Fred C. Fehsenfeld

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.

Ozone precursor relationships in the ambient atmosphere

The concentrations of ozone, nitrogen oxides, and nonmethane hydrocarbons measured near the surface in a variety of urban, suburban, rural, and remote locations are analyzed and compared in order to elucidate the relationships between ozone, its photochemical precursors, and the sources of these precursors. While a large gradient is found among remote, rural, and urban/suburban nitrogen oxide concentrations, the total hydrocarbon reactivity in all continental locations is found to be comparable. Apportionment of the observed hydrocarbon species to mobile and stationary anthropogenic sources and biogenic sources suggests that present-day emission inventories for the United States underestimate the size of mobile emissions. The analysis also suggests a significant role for biogenic hydrocarbon emissions in many urban/suburban locations and a dominant role for these sources in rural areas of the eastern United States. As one moves from remote locations to rural locations and then from rural to urban/suburban locations, ozone and nitrogen oxide concentrations tend to increase in a consistent manner while total hydrocarbon reactivity does not.

NOx And N2O Emissions From Soil

Emission Of NOx (principally NO) and N2O from soils is reviewed with particular emphasis placed on the atmospheric and ecological implications of this source. The photochemistry of these species in the atmosphere is summarized as well as the methods available for the determination of fluxes. Processes which produce and consume both NO and N2O in soils are principally microbiological in nature and are linked directly and indirectly with the chemical and physical factors that control gaseous transport through the soil medium. Linkages among these processes occur over many different temporal and spatial scales which makes interpretation of the available data difficult. A summary of results from laboratory and field studies shows that considerable spatial and temporal variability exists in the emissions. This variability can be related to factors such as temperature, water content, soil composition, nutrient availability, vegetation, disturbances (e.g., burning, agricultural practices), and others. Because NOx and N2O play central roles in many important environmental problems, there is a need for accurate estimates of the magnitude of the soil source, but the large degree of variability in the existing data makes extrapolation highly uncertain. To overcome this uncertainty, models are required which can simulate the processes responsible for production, consumption, and transport of these species at all relevant temporal and spatial scales. Integrated field studies will also be required to validate the model results.

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.

Flow‐drift technique for ion mobility and ion‐molecule reaction rate constant measurements. II. Positive ion reactions of N+, O+, and H2+ with O2 and O+ with N2 from thermal to [inverted lazy s]2 eV

The positive ion‐molecule reactions N++O2→ NO++O→ O2++N, N2++O2→ O2++N2, O++O2→ O2++O, O++N2→ NO++N, were measured in a newly constructed flow‐drift tube apparatus. Reactions (a), (b), and (c) were measured from thermal energy to approximately 2 eV ion kinetic energy in the center of mass. Reaction (d) was measured from 0.3–3 eV ion kinetic energy. The data agree well with previous thermal values at low energies and agree well with beam data at the highest energy. In the case of all the reaction except (a), there is an initial decrease of the rate constant with increasing energy followed by an increase.

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.

Laboratory studies of negative ion reactions with atmospheric trace constituents

A flowing afterglow system has been used to measure 296°K reaction rate constants and equilibrium constants for a number of negative ion reactions with atmospheric constituents. Three‐body association reactions of O−, OH−, O2−, O3−, Cl−, CO3−, OH−(H2O), and O2−(H2O) with H2O have been measured and association rate constants for several ions with CO2 and SO2 have been measured. A number of binary reactions for these ions and their hydrates have been measured with H2O, CO2, SO2, NO2, O3, and NO. Some equilibrium constants for negative ion hydration and some equilibrium constants for solvent (H2O, CO2, and SO2) exchange to several negative ions are reported.

Evaluation of a catalytic reduction technique for the measurement of total reactive odd-nitrogen NOy in the atmosphere

A catalytic reduction technique for the measurement of total reactive odd-nitrogen NOy in the atmosphere was evaluated in laboratory and field tests. NOy component species include NO, NO2, NO3, HNO3, N2O5, CH3COO2NO2(PAN), and particulate nitrate. The technique utilizes the reduction of the higher oxides to NO in reaction with CO on a metal catalyst and the subsequent detection of NO by chemiluminescence produced in reaction with O3. The efficiency and linearity of the conversion of the principal NOy species were examined for mixing ratios in the range of 0.1 to 100 parts per billion by volume (ppbv). Results of tests with Au, Ni, and stainless steel as the catalyst in the temperature range of 25–500°C showed Au to be the preferred catalyst. NH3, HCN, N2O, CH4, and various chlorine and sulfur compounds were checked as possible sources of NOy interference with the Au catalyst. The effects of pressure, O3, and H2O on NOy conversion were also examined. The results of the checks and tests in the laboratory showed the technique to be suitable for initial NOy measurements in the atmosphere. The technique was subsequently tested in ambient air at a remote ground-based field site located near Niwot Ridge, Colorado. The results of conversion and inlet tests made in the field and a summary of the NOy data are included in the discussion.

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.

Flow‐drift technique for ion mobility and ion‐molecule reaction rate constant measurements. I. Apparatus and mobility measurements

The present paper describes the construction and operation of a new experimental device that combines the chemical versatility of a conventional flowing afterglow system with the energy variability of a drift tube. This allows the measurement of both positive and negative ion mobilities not previously measured. Ion mobility measurements offer a significant constraint upon the ion‐neutral intermolecular potential and are therefore of value in testing either empirical or quantum mechanical theory. The mobilities of He+, He2+, H+, D+, O+, N+, Ar+, H2+, H3+, H2+, H−, O−, and OH− in helium and H3+ in H2 are presented in the present paper. The following papers describe positive ion‐neutral and negative ion‐neutral reaction rate constant measurements in the same device.