Large Contribution of Shoreline Wave Breaking to Coastal Cloud Condensation Nuclei Population

SHENGQIAN ZHOU, Xianda Gong, Eduardo de Azevedo, Francisco Reis, Matthew Salter, Timothy Bertram, Jian Wang, Washington University in St. Louis

     Abstract Number: 129
     Working Group: Remote and Regional Atmospheric Aerosol

Abstract
Sea spray aerosol (SSA) emission is one of the largest sources of global aerosol mass budget, but its contribution to cloud condensation nuclei (CCN) concentration is still under debate. For example, a recent study suggests that the SSA contribution to CCN concentration likely has been underestimated. Long-term measurements on islands or in coastal regions are often used to study SSA. The wave breaking near the shoreline, which is distinct from the open-ocean processes, may generate a large flux of SSA. However, the impact of such flux on the observed aerosol properties, especially the concentration and hygroscopicity of fine particles, has rarely been investigated. Here we show enhanced aerosol number and CCN concentrations with abnormally strong fluctuations under the conditions of high waves and onshore wind. The enhanced concentrations and strong fluctuations are due to SSA emission by incoming waves lapping the shoreline and/or swell wave breaking in the surf zone. While wind speed is the key parameter determining the SSA flux in open oceans, we found that the SSA production at shoreline primarily depends on the wave height, and high wave often exists under the conditions of low wind speed. Strong SSA production by shoreline wave breaking occurs ubiquitously and frequently in both rocky and beach shorelines, making a substantial contribution to the CCN population in nearby downwind areas. Using one-year measurements on Graciosa Island in Eastern North Atlantic, we show almost all events when ambient fine aerosols exhibit sea-salt-like hygroscopicity can be explained by the shoreline SSA generation. Therefore, extrapolating the results based on coastal measurements to open oceans without taking into account the shoreline effects may significantly overestimate the contribution of SSA to CCN concentration, and by extension, the impact of SSA on clouds and climate.