This paper presents upper tropospheric methane (CH4) and nitrous oxide (N2O) concentrations retrieved from thermal infrared spectra as observed by the remote sensor IASI (Infrared Atmospheric Sounding Interferometer) on-board the EUMETSAT/MetOp meteorological satellites. The CH4 and N2O mixing ratios are retrieved as side products of the MetOp/IASI retrieval developed for the European Research Council project MUSICA (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water). The MUSICA/IASI CH4 and N2O retrieval strategy is described in detail as well as their characterisation in terms of the vertical resolution and expected errors. Theoretically, we document that MUSICA/IASI products can capture the upper tropospheric CH4 and N2O variability (at ≈ 300–350 hPa) with a precision better than 2 %. We compare the remote sensing data to coincident high precision aircraft vertical profiles taken within the HIAPER Pole-to-Pole Observations (HIPPO) project and empirically estimate a precision of 2.1 % (38.2 ppbv) for each individual IASI CH4 observation (see Figure 1). The precision is improved to 1.7 % (32.1 ppbv) for IASI data that have been averaged within 2° × 2° boxes. For N2O the empirically estimated precision is 2.7 % (8.7 ppbv) for each individual observation and 2.1 % (6.9 ppbv) for the 2° × 2° averages.
In addition, we present a product that combines the CH4 and N2O retrieval estimates. The combination is made a-posteriori and we theoretically and empirically show that the combined product has a much better precision than the individual CH4 and N2O products. For the combined product the theoretical precision is 0.8 % and the comparison with HIPPO data gives an empirical precision estimate of 1.5 % (26.3 ppbv) when considering all individual IASI observations and of 1.2 % (21.8 ppbv) for the 2° ×2° averages (see Figure 1). These results mean that in the case that the horizontal, vertical and temporal variation of N2O can be robustly modeled, we can easily reconstruct CH4 from the combined product and generate high quality IASI CH4 data.
In the next step it should be examined to what extent IASI observations of upper tropospheric CH4 and N2O variations can help to investigate the emission source patterns, transport pathways and sinks of these greenhouse gases (see Figure 2), and whether the provision of a highly precise product ("CH4 combined with N2O") offers additional beneﬁts for such research purposes.
Figure 1: Correlation plots for the comparison between MUSICA/IASI and HIPPO data. Top panel: (a) scatter plot of the IASI N2O VMR versus the HIPPO smoothed N2O VMR at 10 km considering all IASI-HIPPO coincidences (N=1408); (b) same as (a), but for CH4; (c) same as (a), but for the combined CH4 VMR with the simultaneous N2O VMR observations. Bottom panel: same as top panel (d, e, and f), but averaging all the IASI observations within the collocation box of±2◦ latitude/longitude centred at each HIPPO mean location (N=36). The stars indicate the a-priori VMR at 10 km used for the IASI retrievals, while the solid and dashed black lines represent the least squares ﬁts and the diagonals (x=y), respectively. The ﬁt parameters are shown in the legend.
Figure 2: Upper panels: example of the MUSICA/IASI-A N2O global maps at ≈350-300 hPa in winter (16.02.2014). Middle and bottom panels: same as upper panels, but for CH4 and for the combined product, respectively.
Further details about this study can be found at the on-line paper: García, O. E., Sepúlveda, E., Schneider, M., Wiegele, A., Borger, C., Hase, F., Barthlott, S., Blumenstock, T., and de Frutos, Á. M.: Upper tropospheric CH4 and N2O retrievals from MetOp/IASI within the project MUSICA, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2016-326, in review, 2017. http://www.atmos-meas-tech-discuss.net/amt-2016-326/