Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 6 2006 10.5194/acpd-6-12179-2006 http://www.atmos-chem-phys-discuss.net/6/12179/2006/ http://www.atmos-chem-phys-discuss.net/6/12179/2006/acpd-6-12179-2006.html http://www.atmos-chem-phys-discuss.net/6/12179/2006/acpd-6-12179-2006.pdf 12179 12197 2006-11-28 Is aerosol formation in cirrus clouds possible? J. Kazil E. R. Lovejoy E. J. Jensen D. R. Hanson NOAA Earth System Research Laboratory, Boulder, CO, USA NASA Ames Research Center, Moffett Field, CA, USA Augsburg College, Minneapolis, MN, USA The recent observation of ultrafine aerosol particles in cirrus clouds has raised the question whether aerosol formation within cirrus clouds is possible, and if so, what mechanisms are involved. We have developed an aerosol parcel model of neutral and charged H₂SO₄/H₂O aerosol processes, including nucleation from the gas phase and loss onto cirrus ice particles. Laboratory thermodynamic data for sulfuric acid uptake and loss by small neutral and charged clusters are used, allowing for a reliable description of both neutral and charged nucleation down to the very low temperatures occurring in the upper troposphere and lower stratosphere. The model implements a first order scheme for resolving the aerosol size distribution within its geometric size sections, which efficiently suppresses numerical diffusion. We operate the model offline on trajectories generated with a detailed 1-D cirrus model which describes ice crystal nucleation, deposition growth, vertical advection of ice crystals and water vapor, and ice crystal sedimentation. In this paper we explore the possibility of aerosol formation within non-convective cirrus clouds and draw conclusions for aerosol formation in anvil cirrus. We find that sulfate aerosol formation within cirrus clouds can proceed even at high ice surface area concentrations, and depends strongly on the size of the cirrus ice crystals and on the surface area concentration of preexisting aerosol particles.