Sea Spray

Sea spray aerosol is produced by bubbles bursting at the air sea interface. Although it is a common type of aerosol over the oceans and is important for understanding clouds, it's production rate remains uncertain. The physical processes associated with the generation of sea spray are summarized in the Figure below. Breaking waves entrain air, which results in bubbles bursting at the air-sea interface, lofting jet and film drops into the atmosphere, which form sea spray aerosol.

Figure 1. Processes associated with generating sea spray. Source: Wikimedia Commons.

Sea spray particle production depends strongly on the wind speed. However, the details of the process are complex, as sea spray production rates are also modulated by temperature, salinity, organic components in the sea water, and the presence or absence of the sea surface microlayer.

During CAESAR we have the opportunity to sample aerosol close to the sea surface and during high wind speeds associated with cold air outbreaks.

Figure 2. Picture of the sea state sampling inside the boundary layer during CAESAR at a wind speed of 17 m/s. The white areas are regions of wave breaking and sea spray production.

Using a combination of three aerosol size spectrometers we are able to characterize and constrain the aerosol size distribution from 10 to 4000 nm. On one of the flights we sampled aerosol at an altitude of 750 m altitude for a period of 10 min at wind speed exceeding 20 m/s. Total aerosol number concentration varied between 200 and 400 cm⁻³ and were positively correlated with wind speed. The size distribution shows three main modes, one at 20 nm, one at 150 nm, and one at 2000 nm.

Figure 3. Measured aircraft altitude, wind speed, and total aerosol number concentration (left plots, top-to-bottom). The size distribution (right) is measured by a scanning mobility particle sizer (SMPS), an Ultra-High Sensitivity Aerosol Spectrometer (UHSAS), and a Passive Cavity Aerosol Spectrometer Probe (PCASP).

Samples at high wind speed and over the remote ocean are rare and thus critical for understanding the sea spray source function and how sea spray controls the boundary layer aerosol budget. Analyzing observations from the size spectrometers in more detail and across multiple research flights will help us quantify the contribution of sea spray production to the boundary layer particle burden and the influence of these particles on clouds over this region.