An examination of chemistry and transport processes in the tropical lower stratosphere using observations of long-lived and short-lived compounds obtained during STRAT and POLARIS

A suite of compounds with a wide range of photochemical lifetimes (3 months to several decades) was measured in the tropical and midlatitude upper tro posphere and lower stratosphere during the Stratospheric Tracers of Atmosph eric Transport (STRAT) experiment (fall 1995 and winter, summer, and fall 1 996) and the Photochemistry of Ozone Loss in the Arctic Region in Summer (P OLARIS) deployment in late summer 1997. These species include various chlor ofluorocarbons, hydrocarbons, halocarbons, and halons measured in whole air samples and CO measured in situ by tunable diode laser spectroscopy. Mixin g ratio profiles of long-lived species in the tropical lower stratosphere a re examined using a one-dimensional (1-D) photochemical model that includes entrainment from the extratropical stratosphere and is constrained by meas ured concentrations of OH. Profiles of tracers found using the 1-D model ag ree well with all the observed tropical profiles for an entrainment time sc ale of 85(-4)(+6) months, independent of altitude between potential tempera tures of 370 and 500 K. The tropical profile of CO is used to show that the annually averaged ascent rate profile, on the basis of a set of radiative heating calculations, is accurate to approximately +/-44%, a smaller uncert ainty than found by considering the uncertainties in the radiative model an d its inputs. Tropical profiles of ethane and C2Cl4 reveal that the concent ration of C1 is higher than expected on the basis of photochemical model si mulations using standard gas phase kinetics and established relationships b etween total inorganic chlorine and CFC-11. Our observations suggest that s hort-lived organic chlorinated compounds and HCl carried across the tropica l tropopause may provide an important source of inorganic chlorine to the t ropical lower stratosphere that has been largely unappreciated in previous studies. The entrainment timescale found here is considerably less than the value found by a similar study that focused on observations obtained in th e lower stratosphere during 1994. Several possible explanations for this di fference are discussed.