Global distribution of the effective aerosol hygroscopicity parameter for CCN activation

In this study we use the ECHAM/MESSy Atmospheric Chemistry (EMAC) model to simulate global fields of the effective hygroscopicity parameter kappa which approximately describes the influence of chemical composition on the cloud condensation nucleus (CCN) activity of aerosol particles. The obtained global mean values of kappa at the Earth's surface are 0.27 +/- 0.21 for continental and 0.72 +/- 0.24 for marine regions (arithmetic mean +/- standard deviation). These values are the internally mixed kappa calculated across the Aitken and accumulation modes. The mean kappa values are in good agreement with previous estimates based on observational data, but the model standard deviation for continental regions is higher. Over the continents, the regional distribution appears fairly uniform, with kappa values mostly in the range of 0.1-0.4. Lower values over large arid regions and regions of high organic loading lead to reduced continental average values for Africa and South America (0.15-0.17) compared to the other continents (0.21-0.36). Marine regions show greater variability with kappa values ranging from 0.9-1.0 in remote regions to 0.4-0.6 in continental outflow regions where the highly hygroscopic sea spray aerosol mixes with less hygroscopic continental aerosol. Marine kappa values as low as 0.2-0.3 are simulated in the outflow from the Sahara desert. At the top of the planetary boundary layer the kappa values can deviate substantially from those at the surface (up to 30%) - especially in marine and coastal regions. In moving from the surface to the height of the planetary boundary layer, the global average marine kappa value reduces by 20%. Thus, surface observations may not always be representative for the altitudes where cloud formation mostly occurs. In a pre-industrial model scenario, the kappa values tend to be higher over marine regions and lower over the continents, because the anthropogenic particulate matter is on average less hygroscopic than sea-spray but more hygroscopic than the natural continental background aerosol (dust and organic matter). In regions influenced by desert dust the particle hygroscopicity has increased strongly as the mixing of air pollutants with mineral particles typically enhances the kappa values by a factor of 2-3 above the initial value of approximate to 0.005.