A global climatology (2005 – 2020) of sea salt aerosols using MODIS and OMI satellite data

 

Sea salt (SS) are natural strongly scattering coarse aerosols, which yield the largest fraction of aerosol burden over many places on the Earth. They are important to the physics and chemistry of the marine atmosphere, affecting visibility, remote sensing, atmospheric chemistry, and air quality. The production, entrainment, transport and removal of SS aerosol are affected by several meteorological and environmental factors, such as wind speed, surface ocean and air temperature, relative humidity, atmospheric stability, precipitation and sea bottom depth and topography. The key meteorological factor that governs the SS production and life cycle is wind, which causes waves to break, forming whitecaps, thus influencing the injection of SS to upper atmospheric levels and their horizontal transport. Although, most of SS aerosols can be transported with atmospheric circulation only to short distances from their sources, the relatively smaller bubbles can live for a longer time in the atmosphere and thus can be transported not only over oceanic, but also over adjacent continental areas. Sea salt aerosols are highly hygroscopic, adsorbing water, and thus behave as Cloud Condensation Nuclei (CCN), affecting the formation, physical and optical properties of clouds. Therefore, their quantification and spatiotemporal variability is essential for the accurate determination of their climatic ole.

In the present study, SS aerosols are detected on a global scale and for the 16-year period from 2005 to 2020, using a satellite algorithm, which is based on aerosol optical properties. This algorithm uses as input daily spectral Aerosol Optical Depth (AOD) and Aerosol Index (AI) or single scattering albedo (SSA) data from MODIS C6.1 and OMI OMAERUV databases, respectively.  It operates on a daily basis and 1°×1° pixel level and detects the presence of SS aerosols by applying suitable thresholds on Ångström Exponent (AE) (calculated using spectral AOD from MODIS) and AI or SSA. The algorithm outputs the absolute and relative frequency of occurrence of SS aerosols, as well as the associated AOD, on a monthly and annual basis. The results are given on a pixel as well as on regional and global scales. By running the algorithm for each year of the study period, the climatological mean values and the interannual variability and trends of the frequency of occurrence and AOD of SS aerosols are estimated.