**5. Application to CH**<sup>4</sup>

18 Will-be-set-by-IN-TECH

Pressure (hPa)

0 0.005 0.01

Tropical model HLW model

b)

SCO,Δ (ppbv)−1

a)

0 0.005 0.01

Tropical model HLW model

Fig. 12. Panel on the left: (a) Sensitivity of the IASI spectrum channel at 2195 cm−<sup>1</sup> to CO variations for the case of two atmospheric models; (b) as in (a), but now the sensitivity is

These CO profiles are shown in Fig. 13 and compared to the CO reference profile we have used to perform all the radiative transfer calculations needed to estimate the regression coefficients. It is seen from Fig.13 that the reference profile largely differs form those observed in the lower part of the troposphere. Below 400 hPa, the agreement is excellent, just because we use the same climatology as that used by the JAIVEx team. From Fig. 13 it is clearly

> Reference 29 April 2007 (2 profiles) 30 April 2007 (4 profiles) 04 May 2007 (3 profiles)

0 50 100 150 200 250

CO mixing ratio (ppbv)

seen that the CO profiles for 04 May 2007 are just a crude interpolation of that on 29 April 2007. Nevertheless, they have been included in the comparison with columnar CO retrieved from IASI for completeness and because these profiles constitute for that day the best in situ

Having said that, we see that the JAIVEx experiment provides a case study in which the CO reference profile is different from the supposedly correct CO profile corresponding to the JAIVEx campaign (see Fig.13). Thus, we have a case study in which the shape of the profile, and not only the CO integrated amount, differs form that of the reference profile. This situation allows us to check the sensitivity of the methodology to the shape of the CO profile. The results of our methodology applied to the 25 IASI soundings during the JAIVEx experiment are shown in Fig. 14(a) along with the estimation of the columnar amount from in situ measurements. This last estimate has been obtained by integrating the CO mixing ratio profile (see Fig.13) for each day of the JAIVEx experiment. For each day, the corresponding

Fig. 13. CO profiles for two days of the JAIVEx experiemnt and comparison with the CO reference profile within the retrieval methodology to estimate the CO columnar amount.

SCO (ppbv)−1

computed for the corresponding channel of the difference-spectrum.

Pressure (hPa)

estimate of the CO profile.

Pressure (hPa)

Unlike CO and CO2, methane is not a linear molecule, therefore we have no particular hint from its structure about which interval of the interferogram is most sensitive to the variation of this gas. However, methane together with water vapour, is the main absorber within IASI band 2, which means that if we consider the interferogram of IASI band 2 alone, we should be able to isolate a suitable portion of the interferogram signal, which is mostly dominated by CH4. By trial and error this interval has been identified in the segment 1.34-1.352 cm, for a bandwidth of 0.0120 cm. With this reduced bandwidth, according to Eq. 11, we have a noise reduction within the difference spectrum of 12.90. Actually, because of the effect of IASI noise correlation, the reduction factor is even higher. If we consider that for IASI band 2 we have the better signal-to-noise ratio, we have that methane is the gas, which we can retrieve with the highest stability and accuracy. In particular the channels in the spectral segment 1210 to 1220 cm−<sup>1</sup> exhibit the poorest sensitivity to the state vector, but methane.

A good channel is that at *σ* = 1210.75 cm−1. The regression relation between the channel and the methane columnar amount is a polynomial of third order. The regression error is 0.01 ppmv in case of noise-free radiances and ≈ 0.1 ppmv in case of noisy radiances. The regression relation is invariant with the state vector as it is shown in Fig. 15. The CH4 reference profile we use for the radiative transfer calculation is that shown in Fig. 15(d), which gives a columnar amount for methane of 1.65 ppmv. The sensitivity, *SCH*4,Δ(*σ*) of the *d*-spectrum

Observations. Methodological Aspects and Application to IASI 21

Atmospheric Gases Concentrations from High Spectral Resolution Satellite Observations...

<sup>267</sup> Fourier Transform Spectroscopy with Partially Scanned Interferograms as a Tool to Retrieve

Fig. 16. (a)- CH4 integrated amount estimated from IASI. (b)- IASI CH4 for July 2010 over the

Finally, as done for CO2 and CO, for illustrative purposes Fig. 16(b) shows a monthly map of CH4 computed over the Mediterranean area for the month of July 2010. Also for CH4, the map clearly shows a gradient in the North-to-South direction, which is consistent with the

As for the methane, N2O is not a linear molecule, therefore partial interferograms, which are capable of enhancing the variations of this gas with respect to those of other dominant atmospheric parameters, have to be be judiciously found by careful inspections of synthetic interferogram signals generated as a function of N2O columnar amount, q*<sup>N</sup>*2*O*. Using this strategy we have found that the partial interferogram in the range 1.06-1.08 cm for a width of 0.02 cm is largely sensitive to N2O. With this reduced bandwidth, according to Eq. 11, we have a noise reduction within the difference spectrum of 10. Actually, because of the effect of IASI noise correlation, the reduction factor is even higher. However, we have to consider that N2O absorption insists within the IASI band 3, which is that with the worse signal-to-noise ratio. With this in mind we have that accuracy with which we can estimate *qN*2*<sup>O</sup>* is of the order of 10% at the level of single channels. In addition, *good* channels tend to be strongly correlated, therefore there is no advantage in trying to combine them to improve the final accuracy.

The regression relation, which fits to the data with an error of less than 3 ppbv is a polynomial of fourth order. The polynomial is independent of the atmospheric sate vector as it is shown in Fig. 17 which exemplifies the polynomial regression for the case of the *d*-channel at 2165.50

The N2O reference profile we use for the radiative transfer calculation is that shown in Fig. 17(d), which gives a columnar amount for N2O of 306.5 ppbv. The sensitivity, *SN*2*O*,Δ(*σ*) of the *d*-spectrum (channel at *σ* = 2204.5 cm−1) to N2O is shown in 17(c) for the tropical and High Latitude Winter models of atmosphere, whose main atmospheric parameters have been

The philosophy and mechanism of the procedure to estimate N2O columnar amount from IASI observations is the same as that illustrated for CO2 in section 3.3. For N2O, the procedure

Average CH4

concentration for July 2010 (ppmv)

(b) Mediterranean case study

<sup>0</sup> <sup>5</sup> <sup>10</sup> <sup>15</sup> <sup>20</sup> <sup>25</sup> 1.2

Number of IASI Sounding

(a) JAIVEx case study

general circulation of the Mediterranean area.

1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2

Mediterranean area.

**6. Application to N**2**O**

cm−1.

shown in Fig. 4

**6.1 Application to IASI data**

qCH4 (ppbv)

(channel at *σ* = 1210.75 cm−1) to methane is shown in 15(c) for the tropical and High Latitude Winter models of atmosphere, whose main atmospheric parameters have been shown in Fig. 4

Fig. 15. The polynomial fit at 1210.75 cm−<sup>1</sup> exemplified for two models of atmosphere, (a) and (b); the sensitivity to methane (c); the reference methane mixing ratio profile (d).

#### **5.1 Application to IASI data**

The mechanism of the procedure to estimate CH4 columnar amount from IASI observations is the same as that illustrated for CO2 in section 3.3. The procedure for methane has been applied to the 25 IASI spectra and the results are shown in Fig. 16(a). We see that the columnar amount is very stable and varies in between 1.60-1.90 ppmv with an average of (1.70 ± 0.02) ppmv. We remember that this data were acquired in 2007. Today the average global value of methane is credited of a value equal to 1.74 ppmv (Blasing, 2011). During the JAIVEx experiment there were no in situ observations of methane. However, we can perform a consistency check about the observed and computed variability. According to our procedure, the accuracy of the estimates is ≈ 0.094 ppmv. Because the JAIVEx case study consider a limited target area, we have to expect a very low variability as far as the columnar amount of CH4 is considered. This means that the variability we see in Fig. 16(a) has to be largely due to random fluctuations, therefore the standard deviation of the 25 IASI estimates has to be consistent with the computed accuracy of 0.094 ppmv. In fact, this standard deviation gives the value 0.0945 ppmv.

Fig. 16. (a)- CH4 integrated amount estimated from IASI. (b)- IASI CH4 for July 2010 over the Mediterranean area.

Finally, as done for CO2 and CO, for illustrative purposes Fig. 16(b) shows a monthly map of CH4 computed over the Mediterranean area for the month of July 2010. Also for CH4, the map clearly shows a gradient in the North-to-South direction, which is consistent with the general circulation of the Mediterranean area.
