Global distribution of the effective aerosol hygroscopicity parameter for CCN activation 1Max Planck Institute for Chemistry, Mainz, Germany
2The Cyprus Institute, Energy, Environment and Water Research Centre, Nicosia, Cyprus
Received: 29 January 2010 – Published in Atmos. Chem. Phys. Discuss.: 08 March 2010 Abstract. In this study we use the ECHAM/MESSy Atmospheric Chemistry (EMAC)
model to simulate global fields of the effective hygroscopicity
parameter κ which approximately describes the influence of
chemical composition on the cloud condensation nucleus (CCN)
activity of aerosol particles. The obtained global mean values of
κ 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 κ
calculated across the Aitken and accumulation modes. The mean κ
values are in good agreement
with previous estimates based on observational data, but the model
standard deviation for continental regions is higher.
Revised: 01 June 2010 – Accepted: 09 June 2010 – Published: 15 June 2010
Over the continents, the regional distribution appears fairly
uniform, with κ 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 κ 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 κ 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 κ 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 κ 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 κ 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 κ values by a factor of
2–3 above the initial value of ≈0.005.
Citation: Pringle, K. J., Tost, H., Pozzer, A., Pöschl, U., and Lelieveld, J.: Global distribution of the effective aerosol hygroscopicity parameter for CCN activation, Atmos. Chem. Phys., 10, 5241-5255, doi:10.5194/acp-10-5241-2010, 2010.