Ralph J. Cicerone

The Halogen Occultation Experiment

The Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) spacecraft September 12, 1991, and after a period of outgassing, it began science observations October 11. The experiment uses solar occultation to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature versus pressure with an instantaneous vertical field of view of 1.6 km at the Earth limb. Latitudinal coverage is from 80°S to 80°N over the course of 1 year and includes extensive observations of the Antarctic region during spring. The altitude range of the measurements extends from about 15 km to ≈ 60–130 km, depending on channel. Experiment operations have been essentially flawless, and all performance criteria either meet or exceed specifications. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results are in good agreement. Examples of pressure versus latitude cross sections and a global orthographic projection for the September 21 to October 15, 1992, period show the utility of CH4, HF, and H2O as tracers, the occurrence of dehydration in the Antarctic lower stratosphere, the presence of the water vapor hygropause in the tropics, evidence of Antarctic air in the tropics, the influence of Hadley tropical upwelling, and the first global distribution of HCl, HF, and NO throughout the stratosphere. Nitric oxide measurements extend through the lower thermosphere.

Changes in Stratospheric Ozone

The ozone layer in the upper atmosphere is a natural feature of the earths environment. It performs several important functions, including shielding the earth from damaging solar ultraviolet radiation. Far from being static, ozone concentrations rise and fall under the forces of photochemical production, catalytic chemical destruction, and fluid dynamical transport. Human activities are projected to deplete substantially stratospheric ozone through anthropogenic increases in the global concentrations of key atmospheric chemicals. Human-induced perturbations may be occurring already.

NOx Production in Lightning

Abstract The rate of odd nitrogen (NOx) production by electrical discharge through air was theoretically and experimentally estimated to be ∼6 × 1016 NOx molecules per joule. The theoretical treatment employed a cylindrical shock-wave solution to calculate the rate of NOx production in high temperature reactions. The limits obtained were experimentally verified by subjecting a regulated air flow to electrical discharges followed by a measurement of NOx production using chemiluminescence. These measurements also indicated that water vapor content has no detectable effect on the NOx production rate. Our results imply that lightning is a significant source of NOx, producing about 30–40 megatons NOx-N per year and possibly accounting for as much as 50% of the total atmospheric NOx source.

Methane emissions from California rice paddies with varied treatments

Two field experiments in California rice paddies are reported, one with a single treatment of a research plot and the other with varied treatments in a typical commercial rice field. Small total methane emissions, only 11 g CH4/m2, were measured for the entire growing season in the first experiment. In the second experiment, the addition of exogenous organic matter (rice straw), the presence or absence of vegetation, and the nitrogen fertilizer amounts were examined for their influence on methane emissions. The total methane emission over the growing season varied from 1.2 g CH4/m2 (with no added organic matter) to 58.2 g CH4/m2 (with largest organic matter treatments). Added organic matter was the major factor affecting methane emissions. Vegetation did not greatly affect total methane fluxes, but it did influence the mode and timing of release. Nitrogen fertilizer did not greatly affect the amount of methane emitted, but it influenced slightly the time course of the process. A diurnal effect in methane emission was observed during the early ontogeny of the crop. The variation of methane emission with time during the course of the growing season was very unusual in this experiment; only one peak was observed, and it was early in the season. During the period of largest emissions, δ13C values of the methane were measured to be −55.7 ±1.8‰ in plots with added organic matter.

Atmospheric methyl bromide (CH3Br) from agricultural soil fumigations

The treatment of agricultural soils with CH3Br (MeBr) has been suggested to be a significant source of atmospheric MeBr which is involved in stratospheric ozone loss. A field fumigation experiment showed that, after 7 days, 34 percent of the applied MeBr had escaped into the atmosphere. The remaining 66 percent should have caused an increase in bromide in the soil; soil bromide increased by an amount equal to 70 percent of the applied MeBr, consistent with the flux measurements to within 4 percent. Comparison with an earlier experiment in which the escape of MeBr to the atmosphere was greater showed that higher soil pH, organic content and soil moisture, and deeper, more uniform injection of MeBr may in combination reduce the escape of MeBr.

HALOE Antarctic observations in the spring of 1991

HALOE observations of O3, CH4, HF, H2O, NO, NO2, and HCl collected during the October 1991 Antarctic spring period are reported. The data show a constant CH4 mixing ratio of about 0.25 ppmv for the altitude range from 65 km down to about 25 km at the position of minimum wind speed in the vortex: i.e., the vortex center, and depressions in pressure versus longitude contours of NO, NO2, HF, and HCl in this same region. Water vapor, HF, and HCl enhancement are also observed in the vortex center region above ∼25 km. Between 10 and 20 km, the expected mixing ratio signatures exist within the vortex, i.e., low ozone and dehydration. The water vapor increased by 50%, and the ozone level doubled inside the vortex between October 11 and 24 in the 15 to 20 km layer. These changes imply a time constant for recovery from ozone hole conditions or 19 and 30 days for O3 and H2O, respectively. The data further show the presence of air inside the vortex between 3 and 30 mb which has mixing ratios characteristic of mid latitudes.

Modeling atmospheric δ13CH4 and the causes of recent changes in atmospheric CH4 amounts

Inclusion of kinetic isotope effects (KIEs) of methane (CH4) sinks (gaseous OH and Cl, and soil microbes) has a significant effect on modeled distributions of δ13C of atmospheric CH4. For a given scenario of surface sources and corresponding δ13C values of individual CH4 sources, the KIE due to soil uptake enriched δ13C by 1.18‰ in the models northern hemisphere (NH) (40°N) and 1.16‰ in the southern hemisphere (SH) (40°S) under steady state conditions in January. The KIE due to CH4 oxidation by stratospheric Cl radicals further enriched these δ13C values at the surface by 0.99‰ and 1.03‰, respectively. In the vertical direction, during January at 50°N, inclusion of a KIE due to Cl enriched δ13C at 18 km by 0.95‰ compared to the corresponding surface value, whereas the enrichment was only 0.31‰ when this KIE was omitted. These results suggest that modeling of δ13C distributions should include KIEs due to CH4 oxidation by soil and stratospheric chlorine radicals. It is shown that possible oxidation of CH4 in marine boundary layer by Cl radicals can significantly enrich δ13C. However, if a recent theoretical value for the KIE of the Cl and CH4 reaction is correct, then the impact of this reaction is less than the figures quoted above. In the model, monthly variations in OH concentration and interhemispheric exchange transport cannot reproduce the observed seasonal amplitude variation of δ13C in either the NH or SH. It is argued that seasonal variations in individual CH4 fluxes are primarily responsible for this discrepancy. We show that increasing Cl radical concentrations due to continued release of anthropogenic chlorocarbons enrich the δ13C values. The effects of an increase in tropospheric OH concentration due to stratospheric ozone depletion and a cooling of the troposphere due to the eruption of Mt. Pinatubo, with a lowering of water vapor concentration and reduction in isoprene emissions, on δ13C and surface CH4 mixing ratios are investigated. Other model simulations with adjusted surface CH4 fluxes have been performed to study the postulated explanations for recent changes in CH4 surface mixing ratios and δ13C values. A modified version of the Oslo two-dimensional global tropospheric photochemical model was used for all simulations.

A study of the sources and sinks of methane and methyl chloroform using a global three‐dimensional Lagrangian tropospheric tracer transport model

Sources and sinks of methane and methyl chloroform are investigated using a global three-dimensional Lagrangian tropospheric tracer transport model with parameterized hydroxyl and temperature fields. By comparison with methyl chloroform observations a global average tropospheric hydroyl radical concentration of 6.4×105 cm−3 was found to be consistent with published methyl chloroform emission data for the year 1980. Published methyl chloroform emissions data for 1981–1984 were found to be inconsistent with the observed methyl chloroform concentration increases. A large decrease in hydroxyl radical concentrations could explain the disagreement between the emission data and atmospheric methyl chloroform concentrations, but this is unlikely. Using the hydroxyl radical field calibrated to the methyl chloroform observations, the globally averaged release of methane and its spatial and temporal distribution were investigated. Two source function models of the spatial and temporal distribution of the flux of methane to the atmosphere were developed. The first model was based on the assumption that methane is emitted as a proportion of net primary productivity (NPP). With the average hydroxyl radical concentration fixed, the methane source term was computed as ∼623 Tg CH4, giving an atmospheric lifetime for methane ∼8.3 years. The second model identified source regions for methane from rice paddies, wetlands, enteric fermentation, termites, and biomass burning based on high-resolution land use data. This methane source distribution resulted in an estimate of the global total methane source of ∼611 Tg CH4, giving an atmospheric lifetime for methane ∼8.5 years. The most significant difference between the two models were predictions of methane fluxes over China and South East Asia, the location of most of the worlds rice paddies, indicating that either the assumption that a uniform fraction of NPP is converted to methane is not valid for rice paddies, or that NPP is underestimated for rice paddies, or that present methane emission estimates from rice paddies are too high. Using a recent measurement of the reaction rate of hydroxyl radical and methane by Vaghjiani and Ravishankara (G. L. Vaghjiani and A. R. Ravishankara, Rate coefficient for the reaction of OH and CH4: Implications to the atmospheric lifetime and budget of methane, submitted to Nature, 1990) (hereinafter referred to as Vaghjiani and Ravishankara, 1990) leads to estimates of the global total methane source for SF1 of ∼524 Tg CH4 giving an atmospheric lifetime of ∼10.0 years and for SF2 ∼514 Tg CH4 yielding a lifetime of ∼10.2 years. These results are provisional pending any revision of the reaction rate for hydroxyl radical and methyl chloroform.

Factors affecting methane production under rice

To understand why atmospheric methane is increasing worldwide, accurate estimates are needed of the global input from rice fields. We report greenhouse and laboratory studies over three growing seasons to isolate and control factors that might affect methane emission from rice paddies, including soil texture, added exogenous organic matter (OM), nitrogen and sulfate ion, and water management. Without added OM, methane production was relatively low, increasing during the growing season, and continuing after harvest, provided the soil remained water-logged. If ground rice straw was added to the soil prior to planting, methane production began shortly after flooding, with an initial burst of the gas after 3 to 5 weeks, and then a gradual increase to a second peak later in the season (and after harvest), with rates considerably higher than in treatments without added OM.

Spectroscopic Detection of Stratospheric Hydrogen Cyanide

A number of features have been identified as absorption lines of hydrogen cyanide in infrared spectra of stratospheric absorption obtained from a high-altitude aircraft. Column amounts of stratospheric hydrogen cyanide have been derived from spectra recorded on eight flights. The average vertical column amount above 12 kilometers is 7.1 � 0.8 x 1014 molecules per square centimeter, corresponding to an average mixing ratio of 170 parts per trillion by volume.