Seasonal variation of tropospheric methyl bromide concentrations: Constraints on anthropogenic input

Although removal of tropospheric methyl bromide (CH3Br) is dominated by the reaction with the seasonally varying hydroxyl (HO) radical concentration, the anticipated corresponding seasonal dependence of CH3Br, as found for other gases with major HO sinks, has been sought previously without success [WMO, 1995]. Our observations of northern hemispheric boundary layer CH3Br concentrations do reveal substantial seasonal changes. The high latitude CH3Br North/South interhemispheric concentration ratio (IHR) varies from a maximum of 1.35±0.04 (1σ) in March-April to 1.10±0.04 in September, with an equal area and seasonally (EAS) weighted average IHR of 1.21±0.03. These observations suggest northern hemispheric emissions are about 15 kilotons/year less than when an IHR of 1.3 is considered [WMO, 1995]. The observed seasonality also partially explains the differences in the IHR reported by several research groups [WMO, 1995] and places needed constraints on the magnitude and seasonality of sources and sinks of CH3Br

Nonmethane hydrocarbon and halocarbon distributions during Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange, June 1992

Aircraft measurements of selected nonmethane hydrocarbon and halocarbon species were made in the lower troposphere of the NE Atlantic near the Azores, Portugal, during June 1992 as part of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange. In this paper, the impact of continental outflow from both Europe and North America on the study region were assessed. Four main air mass types were characterized from trajectories and trace gas concentrations: clean marine from the Atlantic, and continental air from the Iberian Peninsula, the British Isles and Northern Europe, and North America. Each classification exhibited trace gas concentrations that had been modified en route by photochemical processes and mixing. Comparison with the clean marine boundary layer (MBL) shows that the boundary layer of the predominantly continental air masses were enhanced in hydrocarbons and halocarbons by factors of approximately 2 for ethane, 5 for propane, 2-6 for ethyne and benzene, and 2-3 for C2Cl4. The same air masses also exhibited large ozone enhancements, with 2 to 3 times higher mixing ratios in the continental boundary layer air compared to the clean MBL. This indicates a primarily anthropogenic photochemical source for a significant fraction of the lower tropospheric ozone in this region. Methyl bromide exhibited on average 10-20% higher concentrations in the boundary layer affected by continental outflow than in the clean MBL, and was seen to be enhanced in individual plumes of air of continental origin. This is consistent with significant anthropogenic sources for methyl bromide. In addition, median MBL concentrations of ethene and methyl iodide showed enhancements of approximately a factor of 2 above free tropospheric values, suggesting primarily coastal/oceanic sources for these species. Copyright 1996 by the American Geophysical Union

Unexplained enhancements of CH3Br in the Arctic and sub-Arctic lower troposphere during TOPSE spring 2000

Elevated concentrations of methyl bromide (CH3Br) were observed in the Arctic atmospheric boundary layer (BL) during periods of widespread BL ozone (O3) depletion episodes (ODEs: O3 mixing ratios < 20 × 10-9 or parts per billion by volume, ppbv) particularly during major ODEs (MODES: O3 < 4 ppbv). No other organic gases measured during TOPSE (Tropospheric Ozone Production about the Spring Equinox) exhibited anti-correlations with O3 during these ODEs. Methyl bromide has both natural and anthropogenic sources and contributes ∼ half of the bromine (Br) to the stratosphere, where it can catalytically destroy O3. Several known CH3Br sources are evaluated, but the current knowledge cannot explain the observed enhancements. If the mechanism is direct gasphase photochemical production, a significant portion of the unknown CH3Br source may be found

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 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. Copyright 1996 by the American Geophysical Union

A large terrestrial source of methyl iodide

We have identified terrestrial sources of methyl iodide (CH3I) and assessed their importance in its atmospheric budget using a synthesis of field observations. Measurements include those from NASA DC‐8 research flights over the United States and the North Atlantic, the AIRMAP long‐term ground‐observing network in New England, and a field campaign at Duke Forest, North Carolina. We found an average CH3I flux of ∼2,700 ng m−2 d−1 to the atmosphere from midlatitude vegetation and soils, a value similar in magnitude to previous estimates of the oceanic source strength. The large‐scale aircraft measurements of vertical profiles over the continental U.S. showed CH3I‐mixing ratios comparable to and greater than those observed over the North Atlantic. Overall, midlatitude terrestrial biomes appear to contribute 33 Gg yr−1 to the CH3I global budget

A large terrestrial source of methyl iodide

We have identified terrestrial sources of methyl iodide (CH3I) and assessed their importance in its atmospheric budget using a synthesis of field observations. Measurements include those from NASA DC-8 research flights over the United States and the North Atlantic, the AIRMAP long-term ground-observing network in New England, and a field campaign at Duke Forest, North Carolina. We found an average CH3I flux of ∼2,700 ng m-2 d-1 to the atmosphere from midlatitude vegetation and soils, a value similar in magnitude to previous estimates of the oceanic source strength. The large-scale aircraft measurements of vertical profiles over the continental U.S. showed CH3I-mixing ratios comparable to and greater than those observed over the North Atlantic. Overall, midlatitude terrestrial biomes appear to contribute 33 Gg yr-1 to the CH3I global budget. Copyright 2007 by the American Geophysical Union

Three-dimensional distribution of nonmenthane hydrocarbons and halocarbons over the northwestern Pacific during the 1991 Pacific Exploratory Mission (PEM-West A)

A total of 1667 whole air samples were collected onboard the NASA DC-8 aircraft during the 6-week Pacific Exploratory Mission over the western Pacific (PEM-West A) in September and October 1991. The samples were assayed for 15 C2-C7 hydrocarbons and six halocarbons. Latitudinal (0.5°S to 59.5°N) and longitudinal (114°E to 122°W) profiles were obtained from samples collected between ground level and 12.7 km. Thirteen of the 18 missions exhibited at least one vertical profile where the hydrocarbon mixing ratios increased with altitude. Longitude-latitude color patch plots at three altitude levels and three-dimensional color latitudealtitude and longitude-altitude contour plots exhibit a significant number of middle-upper tropospheric pollution events. These and several lower tropospheric pollution plumes were characterized by comparison with urban data from Tokyo and Hong Kong, as well as with natural gas and the products from incomplete combustion. Elevated levels of nonmethane hydrocarbons (NMHC) and other trace gases in the upper-middle free troposphere were attributed to deep convection over the Asian continent and to typhoon-driven convection near the western Pacific coast of Asia. In addition, NMHCs and CH3CCI3 were found to be useful tracers with which to distinguish hydrocarbon and halocarbon augmented plumes emitted from coastal Asian cities into the northwestern Pacific