulated surface annual mean concentrations of fungal spores are ~ 2.5 × 104 m−3 over continental midlatitudes and 1 × 105 m−3 over tropical forests. Simulated surface concentrations of bacteria are 2.5 × 104 m−3 over most continental regions and 5 × 104 m−3 over grasslands of central Asia and North America. These simulated surface number concentrations of fungal spores and bacteria are broadly in agreement with the limited available observations. We find that fungal spores and bacteria contribute 8 and 5% respectively to simulated continental surface mean supermicron number concentrations, but have very limited impact on CCN concentrations, altering regional concentrations by less than 1%. In agreement with previous global modelling studies, we find that fungal spores and bacteria contribute very little (3 × 10−3%, even when we assume upper limits for ice nucleation activity) to global average immersion freezing ice nucleation rates, which are dominated by soot and dust. However, at lower altitudes (400 to 600 hPa), where warmer temperatures mean that soot and dust may not nucleate ice, we find that PBAP controls the immersion freezing ice nucleation rate. This demonstrates that PBAPs can be of regional importance for IN formation, in agreement with case study observations.
Highlights
Primary biological aerosol particles (PBAPs) include a wide range of biological particles emitted directly from the biosphere including bacteria, viruses, fungal spores, pollen and leaf debris
Simulated concentrations of fungal spores are typically 2 × m−3 over midlatitude continental regions and exceed 1 × m−3 over tropical forests matching the regions of greatest fungal spore emission (Heald and Spracklen, 2009)
We have explored the contribution of fungal spores and bacteria to global aerosol number concentrations
Summary
Primary biological aerosol particles (PBAPs) include a wide range of biological particles emitted directly from the biosphere including bacteria, viruses, fungal spores, pollen and leaf debris. It has been suggested that PBAPs can make a large contribution to atmospheric aerosol (Jaenicke, 2005), influencing climate through scattering and absorbing radiation (the aerosol direct effect) and by altering the properties of clouds (the aerosol indirect effect). The number of PBAPs emitted into the atmosphere is thought to be substantial with estimates as large as 1000 Tg a−1 (Jaenicke, 2005). Previous estimates of the global emission of fungal spores vary from 8 to 186 Tg a−1 (Elbert et al, 2007; Heald and Spracklen, 2009; Jacobson and Streets, 2009; Hoose et al, 2010b; Sesartic and Dallafior, 2011; Després et al, 2012). The global emissions of bacteria are even more uncertain, spanning nearly 2 orders of magnitude from 0.4 to 28.1 Tg a−1 (Burrows et al, 2009b; Hoose et al, 2010b; Jacobson and Streets, 2009; Després et al, 2012)
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