Formation of large NAT particles and denitrification in polar stratosphere: possible role of cosmic rays and effect of solar activity

[摘要] The formation of large nitric acid trihydrate (NAT) particles has importantimplications for denitrification and ozone depletion. Existing theories havedifficulty in explaining the formation of large NAT particles at temperaturesabove the ice frost point, which has been observed recently over wide Arcticregions. Our analyses reveal that high-energy comic ray particles mightinduce the freezing of supercooled HNO₃-H₂O-H₂SO₄droplets when they penetrate these thermodynamically unstable droplets. Thecosmic ray-induced freezing (CRIF) appears to be consistent with theobserved, highly selective formation of NAT particles. We suggest a possiblephysical process behind the CRIF mechanism: the reorientation of polarsolution molecules into the crystalline configuration in the strongelectrical fields of moving secondary ions generated by passing cosmic rays.A simple formula connecting the CRIF rate to cosmic ray flux is derived withan undefined parameter constrained by observed NAT formation rates. Oursimulations indicate that strong solar proton events (SPEs) maysignificantly enhance the formation of large NAT particles anddenitrification. The CRIF mechanism offers a possible explanation for theobserved high correlations between the thin nitrate-rich layers in polar icecores and major SPEs, and the observed enhancement in the aerosol backscatteringratio at PSC layers shortly after an SPE and the significant precipitationvelocity of the enhanced PSC layers. The key uncertainty in the CRIFmechanism is the probability (P) of freezing when a CR particle hits athermodynamically, unstable STS droplet. Further studies are needed to eitherconfirm or reject the CRIF hypothesis.

[学科分类] 大气科学