Thomas M. Stephen

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.

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.

Improvements to air mass calculations for ground-based infrared measurements

High-resolution ground-based infrared solar spectra are routinely recorded at the Network for the Detection of Stratospheric Change (NDSC) stations. These data sets play a key role in providing a long-term record of atmospheric composition and their links to climate change. The analysis of observed infrared spectra involves comparison to a computer-modeled atmosphere where knowledge of the air mass distribution is an essential component. This note summarises improvements made to an existing and widely used computer code (FSCATM) to perform refractive ray-tracing and calculation of the air mass distribution. Changes were made towards higher vertical resolution in the troposphere and increased numerical precision. The revised FSCATM improves the analysis of infrared spectra mostly through the more accurate representation of the temperature profile. Air mass differences with respect to earlier versions are documented and are typically <0.7%, exceptions being extreme cases of inversion layers. The current version provides ray tracing and air mass calculations for any terrestrial observation site. The output files are reported in a format compatible with the SFIT and SFIT2 retrieval algorithms, which are widely used for NDSC infrared atmospheric studies. The improved computer code, documentation, reference profiles, and test cases are available electronically.

Spectroscopic parameters for ozone and its isotopes: recent measurements, outstanding issues, and prospects for improvements to HITRAN

In this article we review ozone spectroscopy from the microwave to the ultraviolet since the release of the 1996 HITRAN database. Uncertainties, deficiencies, areas of potential improvement, and anticipated new spectral line parameters datasets are highlighted.

SPECTROSCOPIC PARAMETERS FOR OZONE AND ITS ISOTOPES: CURRENT STATUS, PROSPECTS FOR IMPROVEMENT, AND THE IDENTIFICATION OF 16O16O17O AND 16O17O16O LINES IN INFRARED GROUND-BASED AND STRATOSPHERIC SOLAR ABSORPTION SPECTRA

Abstract We describe the updates to the spectroscopic parameters of ozone and its isotopes in the 1996 HITRAN compilation. Recent published studies not included in HITRAN are also summarized. Finally, we report the identification of infrared lines of the ν3 bands of 16O16O17O and 16O17O16O in high-resolution solar spectra recorded by stratospheric balloon-borne and ground-based Fourier transform spectrometers.

O2 continuum: A possible explanation for the discrepancies between measured and modeled shortwave surface irradiances

Ground based solar spectra between 1 and 5µm (10,000 and 2000 cm−1) have been obtained with a new, absolutely calibrated spectrometer. These spectra have sufficient spectral resolution (2 cm−1) to allow atmospheric absorption lines to be distinguished from underlying continuum absorption. Continuum absorption due to O2 has been identified around 1.06µm (9366 cm−1), 1.27µm (7882 cm−1), and 1.6µm (6326 cm−1). These continuum absorptions are not included in the HITRAN database. Similar continuum absorption should be present around the 0.762µm (13121 cm−1) and 0.688µm (14525 cm−1) bands of O2.

Isotopic ozone in the 5 μ region from high resolution balloon-borne and ground-based ftir solar spectra

Abstract High resolution (0.002–0.004 cm−1) i.r. solar absorption spectra of the stratosphere obtained during University of Denver balloon flights, and from the ground-based Network for the Detection of Stratospheric Change (NDSC) observatory at Mauna Loa, Hawaii, show numerous spectral features of several isotopic species of O3, in both the 10 μ and 5 μ regions. Many of the 5 μ lines reported here have not been previously observed in atmospheric spectra. The identification and quantification of the lines proceed by combined analyses of the atmospheric spectra, laboratory spectra of enriched samples, and updated line parameter calculations.

Correlation relationships of stratospheric molecular constituents from high spectral resolution, ground‐based infrared solar absorption spectra

Comparisons of chemically active species with chemically inert tracers are useful to quantify transport and mixing and assess the accuracy of model predictions. We report measurements of chemically active species and chemically inert tracers in the stratosphere derived from the analysis of infrared solar absorption spectra recorded with a ground-based Fourier transform spectrometer operated typically at 0.005- to 0.01-cm−1 spectral resolution. The measurements were recorded from Kitt Peak in southern Arizona (latitude 31.9°N, 111.6°W, 2.09 km altitude). Time series of N2O, CH4, O3, and HNO3 vertical profiles have been retrieved from measurements in microwindows. From these results, correlations between N2O and CH4 stratospheric mixing ratios and between O3 and HNO3 lower stratospheric mixing ratios have been derived. The measured correlations between N2O versus CH4 mixing ratios are compact and show little variability with respect to season in quantitative agreement with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) spring and autumn measurements recorded near the same latitude. Lower stratospheric O3 versus HNO3 mixing ratios measured during low to moderate aerosol loading time periods also show a compact relations though the HNO3/O3 slope is a factor of 2 lower than obtained from November 1994 ATMOS measurements near the same latitude. We also compare Kitt Peak and ATMOS N2O versus CH4 and O3 versus HNO3 relations obtained by averaging the measurements over two broad stratospheric layers. This comparison avoids bias from the a priori profiles and the limited vertical resolution of the ground-based observations.