Factors influencing emission fluxes and bacterial enrichment in sea spray aerosols : Insights from laboratory and field studies
Abstract: Sea spray aerosol (SSA) is one of the major natural aerosol sources and is produced when wave breaking entrains air into ocean surface water, which subsequently breaks up into bubbles. These bubbles rise to the surface and can scavenge biogenic material. Once they reach the surface, they burst and produce both a large number of relatively small film drops that result from the disintegration of the bubble film cap and a smaller number of jet drops that result from the collapse of the bubble cavity and are typically larger in size than the film drops. The production of SSA is influenced by several factors, including wind speed, sea state, seawater temperature, salinity, and the physicochemical and biological condition of the ocean. SSA can significantly impact Earth's radiation budget by scattering incoming solar radiation directly and by acting as cloud condensation nuclei. To improve our understanding of the impact of sea spray aerosols on the Earth’s climate, it is critical to understand the physical mechanisms which determine the size-resolved SSA production flux. Furthermore, SSA can be a vector for the emission of primary biological airborne particles (PBAP) from the oceans to the atmosphere. PBAP encompass bacteria, viruses, pollen and spores and can be present in the atmosphere in form of agglomerates, single particles or cell fractions. Although, the abundance of PBAP typically only make up < 0.1% of the number of aerosols, this does not imply their insignificance. On the contrary, PBAP are known to be very efficient cloud- and ice condensation nuclei and thus can influence cloud properties such as cloud phase, albedo and lifetime, thereby affecting the Earth’s climate as well as biogeochemical cycles. As the Earth is 70% covered by oceans, of which most could be characterized as remote, quantifying the PBAP emissions over these waters are important for the enhancement of climate models.The goal of this thesis was to study the factors impacting SSA emissions and the emission of primary biological particles with SSA with particular focus on bacteria emissions. This was done both through laboratory and field experiments in the Baltic Sea and in the Azores archipelago using a plunging jet sea spray simulation chamber and various techniques to characterize aerosol emissions. More specifically, a parameterization for the SSA production flux as a function of salinity and temperature was derived from laboratory experiments and a wind speed and sea state dependent parameterization were derived from ambient eddy covariance (EC) flux measurements in the Baltic Sea. The combination of EC flux measurements and laboratory generated SSA allowed to derive a chamber specific scaling factor that could be applied to derive bacteria emission fluxes ranging between 16-63 cells m−2 s−1 from the Baltic Sea. Bacteria were found to be 13-488 and 9-148 times enriched in SSA compared to the underlying seawater from mesocosm experiments in the Baltic Sea and Azores, respectively. A comparison of single cell abundance estimates from fluorescence microscopy and real-time measurements of PBAP with diameters > 0.8 µm using a bioaerosol sensor revealed that the latter yielded consistently lower concentrations. The discrepancy was explained by differences in the sampling approach and size cut-offs (i.e. single cells versus agglomerates or particle-attached cells). As such, both methods are applicable to different research questions and should be considered complementary.An analysis of the microbial community composition in the aerosols and underlying seawater showed selective aerosolization of certain bacteria taxa. Furthermore, selective growth and a decrease in alpha diversity in the seawater was observed when the mesocosm experiments were operated in a closed mode (meaning that the seawater was not exchanged over the duration of each experiment), which can however be circumvented by continuously replacing the water in the mesocosm.Ambient measurements of PBAP revealed diurnal variations with a peak during the early morning hours that was correlated to changes wind speed, wave height, air temperature, relative humidity, latent and sensitive heat flux.
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