Bioaerosols are relevant for public health and may play an important role in the climate system, but their atmospheric abundance, properties, and sources are not well understood. Here we show that the concentration of air- borne biological particles in a North American forest ecosystem increases significantly during rain and that bioparticles are closely correlated with atmospheric ice nuclei (IN). The greatest increase of bioparticles and IN occurred in the size range of 2–6 μm, which is characteristic for bacterial aggregates and fungal spores.
By DNA analysis we found high diversities of airborne bacteria and fungi, including groups containing human and plant pathogens (mildew, smut and rust fungi, molds, Enterobacteriaceae, Pseudomonadaceae). In addition to detecting known bacterial and fungal IN (Pseudomonas sp., Fusarium sporotrichioides), we discovered two species of IN-active fungi that were not previously known as biological ice nucleators (Isaria farinosa and Acremonium implicatum). Our findings suggest that atmospheric bioaerosols, IN, and rainfall are more tightly coupled than previously assumed.
Our observations indicate that rainfall can trigger intense bursts of bioparticle emission within the forest canopy and massive enhancements of atmospheric bioaerosol concentrations by an order of magnitude or more, from the onset of precipitation through extended periods of high surface wetness after the rainfall (up to one day). The strong contrast against low background concentrations under dry conditions suggests that the repeated bursts of bioparticle release during and after rain may play an important role in the spread and reproduction of microorganisms in certain environments (e.g., Hirst and Stedman, 1963; Fitt et al., 1989; Constantinidou et al., 1990; and Paul et al., 2004), and may also contribute to the atmospheric transmission of pathogenic and allergenic agents (Fig. S1). To quantify these effects, we suggest comprehensive metagenomic analyses and dispersion studies of atmospheric bioaerosols contrasting different meteorological conditions. Follow-up studies in other environments shall elucidate whether the observed rain-related bioaerosol in- crease is a common feature of terrestrial ecosystems or specific for the investigated semi-arid environment.
Three key results of our measurements during rain and dry periods indicate the critical and dynamic role of bioaerosols as IN sources that may strongly influence the evolution of cloud microphysics and precipitation processes: (1) large and closely correlated increases of bioparticles and IN during rain events; (2) similar size distribution patterns of rain-enhanced bioparticles and IN active in the warmest regime of mixed- phase clouds (≥-15 ◦ C); and (3) identification of IN-active bioparticles in aerosol and IN samples collected during rain events. Rainfall that triggers bioparticle emission may seed further precipitation (Bigg and Miles, 1964) by convective lifting of bioparticles into clouds where they can serve as IN, inducing cold rain formation (Hallett and Mossop, 1974; Korolev, 2007), or as GCCN, inducing warm rain formation (Möhler et al., 2007; Pöschl et al., 2010; Després et al., 2012). However, more detailed vertical transport and vertical profile information about rain-related effects will be critical to understanding what the impact of rain-initiated bioaerosol production could mean at the cloud level and for cloud formation.
Recent studies suggested that bioaerosols play crucial roles in the hydrological cycle and evolution of pristine tropical rainforest ecosystems (Prenni et al., 2009; Pöschl et al., 2010; Pöhlker et al., 2012b). The measurement results of this study suggest that bioaerosols may also play an important role in midlatitude semi-arid forest ecosystems (Schumacher et al., 2013), consistent with the recent observation that biogenic emissions significantly impact CCN in the region (Levin et al., 2012). Accordingly, deforestation and changes in land use and biodiversity might have a significant influence on the abundance of IN, the microphysics and dynamics of clouds and precipitation in these regions, and thus on regional and global climate (DeMott et al., 2010). In-cloud measurements of aerosol and hydrometeor composition, aerosol and cloud-resolving process model studies, and earth system model studies capturing potential feedbacks between the atmosphere and biosphere will be required to further quantify the relevance of these effects for climate prediction.