Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment (ALE/GAGE/AGAGE) measurements of CCl4 at five remote surface locations from 1978 to ...1996 are reported. The Scripps Institution of Oceanography (SIO) 1993 absolute calibration scale is used, reducing the concentrations by a factor of 0.77 compared to previous ALE/GAGE reports. Atmospheric concentrations of CCl4 reached a peak in 1989–1990 of 104.4±3.1 parts per trillion (ppt) and have since been decreasing 0.7±0.1 ppt yr−1. Assuming an atmospheric lifetime of 42±12 years, the emissions averaged 94−11+22 × 106 kg from 1979 to 1988 and 49−13+26 × 106 kg frorn 1991 to 1995. The reduction in the emissions in 1989–1990 coincided with a substantial decrease in the global production of the chlorofluorocarbons (CFCs). The total emission of CCl4 from countries that report annual production is estimated to have declined from 11% in 1972 to 4% in 1995 of the CCl4 needed to produce the CFC amounts reported. This implies that nonreporting countries released substantial amounts of CCl4 into the atmosphere in the 1980s and that their releases have exceeded those from the reporting countries since 1991.
An ozone climatology for the period April 1992 to March 1993 and covering pressures from 0.1 to 100 hPa and from 80°N to 80°S is derived from satellite‐based measurements by the Stratospheric Aerosol ...and Gas Experiment (SAGE), the Halogen Occultation Experiment (HALOE), and the Microwave Limb Sounder (MLS). At pressures <1 hPa, separate distributions are given for daytime and nighttime conditions. From 0.46 to 32 hPa the accuracy of the distribution is estimated to be 5%, and the precision is also ∼5%. Estimates of atmospheric variability are provided on the basis of standard deviations of the measurements within months. Distributions of ozone monthly means and standard deviations are also given in a potential temperature, equivalent latitude coordinate system. This data set is included in the UARS reference atmosphere, and it is accessible through that web site.
The time series of differences in coincident measurements of ozone by Stratospheric Aerosol and Gas Experiment (SAGE) and by Solar Backscattered Ultraviolet (SBUV), SBUV/2, Umkehr and Microwave Limb ...Sounder (MLS) are analyzed, and the slopes in the differences are calculated. SAGE ozone measurements are also compared against those by HALOE. The purpose of these comparisons is to look for statistically significant nonzero slopes which could indicate long‐term calibration problems in one or more of the measurement systems. It is found that the slopes are remarkably similar between the Northern and Southern Hemisphere midlatitudes, and, apart from a few exceptions, the slopes are also similar in the tropics. Slopes of MLS‐SAGE differences and HALOE‐SAGE trends from approximately 1992 to 1996 have values of approximately −0.5±0.4% yr−1 (95% confidence limits) in Umkehr layers 7–9 (which are centered at ∼37, 42, and 47 km altitude). Umkehr‐SAGE slopes for 1979–1996, however, are almost all positive and in the range −0.1–0.4% yr−1 for Umkehr layers 4–8, while SBUV‐SAGE slopes for 1979–1989 are essentially zero in layers 4–7 and 0.3–0.4% yr−1 in layers 8 and 9. Averaging all these results with SBUV‐SAGE II slopes from 1985 to 1989, the other sensors minus SAGE slopes are most likely between 0.2 and −0.2% yr−1 from ∼20 to 40 km altitude. The results indicate slightly negative slopes in Umkehr layers 5–7 and positive slopes in the other three layers. There thus appears to be no overall drift in the SAGE ozone measurements from 1979 to 1996, but SAGE sunrise/sunset trend differences >40 km altitude, combined with the more accurate SBUV‐SAGE slopes for 1979–1989, suggest a most likely slope range of 0.4 to −0.4% yr−1 between 40 and 50 km altitude. SBUV/2 measurements from 1989 to 1994 have an upward trend with respect to SAGE measurements of ∼0.7% yr−1 with some altitudinal structure; this slope exceeds the estimated 95% uncertainties on the SBUV/2 trends.
Analyzing coincident observations of ozone profiles at 15 Umkehr stations with Stratospheric Aerosol and Gas Experiment (SAGE) I and SAGE II measurements within 1000 km and 12 hours in low‐aerosol ...conditions between 1979 and 1991, we find improved agreement between Umkehr and SAGE retrievals using the new (1992) Umkehr algorithm compared to the previous (1964) algorithm, but some significant differences remain. The column ozone amounts in layers 4 through 10 for both old Umkehr64 (after adjustment for the scale change from the International Ozone Commission/World Meteorological Organization 1968 scale to the Bass and Paur 1985 scale) and new Umkehr92 retrievals are approximately 5–6% lower than SAGE column amounts. The structure of the aerosol‐corrected Umkehr92 profiles compares much more favorably to SAGE than did the Umkehr64 profiles in layers 4 to 7 (20–35 km), with considerable consistency in the vertical structure of differences across most sites. The layer‐ozone differences, however, increase from zero in layer 4 to −15% (Umkehr92 low) in layer 8. Belsk and Sapporo are the only 2 sites of the 15 analyzed here that exhibit somewhat dissimilar vertical structure in their differences versus SAGE. The Umkehr92 a priori climatology contains less ozone in the lower layers (2–5) than does SAGE and somewhat more in the upper layers (7–9). However, these differences in a priori climatology do not significantly affect the broad altitude structure in the SAGE‐Umkehr layer‐mean ozone differences. On average, the Umkehr92 profiles possess a correlation between 0.3 and 0.5 (higher at some stations) with the SAGE‐measured ozone in individual layers 4 to 8. The time series of the Umkehr92 and SAGE measurements typically exhibit similar trends except for discontinuous changes noted at Mauna Loa and Kagoshirna.
Tunable diode laser absorption spectroscopy has been employed to measure the amount of N2O produced from laser flash photolysis of O3/N2/O2 mixtures at 266 and 532 nm. In the 532 nm photolysis ...experiments very little N2O is observed, thus allowing an upper limit yield of 7 × 10-8 to be established for the process O3 † + N2 → N2O + O2, where O3 † is nascent O3 that is newly formed via O(3P J ) + O2 recombination (with vibrational excitation near the dissociation energy of O3). The measured upper limit yield is a factor of ∼600 smaller than a previous literature value and is approximately a factor of 10 below the threshold for atmospheric importance. In the 266 nm photolysis experiments, significant N2O production is observed and the N2O quantum yield is found to increase linearly with pressure over the range 100−900 Torr in air bath gas. The source of N2O in the 266 nm photolysis experiments is believed to be the addition reaction O(1D2) + N2 + M N2O + M, although reaction of (very short-lived) electronically excited O3 with N2 cannot be ruled out by the available data. Assuming that all observed N2O comes from the O(1D2) + N2 + M reaction, the following expression describes the temperature dependence of k 6 (in its third-order low-pressure limit) that is consistent with the N2O yield data: k 6 = (2.8 ± 0.1) × 10-36(T/300)-(0.88±0.36) cm6 molecule-2 s-1, where the uncertainties are 2σ and represent precision only. The accuracy of the reported rate coefficients at the 95% confidence level is estimated to be 30−40% depending on the temperature. Model calculations suggest that gas phase processes initiated by ozone absorption of a UV photon represent about 1.4% of the currently estimated global source strength of atmospheric N2O. However, these processes could account for a significant fraction of the oxygen mass-independent enrichment observed in atmospheric N2O, and they appear to be the first suggested photochemical mechanism that is capable of explaining the altitude dependence of the observed mass-independent isotopic signature.
A contour advection technique, contour advection with surgery (CAS), is applied to the Northern Hemisphere Arctic vortex during several dynamically active periods in midwinter and at several ...different levels in the stratosphere. The ability of the technique to accurately depict vortex evolution is assessed. Isentropic potential vorticity (PV) is used as the dynamic tracer, and observed winds on isentropic surfaces are used to advect the PV contours. Results of the current study show that while it may provide a continuous view of vortex evolution that is spatially and temporally consistent, quantitative information regarding vortex area change derived from CAS is of limited utility. The results are shown to be somewhat sensitive to subtle differences in the wind and PV fields for quantities (such as area) that develop slowly in time. Increasing the temporal resolution of the advecting wind field does not appear to improve the agreement between CAS results and model or observed data. The poor correspondence between the area changes from CAS and those derived from analyzed data suggests that diabatic effects during most of these events are important. Percentage differences between the CAS and the analyzed PV contours are more pronounced above the lower region of the stratosphere, and a large part of the differences are acquired in the first day of a CAS calculation. Contour length parameters such as e-folding time are relatively insensitive to uncertainties in the initial conditions, suggesting that quantities that change rapidly (as length does) are better determined by CAS; however, the filamentary structure contributes little to vortex area results. (Author)
Frequent atmospheric measurements of the anthropogenic compound methylchloroform that were made between 1978 and 1985 indicate that this species is continuing to increase significantly around the ...world. Reaction with the major atmospheric oxidant, the hydroxyl radical (OH), is the principal sink for this species. The observed mean trends for methylchloroform are 4.8, 5.4, 6.4, and 6.9 percent per year at Aldrigole (Ireland) and Cape Meares (Oregon), Ragged Point (Barbados), Point Matatula (American Samoa), and Cape Grim (Tasmania), respectively, from July 1978 to June 1985. These measured trends, combined with knowledge of industrial emissions, were used in an optimal estimation inversion scheme to deduce a globally averaged methylchloroform atmospheric lifetime of 6.3 (+1.2, -0.9) years (1σ uncertainty) and a globally averaged tropospheric hydroxyl radical concentration of (7.7 $\pm $ 1.4) $\times $ 10$^{5}$ radicals per cubic centimeter (1σ uncertainty). These 7 years of gas chromatographic measurements, which comprise about 60,000 individual calibrated real-time air analyses, provide the most accurate estimates yet of the trends and lifetime of methylchloroform and of the global average for tropospheric hydroxyl radical levels. Accurate determination of hydroxyl radical levels is crucial to understanding global atmospheric chemical cycles and trends in the levels of trace gases such as methane.
The first in‐situ measurements by automated gas chromatograph‐mass spectrometer are reported for 1,1,1,2‐tetrafluoroethane (HFC‐134a), 1,1‐dichloro‐1‐fluoroethane, (HCFC‐141b), and ...1‐chloro‐1,1‐difluoroethane, (HCFC‐142b). These compounds are steadily replacing the chlorofluorocarbons (CFCs) as refrigerants, foam‐blowing agents, and solvents. The concentrations of all three compounds are shown to be rapidly increasing in the atmosphere, with 134a increasing at a rate of 2.05±0.02 ppt yr−1 over the 30 months of observations. Similarly, 141b and 142b increased at rates of 2.49±0.03 and 1.24±0.02 ppt yr−1, respectively, over the same period. The concentrations recorded at the atmospheric research station at Mace Head, Ireland, on January 1, 1996, the midpoint of the time series, were 3.67 ppt (134a), 7.38 ppt (141b), and 8.78 ppt (142b). From these observations we optimally estimate the HCFC and HFC emissions using a 12‐box global model and OH concentrations derived from global 1,1,1‐trichloroethane (CCl3CH3) measurements. Comparing two methods of estimating emissions with independent industry estimates shows satisfactory agreement for 134a and 141b, while for 142b, industry estimates are less than half those required to explain our observations.
A chemical box model constrained by Upper Atmosphere Research Satellite (UARS) measurements of CH4, H2O, NOx, and temperature in 1992–1994 has been used in an attempt to simulate ozone trend ...measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) at 43 km, 45° latitude and their hemispheric differences. The model is successful in simulating the seasonal cycle in ozone mixing ratios observed by the Microwave Limb Sounder (MLS) and the Solar Backscattered Ultraviolet (SBUV) instrument including the 15% larger values in the Southern Hemisphere at the wintertime ozone maximum. The model indicates that this hemispheric asymmetry is associated with hemispheric temperature differences of approximately 10°K; in contrast, hemispheric differences in methane and the resulting effects on ClOx make only a small contribution to this ozone asymmetry. The model predicts that, based on observed Cly increases, the ozone decrease in the Southern Hemisphere should have exceeded that in the Northern Hemisphere by approximately 1%/decade. This is consistent with an observed ozone hemispheric trend difference of 1.7 ± 2.1%/decade (1σ) for 1979–1997 and −0.8 ± 2.8%/decade for 1985–1997 from SAGE I and SAGE II (version 6.1) measurements. These trend differences are caused by hemispheric differences in temperature in early winter and in CH4 in late winter. The model, based on changes in Cly but not in other specified parameters, calculates a larger downtrend of ozone than is observed by SAGE II in 1985–1997 by approximately 3.1 ± 1.4%/decade (1σ), but it qualitatively simulates the observed ozone trend variations over the period 1979–1997. The amplitude of the model calculated downtrend would be reduced by approximately 1%/decade for a methane increase of 1%/yr. Observed temperature decreases of approximately 0.1°K/yr would have decreased the magnitudes of the calculated ozone downtrends on pressure surfaces by approximately 1%/decade but could have affected SAGE trends by a small amount in the opposite direction because of associated geopotential height changes.
A contour advection technique, contour advection with surgery (CAS), is applied to the Northern Hemisphere Arctic vortex during several dynamically active periods in midwinter and at several ...different levels in the stratosphere. The ability fo the technique to accurately depict vortex evolution is assessed.
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