990 research outputs found
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Aircraft measurements of the latitudinal, vertical, and seasonal variations of NMHCs, methyl nitrate, methyl halides, and DMS during the First Aerosol Characterization Experiment (ACE 1)
Canister sampling for the determination of atmospheric mixing ratios of nonmethane hydrocarbons (NMHCs), selected halocarbons, and methyl nitrate was conducted aboard the National Center for Atmospheric Research (NCAR) C-130 aircraft over the Pacific and Southern Oceans as part of the First Aerosol Characterization Experiment (ACE 1) during November and December 1995. A latitudinal profile, flown from 76°N to 60°S, revealed latitudinal gradients for most trace gases. NMHC and halocarbon gases with predominantly anthropogenic sources, including ethane, ethyne, and tetrachloroethene, exhibited significantly higher mixing ratios in the northern hemisphere at all altitudes. Methyl chloride exhibited its lowest mixing ratios at the highest northern hemisphere latitudes, and the distributions of methyl nitrate and methyl iodide were consistent with tropical and subtropical oceanic sources. Layers containing continental air characteristic of aged biomass burning emissions were observed above about 3 km over the remote southern Pacific and near New Zealand between approximately 19°S and 43°S. These plumes originated from the west, possibly from fires in southern Africa. The month-long intensive investigation of the clean marine southern midlatitude troposphere south of Australia revealed decreases in the mixing ratios of ethane, ethyne, propane, and tetrachloroethene, consistent with their seasonal mixing ratio cycle. By contrast, increases in the average marine boundary layer concentrations of methyl iodide, methyl nitrate, and dimethyl sulfide (DMS) were observed as the season progressed to summer conditions. These increases were most appreciable in the region south of 44°S over Southern Ocean waters characterized as subantarctic and polar, indicating a seasonal increase in oceanic productivity for these gases. Copyright 1999 by the American Geophysical Union
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Aircraft Measurements of Dimethyl Sulfide (DMS) Using a Whole Air Sampling Technique
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Chemical signatures of aged Pacific marine air: Mixed layer and free troposphere as measured during PEM-West A
The Pacific Ocean is one of the few remaining regions of the northern hemisphere that is relatively free of direct anthropogenic emissions. However, long-range transport of air pollutants is beginning to have a significant impact on the atmosphere over the Pacific. In September and October 1991, NASA conducted the Pacific Exploratory Mission-West A expedition to study the atmospheric chemistry and background budgets of key atmospheric trace species. Aircraft sampling centered on the northern Pacific, 0° to 40°N and 115° to 180°E. The paper summarizes the chemical signature of relatively well-aged Pacific marine air (residence time ≥10 days over the ocean). The chemical signatures show that marine air is not always devoid of continental influences. Aged marine air which circulates around the semipermanent subtropical anticyclone located off the Asian continent is influenced by infusion of continental air with anthropogenic emissions. The infusion occurs as the result of Asian outflow swept off the continent behind eastward moving cold fronts. When compared to aged marine air with a more southerly pathway, this infusion results in enhancements in the mixing ratio of many anthropogenic/continental species and typically those with lifetimes of weeks in the free troposphere. Less enhancement is seen for the short-lived species with lifetimes of a few days as infused continental emissions are depleted during transport (about a week) around the semipermanent subtropical high. Copyright 1996 by the American Geophysical Union
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Carbonyl sulfide and carbon disulfide: Large-scale distributions over the western Pacific and emissions from Asia during TRACE-P
An extensive set of carbonyl sulfide (OCS) and carbon disulfide (CS 2) observations were made as part of the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) project, which took place in the early spring 2001. TRACE-P sampling focused on the western Pacific region but in total included the geographic region 110°E to 290°E longitude, 5°N to 50°N latitude, and 0-12 km altitude. Substantial OCS and CS2 enhancements were observed for a great many air masses of Chinese and Japanese origin during TRACE-P. Over the western Pacific, mean mixing ratios of long-lived OCS and shorter-lived CS2 showed a gradual decrease by about 10% and a factor of 5-10, respectively, from the surface to 8-10 km altitude, presumably because land-based sources dominated their distribution during February through April 2001. The highest mean OCS and CS 2 levels (580 and 20 pptv, respectively, based on 2.5° × 2.5° latitude bins) were observed below 2 km near the coast of Asia, at latitudes between 25°N and 35°N, where urban Asian outflow was strongest. Ratios of OCS versus CO for continental SE Asia were much lower compared to Chinese and Japanese signatures and were strongly associated with biomass burning/biofuel emissions. We present a new inventory of anthropogenic Asian emissions (including biomass burning) for OCS and CS2 and compare it to emission estimates based on regional relationships of OCS and CS 2 to CO and CO2. The OCS and CS2 results for the two methods compare well for continental SE Asia and Japan plus Korea and also for Chinese CS2 emissions. However, it appears that the inventory underestimates Chinese emissions of OCS by about 30-100%. This difference may be related to the fact that we did not include natural sources such as wetland emissions in our inventory, although the contributions from such sources are believed to be at a seasonal low during the study period. Uncertainties in OCS emissions from Chinese coal burning, which are poorly characterized, likely contribute to the discrepancy. Copyright 2004 by the American Geophysical Union
Chemical characteristics of Pacific tropospheric air in the region of the Intertropical Convergence Zone and South Pacific Convergence Zone
The Pacific Exploratory Mission (PEM)-Tropics provided extensive aircraft data to study the atmospheric chemistry of tropospheric air in Pacific Ocean regions, extending from Hawaii to New Zealand and from Fiji to east of Easter Island. This region, especially the tropics, includes some of the cleanest tropospheric air of the world and, as such, is important for studying atmospheric chemical budgets and cycles. The region also provides a sensitive indicator of the global-scale impact of human activity on the chemistry of the troposphere, and includes such important features as the Pacific "warm pool," the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), and Walker Cell circulations. PEM-Tropics was conducted from August to October 1996. The ITCZ and SPCZ are major upwelling regions within the South Pacific and, as such, create boundaries to exchange of tropospheric air between regions to the north and south. Chemical data obtained in the near vicinity of the ITCZ and the SPCZ are examined. Data measured within the convergent zones themselves are not considered. The analyses show that air north and south of the convergent zones have different chemical signatures, and the signatures are reflective of the source regions and transport histories of the air. Air north of the ITCZ shows a modest urban/industrialized signature compared to air south of the ITCZ. The chemical signature of air south of the SPCZ is dominated by combustion emissions from biomass burning, while air north of the SPCZ is relatively clean and of similar composition to ITCZ south air. Chemical signature differences of air north and south of the zones are most pronounced at altitudes below 5 km, and, as such, show that the ITCZ and SPCZ are effective low-altitude barriers to the transport of tropospheric air. At altitudes of 8 to 10 km, chemical signatures are less dissimilar, and air backward trajectories (to 10 days) show cross-convergent-zone flow. At altitudes below about 5 km, little cross-zonal flow is observed. Chemical signatures presented include over 30 trace chemical species including ultrafine, fine, and heated-fine (250°C) aerosol. Copyright 1999 by the American Geophysical Union
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An examination of chemistry and transport processes in the tropical lower stratosphere using observations of long-lived and short-lived compounds obtained during STRAT and POLARIS
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. Copyright 1999 by the American Geophysical Union
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