Abstract
Nonmethane hydrocarbons and halocarbons were measured during two Lagrangian experiments conducted in the lower troposphere of the North Atlantic as part of the June 1992, Atlantic Stratosphere Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) expedition. The first experiment was performed in very clean marine air. Meteorological observations indicate that the height of the marine boundary layer rose rapidly, entraining free tropospheric air. However, the free tropospheric and marine boundary layer halocarbon concentrations were too similar to allow this entrainment to be quantified by these measurements. The second Lagrangian experiment took place along the concentration gradient of an aged continental air mass advecting from Europe. The trace gas measurements confirm that the National Center for Atmospheric Research (NCAR) Electra aircraft successfully intercepted the same air mass 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 different from that of the mixed layer, making entrainment from the free troposphere evident. Concentrations of the nonmethane hydrocarbons in the Lagrangian surface layer were observed to become depleted relative to the longer‐lived tetrachloroethene. A best fit to the observations was calculated using various combinations of the three parameters, loss by reaction with hydroxyl, loss by reaction with chlorine, and/or dilution from the mixed layer. These calculations provided estimated average concentrations in the surface layer for a 5‐hour period from dawn to 11 UT of 0.3 ± 0.5 × 106 molecules cm−3 for HO, and 3.3 ± 1.1 × 104 molecules cm−3 for Cl. Noontime concentration estimates were 2.6 ± 0.7 × 106 molecules cm−3 for HO and 6.5 ± 1.4 × 104 molecules cm−3 for Cl.
Highlights
Despite their low atmosphericabundance,hydroxyl radicals (HO) play a crucial role in the oxidation of many important trace atmosphericgases including hydrocarbons,hydrogencontaining halocarbons, and dimethyl sulfide
Whole air samples were collected onboard the National valuesfor six samplescollectedin the MBL duringflight 4: Centerfor AtmosphericResearch(NCAR) Electraaircraft.This Ethane 1115ñ5, propane 89.3ñ1.7, n-butane 7.6ñ1.4, ipaperdealswith the lower troposphericmeasurementosf two anthropogenichalocarbonsand five C2-C5 nonmethane hydrocarbon (NMHC) made during the sevenflights comprisingthe two ASTEX/MAGE Lagrangianexperiments.Theseflightstook placeduringthe periods, June 12-13 and June 18-20, 1992, in the North Atlantic east of the Azores from 30øto 41øN, and 20øto 26øW
The fact that to estimatea C2C14deficitfor eachsamplea, sshownin Figure flight 11 was made in a region with a characteristic
Summary
Hydrocarbon and halocarbon measurements as photochemical and dynamical indicators of atmospheric hydroxyl, atomic chlorine, and vertical mixing obtained during Lagrangian flights.
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