Drivers of the fungal spore bioaerosol budget: observational analysis and global modeling

Abstract

Abstract. Bioaerosols are produced by biological processes and directly emitted into\nthe atmosphere, where they contribute to ice nucleation and the formation of\nprecipitation. Previous studies have suggested that fungal spores constitute\na substantial portion of the atmospheric bioaerosol budget. However, our\nunderstanding of what controls the emission and burden of fungal spores on\nthe global scale is limited. Here, we use a previously unexplored source of\nfungal spore count data from the American Academy of Allergy, Asthma, and\nImmunology (AAAAI) to gain insight into the drivers of their emissions.\nFirst, we derive emissions from observed concentrations at 66 stations by\napplying the boundary layer equilibrium assumption. We estimate an annual\nmean emission of 62\u2009±\u200931\u2009m−2\u2009s−1 across the USA. Based on\nthese pseudo-observed emissions, we derive two models for fungal spore\nemissions at seasonal scales: a statistical model, which links fungal spore\nemissions to meteorological variables that show similar seasonal cycles (2\u2009m\nspecific humidity, leaf area index and friction velocity), and a population\nmodel, which describes the growth of fungi and the emission of their spores\nas a biological process that is driven by temperature and biomass density.\nBoth models show better skill at reproducing the seasonal cycle in fungal\nspore emissions at the AAAAI stations than the model previously developed by\nHeald and Spracklen (2009) (referred to as HS09). We implement all three\nemissions models in the chemical transport model GEOS-Chem to evaluate\nglobal emissions and burden of fungal spore bioaerosol. We estimate annual\nglobal emissions of 3.7 and 3.4\u2009Tg\u2009yr−1 for the statistical model and\nthe population model, respectively, which is about an order of magnitude\nlower than the HS09 model. The global burden of the statistical and the\npopulation model is similarly an order of magnitude lower than that of the\nHS09 model. A comparison with independent datasets shows that the new models\nreproduce the seasonal cycle of fluorescent biological aerosol particle\n(FBAP) concentrations at two locations in Europe somewhat better than the\nHS09 model, although a quantitative comparison is hindered by the ambiguity\nin interpreting measurements of fluorescent particles. Observed vertical\nprofiles of FBAP show that the convective transport of spores over source\nregions is captured well by GEOS-Chem, irrespective of which emission scheme\nis used. However, over the North Atlantic, far from significant spore\nsources, the model does not reproduce the vertical profiles. This points to\nthe need for further exploration of the transport, cloud processing and wet\nremoval of spores. In addition, more long-term observational datasets are\nneeded to assess whether drivers of seasonal fungal spore emissions are\nsimilar across continents and biomes.\n