Sea spray

Sea spray refers to aerosol particles that are formed directly from the ocean, mostly by ejection into the atmosphere by bursting bubbles at the air-sea interface. Sea spray contains both organic matter and inorganic salts that form sea salt aerosol (SSA). SSA has the ability to form cloud condensation nuclei (CCN) and remove anthropogenic aerosol pollutants from the atmosphere. Sea spray is directly (and indirectly, through SSA) responsible for a significant degree of the heat and moisture fluxes between the atmosphere and the ocean, affecting global climate patterns and tropical storm intensity. Sea spray also influences plant growth and species distribution in coastal ecosystems and increases corrosion of building materials in coastal areas. Sea spray refers to aerosol particles that are formed directly from the ocean, mostly by ejection into the atmosphere by bursting bubbles at the air-sea interface. Sea spray contains both organic matter and inorganic salts that form sea salt aerosol (SSA). SSA has the ability to form cloud condensation nuclei (CCN) and remove anthropogenic aerosol pollutants from the atmosphere. Sea spray is directly (and indirectly, through SSA) responsible for a significant degree of the heat and moisture fluxes between the atmosphere and the ocean, affecting global climate patterns and tropical storm intensity. Sea spray also influences plant growth and species distribution in coastal ecosystems and increases corrosion of building materials in coastal areas. When wind, whitecaps, and breaking waves mix air into the sea surface, the air regroups to form bubbles, floats to the surface, and bursts at the air-sea interface. When they burst, they release up to a thousand particles of sea spray, which range in size from nanometers to micrometers and can be expelled up to 20 cm from the sea surface. Film droplets make up the majority of the smaller particles created by the initial burst, while jet droplets are generated by a collapse of the bubble cavity and are ejected from the sea surface in the form of a vertical jet. In windy conditions, water droplets are mechanically torn off from crests of breaking waves. Sea spray droplets generated via such a mechanism are called spume droplets and are typically larger in size and have less residence time in air. Impingement of plunging waves on sea surface also generates sea spray in the form of splash droplets . The composition of the sea spray depends primarily on the composition of the water from which it is produced, but broadly speaking is a mixture of salts and organic matter. Several factors determine the production flux of sea spray, especially wind speed, swell height, swell period, humidity, and temperature differential between the atmosphere and the surface water. Production and size distribution rate of SSAs are thus sensitive to the mixing state. A lesser studied area of sea spray generation is the formation of sea spray as a result of rain drop impact on the sea surface . In addition to the local conditions that influence sea spray formation, there are also consistent spatial patterns in sea spray production and composition. Because sea spray is generated when air is mixed into the ocean, formation gradients are established by turbulence of the surface water. Wave action along coastal shorelines is generally where turbulence is greatest, so this is where sea spray production is the highest. Particles generated in turbulent coastal areas can travel horizontally up to 25 km within the planetary boundary layer. As distance from shore decreases, sea spray production declines to a level sustained almost exclusively by white caps. The proportion of the ocean surface area that is turbulent enough to produce significant sea spray is called the white cap fraction. The only other production mechanism of sea spray in the open ocean is through direct wind action, where strong winds actually break the surface tension of the water and lift particles into the air. However, particles of seawater generated in this way are often too heavy to remain suspended in the atmosphere and usually are deposited back to the sea within a few dozen meters of transport. During winter months, the ocean typically experiences stormy, windy conditions that generate more air inundation into the sea and therefore more sea spray. Calmer summer months result in lower overall production of sea spray. During peak primary productivity in the summer, increased organic matter in the surface ocean drives subsequent increases in sea spray. Given that sea spray retains the properties of the water from which it was produced, the composition of sea spray experiences extreme seasonal variation. During the summer, dissolved organic carbon (DOC) can constitute 60-90% of sea spray mass. Even though much more sea spray is produced during the stormy winter season, the composition is nearly all salt because of the low primary production. The organic matter in sea spray consists of dissolved organic carbon (DOC) and even microbes themselves, like bacteria and viruses. The amount of organic matter in sea spray depends on microbiological processes, though the total effect of these processes is still unknown. Chlorophyll-a is often used as a proxy for primary production and organic matter content in sea spray, but its reliability for estimating DOC concentrations is controversial. Biomass often enters sea spray through the death and lysis of algal cells, often caused by viral infections. Cells are broken apart into DOC that is propelled into the atmosphere when surface bubbles pop. When primary productivity peaks during the summer, algal blooms can generate an enormous amount of organic matter that is eventually incorporated into sea spray. In the right conditions, aggregation of the DOC can also form surfactant or sea foam. At high winds the droplet evaporation layer (DEL) influences the surface energy heat exchange of the ocean. The latent heat flux of sea spray generated at the DEL has been cited as an important addition to climate modeling efforts, particularly in simulations assessing air/sea heat balance as related to hurricanes and cyclones formed during high wind events. During the formation of whitecaps, sea spray droplets exhibit the same properties as the ocean surface, but rapidly adapt to surrounding air. Some sea spray droplets immediately reabsorb into the sea while others evaporate entirely and contribute salt particles like dimethyl sulfide (DMS) to the atmosphere where they can be transported via turbulence to cloud layers and serve as CCN. The formation of these CCN like DMS have climate implications as well, due to their influence on cloud formation and interaction with solar radiation. Additionally, the contribution of sea spray DMS to the atmosphere is linked to the global sulfur cycle. Understanding total forcing from natural sources like sea spray can illuminate critical constraints posed by anthropogenic influence and can be coupled with ocean chemistry, biology and physics to predict future ocean and atmospheric variability. The proportion of organic matter in sea spray can impact reflectance, determine the overall cooling effect of SSAs, and slightly alter the capacity for SSAs to form CCN (17). Even small changes in SSA levels can affect the global radiation budget leading to implications for global climate. SSA has a low albedo, but its presence overlaid on the darker ocean surface affects absorption and reflectance of incoming solar radiation. The influence of sea spray on the surface heat and moisture exchange peaks during times of greatest difference between air and sea temperatures. When air temperature is low, sea spray sensible heat flux can be nearly as great as the spray latent heat flux at high latitudes. In addition, sea spray enhances the air/sea enthalpy flux during high winds as a result of temperature and humidity redistribution in the marine boundary layer. Sea spray droplets injected into the air thermally equilibrate ~1% of their mass. This leads to the addition of sensible heat prior to ocean reentry, enhancing their potential for significant enthalpy input. The effects of sea spray transport in the atmospheric boundary layer is not yet completely understood . Sea spray droplets alter the air-sea momentum fluxes by being accelerated and decelerated by the winds . In hurricane-force winds, it is observed that there is some reduction in the air/sea momentum flux . This reduction in momentum flux manifests as saturation of air/sea drag coefficient. Some studies have identified spray effects as one of the potential reasons for the air/sea drag coefficient saturation. It has been shown through several numerical and theoretical studies that sea spray, if present in significant amounts in the atmospheric boundary layer, leads to saturation of air-sea drag coefficients.