William C. Kuster

Emissions lifetimes and ozone formation in power plant plumes

The concept of ozone production efficiency (OPE) per unit NOx is based on photochemical models and provides a tool with which to assess potential regional tropospheric ozone control strategies involving NOx emissions reductions. An aircraft study provided data from which power plant emissions removal rates and measurement-based estimates of OPE are estimated. This study was performed as part of the Southern Oxidants Study-1995 Nashville intensive and focuses on the evolution of NOx, SO2, and ozone concentrations in power plant plumes during transport. Two approaches are examined. A mass balance approach accounts for mixing effects within the boundary layer and is used to calculate effective boundary layer removal rates for NOx and SO2 and to estimate net OPE. Net OPE is more directly comparable to photochemical model results than previous measurement-based estimates. Derived net production efficiencies from mass balance range from 1 to 3 molecules of ozone produced per molecule of NOx emitted. A concentration ratio approach provides an estimate of removal rates of primary emissions relative to a tracer species. This approach can be combined with emissions ratio information to provide upper limit estimates of OPE that range from 2 to 7. Both approaches illustrate the dependence of ozone production on NOx source strength in these large point source plumes. The dependence of total ozone production, ozone production efficiency, and the rate of ozone production on NOx source strength is examined. These results are interpreted in light of potential ozone control strategies for the region.

Hydrocarbon measurements in the southeastern United States: The Rural Oxidants in the Southern Environment (ROSE) Program 1990

An automated gas Chromatographic system was employed at a rural site in western central Alabama to measure atmospheric hydrocarbons and oxygenated hydrocarbons (oxy-hydrocarbons) on an hourly basis from June 8 to July 19, 1990. The location, which was a designated site for the Southern Oxidant Study (SOS), was instrumented for a wide variety of measurements allowing the hydrocarbon and oxy-hydrocarbon measurements to be interpreted both in terms of meteorological data and as part of a large suite of gas phase measurements. Although the site is situated in a Loblolly pine plantation, isoprene was observed to be the dominant hydrocarbon during the daytime with afternoon maxima of about 7 parts per billion by volume (ppbv). Decrease of isoprene after sunset was too rapid to be accounted for solely on the basis of gas phase chemistry. During the nighttime, α-pinene and β-pinene were the dominant hydrocarbons of natural origin. The ratio of α-pinene to β-pinene showed a well-defined diurnal pattern, decreasing by more than 30% during the night; a decrease that could be understood on the basis of local gas phase chemistry. Oxy-hydrocarbons, dominated by methanol and acetone, were the most abundant compounds observed. On a carbon atom basis, the oxy-hydrocarbons contributed about 46% of the measured atmospheric burden during the daytime and about 40% at night. The similarity of the observed diurnal methanol variation to that of isoprene and subsequent measurements [McDonald and Fall, 1993] indicate that much of the observed methanol was of local biogenic origin. Correlation of acetone with methanol suggests that it, also, has a significant biogenic source. In spite of the sites rural location, anthropogenic hydrocarbons constituted, on a carbon atom basis, about 21% of the hydrocarbon burden measured during the daytime and about 55% at night. Significant diurnal variations of the anthropogenic hydrocarbons, with increases at night, appeared to be driven by the frequent formation of a shallow nocturnal boundary layer.

Proton-Transfer-Reaction Mass Spectrometry as a New Tool for Real Time Analysis of Root-Secreted Volatile Organic Compounds in Arabidopsis

Plant roots release about 5% to 20% of all photosynthetically-fixed carbon, and as a result create a carbon-rich environment for numerous rhizosphere organisms, including plant pathogens and symbiotic microbes. Although some characterization of root exudates has been achieved, especially of secondary metabolites and proteins, much less is known about volatile organic compounds (VOCs) released by roots. In this communication, we describe a novel approach to exploring these rhizosphere VOCs and their induction by biotic stresses. The VOC formation of Arabidopsis roots was analyzed using proton-transfer-reaction mass spectrometry (PTR-MS), a new technology that allows rapid and real time analysis of most biogenic VOCs without preconcentration or chromatography. Our studies revealed that the major VOCs released and identified by both PTR-MS and gas chromatography-mass spectrometry were either simple metabolites, ethanol, acetaldehyde, acetic acid, ethyl acetate, 2-butanone, 2,3,-butanedione, and acetone, or the monoterpene, 1,8-cineole. Some VOCs were found to be produced constitutively regardless of the treatment; other VOCs were induced specifically as a result of different compatible and noncompatible interactions between microbes and insects and Arabidopsis roots. Compatible interactions of Pseudomonas syringae DC3000 and Diuraphis noxia with Arabidopsis roots resulted in the rapid release of 1,8-cineole, a monoterpene that has not been previously reported in Arabidopsis. Mechanical injuries to Arabidopsis roots did not produce 1,8-cineole nor any C6 wound-VOCs; compatible interactions between Arabidopsis roots and Diuraphis noxia did not produce any wound compounds. This suggests that Arabidopsis roots respond to wounding differently from above-ground plant organs. Trials with incompatible interactions did not reveal a set of compounds that was significantly different compared to the noninfected roots. The PTR-MS method may open the way for functional root VOC analysis that will complement genomic investigations in Arabidopsis.

Isoprene and its oxidation products, methyl vinyl ketone and methacrolein, in the rural troposphere

The mixing ratios of methyl vinyl ketone (CH2=CHCOCH3) and methacrolein (CH2=C(CH3)COH) were measured at a site located in the Kinterbish Wildlife Management Area in western Alabama. The measurements were made between June 15 and July 20, 1990. Considering all the data over the whole measurement period, the concentrations of these two carbonyls were approximately equal at this isolated rural site. The average mixing ratios for methyl vinyl ketone and methacrolein were 0.98 parts per billion by volume (ppbv) and 0.66 ppbv, respectively, while the medians were 0.87 ppbv and 0.57 ppbv. The methyl vinyl ketone mixing ratio varied from 3.4 ppbv to the detection limit of the instrument, ≈0.01 ppbv, while the methacrolein mixing ratio varied from 2.6 ppbv to 0.027 ppbv. These carbonyls constituted a significant fraction of the volatile organic compounds observed at the site: their mixing ratios, measured 2 m above the top of the forest canopy, were less than that of the dominant compound isoprene but were considerably greater than the mixing ratios of anthropogenic compounds (e.g., benzene). The mixing ratios of methyl vinyl ketone and methacrolein were found to be highly correlated and exhibited a systematic variation with respect to each other. On average, during the day, methyl vinyl ketone was larger than methacrolein, while methacrolein tended to be slightly larger during the night. The systematic behavior of these compounds with respect to each other and other compounds measured at the site were simulated using a one-dimensional photochemical model. These observations were consistent with the production and loss of isoprene, methyl vinyl ketone, and methacrolein by photochemical oxidation reactions.

Source Signature of Volatile Organic Compounds from Oil and Natural Gas Operations in Northeastern Colorado

An extensive set of volatile organic compounds (VOCs) was measured at the Boulder Atmospheric Observatory (BAO) in winter 2011 in order to investigate the composition and influence of VOC emissions from oil and natural gas (O&NG) operations in northeastern Colorado. BAO is 30 km north of Denver and is in the southwestern section of Wattenberg Field, one of Colorados most productive O&NG fields. We compare VOC concentrations at BAO to those of other U.S. cities and summertime measurements at two additional sites in northeastern Colorado, as well as the composition of raw natural gas from Wattenberg Field. These comparisons show that (i) the VOC source signature associated with O&NG operations can be clearly differentiated from urban sources dominated by vehicular exhaust, and (ii) VOCs emitted from O&NG operations are evident at all three measurement sites in northeastern Colorado. At BAO, the reactivity of VOCs with the hydroxyl radical (OH) was dominated by C(2)-C(6) alkanes due to their remarkably large abundances (e.g., mean propane = 27.2 ppbv). Through statistical regression analysis, we estimate that on average 55 ± 18% of the VOC-OH reactivity was attributable to emissions from O&NG operations indicating that these emissions are a significant source of ozone precursors.

Sources of particulate matter in the northeastern United States in summer: 1. Direct emissions and secondary formation of organic matter in urban plumes

[1] Ship and aircraft measurements of aerosol organic matter (OM) and water-soluble organic carbon (WSOC) were made in fresh and aged pollution plumes from major urban areas in the northeastern United States in the framework of the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. A large part of the variability in the data was quantitatively described by a simple parameterization from a previous study that uses measured mixing ratios of CO and either the transport age or the photochemical age of the sampled air masses. The results suggest that OM was mostly due to secondary formation from anthropogenic volatile organic compound (VOC) precursors in urban plumes. Approximately 37% of the secondary formation can be accounted for by the removal of aromatic precursors using newly published particulate mass yields for low-NOx conditions, which are significantly higher than previous results. Of the secondary formation, 63% remains unexplained and is possibly due to semivolatile precursors that are not measurable by standard gas chromatographic methods. The observed secondary OM in urban plumes may account for 35% of the total source of OM in the United States and 8.5% of the global OM source. OM is an important factor in climate and air quality issues, but its sources and formation mechanisms remain poorly quantified.

The observation of a C5 alcohol emission in a North American pine forest

During a recent study carried out at an isolated site in the Colorado mountains, a C5 alcohol, 2-methyl-3-buten-2-ol, was found to be the most abundant volatile organic compound of biogenic origin present in the atmosphere. This finding, if generally characteristic of the natural chemical species present in the atmosphere in forested areas, has important implications. First, the presence in large quantities of a reactive chemical compound at these high levels can significantly influence the local atmospheric chemistry. Secondly, this compound, although previously identified as a pheromone for Ips typographus (spruce bark beetle), an insect predator responsible for major forest die-backs in this region, is strongly correlated with isoprene. Since isoprene is known to be emitted by the local vegetation, the observed 2-methyl-3-buten-2-ol appears also to have a vegetative rather than entomological source.

Gasoline emissions dominate over diesel in formation of secondary organic aerosol mass

Although laboratory experiments have shown that organic compounds in both gasoline fuel and diesel engine exhaust can form secondary organic aerosol (SOA), the fractional contribution from gasoline and diesel exhaust emissions to ambient SOA in urban environments is poorly known. Here we use airborne and ground-based measurements of organic aerosol (OA) in the Los Angeles (LA) Basin, California made during May and June 2010 to assess the amount of SOA formed from diesel emissions. Diesel emissions in the LA Basin vary between weekdays and weekends, with 54% lower diesel emissions on weekends. Despite this difference in source contributions, in air masses with similar degrees of photochemical processing, formation of OA is the same on weekends and weekdays, within the measurement uncertainties. This result indicates that the contribution from diesel emissions to SOA formation is zero within our uncertainties. Therefore, substantial reductions of SOA mass on local to global scales will be achieved by reducing gasoline vehicle emissions.

Photochemical modeling of hydroxyl and its relationship to other species during the Tropospheric OH Photochemistry Experiment

Because of the extremely short photochemical lifetime of tropospheric OH, comparisons between observations and model calculations should be an effective test of our understanding of the photochemical processes controlling the concentration of OH, the primary oxidant in the atmosphere. However, unambiguous estimates of calculated OH require sufficiently accurate and complete measurements of the key species and physical variables that determine OH concentrations. The Tropospheric OH Photochemistry Experiment (TOHPE) provides an extremely complete set of measurements, sometimes from multiple independent experimental platforms, that allows such a test to be conducted. When the calculations explicitly use observed NO, NO2, hydrocarbons, and formaldehyde, the photochemical model consistently overpredicts in situ observed OH by ∼50% for the relatively clean conditions predominantly encountered at Idaho Hill. The model bias is much higher when only CH4-CO chemistry is assumed, or NO is calculated from the steady state assumption. For the most polluted conditions encountered during the campaign, the model results and observations show better agreement. Although the comparison between calculated and observed OH can be considered reasonably good given the ±30% uncertainties of the OH instruments and various uncertainties in the model, the consistent bias suggests a fundamental difference between theoretical expectations and the measurements. Several explanations for this discrepancy are possible, including errors in the measurements, unidentified hydrocarbons, losses of HOx to aerosols and the Earths surface, and unexpected peroxy radical chemistry. Assuming a single unidentified type of hydrocarbon is responsible, the amount of additional hydrocarbon needed to reduce theoretical OH to observed levels is a factor of 2 to 3 greater than the OH-reactivity-weighted hydrocarbon content measured at the site. Constraints can be placed on the production and yield of various radicals formed in the oxidation sequence by considering the observed levels of certain key oxidation products such as formaldehyde and acetaldehyde. The model results imply that, under midday clean westerly flow conditions, formaldehyde levels are fairly consistent with the OH and hydrocarbon observations, but observed acetaldehyde levels are a factor of 4 larger than what is expected and also imply a biogenic source. Levels of methacrolein and methylvinylketone are much lower than expected from steady state isoprene chemistry, which implies important removal mechanisms or missing information regarding the kinetics of isoprene oxidation within the model. In a prognostic model application, additional hydrocarbons are added to the model in order to force calculated OH to observed levels. Although the products and oxidation steps related to pinenes and other biogenic hydrocarbons are somewhat uncertain, the addition of a species with an oxidation mechanism similar to that expected from C10 pinenes would be consistent with the complete set of observations, as opposed to naturally emitted isoprene or any of the anthropogenic hydrocarbons examined in the model. Further constraints on the abundance of peroxy radicals are necessary in order to fill the gaps in our understanding of OH photochemistry for the clean continental conditions typical of Idaho Hill.

The measurement of natural sulfur emissions from soils and vegetation: three sites in the eastern United States revisited.

Sulfur fluxes from bare soils, naturally vegetated surfaces and from several agricultural crops were measured at two mid-continent sites (Ames, Iowa and Celeryville, Ohio) and from one salt water marsh site (Cedar Island, North Carolina) during a field program conducted jointly by the NOAA Aeronomy Laboratory, Washington State University Laboratory for Atmospheric Research and University of Idaho Department of Chemistry during July and August 1985. The sites were chosen specifically because they had been characterized by previous studies (Anejaet al., 1979; Adamset al., 1980, 1981). The NOAA gas chromatographic/dynamic-enclosure measurements yielded bare soil surfaces fluxes from the mid-continent sites composed predominantly of COS, H2S, CH3−S−CH3 (DMS) and CS2, all of which were strongly correlated with air temperature. Net fluxes of approximately 5 and 15 ng S/m2 min were observed in Iowa and Ohio, respectively, at appropriate weighted mean July temperatures. These fluxes are roughly a factor of 10 smaller than the earlier measurements, the greatest difference being in the measurement of the H2S flux. The presence of growing vegetation was observed to measurably increase the flux of H2S, significantly increase that of DMS and to decrease that of COS. Sulfur fluxes in the Cedar Island environs were observed to be both spatially and temporally much more variable and to include CH3SH as a measurable contributor. Net fluxes, composed predominantly of DMS and H2S, were estimated to be about 300 ngS/m2min during August; again about a factor of 10 lower than previous estimates. All measurements were corroborated to within about a factor of 2 by those of the other participating laboratories.