Lake spray aerosol generation: A method for producing representative particles from freshwater wave breaking

Abstract. Wave breaking action in bodies of freshwater produces atmospheric aerosols via a similar mechanism to sea spray aerosol (SSA) from seawater. The term lake spray aerosol (LSA) is proposed to describe particles formed by this mechanism, which have been observed over the Laurentian Great Lakes. Though LSA has been identified from size distribution measurements during a single measurement campaign, no measurements of LSA composition or relationship to bubble bursting dynamics have been conducted. A LSA generator utilizing a plunging jet, similar to many SSA generators, was constructed for the generation of aerosol from freshwater samples and model salt solutions. To evaluate this new generator, bubble and aerosol number size distributions were measured for salt solutions representative of freshwater (CaCO3) and seawater (NaCl) at concentrations ranging from that of freshwater to seawater (0.05–35 g L−1), synthetic seawater (inorganic), synthetic freshwater (inorganic), and a freshwater sample from Lake Michigan. Following validation of the bubble and aerosol size distributions using synthetic seawater, a range of salt concentrations was investigated. Decreasing salt concentrations from seawater to freshwater led to greater bubble coalescence and formation of larger bubbles, which generated larger particles and lower aerosol number concentrations. The systematic studies of the model salts, synthetic freshwater, and Lake Michigan sample indicate that LSA is characterized by a larger bubble size distribution, compared to seawater, with a peak near 300 μm. This resulted in a bimodal aerosol size distribution with a primary mode (180 ± 20 nm) larger than that of SSA, and a secondary mode (46 ± 6 nm) smaller than that of SSA. This new method for studying LSA under isolated conditions is needed as models, at present, utilize SSA parametrizations for freshwater systems, which are not accurate for predicting climate properties of the different size distributions observed for LSA. Given the abundance of freshwater globally, this potentially important source of aerosol needs to be thoroughly characterized, as the sizes produced are relevant to light scattering, cloud condensation nuclei (CCN), and ice nuclei (IN) concentrations over the bodies of freshwater.