Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 2: Methane

Abstract

Abstract. Carbon dioxide (CO 2 ) and methane (CH 4 ) are the two most important anthropogenic greenhouse gases. SCIAMACHY on ENVISAT is the first satellite instrument whose measurements are sensitive to concentration changes of the two gases at all altitude levels down to the Earth's surface where the source/sink signals are largest. We have processed three years (2003)(2004)(2005) of SCIAMACHY nearinfrared nadir measurements to simultaneously retrieve vertical columns of CO 2 (from the 1.58 µm absorption band), CH 4 (1.66 µm) and oxygen (O 2 A-band at 0.76 µm) using the scientific retrieval algorithm WFM-DOAS. We show that the latest version of WFM-DOAS, version 1.0, which is used for this study, has been significantly improved with respect to its accuracy compared to the previous versions while essentially maintaining its high processing speed (∼1 min per orbit, corresponding to ∼6000 single measurements, and per gas on a standard PC). The greenhouse gas columns are converted to dry air column-averaged mole fractions, denoted XCO 2 (in ppm) and XCH 4 (in ppb), by dividing the greenhouse gas columns by simultaneously retrieved dry air columns. For XCO 2 dry air columns are obtained from the retrieved O 2 columns. For XCH 4 dry air columns are obtained from the retrieved CO 2 columns because of better cancellation of light path related errors compared to using O 2 columns retrieved from the spectrally distant O 2 Aband. Here we focus on a discussion of the XCH 4 data set. The XCO 2 data set is discussed in a separate paper (Part 1). For 2003 we present detailed comparisons with the TM5 model which has been optimally matched to highly accurate but sparse methane surface observations. After accounting for a systematic low bias of ∼2% agreement with TM5 is typically within 1-2%. We investigated to what extent the SCIAMACHY XCH 4 is influenced by the variability Correspondence to: M. Buchwitz (michael.buchwitz@iup.physik.unibremen.de) of atmospheric CO 2 using global CO 2 fields from NOAA's CO 2 assimilation system CarbonTracker. We show that the CO 2 corrected and uncorrected XCH 4 spatio-temporal pattern are very similar but that agreement with TM5 is better for the CarbonTracker CO 2 corrected XCH 4 . In line with previous studies (e.g., Frankenberg et al., 2005b) we find higher methane over the tropics compared to the model. We show that tropical methane is also higher when normalizing the CH 4 columns with retrieved O 2 columns instead of CO 2 . In consistency with recent results of Frankenberg et al. (2008b) it is shown that the magnitude of the retrieved tropical methane is sensitive to the choice of the spectroscopic line parameters of water vapour. Concerning inter-annual variability we find similar methane spatio-temporal pattern for 2003 and 2004. For 2005 the retrieved methane shows significantly higher variability compared to the two previous years, most likely due to somewhat larger noise of the spectral measurements.

Greenhouse gas relationships in the Indian summer monsoon plume measured by the CARIBIC passenger aircraft

“…High summer time mixing ratios over South Asia were reported for CO (Fu et al, 2006;Park et al, 2008), for H 2 O (Park et al, 2004;Fu et al, 2006;Randel and Park, 2006), and for CH 4 (Park et al, 2004;Schneising et al, 2009;Xiong et al, 2009). Other trace gases that were observed to be enhanced in the monsoon region included NO x (Park et al, 2004), HCN, C 2 H 6 , and C 2 H 2 (Park et al, 2008).…”

Influence of Meteorology and interrelationship with greenhouse gases (CO<sub>2</sub> and CH<sub>4</sub>) at a suburban site of India

“…CH 4 shows a positive relationship during monsoon and postmonsoon and an inverse relationship during pre-monsoon and winter, while a reverse relationship exists for CO 2 . During the wet season aeration is restricted (Smith et al, 2003); hence soil respiration is limited, which decrease CO 2 flux. This may be one of the factors leading to low values of CO 2 during monsoon months; during dry months soil may act as a sink of CH.…”

“…The MODIS was launched in December 1999 on the polar-orbiting NASA-EOS Terra platform (Salomonson et al, 1989;King et al, 1992). It has 36 spectral channels and acquires data in three spatial resolutions of 250, 500, and 1 km (channels 8-36), covering the visible, nearinfrared, shortwave infrared, and thermal-infrared bands.…”

“…The major sources for CH 4 in the Indian region are rice, paddies, wetlands, and ruminants (Schneising et al, 2009). The annual CH 4 concentration over study area is observed to be 1.92 ± 0.07 ppm, with a maximum (2.02 ± 0.01 ppm) observed in post-monsoon and minimum (1.85 ± 0.03 ppm) in monsoon.…”

On the efficient treatment of temperature profiles for the estimation of atmospheric transmittance under scattering conditions

“…Consequently, many forward simulation modules employed in retrieval algorithms in the shortwave region are based on pre-calculated look-up tables of the desired radiative quantity, which is tabulated as a function of the measurement geometry and any influencing geophysical parameter (see e.g. Gao and Kaufman, 2003;Schneising et al, 2009;Lindstrot et al, 2012).…”