Barkley Cushing Sive

Monoaromatic compounds in ambient air of various cities: a focus on correlations between the xylenes and ethylbenzene

Speciation of o-xylene, m-xylene, p-xylene and ethylbenzene was performed by gas chromatography from ambient air and liquid fuel samples collected at various locations in 19 cities in Europe, Asia and South America. The xylenes mixing ratios were compared to each other from the various locations, which included urban air, traffic air and liquid fuel. For all samples, the xylenes exhibited robust correlations, and the slopes remained constant. The m-xylene/p-xylene ratio was found to be 2.33±0.30, and the m-xylene/o-xylene ratio was found to be 1.84±0.25. These ratios remain persistent even in biomass combustion experiments (in South America and South Africa). Comparing the xylenes to toluene and benzene indicate that combustion, but not fuel evaporation, is the major common source of the xylenes in areas dominated by automotive emissions. Although a wide range of combustion types and combustion efficiencies were encountered throughout all the locations investigated, xylenes and ethylbenzene ratios remained persistent. We discuss the implications of the constancies in the xylenes and ethylbenzene ratios on atmospheric chemistry.

Tropospheric hydroxyl and atomic chlorine concentrations, and mixing timescales determined from hydrocarbon and halocarbon measurements made over the Southern Ocean

Author(s): Wingenter, OW; Blake, DR; Blake, NJ; Sive, BC; Rowland, FS; Atlas, E; Flocke, F | Abstract: During the First Aerosol Characterization Experiment (ACE 1) field campaign, 1419 whole air samples were collected over the Southern Ocean, of which approximately 700 samples were collected in the marine boundary layer (MBL), 300 samples were taken in the free troposphere (FT), and the remainder were collected in the buffer layer (BuL), the layer between the MBL and FT. Concentrations of tetrachloroethene, ethane, ethyne, and propane decayed over the 24 day duration of the intensive portion of the field campaign, which began November 18, 1995. This decline was consistent with what is known about seasonal increase of HO and the seasonal decrease in biomass burning. Using a simple empirical model, the best fit to the observations was obtained when the average [HO] was 6.1 ± 0.3 × 105 HO cm-3, and an average [Cl] of 720 ± 100 Cl cm-3. The corresponding exchange times were 14 ± 2 days between the MBL and FT, and 49 +40/-13 days between the MBL in the intensive campaign region and the MBL region to the north (nMBL). Copyright 1999 by the American Geophysical Union.

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.

Large‐scale latitudinal and vertical distributions of NMHCs and selected halocarbons in the troposphere over the Pacific Ocean during the March‐April 1999 Pacific Exploratory Mission (PEM‐Tropics B)

Nonmethane hydrocarbons (NMHCs) and selected halocarbons were measured in whole air samples collected over the remote Pacific Ocean during NASAs Global Tropospheric Experiment (GTE) Pacific Exploratory Mission-Tropics B (PEM-Tropics B) in March and early April 1999. The large-scale spatial distributions of NMHCs and C2Cl4 reveal a much more pronounced north-south interhemispheric gradient, with higher concentrations in the north and lower levels in the south, than for the late August to early October 1996 PEM-Tropics A experiment. Strong continental outflow and winter-long accumulation of pollutants led to seasonally high Northern Hemisphere trace gas levels during PEM-Tropics B. Observations of enhanced levels of Halon 1211 (from developing Asian nations such as the PRC) and CH3Cl (from SE Asian biomass burning) support a significant southern Asian influence at altitudes above 1 km and north of 10°N. By contrast, at low altitude over the North Pacific the dominance of urban/industrial tracers, combined with low levels of Halon 1211 and CH3Cl, indicate a greater influence from developed nations such as Japan, Europe, and North America. Penetration of air exhibiting aged northern hemisphere characteristics was frequently observed at low altitudes over the equatorial central and western Pacific south to ∼5°S. The relative lack of southern hemisphere biomass burning sources and the westerly position of the South Pacific convergence zone contributed to significantly lower PEM-Tropics B mixing ratios of the NMHCs and CH3Cl south of 10°S compared to PEM-Tropics A. Therefore the trace gas composition of the South Pacific troposphere was considerably more representative of minimally polluted tropospheric conditions during PEM-Tropics B.

The seasonal evolution of NMHCs and light alkyl nitrates at middle to high northern latitudes during TOPSE

[1]xa0The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment was designed to follow the role of photochemistry in the evolution of the springtime maximum of tropospheric ozone (O3) in the Northern Hemisphere (NH) at high latitudes. Determination of the composition and seasonal evolution of volatile organic carbon (VOC) species, which take part in and are good indicators for photochemical processes in the troposphere, was an important part of this study. We report measurements of a large number of C2–C10 nonmethane hydrocarbons (NMHCs), selected C1–C2 halocarbons, and C1–C4 alkyl nitrates. These gases were quantified in whole air samples collected aboard the National Center for Atmospheric Research (NCAR) C-130 aircraft at altitudes between 30 m and 8 km. Seven TOPSE sampling trips were flown between early February and mid-May 2000 covering the region from Colorado (40°N) to Churchill (in Manitoba, Canada), Thule (in northern Greenland), and as far north as 85°N. These measurements represent the most comprehensive spatial characterization of the North American Arctic to date and revealed strong latitudinal, vertical, and temporal NMHC gradients. In the midtroposphere north of Churchill (58°N), ΣNMHCs decreased by ∼6.2 ppbC between February and May (1.6 ppbC month−1) and the magnitude of this change diminished with altitude. Over the same period, midtropospheric O3 levels increased by ∼16 ppbv (4.2 ppbv month−1). Free tropospheric NMHC decreases were consistent with removal by hydroxyl (OH) radicals at an average mixing ratio for mid-March to mid-May of 4.1 × 105 mol cm−3. The alkyl nitrates, which are a reservoir species for tropospheric reactive odd nitrogen (NOY), revealed similar latitudinal, vertical, and temporal gradients to their parent NMHCs. Their total decreased by ∼4 pptv month−1, representing 10% or less of NOY. In conjunction with meteorological trajectory analysis, different trace gas signatures provided significant clues to the origins of individual polluted air masses. Several of these air masses were rapidly advected over the Pole from source regions in northeastern and western Europe as well as an air mass that originated over the southwestern United States/Baja California that contained unusually high levels of alkanes. In addition, episodes of low boundary layer (BL) O3 associated with low NMHC mixing ratios and trajectories from over the Arctic Ocean were frequently sampled toward the latter part of the experiment. The TOPSE data described here provide a unique picture of NH trace gas evolution from winter to summer that will be invaluable to models investigating the role that anthropogenic emissions play in high latitude O3 chemistry.

Using stable isotopes of hydrogen to quantify biogenic and thermogenic atmospheric methane sources: A case study from the Colorado Front Range

Global atmospheric concentrations of methane (CH4), a powerful greenhouse gas, are increasing, but because there are many natural and anthropogenic sources of CH4, it is difficult to assess which sources may be increasing in magnitude. Here we present a dataset of δ2H-CH4 measurements of individual sources and air in the Colorado Front Range, USA. We show that δ2H-CH4, but not δ13C, signatures are consistent in air sampled downwind of landfills, cattle feedlots, and oil and gas wells in the region. Applying these source signatures to air in ground and aircraft samples indicates that at least 50% of CH4 emitted in the region is biogenic, perhaps because regulatory restrictions on leaking oil and natural gas wells are helping to reduce this source of CH4. Source apportionment tracers such as δ2H may help close the gap between CH4 observations and inventories, which may underestimate biogenic as well as thermogenic sources.

Nonmethane hydrocarbon measurements in the North Atlantic Flight Corridor during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment

Mixing ratios of nonmethane hydrocarbons (NMHCs) were not enhanced in whole air samples collected within the North Atlantic Flight Corridor (NAFC) during the fall of 1997. The investigation was conducted aboard NASAs DC-8 research aircraft, as part of the Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX). NMHC enhancements were not detected within the general organized tracking system of the NAFC, nor during two tail chases of the DC-8s own exhaust. Because positive evidence of aircraft emissions was demonstrated by enhancements in both nitrogen oxides and condensation nuclei during SONEX, the NMHC results suggest that the commercial air traffic fleet operating in the North Atlantic region does not contribute at all or contributes negligibly to NMHCs in the NAFC.

Source characterization of volatile organic compounds in the Colorado Northern Front Range Metropolitan Area during spring and summer 2015

Hourly measurements of 46 volatile organic compounds (VOCs) from the Boulder Atmospheric Observatory in Erie, CO, were collected over 16u2009weeks in spring and summer 2015. Average VOC reactivity (1.2u2009s−1 in spring and 2.4u2009s−1 in summer) was lower than most other U.S. urban sites. Positive matrix factorization analysis identified five VOC factors in the spring, corresponding to sources from (1) long-lived oil and natural gas (ONG-long lived), (2) short-lived oil and natural gas (ONG-short lived), (3) traffic, (4) background, and (5) secondary chemical production. In the summer, an additional biogenic factor was dominated by isoprene. While ONG-related VOCs were the single largest contributor (40–60%) to the calculated VOC reactivity with hydroxyl radicals (OH) throughout the morning in both spring and summer, the biogenic factor substantially enhanced afternoon and evening (2–10u2009P.M. local time) VOC reactivity (average of 21%; maxima of 49% of VOC reactivity) during summertime. These results contrast with a previous summer 2012 campaign which showed that biogenics contributed only 8% of VOC reactivity on average. The interannual differences suggest that the role of biogenic VOCs in the Colorado Northern Front Range Metropolitan Area (NFRMA) varies with environmental conditions such as drought stress. Overall, the NFRMA was more strongly influenced by ONG sources of VOCs than other urban and suburban regions in the U.S.

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

[1]xa0Elevated 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 gas-phase photochemical production, a significant portion of the unknown CH3Br source may be found.

A characterization of volatile organic compounds and secondary organic aerosol at a mountain site in the Southeastern United States

Mean temperature anomalies in the Southeastern United States (SEUS) over the past century have reflected regional cooling hypothesized to be a result of an enhancement of warm season aerosol optical thickness caused by the oxidation of biogenic volatile organic compounds (VOCs). Aerosol and gas-phase VOC measurements were made at the Appalachian Atmospheric Interdisciplinary Research (AppalAIR) site in the southern Appalachian mountains of North Carolina during the summer of 2013 in an effort to characterize warm season chemistry. Organic aerosol (OA) chemistry was characterized through a positive matrix factorization analysis resolving a low-volatility, semi-volatile, and isoprene oxidation factor contributing 34u2009±u200915, 24u2009±u200912, and 42u2009±u200917xa0%, respectively to the total observed OA. Volatile organic compound characterization described chemistry that was typical of rural background levels with periods of elevated hydrocarbon and urban tracer loading that varied with synoptic flow. Chemical, meteorological, and aerosol optical property data suggested that measurements made at the AppalAIR site are representative of background atmospheric chemistry in the SEUS. Annual background secondary organic aerosol (SOA) production in the SEUS was estimated to be 0.15–0.50 GgC yr−1. Estimates of total and background SOA from this study provide evidence that the SEUS is a region of global significance in the context of global SOA budgets, and can be useful in understanding the extent of anthropogenic enhancement of summertime SOA compared to background levels.