F. J. Murcray

Atmospheric sulfur hexafluoride: Sources, sinks and greenhouse warming

Model calculations using estimated reaction rates of sulfur hexafluoride (SF6) with OH and O(1D) indicate that the atmospheric lifetime due to these processes may be very long (25,000 years). An upper limit for the UV cross section would suggest a photolysis lifetime much longer than 1000 years. The possibility of other removal mechanisms are discussed. The estimated lifetimes are consistent with other estimated values based on recent laboratory measurements. There appears to be no known natural source of SF6. An estimate of the current production rate of SF6 is about 5 kt/yr. Based on historical emission rates, we calculated a present-day atmospheric concentrations for SF6 of about 2.5 parts per trillion by volume (pptv) and compared the results with available atmospheric measurements. It is difficult to estimate the atmospheric lifetime of SF6 based on mass balance of the emission rate and observed abundance. There are large uncertainties concerning what portion of the SF6 is released to the atmosphere. Even if the emission rate were precisely known, it would be difficult to distinguish among lifetimes longer than 100 years since the current abundance of SF6 is due to emission in the past three decades. More information on the measured trends over the past decade and observed vertical and latitudinal distributions of SF6 in the lower stratosphere will help to narrow the uncertainty in the lifetime. Based on laboratory-measured IR absorption cross section for SF6, we showed that SF6 is about 3 times more effective as a greenhouse gas compared to CFC 11 on a per molecule basis. However, its effect on atmospheric warming will be minimal because of its very small concentration. We estimated the future concentration of SF6 at 2010 to be 8 and 10 pptv based on two projected emission scenarios. The corresponding equilibrium warming of 0.0035°C and 0.0043°C is to be compared with the estimated warming due to CO2 increase of about 0.8°C in the same period.

Long‐term trends of inorganic chlorine from ground‐based infrared solar spectra: Past increases and evidence for stabilization

Long-term time series of hydrogen chloride (HCl) and chlorine nitrate (ClONO2) total column abundances has been retrieved from high spectral resolution ground-based solar absorption spectra recorded with infrared Fourier transform spectrometers at nine NDSC (Network for the Detection of Stratospheric Change) sites in both Northern and Southern Hemispheres. The data sets span up to 24 years and most extend until the end of 2001. The time series of Cl-y (defined here as the sum of the HCl and ClONO2 columns) from the three locations with the longest time-span records show rapid increases until the early 1990s superimposed on marked day-to-day, seasonal and inter-annual variability. Subsequently, the buildup in Cl-y slows and reaches a broad plateau after 1996, also characterized by variability. A similar time evolution is also found in the total chlorine concentration at 55 km altitude derived from Halogen Occultation Experiment (HALOE) global observations since 1991. The stabilization of inorganic chlorine observed in both the total columns and at 55 km altitude indicates that the near-global 1993 organic chlorine (CCly) peak at the Earths surface has now propagated over a broad altitude range in the upper atmosphere, though the time lag is difficult to quantify precisely from the current data sets, due to variability. We compare the three longest measured time series with two-dimensional model calculations extending from 1977 to 2010, based on a halocarbon scenario that assumes past measured trends and a realistic extrapolation into the future. The model predicts broad Cl-y maxima consistent with the long-term observations, followed by a slow Cl-y decline reaching 12-14% relative to the peak by 2010. The data reported here confirm the effectiveness of the Montreal Protocol and its Amendments and Adjustments in progressively phasing out the major man-related perturbations of the stratospheric ozone layer, in particular, the anthropogenic chlorine-bearing source gases. (Less)

Infrared Solar Spectroscopic Measurements of Free Tropospheric CO, C2H6, and HCN above Mauna Loa, Hawaii: Seasonal Variations and Evidence for Enhanced Emissions from the Southeast Asian Tropical Fires of 1997-1998

High spectral resolution (0.003/ cm) infrared solar absorption measurements of CO, C2H6, and HCN have been recorded at the Network for the Detection of Stratospheric Change station on Mauna Loa, Hawaii, (19.5 deg N, 155.6 deg W, altitude 3.4 km). The observations were obtained on over 250 days between August 1995 and February 1998. Column measurements are reported for the 3.4 - 16 km altitude region, which corresponds approximately to the free troposphere above the station. Average CO mixing ratios computed for this layer have been compared with flask sampling CO measurements obtained in situ at the station during the same time period. Both show asymmetrical seasonal cycles superimposed on significant variability. The first two years of observations exhibit a broad January-April maximum and a sharper CO minimum during late summer. The C2H6 and CO 3.4 - 16 km columns were highly correlated throughout the observing period with the C2H6/CO slope intermediate between higher and lower values derived from similar infrared spectroscopic measurements at 32 deg N and 45 deg S latitude, respectively. Variable enhancements in CO, C2H6, and particularly HCN were observed beginning in about September 1997. The maximum HCN free tropospheric monthly mean column observed in November 1997 corresponds to an average 3.4 - 16 km mixing ratio of 0.7 ppbv (1 ppbv = 10(exp -9) per unit volume), more than a factor of 3 above the background level. The HCN enhancements continued through the end of the observational series. Back-trajectory calculations suggest that the emissions originated at low northern latitudes in southeast Asia. Surface CO mixing ratios and the C2H6 tropospheric columns measured during the same time also showed anomalous autumn 1997 maxima. The intense and widespread tropical wild fires that burned during 3 the strong El Nino warm phase of 1997-1998 are the likely source of the elevated emission products.

Multiyear infrared solar spectroscopic measurements of HCN, CO, C2H6, and C2H2 tropospheric columns above Lauder, New Zealand (45°S latitude)

[i] Near-simultaneous, 0.0035 or 0.007 cm -1 resolution infrared solar absorption spectra of tropospheric HCN, C 2 H 2 , CO, and C 2 H 6 have been recorded from the Network for the Detection of Stratospheric Change station in Lauder, New Zealand (45.04°S, 169.68°E, 0.37 km altitude). All four molecules were measured on over 350 days with HCN and C 2 H 2 reported for the first time based on a new analysis procedure that significantly increases the effective signal-to-noise of weak tropospheric absorption features in the measured spectra. The CO measurements extend by 2.5 years a database of measurements begun in January 1994 for CO with improved sensitivity in the lower and middle troposphere. The C 2 H 6 measurements lengthen a time series begun in July 1993 with peak sensitivity in the upper troposphere. Retrievals of all four molecules were obtained with an algorithm based on the semiempirical application of the Rodgers optimal estimation technique. Columns are reported for the 0.37- to 12-km-altitude region, approximately the troposphere above the station. The seasonal cycles of all four molecules are asymmetric, with minima in March-June and sharp peaks and increased variability during August-November, which corresponds to the period of maximum biomass burning near the end of the Southern Hemisphere tropical dry season. Except for a possible HCN column decrease, no evidence was found for a statistically significant long-term trend.

Stratospheric N 2 O mixing ratio profile from high-resolution balloon-borne solar absorption spectra and laboratory spectra near 1880 cm −1

A nonlinear least-squares fitting procedure has been used to derive the stratospheric N(2)O mixing ratio profile from balloon-borne solar absorption spectra and laboratory spectra near 1880 cm(-1). The atmospheric spectra were recorded during sunset from a float altitude of 33 km with the University of Denver 0.02-cm(-1) resolution interferometer near Alamogordo, N.M. (33 degrees N), on 10 Oct. 1979. The laboratory data were used to determine the N(2)O line intensities. The measurements indicate an N(2)O mixing ratio of 264 ppbv near 15 km decreasing to 155 ppbv near 28 km.

Observed atmospheric collision-induced absorption in near-infrared oxygen bands

A recent high-resolution measurement of surface solar radiance taken under cloud-free conditions by the Absolute Solar Transmittance Interferometer shows clear indication of continuum absorption associated with the three strongest O2 a1 Δg ← X3Σg− transitions. The differences between these measurements and a calculation by the line-by-line radiative transfer model (LBLRTM) were used to determine the properties of these collision-induced bands and, for two of the bands, led to parameterizations of the spectral behavior of the absorption coefficients. The results indicate that these continuum bands absorb 0.84 W/m2 at the 71.5° solar zenith angle associated with the observation. For the observed 3000–9965 cm−1 spectrum, with the exception of the spectral range in which this collision-induced absorption occurs, there is good agreement between the measured and calculated radiance values, with no evidence for discrete absorption by unknown gases.

Measurements of the Downward Longwave Radiation Spectrum over the Antarctic Plateau and Comparisons with a Line-by-Line Radiative Transfer Model for Clear Skies

A 1-year field program was conducted at South Pole Station in 1992 to measure the downward infrared radiance spectrum at a resolution of 1 cm−1 over the spectral range 550–1667 cm−1. The atmosphere over the Antarctic Plateau is the coldest and driest on Earth, where in winter, surface temperatures average about −60°C, the total column water vapor is as low as 300 μm of precipitable water, and the clear-sky downward longwave flux is usually less than 80 W m−2. Three clear-sky test cases are selected, one each for summer, winter, and spring, for which high-quality radiance data are available as well as ancillary data to construct model atmospheres from radiosondes, ozonesondes, and other measurements. The model atmospheres are used in conjunction with the line-by-line radiative transfer model (LBLRTM) to compare model calculations with the spectral radiance measurements. The high-resolution calculations of LBLRTM (≈0.001 cm−1) are matched to the lower-resolution measurements (1 cm−1) by adjusting their spectral resolution and by applying a correction for the finite field of view of the interferometer. In summer the uncertainties in temperature and water vapor profiles dominate the radiance error in the LBLRTM calculations. In winter the uncertainty in viewing zenith angle becomes important as well as the choice of atmospheric levels in the strong near-surface temperature inversion. The spectral radiance calculated for each of the three test cases generally agrees with that measured, to within twice the total estimated radiance error, thus validating LBLRTM to this level of accuracy for Antarctic conditions. However, the discrepancy exceeds twice the estimated error in the gaps between spectral lines in the region 1250–1500 cm−1, where emission is dominated by the foreign-broadened water vapor continuum.

Spectral least squares quantification of several atmospheric gases from high resolution infrared solar spectra obtained at the South Pole

Abstract Spectral least squares fitting has been used to analyze high resolution (0.02 cm -1 ) i.r. solar spectra obtained at the South Pole in 1980. The spectral regions analyzed allow the simultaneous quantification of CO 2 , H 2 O, N 2 O, CH 4 , and O 3 . Information is obtained on the column amount and on the vertical distribution.

The ν1, 2ν2, and ν3 interacting bands of 14N16O2: Line positions and intensities

Abstract High-resolution Fourier transform spectra have been used in the 1200–1850 cm −1 spectral region to measure the positions of lines with K a = 0–15 for the ν 1 and ν 3 bands of 14 N 16 O 2 , as well as lines with K a = 0, 1, 2, and 6 for the 2 ν 2 band up to very high N values. The spin-rotation energy levels were very satisfactorily reproduced using a theoretical model which takes explicitly into account both the Coriolis interactions between the spin-rotation levels of the (001) vibrational state with those of (100) and (020), and the spin-rotation resonances in each vibrational state. In this analysis, precise band centers and rotational, spin-rotation, and coupling constants were obtained for the triad {(100), (020), (001)} of interacting states for 14 N 16 O 2 . In addition, using a large set of individual line intensities we have determined precisely the ν 1 , 2 ν 2 , and ν 3 transition moment operators of 14 N 16 O 2 . Finally, a comprehensive list of line positions and intensities of the interacting ν 1 , 2 ν 2 , and ν 3 bands of 14 N 16 O 2 has been generated.

Mt. Pinatubo SO2 Column Measurements From Mauna Loa

Absorption features of the v1 band of SO2 have been identified in high resolution infrared solar absorption spectra recorded from Mauna Loa, Hawaii, on July 9 and 12, 1991, shortly after the arrival of the first eruption plume from the Mt. Pinatubo volcano in the Phillipines. A total SO2 vertical column amount of (5.1 ± 0.5) × 1016 molecules cm−2 on July 9 has been retrieved based on nonlinear least-squares spectral fittings of 9 selected SO2 absorption features with an updated set of SO2 spectral parameters. A SO2 total column upper limit of 0.9 × 1016 molecules cm−2 deduced from measurements on September 20–24, 1991, is consistent with the dispersion of the SO2 cloud and the rapid conversion of the SO2 vapor into volcanic aerosol particles.