A GCM study of organic matter in marine aerosol and its potential contribution to cloud drop activation

Abstract

With the global aerosol-climate model ECHAM5-HAM we investigate the potential influence of organic aerosol originating from the ocean on aerosol mass and chemical composition and the droplet concentration and size of marine clouds. We present sensitivity simulations in which the uptake of organic matter in the marine aerosol is prescribed for each aerosol mode with varying organic mass and mixing state, and with a geographical distribution and seasonality similar to the oceanic emission of dimethyl sulfide. Measurements of aerosol mass and chemical composition serve to evaluate the representativity of the model initializations. Good agreement with the measurements is obtained when organic matter is added to the Aitken, accumulation and coarse modes simultaneously. Representing marine organics in the model leads to higher cloud drop number concentrations, smaller cloud drop effective radii, and a better agreement with remote sensing measurements. The mixing state of the organics and the other aerosol matter, i.e., internal or external depending on the formation process of aerosol organ ics, is an important factor for this. We estimate that globally about 75 TgCyr−1 of organic matter from marine origin enters the aerosol phase. An approximate 35% of this occurs through formation of secondary organic aerosol and 65% through emission of primary particles.