Stratosphere-troposphere exchange: Chemical sensitivity to mixing

We present examples of how chemical evolution can exhibit sensitivity to mi xing arising from stratosphere-troposphere exchange. A chemical transport m odel is used to survey the chemical contrasts on isentropic surfaces that i ntersect the tropopause. Significant cross-tropopause gradients in both ozo ne and water vapor are shown to exist between 300 and 340 K, Back trajector ies are used to confirm that air parcels with widely varying chemical prope rties are rapidly brought together in a typical quasi-isentropic stratosphe re-troposphere exchange event. A two-box model is used to investigate the c hemical evolution of stratospheric and tropospheric air parcels and to dete rmine the effect of mixing between them. Mixing of stratospheric ozone and tropospheric water vapor is shown to lead to enhanced hydroxyl (OH) radical concentrations compared with background tropospheric and stratospheric val ues. The oxidation of CO, methane, and higher hydrocarbons is corresponding ly increased, and NOx is also lost more rapidly with faster mixing. Also, i n low NOx conditions, the rate of O-3 loss is found to increase with faster mixing, The consequences of this anomalous chemistry for chemical transpor t in stratosphere-troposphere exchange events are discussed. It is also not ed that if NOx levels in either parcel are very high initially, mixing of N Ox can become more important than O-3 or H2O in determining OH levels.