M. Trainer

Emissions of volatile organic compounds from vegetation and the implications for atmospheric chemistry

Vegetation provides a major source of reactive carbon entering the atmosphere. These compounds play an important role in (1) shaping global tropospheric chemistry, (2) regional photochemical oxidant formation, (3) balancing the global carbon cycle, and (4) production of organic acids which contribute to acidic deposition in rural areas. Present estimates place the total annual global emission of these compounds between approximately 500 and 825 Tg yr−1. The volatile olefinic compounds, such as isoprene and the monoterpenes, are thought to constitute the bulk of these emissions. However, it is becoming increasingly clear that a variety of partially oxidized hydrocarbons, principally alcohols, are also emitted. The available information concerning the terrestrial vegetation as sources of volatile organic compounds is reviewed. The biochemical processes associated with these emissions of the compounds and the atmospheric chemistry of the emitted compounds are discussed.

Correlation of ozone with NOy in photochemically aged air

During the summer of 1988, measurements of photochemical trace species were made at a coordinated network of seven rural sites in the eastern United States and Canada. At six of these sites concurrent measurements of ozone and the sum of the reactive nitrogen species, NOy, were made, and at four of the sites a measure for the reaction products of the NOx oxidation was obtained. Common to all sites, ozone, in photochemically aged air during the summer, shows an increase with increasing NOy levels, from a background value of 30–40 parts per billion by volume (ppbv) at NOy mixing ratios below 1 ppbv to values between 70 to 100 ppbv at NOy levels of 10 ppbv. Ozone correlates even more closely with the products of the NOx oxidation. The correlations from the different sites agree closely at mixing ratios of the oxidation products below 5 ppbv, but systematic differences appear at higher levels. Variations in the biogenic hydrocarbon emissions may explain these differences.

Export of North American Ozone Pollution to the North Atlantic Ocean

Measurement of the levels of ozone and carbon monoxide (a tracer of anthropogenic pollution) at three surface sites on the Atlantic coast of Canada allow the estimation of the amount of ozone photochemically produced from anthropogenic precursors over North America and transported to the lower troposphere over the temperate North Atlantic Ocean. This amount is greater than that injected from the stratosphere, the primary natural source of ozone. This conclusion supports the contention that ozone derived from anthropogenic pollution has a hemisphere-wide effect at northern temperate latitudes.

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.

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.

A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver‐Julesburg Basin

Emissions of methane (CH4) from oil and natural gas (O&G) operations in the most densely drilled area of the Denver-Julesburg Basin in Weld County located in northeastern Colorado are estimated for 2 days in May 2012 using aircraft-based CH4 observations and planetary boundary layer height and ground-based wind profile measurements. Total top-down CH4 emission estimates are 25.8 ± 8.4 and 26.2 ± 10.7 t CH4/h for the 29 and 31 May flights, respectively. Using inventory data, we estimate the total emissions of CH4 from non-O&G gas-related sources at 7.1 ± 1.7 and 6.3 ± 1.0 t CH4/h for these 2 days. The difference in emissions is attributed to O&G sources in the study region, and their total emission is on average 19.3 ± 6.9 t/h, close to 3 times higher than an hourly emission estimate based on Environmental Protection Agencys Greenhouse Gas Reporting Program data for 2012. We derive top-down emissions estimates for propane, n-butane, i-pentane, n-pentane, and benzene from our total top-down CH4 emission estimate and the relative hydrocarbon abundances in aircraft-based discrete air samples. Emissions for these five nonmethane hydrocarbons alone total 25.4 ± 8.2 t/h. Assuming that these emissions are solely originating from O&G-related activities in the study region, our results show that the state inventory for total volatile organic compounds emitted by O&G activities is at least a factor of 2 too low for May 2012. Our top-down emission estimate of benzene emissions from O&G operations is 173 ± 64 kg/h, or 7 times larger than in the state inventory.

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.

A study of the photochemistry and ozone budget during the Mauna Loa Observatory Photochemistry Experiment

Extensive measurements of trace species and parameters that are important to the photochemical production and loss of ozone have been made at Mauna Loa during the Mauna Loa Observatory Photochemistry Experiment experiment. These measurements are used as inputs as well as constraints in a model study of the photochemical budgets of ozone and five other trace species (CH2O, CH3OOH, H2O2, NO, and NOx) that are closely coupled to the photochemical production and loss of ozone. The study shows that there are significant discrepancies in the photochemical budgets of these trace species in this region and suggests that some important uncertainties exist in our understanding of the odd hydrogen photochemical processes.

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.

Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode

A 3-day period of strong, synoptic-scale stagnation, in which daytime boundary-layer winds were light and variable over the region, occurred in mid July of the 1995 Southern Oxidants Study centered on Nashville, Tennessee. Profiler winds showed light and variable flow throughout the mixed layer during the daytime, but at night in the layer between 100 and 2000 m AGL (which had been occupied by the daytime mixed layer) the winds accelerated to 5-10 m s-1 as a result of nocturnal decoupling from surface friction, which producect inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (03). The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled 03 in the boundary layer as the airplane flew along. Vertical cross sections showed that 03 concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the 03 hardly moved at all horizontally, instead forming a dome of pollution over or near the city. The analysis concentrates on four meteorological processes that determine the 3-D spatial distribution of 03 and the interaction between urban and rural pollution: (1) daytime buildup of 03 over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak 03 concentrations, (3) nighttime transport by the accelerated winds above the surface, and (4) vertical mixing of pollution layers the next day. Other consequences of very light-wind conditions were intra-regional differences in daytime mixed-layer depth over distances of 50 km or less, and indications of an urban heat-island circulation.