D. W. Fahey

Removal of Stratospheric O3 by Radicals: In Situ Measurements of OH, HO2, NO, NO2, ClO, and BrO

Simultaneous in situ measurements of the concentrations of OH, HO2, ClO, BrO, NO, and NO2 demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO2 with ozone, accounted for nearly one-half of the total O3 removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O3; reactions involving BrO account for one-half of this loss. Catalytic destruction by NO2, which for two decades was considered to be the predominant loss process, accounted for less than 20 percent of the O3 removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO2 and ClO are inversely correlated with those of NO and NO2. The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O3 removal was inversely correlated with total NOx, loading.

Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere

Reactive nitrogen (NOy) and O3 were measured simultaneously from a NASA ER-2 aircraft during 1987 through 1989. These high resolution measurements cover a broad range of latitudes in the upper troposphere and lower stratosphere. The data show a striking positive correlation between NOy and O3 in the lower stratosphere at all scales sampled. The ratio NOy/O3 is nearly independent of altitude from the tropopause to above 20 km, with ratios in the stratosphere of 0.0015–0.002 in the tropics and 0.0025–0.004 elsewhere. The ratio is much more constant than either individual species, thus providing an excellent reference point for comparing limited data sets with models. Two-dimensional models reproduce general features of the vertical profile of NOy/O3 but not the gradient in the lower stratosphere between tropics and mid-latitudes. NOy and O3 are better correlated in the lower stratosphere than in the upper troposphere. The magnitude and variability of both NOy mixing ratios and NOy/O3 ratios indicate a source of NOy in the upper troposphere. The most plausible source in the tropics is lightning production of NOx. Condensation of NOy onto aerosol particles is often possible in the tropical upper troposphere and may play a role in determining the vertical distribution of NOy. In the mid-latitude upper troposphere the data suggest long-range transport of NOy. NOy mixing ratios in the tropical upper troposphere were usually between 150 and 600 pptv, enough so that upward transport can affect the NOy abundance in the tropical lower stratosphere.

The importance of the Montreal Protocol in protecting climate

The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer is a landmark agreement that has successfully reduced the global production, consumption, and emissions of ozone-depleting substances (ODSs). ODSs are also greenhouse gases that contribute to the radiative forcing of climate change. Using historical ODSs emissions and scenarios of potential emissions, we show that the ODS contribution to radiative forcing most likely would have been much larger if the ODS link to stratospheric ozone depletion had not been recognized in 1974 and followed by a series of regulations. The climate protection already achieved by the Montreal Protocol alone is far larger than the reduction target of the first commitment period of the Kyoto Protocol. Additional climate benefits that are significant compared with the Kyoto Protocol reduction target could be achieved by actions under the Montreal Protocol, by managing the emissions of substitute fluorocarbon gases and/or implementing alternative gases with lower global warming potentials.

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.

Evaluation of source gas lifetimes from stratospheric observations

Simultaneous in situ measurements of the long-lived trace species N2O, CH4, 12, CFC-113, CFC-11, CCl4, CH3CCl3, H-1211, and SF6 were made in the lower stratosphere and upper troposphere on board the NASA ER-2 high-altitude aircraft during the 1994 campaign Airborne Southern Hemisphere Ozone Experiment/ Measurements for Assessing the Effects of Stratospheric Aircraft. The observed extratropical tracer abundances exhibit compact mutual correlations that show little interhemispheric difference or seasonal variability except at higher altitudes in southern hemisphere spring. The environmental impact of the measured source gases depends, among other factors, on the rate at which they release ozone-depleting chemicals in the stratosphere, that is, on their stratospheric lifetimes. We calculate the mean age of the air from the SF6 measurements and show how stratospheric lifetimes of the other species may be derived semiempirically from their observed gradients with respect to mean age at the extratropical tropopause. We also derive independent stratospheric lifetimes using the CFC-11 lifetime and the slopes of the tracers correlations with CFC-11. In both cases, we correct for the influence of tropospheric growth on stratospheric tracer gradients using the observed mean age of the air, time series of observed tropospheric abundances, and model-derived estimates of the width of the stratospheric age spectrum. Lifetime results from the two methods are consistent with each other. Our best estimates for stratospheric lifetimes are 122±24 years for N2O, 93±18 years for CH4, 87±17 years for CFC-12, 100±32 years for CFC-113, 32±6 years for CCl4, 34±7 years for CH3CCl3, and 24±6 years for H-1211. Most of these estimates are significantly smaller than currently recommended lifetimes, which are based largely on photochemical model calculations. Because the derived stratospheric lifetimes are identical to atmospheric lifetimes for most of the species considered, the shorter lifetimes would imply a faster recovery of the ozone layer following the phaseout of industrial halocarbons than currently predicted.

The large contribution of projected HFC emissions to future climate forcing

The consumption and emissions of hydrofluorocarbons (HFCs) are projected to increase substantially in the coming decades in response to regulation of ozone depleting gases under the Montreal Protocol. The projected increases result primarily from sustained growth in demand for refrigeration, air-conditioning (AC) and insulating foam products in developing countries assuming no new regulation of HFC consumption or emissions. New HFC scenarios are presented based on current hydrochlorofluorocarbon (HCFC) consumption in leading applications, patterns of replacements of HCFCs by HFCs in developed countries, and gross domestic product (GDP) growth. Global HFC emissions significantly exceed previous estimates after 2025 with developing country emissions as much as 800% greater than in developed countries in 2050. Global HFC emissions in 2050 are equivalent to 9–19% (CO2-eq. basis) of projected global CO2 emissions in business-as-usual scenarios and contribute a radiative forcing equivalent to that from 6–13 years of CO2 emissions near 2050. This percentage increases to 28–45% compared with projected CO2 emissions in a 450-ppm CO2 stabilization scenario. In a hypothetical scenario based on a global cap followed by 4% annual reductions in consumption, HFC radiative forcing is shown to peak and begin to decline before 2050.

The photochemistry of acetone in the upper troposphere: A source of odd-hydrogen radicals

This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO_(2), odd-nitrogen, and other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO_(2) concentrations within 2 to 5 km layers just below the tropopause are ∼50% larger than expected from O_(3), CH_(4), and H_(2)O chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HO_(x) (HO+HO_(2)), but influences NO_(y) partitioning. A major fraction of HO_(x) production by acetone is through CH_(2)O formation, and the HO_(x) discrepancy can also be explained by CH_(2)O levels in the 20 to 50 pptv range, regardless of the source.

An Inter-Comparison of Instruments Measuring Black Carbon Content of Soot Particles

Inter-comparison studies of well-characterized fractal soot particles were conducted using the following four instruments: Aerosol Mass Spectrometer-Scanning Mobility Particle Sizer (AMS-SMPS), Single Particle Soot Photometer (SP2), Multi-Angle Absorption Photometer (MAAP), and Photoacoustic Spectrometer (PAS). These instruments provided measurements of the refractory mass (AMS-SMPS), incandescent mass (SP2) and optically absorbing mass (MAAP and PAS). The particles studied were in the mobility diameter range from 150 nm to 460 nm and were generated by controlled flames with fuel equivalence ratios ranging between 2.3 and 3.5. The effect of organic coatings (oleic acid and anthracene) on the instrument measurements was determined. For uncoated soot particles, the mass measurements by the AMS-SMPS, SP2, and PAS instruments were in agreement to within 15%, while the MAAP measurement of optically-absorbing mass was higher by ∼ 50%. Thin organic coatings (∼ 10 nm) did not affect the instrument readings. A thicker (∼ 50 nm) oleic acid coating likewise did not affect the instrument readings. The thicker (∼60 nm) anthracene coating did not affect the readings provided by the AMS-SMPS or SP2 instruments but increased the reading of the MAAP instrument by ∼ 20% and the reading of the PAS by ∼ 65%. The response of each instrument to the different particle types is discussed in terms of particle morphology and coating material.

Quantifying Transport Between the Tropical and Mid-Latitude Lower Stratosphere

Airborne in situ observations of molecules with a wide range of lifetimes (methane, nitrous oxide, reactive nitrogen, ozone, chlorinated halocarbons, and halon-1211), used in a tropical tracer model, show that mid-latitude air is entrained into the tropical lower stratosphere within about 13.5 months; transport is faster in the reverse direction. Because exchange with the tropics is slower than global photochemical models generally assume, ozone at mid-latitudes appears to be more sensitive to elevated levels of industrial chlorine than is currently predicted. Nevertheless, about 45 percent of air in the tropical ascent region at 21 kilometers is of mid-latitude origin, implying that emissions from supersonic aircraft could reach the middle stratosphere.

Mixing of polar vortex air into middle latitudes as revealed by tracer‐tracer scatterplots

The occurrence of mixing of polar vortex air with midlatitude air is investigated by examining the scatterplots of insitu measurements of long-lived tracers from the NASA ER-2 aircraft during the Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE, April, May 1993; northern hemisphere) and the Airborne Southern Hemisphere Ozone Experiment / Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA, March-October 1994; southern hemisphere) campaigns. The tracer-tracer scatterplots from SPADE form correlation curves which differ from those measured during previous aircraft campaigns (Airborne Antarctic Ozone Experiment (AAOE), Airborne Arctic Stratospheric Experiments I (AASE I) and II (AASE II)). It is argued that these anomalous linear correlation curves are mixing lines resulting from the recent mixing of polar vortex air into the middle latitude environment. Further support for this mixing scenario is provided by contour advection calculations and calculations with a simple one-dimensional strain-diffusion model. The scatterplots from the midwinter deployments of ASHOE/MAESA are consistent with those from previous midwinter measurements (i.e., no mixing lines), but the spring CO 2 :N 2 O scatterplots form altitude-dependent mixing lines which indicate that air from the vortex edge region (but not from the inner vortex) is mixing with midlatitude air during this period. These results suggest that at altitudes above about 16 km the mixing of polar vortex air into middle latitudes varies with season: in northern and southern midwinter this mixing rarely occurs, in southern spring mixing of vortex-edge air occurs, and after the vortex breakup mixing of inner vortex air occurs.