Global-Scale Measurements of CCN-Sized Particles in the Remote Marine Boundary Layer: Results from the Atmospheric Tomography Mission

Clouds in the remote marine lower troposphere are important components of the climate system. In this region, cloud condensation nuclei (CCN) are limited in abundance, and the radiative properties of low-altitude clouds are particularly sensitive to changes in CCN concentrations. CCN in the marine boundary layer (MBL) originate from sea-spray generation, from the condensation of sulfur and organic species onto new particles formed from gas-to-particle nucleation, and from transport from continental sources.

We report results from airborne measurements of particle size distributions, volatility, and composition made from ~80 ^oN to ~65 ^oS over the central Pacific and Atlantic Oceans during the Atmospheric Tomography (ATom) mission. During ATom, nearly continuous profiles were made from ~150 m to ~12 km altitude. The resulting dataset provides short periods of measurements within the remote MBL spaced roughly every 5 degrees of latitude in each ocean basin, with one set of measurements in August 2016 and the second in February 2017. A non-volatile sea-spray mode dominated aerosol mass, including submicron mass, over most of the ocean regions. In contrast, particles with diameters <0.4 µm usually dominated the number concentration of CCN-sized particles. These particles were volatile at 300 ^oC and were composed primarily of sulfate species in the Pacific. In the Northern Hemisphere, especially in the North Atlantic, organics contributed significantly this volatile CCN mode. Biomass burning was a significant source for this organic material. Taken together, these observations emphasize that it is essential to account for new particle formation in the free troposphere and growth of secondary aerosol species to accurately simulate the concentrations of CCN in the MBL over large areas of the world's oceans.