Simulated Chemical Interactions and Air-sea Fluxes Associated with Spume Droplets Under High Wind Conditions

Marine aerosols represent an important multiphase chemical reaction medium that influences the phase partitioning and lifetimes of soluble gases as well as the chemical and physical evolution of marine air. Under certain conditions, large seawater droplets produced at the sea surface under high wind conditions, known as spume droplets, dominate the mass and volume of liquid marine aerosol in the atmospheric mixed layer. These droplets significantly influence air-sea transfer of energy and mass but their role in the chemistry of the marine atmosphere and the associated air-sea exchange of gases such as nitrogen oxides, CO₂, and atmospheric sulfur is very uncertain. The reactivity of these droplets is dependent on their composition and that of the overlying atmosphere, as well as their size and lifetime against deposition. Reliable simulation requires a comprehensive chemical scheme that accounts for gas and aqueous reactions, mass transfer, and liquid diffusion at a fine timescale. For this project, we modified the Max Planck Institute for Chemistry's atmospheric chemistry boxmodel CAABA (Chemistry as a Boxmodel Application; version 3.0) for three sets of numerical experiments: (1) Using CAABA in Lagrangian trajectory mode, chemical processing of individual droplets of varying lifetimes and sizes was modeled as they fell through the background atmosphere in a wave trough as individual particles. (2) Using CAABA as a standard boxmodel, a high-wind marine aerosol population and background atmospheric composition representative of the eastern North Atlantic was used to investigate the chemical reactivity and air-sea exchange of gases through large droplets within a wave trough. (3) CAABA was initialized as in (2), but for the entire marine boundary layer. For the first experiment, the spume droplets were generated on the front face of breaking wave crests. The particles were subsequently transported in the turbulent, stratified atmospheric boundary layer using a Stochastic Lagrangian approach to model the drop trajectory while concurrently solving for the microphysical temperature and evaporation/condensation of the drops. To account for the liquid-side diffusion limitation for the chemistry solution involving large short-lived droplets, CAABA was modified to account for liquid diffusion and mixing due to Stokes' flow in particles with a radius larger than the diffusion-rate and timestep- or lifetime- (whichever was shorter) dependent diffusion length. Re-emission of deposited gases from the ocean back into the atmosphere was not considered. Results indicate a strong sensitivity of chemistry and air-sea exchange to the physics of spume droplets, in particular, to liquid diffusion. As with smaller, longer lived marine aerosols, the composition (in particular, the acidity) of the atmosphere determines multiphase processing of many gases, though the large droplets do not reach chemical equilibrium. Evaporation and associated latent cooling of spume droplets showed a non-negligible impact on gas-aerosol exchange processes although in a cleaner background atmosphere, these processes become more important. The significance of spume droplets in the uptake or emission and aqueous air-sea flux of CO₂, other inorganic chemical species of anthropogenic origin, and reactive intermediates will be presented.012-->