Abstract
A suite of compounds with a wide range of photochemical lifetimes (3 months to several decades) was measured in the tropical and midlatitude upper troposphere and lower stratosphere during the Stratospheric Tracers of Atmospheric Transport (STRAT) experiment (fall 1995 and winter, summer, and fall 1996) and the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) deployment in late summer 1997. These species include various chlorofluorocarbons, hydrocarbons, halocarbons, and halons measured in whole air samples and CO measured in situ by tunable diode laser spectroscopy. Mixing ratio profiles of long‐lived species in the tropical lower stratosphere are examined using a one‐dimensional (1‐D) photochemical model that includes entrainment from the extratropical stratosphere and is constrained by measured concentrations of OH. Profiles of tracers found using the 1‐D model agree well with all the observed tropical profiles for an entrainment time scale of 8.5−4+6 months, independent of altitude between potential temperatures 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 uncertainty than found by considering the uncertainties in the radiative model and its inputs. Tropical profiles of ethane and C2Cl4 reveal that the concentration of Cl is higher than expected on the basis of photochemical model simulations using standard gas phase kinetics and established relationships between total inorganic chlorine and CFC‐11. Our observations suggest that short‐lived organic chlorinated compounds and HCl carried across the tropical tropopause may provide an important source of inorganic chlorine to the tropical 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 the lower stratosphere during 1994. Several possible explanations for this difference are discussed.
Highlights
We havesimulatedthe observedvmr profilesof a suiteof 12 temperaturelevel using 24-hour average photolysisrates and concentratioonf Os lD andC1resultinfgromtheaveragoef full diurnal runs of the PSS model for tropical solar conditionsof each season.the growth rate term in (1), YZ, was short- and long-lived tracers:CO, ethane, C2C14,CH3Br, the calculatedfrom global mean tropospherictrends during the halonsH-1211 and H-2402, CH3C1,CFC-11, CFC-12, CFC-113, period 1994-1997 [Alternative Fluorocarbons Environmental
In thelowermostropicalstratospherbey nearlya factorof three
Reaction).This multiplicativefactoris unlikely to be realisticfor the higher altitudes (e.g., 0>460 K) because theory and observationof OH are in much better agreementand the fractionaluncertaintyin Cl:, is much smaller.The large differencesin the two PSS model profiles for C1 illustratedin Figure 1 demonstrate that our ability to estimate the concentrationof atomicchlorineusingthe PSS model is rather limited for the tropical lowermoststratosphereW. e derivedan optimizedC1concentrationprofileby runningour 1D model (e.g., equation (1)) to match the observed altitude profiles of the short-lived tracers
Summary
1996;Moteet al., 1996;Murphyet al., 1993;Randelet al., 1993; Trepteand Hitchman,1992; Wofsyet al., 1994]. FLOCKEET AL.: LOWERSTRATOSPHEREPROCESSES uses a unique set of tracer observationsobtained during the relativeresponsesb,ut anoverallestimateof betterthan5% or the StratosphericTracersof AtmosphericTransport(STRAT) and Photochemistryof OzoneLossin the Arctic Region in Summer (POLARIS) missions on board the NASA ER-2 aircraft [Newmanet al., thisissue]by the Whole Air Sampler(WAS) and the Aircraft Laser Infrared AbsorptionSpectrometer(ALIAS). We use a one-dimensional(l-D) photochemicaltransport model to simulatethe altitude profiles of the volume mixing ratios(referredto as "vmr profile" below) of tracersobservedat or the canisterwalls)ratherthanby the LOD of the analytical instrument.We will discussthesebackgroundissuesand their effectonthemodelanalyseslaterin thepaper. Becauseof the limited number of tracer data, we do not ALIAS is a high-resolution,midinfrared absorption treat seasonal variations in this model. For CO the instrumentprecision (+1 standard lower stratosphereand in situ observationsof radicalsmade on deviation)is +0.7 partsper billion by volume(ppbv)for a 3-s board the ER-2 to estimate photochemical removal rates.
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