Measurements of methane and nitrous oxide distributions by the improved stratospheric and mesospheric sounder: Retrieval and validation

Abstract

The improved stratospheric and mesospheric sounder (ISAMS) used the pressure modulation technique to observe infrared emissions from the atmosphere at 7.4/•m and 7.8/•m. The target gases, methane and nitrous oxide, both emit at these wavelengths and so their concentrations have been determined by joint retrieval from the available signals. This paper describes the measurement technique, the current retrieval scheme (version 10), and the data set produced. The version 10 data set comprises typically 2600 profiles per day on 180 days between September 26, 1991, and July 29, 1992. Retrieved profiles extend in altitude from 7 mbar to 0.08 mbar for methane and from 7 mbar to 0.8 mbar for nitrous oxide. The precision of the data is better than 20% over much of the range and estimated systematic uncertainties are less than 30%. Comparisons with coincident measurements show that the systematic uncertainties are a reasonable estimate for the methane data but reveal a much larger positive bias for the nitrous oxide data relative to other measurements. Accounting for aerosol contamination effects and a priori biases, ISAMS methane data are recommended for use in scientific studies at altitudes between 7 mbar (5 mbar in the tropics) and 0.1 mbar. ISAMS nitrous oxide data are recommended for use between 7 mbar and 1.0 mbar where relative rather than absolute values are required. Introduction Observations of methane and nitrous oxide in the middle atmosphere have proved useful for a number of reasons. Both gases are produced almost entirely at the Earth's surface and are well-mixed in the troposphere. In the stratosphere, they have relatively long photochemical lifetimes which decrease rapidly with height from over 100 years at 20 km to a few months or less at 40 km [Solomon et al., 1986; Stanford and Ziemke, 1991]. Particularly in the lower and middle stratosphere, these lifetimes are much greater than the timescales of dynamical activity, so that these gases act as long-lived tracers of the atmospheric motions. The end prod-1Atmospheric, ucts of their destruction mechanisms are also important, the reaction of N20 with O( • D) providing the primary source of odd nitrogen in the stratosphere [WMO, 1986], and CH4 oxidation dominating the formation of stratospheric water vapor [Brasseur and Solomon, 1986]. The only global satellite observations of CH4 and N20 available before the launch of the Upper Atmosphere Research Satellite (UARS) [Reber et al., 1993] were those from the stratospheric and mesospheric sounder (SAMS) [Drummond et al., 1980]. These data, as described by Jones and Pyle [1984], revealed a number of interesting features of the atmospheric circulation, including the "double peak" observed at the equinoxes in the middle stratosphere and, subsequently, shown to be related to the semi-annual oscillation [Gray and Pyle, 1986, 1987]. The SAMS data were also used to explore the chemistry of the middle atmosphere, e.g., the relationship between CH4 oxidation and water vapor [Jones, 1984], but nonetheless important questi...

Validation of the Improved Limb Atmospheric Spectrometer‐II (ILAS‐II) Version 1.4 nitrous oxide and methane profiles

“…The Cryogenic Limb Array Etalon Spectrometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS) measured N 2 O and CH 4 [e.g., Roche et al , 1996] as well as other species. The Improved Stratospheric and Mesospheric Sounder (ISAMS) on board UARS produced profiles of N 2 O and CH 4 between 7 and 0.08 hPa from September 1991 to July 1992 [e.g., Remedios et al , 1996]. The Halogen Occultation Experiment (HALOE), also on board UARS, measured CH 4 as well as other species since October 1991 [e.g., Park et al , 1996].…”

Validation of ACE-FTS N<sub>2</sub>O measurements

“…This was followed by the Improved Stratospheric and Mesospheric Sounder (ISAMS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosphere Research Satellite (UARS). ISAMS also used pressure modulator radiometers, operating from 4.6 to 16.3 µm, and provided N 2 O profiles between October 1991 and July 1992 (Taylor et al, 1993;Ruth et al, 1994;Remedios et al, 1996). CLAES also measured N 2 O using thermal limb emission, from 3.5 to 13 µm, between October 1991 and May 1993 .…”

Validation of ACE-FTS v2.2 methane profiles from the upper troposphere to the lower mesosphere

“…nasa.gov/atmos) that was deployed four times on the Space Shuttle (April, 1985, March, 1992, April, 1993, and that can be considered a predecessor of the ACE-FTS instrument (Gunson et al, 1990 andAbrams et al, 1996). Following ATMOS, observations of CH 4 vertical profiles in the stratosphere and upper troposphere have been carried out by the ISAMS (Improved Stratospheric and Mesospheric Sounder) (Remedios et al, 1996), CLAES (Cryogenic Limb Array Etalon Spectrometer) and HALOE (Halogen Occultation Experiment) (Russell et al, 1993; instruments onboard UARS, the Upper Atmosphere Research Satellite (http://uars.gsfc.nasa.gov/www root/homepage/uars-science.html; Reber et al, 1993), that operated between mid-September 1991 and mid-December 2005. ISAMS and CLAES stopped observations in July 1992, and May 1993, respectively.…”