HYDROCARBON AND HALOCARBON MEASUREMENTS AS PHOTOCHEMICAL AND DYNAMICAL INDICATORS OF ATMOSPHERIC HYDROXYL, ATOMIC CHLORINE, AND VERTICAL MIXING OBTAINED DURING LAGRANGIAN FLIGHTS

Nonmethane hydrocarbons and halocarbons were measured during two Lagra ngian experiments conducted in the lower troposphere of the North Atla ntic as part of the June 1992, Atlantic Stratosphere Transition Experi ment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) expedition. The firs t experiment was performed in very clean marine air. Meteorological ob servations indicate that the height of the marine boundary layer rose rapidly, entraining free tropospheric air. However, the free troposphe ric and marine boundary layer halocarbon concentrations were too simil ar to allow this entrainment to be quantified by these measurements. T he second Lagrangian experiment took place along the concentration gra dient of an aged continental air mass advecting from Europe. The trace gas measurements confirm that the National Center for Atmospheric Res earch (NCAR) Electra aircraft successfully intercepted the same air ma ss on consecutive days. Two layers, a surface layer and a mixed layer with chemically distinct compositions, were present within the marine boundary layer. The composition of the free troposphere was very diffe rent from that of the mixed layer, making entrainment from the free tr oposphere evident. Concentrations of the nonmethane hydrocarbons in th e Lagrangian surface layer were observed to become depleted relative t o the longer-lived tetrachloroethene. A best fit to the observations w as calculated using various combinations of the three parameters, loss by reaction with hydroxyl, loss by reaction with chlorine, and/or dil ution from the mixed layer. These calculations provided estimated aver age concentrations in the surface layer for a 5-hour period from dawn to 11 UT of 0.3+/-0.5 x10(6) molecules cm(-3) for HO, and 3.3+/-1.1 x1 0(4) molecules cm(-3) for Cl. Noontime concentration estimates were 2. 6+/-0.7 x10(6) molecules cm(-3) for HO and 6.5+/-1.4 x10(4) molecules cm(-3) for Cl.