Replies to referee comments

Abstract

Atmospheric aerosols can act as ice nucleating particles (INPs) and thereby influence the formation and the microphysical properties of cirrus clouds resulting in distinct climate modifications. From laboratory experiments several types of aerosol particles have been identified as effective INPs at cirrus conditions. However, the global atmospheric distribution of INPs in the cirrus regime is still highly uncertain as in situ observations are scarce and limited in space and time. To study the influence of INPs on cirrus clouds and climate on the global scale these particles have been simulated with global chemistry-climate models. Typically, mineral dust and soot particles, which are known to initiate ice nucleation in cirrus clouds, have been considered in these models. In addition, laboratory studies suggest crystalline ammonium sulfate and glassy organic particles as effective INPs in the cirrus regime. However, the representation of these particles in global models is challenging as their phase state, i.e. crystalline or glassy, needs to be simulated. In turn, crystalline ammonium sulfate and glassy organics have only rarely been considered in global model studies and their impact on the global scale is still uncertain. Here, we present and analyse a global climatology of INPs derived from global model simulations performed with the ECHAM/MESSy Atmospheric Chemistry (EMAC) general circulation model including the aerosol microphysics submodel MADE3 (Modal Aerosol Dynamics model for Europe, adapted for global applications, third generation) coupled to a two-moment cloud microphysical scheme and a parametrization for aerosol-induced ice formation in cirrus clouds. This global INP-climatology comprises mineral dust and soot particles, as well as crystalline ammonium sulfate and glassy organics, including a simplified formulation of the particle phase state for the latter. By coupling the different INP-types to the microphysical cirrus cloud scheme, their ice nucleation potential at cirrus conditions is analysed, considering possible competition mechanisms between different INPs. The simulated INP concentrations in the range of about 1 to 100 L−1 agree well with in situ observations and other global model studies. We show that INP concentrations of glassy organics and crystalline ammonium sulfate are strongly related to the ambient conditions which often inhibit the glassy or crystalline phase, respectively. Our model results suggest that glassy organic particles probably have only minor influence, as typical INP concentrations are mostly low in the cirrus regime. On the other hand, crystalline ammonium sulfate often shows large INP concentrations, has the potential to influence ice nucleation in cirrus clouds, and should be taken into account in future model applications.