Wind-driven spume droplet production and the transport ofPseudomonas syringaefrom aquatic environments

Abstract

Natural aquatic environments such as oceans, lakes, and rivers are home to a tremendous diversity of microorganisms. Some may cross the air-water interface within droplets and become airborne, with the potential to impact the Earth’s radiation budget, precipitation processes, and spread of disease. Larger droplets are likely to return to the water or adjacent land, but smaller droplets may be suspended in the atmosphere for transport over long distances. Here, we report on a series of controlled laboratory experiments to quantify wind-driven droplet production from a freshwater source for low wind speeds. The rate of droplet production increased quadratically with wind speed above a critical value (10-m equivalent 5.7 m/s) where droplet production initiated. Droplet diameter and ejection speeds were fit by a gamma distribution. The droplet mass flux and momentum flux increased with wind speed. Two mechanisms of droplet production, bubble bursting and fragmentation, yielded different distributions for diameter, speed, and angle. At a wind speed of about 3.5 m/s, aqueous suspensions of the ice-nucleating bacterium Pseudomonas syringae were collected at rates of 283 cells m−2 s−1 at 5 cm above the water surface, and at 14 cells m−2 s−1 at 10 cm above the water surface. At a wind speed of about 4.0 m/s, aqueous suspensions of P. syringae were collected at rates of 509 cells m−2 s−1 at 5 cm above the water surface, and at 81 cells m−2 s−1 at 10 cm above the water surface. The potential for microbial flux into the atmosphere from aquatic environments was calculated using known concentrations of bacteria in natural freshwater systems. Up to 3.1 × 104 cells m−2 s−1 of water surface were estimated to leave the water in potentially suspended droplets (diameters <100 µm). Understanding the sources and mechanisms for bacteria to aerosolize from freshwater aquatic sources may aid in designing management strategies for pathogenic bacteria, and could shed light on how bacteria are involved in mesoscale atmospheric processes.

Ice Nucleation Activity of Alpine Bioaerosol Emitted in Vicinity of a Birch Forest

“…In contrast to the aerosol samples of this study, however, the freezing onset temperature of the samples was ≥−8 • C. The aerosolization of microbes from waters occurs either through bubble bursting or fragmentation mechanisms. For fresh waters, strong winds (about 3-4 m•s −1 ) are necessary to induce the formation and breaking of waves [25]. In this study, however, the average wind speed at 10 m altitude was lower than 1.2 m•s −1 each day.…”

“…The alpine region is rich with lakes and rivers. Icenucleating microbes from these aquatic environments can be aerosolized via different mechanisms such as bubble bursting [22,23] or fragmentation [24] possibly induced by strong winds [25]. For example, Powers, et al [26] collected aerosols directly above a freshwater lake in Dublin, VA, USA, with a fixed-wing drone and detected culturable microorganisms, some of which were ice nucleation active.…”

Fragmentation versus Cohesion

“…O'Dowd & de Leeuw (2007) and Veron (2015)). If they all contribute to the global air-sea exchanges, the fate of each droplet in this distribution is not identical: the smallest aerosols are long lived in the atmosphere, carry salt and diverse chemical/biological substances (Cochran et al 2017;Pietsch et al 2018) over large distances inland while the biggest spume droplets carry momentum, heat and produce moisture by evaporation, feeding hurricanes for example, before settling by gravity.…”

Increasing Freshwater Salinity Impacts Aerosolized Bacteria

“…Freshwater salinization has both an instant impact on spray aerosol generation from breaking waves , and a more prolonged effect by altering aquatic bacterial community structure. ,, Aqueous bacterial communities can be aerosolized by the action of breaking waves and the ensuing bubble-bursting mechanism. − As they rise to the surface, entrained air bubbles can scavenge suspended bacterial cells in the bulk water mainly by interception, − with bubble-to-cell size ratio playing a major role in scavenging (collision) efficiency . Once settled at the surface, bacteria-laden surface bubbles burst, ejecting tiny “film drops” during the shattering of the bubble cap and bigger “jet drops” during the ensuing cavity collapse .…”