Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 1 2004 10.5194/acpd-4-1037-2004 http://www.atmos-chem-phys-discuss.net/4/1037/2004/ http://www.atmos-chem-phys-discuss.net/4/1037/2004/acpd-4-1037-2004.html http://www.atmos-chem-phys-discuss.net/4/1037/2004/acpd-4-1037-2004.pdf 1037 1062 2004-02-11 Formation of large NAT particles and denitrification in polar stratosphere: possible role of cosmic rays and effect of solar activity F. Yu yfq@asrc.cestm.albany.edu Atmospheric Sciences Research Center, State University of New York at Albany, Albany, New York, USA The formation of large nitric acid trihydrate (NAT) particles has important implications for denitrification and ozone depletion. Existing theories can't explain the recent observations of large NAT particles over wide Arctic regions at temperature above ice frost point. Our analyses reveal that high-energy comic rays may induce the freezing of supercooled HNO₃−H₂O–H₂SO₄ droplets when they penetrate these thermodynamically unstable droplets. The cosmic ray-induced freezing (CRIF) is consistent with the observed highly selective formation of NAT particles. We suggest that the physics behind the CRIF mechanism is the reorientation of polar solution molecules into the crystalline configuration in the strong electrical fields of moving secondary ions generated by passing cosmic rays. Our simulations indicate that strong solar proton events (SPEs) may significantly enhance the formation of large NAT particles and denitrification. The CRIF mechanism can explain the high correlations between the thin nitrate-rich layers in polar ice cores and major SPEs. The observed enhancement in aerosol backscattering ratio at PSC layers shortly after an SPE and the significant precipitation velocity of the enhanced PSC payers also provide strong support for the CRIF mechanism.