Global model studies on the distribution and composition of potential atmospheric ice nuclei

Kurzfassung

Black carbon (BC) and mineral dust particles are among the most important atmospheric aerosol types forming ice crystals by heterogeneous nucleation, the so called potential ice nuclei (PIN). When emitted, most BC and dust particles are externally mixed with other aerosol compounds. Through coagulation with other particles and condensation of gases, externally mixed particles gain a liquid coating and are, therefore, transferred to an internal mixture. This ageing process is essential for the direct and indirect effect of BC and dust particles on the climate, since the coating changes their radiative and hygroscopic properties and consequently their cloud activation ability and lifetime. Moreover, laboratory studies have shown that a liquid coating influences the freezing properties of the particles and hence their behavior as ice nuclei. Due to large computational resources required, global climate models mostly parameterize the particle ageing by using estimated turnover times rather than simulating the ageing processes explicitly. In the present study the population of PIN in the global upper troposphere and lowermost stratosphere (UTLS) is characterized. To reach this goal the new aerosol model ECHAM5/MESSy-MADEsoot is developed. The aerosol module MADEsoot is able to simulate separately BC and dust particles in their different states of mixing (internally or externally mixed) and BC and dust free aerosols, as well as the relevant ageing processes of externally mixed particles. MADEsoot is implemented in the global climate model ECHAM5/MESSY. The resulting system is evaluated with aircraft and surface measurements and performs well both in the boundary layer and in the UTLS. ECHAM5/MESSy-MADEsoot (E5/M-MADEsoot) is the only existing model able to resolve the mixing state of BC and dust particles while giving a reliable representation of the UTLS. E5/M-MADEsoot is applied to characterize the PIN through their number and mass concentration, composition and mixing state. The results of this study show that PIN contribute only up to 0.7% to the total aerosol number concentration in the UTLS. At surface level PIN contribute between 10% and 50% to the total aerosol number concentration, where the highest values are reached over the major emission areas. Nearly all PIN in the UTLS are internally mixed with soluble material, while only up to the 3% of PIN is externally mixed. E5/M-MADEsoot allows also for the investigation of the ageing process of BC and dust particles and for the determination of its timescale, showing that the ageing process is mainly driven by the condensation of vapor. The timescale of the ageing process is calculated with E5/M-MADEsoot to be around some hours at surface level and some days in the UTLS, but shows a high geographical variability, especially in the boundary layer and in the lower troposphere. The timescale of the ageing process shows also a less pronounced seasonality, with higher values in winter and lower in summer.