Hydrocarbon and halocarbon measurements as photochemical and dynamical indicators of atmospheric hydroxyl, atomic chlorine, and vertical mixing obtained during Lagrangian flights

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.

Changing concentrations of CO, CH 4 , C 5 H 8 , CH 3 Br, CH 3 I, and dimethyl sulfide during the Southern Ocean Iron Enrichment Experiments

“…The strong correlations of isoprene with biological indicators suggest that its production is dominated by processes associated with biological productivity. Nonmethane hydrocarbons (NMHCs) of marine origin can significantly impact the oxidative capacity of the natural atmospheric marine boundary layer, reacting with hydroxyl (HO) and atomic chlorine (22,23). As a result, increases in NMHCs, such as isoprene, can increase the local lifetimes of short-lived gases, such as DMS, by competing for oxidants.…”

Reactions of OH and Cl with isopropyl formate, isobutyl formate, n-propyl isobutyrate and isopropyl isobutyrate

“…Thus, only the present kinetic data for OH and Cl reactions were used to estimate the tropospheric lifetimes of the studied esters. Using tropospheric [OH] = 2 Â 10 6 radicals cm À3 (12-h daytime average [15]) and [Cl] = 1 Â 10 4 atoms cm À3 (24-h average [16], the tropospheric lifetimes (s = 1/k[X] with X = OH and Cl) with respect to reactions of esters with OH and Cl are estimated to be around 1-3 days and 10-82 days, respectively. These calculations confirm that the daytime OH reactions constitute the dominant chemical removal pathway for these esters, even if Cl reactions are not negligible and can be important and ubiquitous in the atmosphere.…”

Reactive chlorine in the marine boundary layer in the outflow of polluted continental air: A model study

“…[4] By means of indirect methods typical noon-time Cl radical concentrations in the MBL have been inferred to range roughly between 10 3 and 10 5 cm À3 depending on location and time [e.g., Singh et al, 1996;Wingenter et al, 1996;Rudolph et al, 1997]. Very recently, reported average Cl atom concentrations of 2.2À5.6 Â 10 4 cm À3 for Appledore Island, 10 km off the coast of Maine, during the CHAiOS (Chemistry of Halogens at the Isles of Shoals) field campaign (July/August 2004), which was part of the larger campaign International Consortium for Research on Transport and Transformation (ICARTT) [ Fehsenfeld et al, 2006].…”