Global Distributions of Coarse-Mode Aerosol and Clouds during the Atmospheric Tomography Mission (ATom)

Although, coarse-mode aerosol (>1 µm diameter), composed mainly of mineral dust and sea-salt, is highly abundant over large regions of the world, these particles form a particularly poorly understood and characterized subset of atmospheric aerosol constituents.

During three ATom missions, we studied the global coarse-mode aerosol and cloud distribution between the high Arctic and the Southern Ocean close to Antarctica in the Pacific and the Atlantic through continuously profiling between ~0.2 and ~13 km.

Although coarse mode sea salt was present in both, the Atlantic and the Pacific, the measurements showed clear differences in the vertical distribution of coarse mode aerosol over the Pacific and in the Atlantic. The Pacific Ocean was characterized by enhanced coarse mode aerosol concentrations at altitudes up to 2 km. In addition, elevated coarse mode aerosol layers were occasionally found at higher altitudes. In contrast, very pronounced biomass burning layers containing significant amounts of large super-micron particles were present over large regions in the Atlantic Ocean. North of the equator, the Atlantic Ocean was characterized by the outflow of the Saharan Air Layer (SAL) which extended up to 6-7 km altitude in summer (August 2016) and up to 4-5 km in winter (February 2017). The mineral dust size distribution measured in the SAL showed a significant fraction of particles in the 20-30 µm size range and the was very similar to observations during dedicated mineral dust experiments such as the Saharan Aerosol Long-Range Transport and Aerosol Cloud Interaction Experiment (SALTRACE, 2013) or the Saharan Mineral Dust Experiment (SAMUM, 2008).

In this study, we will present global distributions of coarse-mode aerosol and clouds observed during ATom. We will explore the origins of coarse mode aerosol in the upper troposphere, discuss the contribution of coarse mode aerosol to total aerosol mass, and assess the importance of coarse mode aerosol as a source of ice nucleating particles in remote regions over the Pacific and the Atlantic.