Validation of Halogen Occultation Experiment CH₄ measurements from the UARS

Abstract

Global distributions of CH 4 in the mesosphere and stratosphere have been measured continuously since October 11, 1991, by the Halogen Occultation Experiment (HALOE) onboard the UARS. CH 4 mixing ratio is obtained using the gas filter correlation technique operating in the 3.3-grn region. Since measurements are made during solar occultation in the 57 ø inclination orbit, data are collected 15 times daily for both sunrises and sunsets. This provides coverage of one hemisphere in a month period. One complete hemispheric sweep (from equator to -80 ø latitude) is made during the spring and summer seasons of two hemispheres, and a partial sweep (from equator to around 50 ø latitude) is made during the fall and winter seasons of two hemispheres. HALOE CH 4 measurements are validated using direct comparisons with correlative data and internal consistency checks using other HALOE-measured tracers, HF, and aerosols. It is estimated for the 0.3-to 50-mbar region that the total error, including systematic and random components, is less than 15 % and that the precision is better than 7%. The CH4 gas filter channel does not depend significantly on the Pinatubo aerosol extinction. An experimentally accurate measurement of CH 4 is very important because CH 4 is a primary interfering gas in the HALOE HC1 channel and, subsequently, can cause HC1 measurement error. Simultaneous measurements of CH4 and other HALOE species (03, H20, NO, NO2, HC1, HF, and aerosol extinction coefficients) provide important information on atmospheric dynamic and chemical processes, since CH4 can be used as a tracer and an indicator of atmospheric transport processes. Several new pieces of infolsnation on previously unreported HALOE-observed features are also presented. Introduction Atmospheric methane, CH 4, naturally produced and released from the surface, has an important role in the greenhouse effect, causing a global warming, and in atmospheric chemistry, by removing hydroxyl radicals in the troposphere and in the stratosphere by converting reactive chlorine atoms to HC1 and producing hydrogen species to form water vapor by oxidation (see summary [World Meteorological Organization (WMO), 1982]). Methane can be used as a tracer to study atmospheric transport processes when enough measurements are made on a regional or global scale. For example, the features of CH 4 and other species observed by the Halogen Occultation Experiment (HALOE) were reported by Russell et al. [1993a] for the springtime Antarctic and by Park and Russell [1994] for the summertime polar region. Both papers used CH 4 as a tracer, and in the latter paper it was used to separate dynamical effects and to assess the importance of chemical processes for ozone deficiency in the polar regions. Methane measurements in the stratosphere before the Upper Atmosphere Research Satellite (UARS) [Reber, 1993] include the 1979 global measurements by stratospheric and mesospheric sounder (SAMS) on Nimbus 7 [Rodgers et al., 1984] and otherwise sparse individual profiles obtained by in situ sampli...

Impact of land convection on troposphere-stratosphere exchange in the tropics

“…The HALOE instrument onboard the UARS satellite measured CH 4 from October 1991 to November 2005 (Russell et al, 1993 (Park et al, 1996) in the 50-100 hPa domain whilst the precision (including random error) is ∼7%. The vertical resolution is ∼2-3 km.…”

Evaluation of transport in the lower tropical stratosphere in a global chemistry and transport model

“…The vertical resolution for methane profiles is about 4 km; between 50 hPa and 0.2 hPa the total error is less than 15% [Park et al, 1996] In situ observations of ozone [Profitt et al, 1989] and reactive nitrogen (NOy) [Fahey et al, 1989] The data show that the interior of the tropics between 50 and 70 hPa is strongly isolated from the middle latitudes [Fahey et al, 1996]. The gradient in the observed ratio NOy/O3 is weaker at middle latitudes and sharper in the subtropics than the gradient of either NOy or 03.…”

Methane as a diagnostic tracer of changes in the Brewer–Dobson circulation of the stratosphere

“…This concern is explored as follows. First, it is noted that major advantages of solar occultation measurements are the high signal-to-noise ratios for their limb transmission profiles and their inherently good vertical resolution; each retrieved HALOE CH 4 profile is quite accurate and has a vertical resolution of about 4 km (Park et al, 1996). Still, for the lower latitudes HALOE typically made its measurements across a given latitude bin over just a few days, due to the alternating nature of the time sequence of its SR and SS occultations from the UARS satellite.…”