Sea Spray Fluxes from the Southwest Coast of the United Kingdom – Dependence on Wind Speed and Wave Height

Abstract

Fluxes of sea spray aerosols were measured with the eddy covariance technique from the Penlee Point Atmospheric 10 Observatory (PPAO) on the southwest coast of the United Kingdom over several months from 2015 to 2017. Two different fastresponding aerosol instruments were employed: an ultra-fine condensation particle counter (CPC) that detects aerosols with radius above ca. 1.5 nm, and a compact lightweight aerosol spectrometer probe (CLASP) that provides a size distribution between ca. 0.1 and 6 µm. The aerosol fluxes were almost always from sea to air, indicating sea spray emission. Fluxes from the CPC and from the CLASP (integrated over all sizes) were generally comparable, implying a reasonable closure in the aerosol 15 number flux. Compared to most previous observations over the open ocean, at the same wind speed the mean sea spray number fluxes at PPAO are much greater. Significant wave height and wave Reynolds number explain more variability in sea spray fluxes than does wind speed, implying that enhanced wave breaking resulting from shoaling in shallow coastal waters is a dominant control on sea spray emission. Comparisons between two different wind sectors (open water vs. fetch-limited Plymouth Sound) and between two sets of sea states (growing vs. falling seas) further confirm the importance of wave 20 characteristics on sea spray fluxes. These results suggest that spatial variability in wave characteristics need to be taken into account in predictions of coastal sea spray productions and also aerosol loading.for heterogeneous reactions (Sievering et al. 1991;Kim et al. 2014). In coastal regions where roughly half of the world's population resides, sea spray constitutes a major component of particulate matter in the marine atmospheric boundary layer. 30Particulate matter is directly relevant for human health and is subject to air quality regulations.Breaking waves entrain air into water, resulting in plumes of bubbles within the top meters of the ocean and the appearance of whitecaps (Thorpe, 1992). Sea spray aerosols are primarily formed from the bursting of those bubbles. Two modes of sea spray aerosols are commonly observed: the film drop mode (predominantly submicron) arising from disintegration of relatively large bubbles, and the jet drop mode (≥500 nm to a few microns in radius) from relatively small bubbles (e.g.35 Blanchard, 1963). Tearing of the wave crest under conditions of very high winds can also lead to large spume droplets (tens of microns to millimeters; Monahan et al., 1983), which may contribute significantly to the total sea spray mass flux but not to the number flux. The causal relationship between bubbles and sea spray has led to the application of the "whitecap" method for the estimation of sea spray flux over the ocean: in which the spray production flux from unit area of a foam surface is prescribed based on other measurements and scaled up by the whitecap fraction (e.g. Monahan et al. 1986; Martensson et al. 2003; Clarke et 40 al. 2006). However, the parameterization of whit...

Ocean bubbles under high wind conditions – Part 1: Bubble distribution and development

“…The wind-wave Reynolds number R Hw = u * H s /υ W is a dimensionless number introduced by Zhao and Toba (2001) which combines a measure of the sea state (significant wave height H s ) with friction velocity (u * ) and water kinematic viscosity (v W ). It has been successfully used to parameterise CO 2 gas transfer (Brumer et al, 2017a) and is better than wind speed alone at explaining the variability of sea-spray aerosol flux (Norris et al, 2013;Yang et al, 2019). Here we directly evaluate the relationship between R Hw and deeper bubble plumes and investigate whether knowledge of surface processes alone is sufficient to predict subsurface bubble behaviour.…”

Oceanic Supply of Ice‐Nucleating Particles and Its Effect on Ice Cloud Formation: A Case Study in the Arctic Ocean During a Cold‐Air Outbreak in Early Winter

“…The trend of increasing cyclonic activity over the ice‐free Arctic Ocean is one potential reason for the observed trend in (Waseda et al., 2021). In comparison with wind speed and , the wave Reynolds number is more indicative of sea spray emissions (Yang et al., 2019). Thus, an additional supply of sea spray aerosols would be expected during autumn and early winter.…”