Abstract
Abstract. Bioaerosols are produced by biological processes and directly emitted into the atmosphere, where they contribute to ice nucleation and the formation of precipitation. Previous studies have suggested that fungal spores constitute a substantial portion of the atmospheric bioaerosol budget. However, our understanding of what controls the emission and burden of fungal spores on the global scale is limited. Here, we use a previously unexplored source of fungal spore count data from the American Academy of Allergy, Asthma, and Immunology (AAAAI) to gain insight into the drivers of their emissions. First, we derive emissions from observed concentrations at 66 stations by applying the boundary layer equilibrium assumption. We estimate an annual mean emission of 62 ± 31 m−2 s−1 across the USA. Based on these pseudo-observed emissions, we derive two models for fungal spore emissions at seasonal scales: a statistical model, which links fungal spore emissions to meteorological variables that show similar seasonal cycles (2 m specific humidity, leaf area index and friction velocity), and a population model, which describes the growth of fungi and the emission of their spores as a biological process that is driven by temperature and biomass density. Both models show better skill at reproducing the seasonal cycle in fungal spore emissions at the AAAAI stations than the model previously developed by Heald and Spracklen (2009) (referred to as HS09). We implement all three emissions models in the chemical transport model GEOS-Chem to evaluate global emissions and burden of fungal spore bioaerosol. We estimate annual global emissions of 3.7 and 3.4 Tg yr−1 for the statistical model and the population model, respectively, which is about an order of magnitude lower than the HS09 model. The global burden of the statistical and the population model is similarly an order of magnitude lower than that of the HS09 model. A comparison with independent datasets shows that the new models reproduce the seasonal cycle of fluorescent biological aerosol particle (FBAP) concentrations at two locations in Europe somewhat better than the HS09 model, although a quantitative comparison is hindered by the ambiguity in interpreting measurements of fluorescent particles. Observed vertical profiles of FBAP show that the convective transport of spores over source regions is captured well by GEOS-Chem, irrespective of which emission scheme is used. However, over the North Atlantic, far from significant spore sources, the model does not reproduce the vertical profiles. This points to the need for further exploration of the transport, cloud processing and wet removal of spores. In addition, more long-term observational datasets are needed to assess whether drivers of seasonal fungal spore emissions are similar across continents and biomes.
Highlights
Bioaerosols are omnipresent in the global atmosphere (DeLeon-Rodriguez et al, 2013; Després et al, 2012; Fröhlich-Nowoisky et al, 2016)
Spore emissions are implemented as a Harvard–NASA Emission Component (HEMCO; Keller et al, 2014) extension, which uses the model meteorology at either the surface or the lowest vertical level, and MODIS leaf area index (LAI) product from Yuan et al (2011) for the year 2008 to calculate emissions
Calculated spore concentrations from the population and the statistical model capture the seasonal variations in fluorescent biological aerosol particle (FBAP) concentrations with skill comparable to the HS09 model, assumptions on the temperature threshold below which no emissions occur have a large influence on the performance of the former two models
Summary
Bioaerosols are omnipresent in the global atmosphere (DeLeon-Rodriguez et al, 2013; Després et al, 2012; Fröhlich-Nowoisky et al, 2016). We focus on fungal spores, as they have a smaller size than pollen, which implies that they are more likely to be transported over longer distances, and to contribute significantly to the organic aerosol budget on the regional and global scale They can produce large quantities of submicrometer fragments after rupturing in the atmosphere and thereby contribute to CCN and INP populations (China et al, 2016; O’Sullivan et al, 2015). Since the sources of fungal spores are diverse, it is challenging to develop a mechanistic description of their atmospheric emissions, and emissions are usually based on extrapolation of the limited number of available observations These estimated emissions of fungal spores range widely for different methods, including both models and educated guesses, from 50 Tg yr−1 (Elbert et al, 2007), 28 Tg yr−1 (HS09), 186 Tg yr−1 (Jacobson and Streets, 2009) to 79 Tg yr−1 (Sesartic and Dallafior, 2011). We evaluate the ability of both emission schemes to simulate spatial and seasonal variations in observed fungal spore concentrations and compare results from the new schemes to those from the previously developed Heald and Spracklen (2009) scheme (Sect. 3.4)
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