Atmospheric implications of large C2-C5 alkane emissions from the U.S. oil and gas industry.

Emissions of C2-C5 alkanes from the U.S. oil and gas sector have changed rapidly over the last decade. We use a nested GEOS-Chem simulation driven by updated 2011NEI emissions with aircraft, surface and column observations to 1) examine spatial patterns in the emissions and observed atmospheric abundances of C2-C5 alkanes over the U.S., and 2) estimate the contribution of emissions from the U.S. oil and gas industry to these patterns. The oil and gas sector in the updated 2011NEI contributes over 80% of the total U.S. emissions of ethane (C2H6) and propane (C3H8), and emissions of these species are largest in the central U.S. Observed mixing ratios of C2-C5 alkanes show enhancements over the central U.S. below 2 km. A nested GEOS-Chem simulation underpredicts observed C3H8 mixing ratios in the boundary layer over several U.S. regions and the relative underprediction is not consistent, suggesting C3H8 emissions should receive more attention moving forward. Our decision to consider only C4-C5 alkane emissions as a single lumped species produces a geographic distribution similar to observations. Due to the increasing importance of oil and gas emissions in the U.S., we recommend continued support of existing long-term measurements of C2-C5 alkanes. We suggest additional monitoring of C2-C5 alkanes downwind of northeastern Colorado, Wyoming and western North Dakota to capture changes in these regions. The atmospheric chemistry modeling community should also evaluate whether chemical mechanisms that lump larger alkanes are sufficient to understand air quality issues in regions with large emissions of these species.

Highly elevated atmospheric levels of volatile organic compounds in the Uintah Basin, Utah.

Oil and natural gas production in the Western United States has grown rapidly in recent years, and with this industrial expansion, growing environmental concerns have arisen regarding impacts on water supplies and air quality. Recent studies have revealed highly enhanced atmospheric levels of volatile organic compounds (VOCs) from primary emissions in regions of heavy oil and gas development and associated rapid photochemical production of ozone during winter. Here, we present surface and vertical profile observations of VOC from the Uintah Basin Winter Ozone Studies conducted in January-February of 2012 and 2013. These measurements identify highly elevated levels of atmospheric alkane hydrocarbons with enhanced rates of C2-C5 nonmethane hydrocarbon (NMHC) mean mole fractions during temperature inversion events in 2013 at 200-300 times above the regional and seasonal background. Elevated atmospheric NMHC mole fractions coincided with build-up of ambient 1-h ozone to levels exceeding 150 ppbv (parts per billion by volume). The total annual mass flux of C2-C7 VOC was estimated at 194 ± 56 × 10(6) kg yr(-1), equivalent to the annual VOC emissions of a fleet of ∼100 million automobiles. Total annual fugitive emission of the aromatic compounds benzene and toluene, considered air toxics, were estimated at 1.6 ± 0.4 × 10(6) and 2.0 ± 0.5 × 10(6) kg yr(-1), respectively. These observations reveal a strong causal link between oil and gas emissions, accumulation of air toxics, and significant production of ozone in the atmospheric surface layer.

North American acetone sources determined from tall tower measurements and inverse modeling.

We apply a full year of continuous atmospheric acetone measurements from the University of Minnesota tall tower Trace Gas Observatory (KCMP tall tower; 244 m a.g.l.), with a 0.5° × 0.667° GEOS-Chem nested grid simulation to develop quantitative new constraints on seasonal acetone sources over North America. Biogenic acetone emissions in the model are computed based on the MEGANv2.1 inventory. An inverse analysis of the tall tower observations implies a 37% underestimate of emissions from broadleaf trees, shrubs, and herbaceous plants, and an offsetting 40% overestimate of emissions from needleleaf trees plus secondary production from biogenic precursors. The overall result is a small (16%) model underestimate of the total primary + secondary biogenic acetone source in North America. Our analysis shows that North American primary + secondary anthropogenic acetone sources in the model (based on the EPA NEI 2005 inventory) are accurate to within approximately 20%. An optimized GEOS-Chem simulation incorporating the above findings captures 70% of the variance (R = 0.83) in the hourly measurements at the KCMP tall tower, with minimal bias. The resulting North American acetone source is 11 Tg a-1, including both primary emissions (5.5 Tg a-1) and secondary production (5.5 Tg a-1), and with roughly equal contributions from anthropogenic and biogenic sources. The North American acetone source alone is nearly as large as the total continental volatile organic compound (VOC) source from fossil fuel combustion. Using our optimized source estimates as a baseline, we evaluate the sensitivity of atmospheric acetone and peroxyacetyl nitrate (PAN) to shifts in natural and anthropogenic acetone sources over North America. Increased biogenic acetone emissions due to surface warming are likely to provide a significant offset to any future decrease in anthropogenic acetone emissions, particularly during summer.

Isoprene biosynthesis in Bacillus subtilis via the methylerythritol phosphate pathway.

Isoprene (2-methyl-1,3-butadiene), an abundant natural product of unknown function in plants, has recently been found to be one of the major volatiles formed by Bacillus subtilis. To understand the metabolic origins of isoprene in B. subtilis, we used (13)C- and (2)H-labeling methods with GC-MS analysis of released isoprene. The results indicate that, in this bacterium, isoprene is not formed by the mevalonate pathway or from catabolism of leucine, but, as in plant systems, it is a product of the methylerythritol phosphate pathway of isoprenoid synthesis. This work supports the idea that B. subtilis could be used as a microbial model for studying the biochemistry of isoprene formation.

Biogenic volatile organic compound emissions (BVOCs). II. Landscape flux potentials from three continental sites in the U.S.

Landscape flux potentials for biogenic volatile organic compounds (BVOCs) were derived for three ecosystems in the continental U.S. (Fernbank Forest, Atlanta, GA; Willow Creek, Rhinelander, WI; Temple Ridge, CO). Analytical data from branch enclosure measurements were combined with ecological survey data for plant species composition and biomass. Other quantitative flux measurements at the leaf and landscape level were incorporated to scale the results from the enclosure measurements to the landscape level. Flux estimates were derived by using a one week ambient temperature and light record (30 min time resolution) and adjusting all emission rates to these conditions with temperature and light correction algorithms.

Biogenic volatile organic compound emissions (BVOCs). I. Identifications from three continental sites in the U.S.

Vegetation composition and biomass were surveyed for three specific sites in Atlanta, GA; near Rhinelander, WI; and near Hayden, CO. At each research site emissions of biogenic volatile organic compounds (BVOCs) from the dominant vegetation species were sampled by enclosing branches in bag enclosure systems and sampling the equilibrium head space onto multi-stage solid adsorbent cartridges. Analysis was performed using a thermal desorption technique with gas chromatography (GC) separation and mass spectrometry (MS) detection. Identification of BVOCs covering the GC retention index range (stationary phase DB-1) from approximately 400 to 1400 was achieved (volatilities C4-C14).

Leaf, branch, stand and landscape scale measurements of volatile organic compound fluxes from U.S. woodlands.

Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 1993. Fluxes from individual leaves and branches were estimated with enclosure techniques and used to initialize and evaluate VOC emission model estimates. Ambient measurements were used to estimate above canopy fluxes for entire stands and landscapes. The branch enclosure experiments revealed 78 VOCs. Hexenol derivatives were the most commonly observed oxygenated compounds. The branch measurements also revealed high rates of isoprene emission from three genera of plants (Albizia, Chusqua and Mahonia) and high rates of monoterpene emission from three genera (Atriplex, Chrysthamnus and Sorbus) for which VOC emission rates have not been reported. Measurements on an additional 34 species confirmed previous results. Leaf enclosure measurements of isoprene emission rates from Quercus were substantially higher than the rates used in existing emission models. Model predictions of diurnal variations in isoprene fluxes were generally within +/- 35% of observed flux variations. Measurements with a fast response analyzer demonstrated that 60 min is a reasonable time resolution for biogenic emission models. Average daytime stand scale (hundreds of m) flux measurements ranged from about 1.3 mg C m(-2) h(-1) for a shrub oak stand to 1.5-2.5 mg C m(-2) h(-1) for a mixed forest stand. Morning, evening and nighttime fluxes were less than 0.1 mg C m(-2) h(-1). Average daytime landscape scale (tens of km) flux measurements ranged from about 3 mg C m(-2) h(-1) for a shrub oak-aspen and rangeland landscape to about 7 mg C m(-2) h(-1) for a deciduous forest landscape. Fluxes predicted by recent versions (BEIS2, BEIS2.1) of a biogenic emission model were within 10 to 50% of observed fluxes and about 300% higher than those predicted by a previous version of the model (BEIS).

Organic chemicals in the air at Whitaker's Forest/Sierra Nevada Mountains, California.

Air samples from a rural forested site in the Sierra Nevada Mountains were analyzed for volatile organic compounds by a thermodesorption GC/MS technique. Approximately 120 compounds were characterized by their mass spectra, with identification achieved for about 70 of these substances. A high proportion of biogenically emitted substances was found and the concentration of anthropogenic chemicals was relatively low. Twenty-one terpenoid compounds (C10H16 and oxygenated derivatives) and p-cymene were identified, and a single sesquiterpene, tentatively identified as longifolene, was also found. Nopinone (bicyclo[3.1.1]-heptan-2-one, 6,6-dimethyl-) and alpha-pinene oxide (3-oxatricyclo-[4.1.1.02,4]-octane, 2,7,7-trimethyl), both possible pinene degradation products, were among the compounds detected, although it is possible that the observed levels of one or both of these included contributions from formation during the sampling. The strong influence of local vegetative emissions on the composition of the forest air was confirmed by quantitative analysis. Concentrations of single monoterpenes up to 6.8 micrograms m-3 were found, whereas toluene was in the range of 0.3 micrograms m-3. The results from the forest site are compared with urban air samples collected in Riverside, California.

Bioassay-directed fractionation of mutagenic PAH atmospheric photooxidation products and ambient particulate extracts.

Simulated atmospheric gas-phase reactions of naphthalene, fluorene and phenanthrene have been carried out in an environmental chamber with bioassay-directed chemical analysis of the reaction products. Nitro-PAH were found to be the most significant mutagens formed from the reactions of naphthalene and fluorene. The mutagram (bar graph of mutagenic activity versus HPLC fraction) of the phenanthrene reaction products closely resembled that of an ambient air particulate extract with the most mutagenic activity being in a fraction more polar than that in which the nitro-PAH elute. Nitrophenanthrene lactones (nitro-6H-dibenzo[b,d]pyran-6-ones) were found to account for the observed activity of this polar fraction of the phenanthrene reaction products. It has been shown that the utilization of an environmental chamber with a known PAH-starting material and the ability to produce sufficient product for isomer-specific identifications of mutagens is a promising complement to bioassay-directed fractionation of ambient air particulate extracts.