**3.3 Application to IASI data for CO**<sup>2</sup> **estimation**

The mechanics of the procedure to estimate the columnar amount of CO2 can be summarized as follow,


The best estimate of the state vector can be obtained in many modes e.g. colocated ECMWF analysis, radiosonde, inversion of IASI spectral radiances. For the work here shown, the state vector is normally obtained from IASI spectral radiances themselves. As said in section 1 we use a retrieval methodology, which we refer to as *ϕ*-IASI, whose mathematical aspects and validation have been largely documented (see e.g. Amato et al. (2002); Carissimo et al. (2005); Grieco et al. (2007); Masiello & Serio (2004); Masiello et al. (2009; 2011)).

To exemplify the procedure Fig. 8 shows the CO2 columnar amount estimated for the 25 IASI soundings of the JAIVEx experiment. Figure 8(a) also provides a comparison with the ECMWF analysis (Engelen et al., 2009). Although the two analysis are largely in agreement, we see that ECMWF tends to overestimate the CO2 amount in comparison to the findings provided in this work. A comparison with aircraft in situ profiles (Grieco et al., 2011) shows that the analysis provided by our methodology is that correct. Finally, for illustrative purposes Fig. 8(b) shows a monthly map of CO2 computed for the Mediterranean area for the month of July 2010. This has been obtained by processing IASI spectra for July 2010 for sea surface. The spectra were checked for clear sky using the IASI stand alone cloud detection developed by Grieco et al. (2007); Masiello et al. (2002; 2003; 2004); Serio et al. (2000).

Figure 8(b) clearly shows that the CO2 amount is not a constant in the atmosphere. The North-to-South-East trend evidenced in the figure is consistent with the general circulation of the Mediterranean area, and is likely a consequence of the summer African anticyclone, which in July begins to extend its influence in the Mediterranean area. This possible effect is currently under investigation with IASI data.

Fig. 8. (a)- CO2 amount estimated from IASI (this work) and ECMWF analysis. (b)- IASI CO2 for July 2010 over the Mediterranean area.

However, it is also important to stress that with a polar satellite such as METOP/1, the time-space data coverage is not uniform over the Mediterranean area, so that the spatial gradient has to be considered with some care and its assessment needs a suitable transport model. However, our finding is in agreement with similar maps derived by the Atmospheric Infrared Radiometer Sounder (AIRS) for the month of July (Chahine et al, 2008).

#### **4. Application to CO**

14 Will-be-set-by-IN-TECH

shows that the *dj*-channels are sensitive to a large part of the troposphere, from to 900 to 100 hPa, which for the case of a tropical model of atmosphere encompasses all the troposphere above the Planetary Boundary Layer. Conversely, the *dj* channels are almost insensitive to what happen in the boundary layer either for CO2 or e.g. temperature, which is good because the lower troposphere is where we expect to have a larger uncertainty to the atmospheric state. In other words, also in case we implement the technique with a state vector which largely differs from the *truth* in the lower troposphere, we can still have valuable estimates for

The mechanics of the procedure to estimate the columnar amount of CO2 can be summarized

• for a given IASI spectrum, choose the possibly best estimate of the state vector

• With this state vector perform the radiative transfer calculations to yield synthetic IASI spectra (normally 4 are enough) corresponding to different CO2 columnar amounts in the

• Transform the IASI spectra to difference spectra in order to estimate the regression

• Compute the observed *dj* values from the IASI spectrum and input them to Eq. 15 to obtain

The best estimate of the state vector can be obtained in many modes e.g. colocated ECMWF analysis, radiosonde, inversion of IASI spectral radiances. For the work here shown, the state vector is normally obtained from IASI spectral radiances themselves. As said in section 1 we use a retrieval methodology, which we refer to as *ϕ*-IASI, whose mathematical aspects and validation have been largely documented (see e.g. Amato et al. (2002); Carissimo et al. (2005);

To exemplify the procedure Fig. 8 shows the CO2 columnar amount estimated for the 25 IASI soundings of the JAIVEx experiment. Figure 8(a) also provides a comparison with the ECMWF analysis (Engelen et al., 2009). Although the two analysis are largely in agreement, we see that ECMWF tends to overestimate the CO2 amount in comparison to the findings provided in this work. A comparison with aircraft in situ profiles (Grieco et al., 2011) shows that the analysis provided by our methodology is that correct. Finally, for illustrative purposes Fig. 8(b) shows a monthly map of CO2 computed for the Mediterranean area for the month of July 2010. This has been obtained by processing IASI spectra for July 2010 for sea surface. The spectra were checked for clear sky using the IASI stand alone cloud detection developed

Figure 8(b) clearly shows that the CO2 amount is not a constant in the atmosphere. The North-to-South-East trend evidenced in the figure is consistent with the general circulation of the Mediterranean area, and is likely a consequence of the summer African anticyclone, which in July begins to extend its influence in the Mediterranean area. This possible effect is

• Finally, combine these estimates according to Eq. 17 and get the accuracy from Eq. 18.

Grieco et al. (2007); Masiello & Serio (2004); Masiello et al. (2009; 2011)).

by Grieco et al. (2007); Masiello et al. (2002; 2003; 2004); Serio et al. (2000).

currently under investigation with IASI data.

CO2 provided the state vector is sufficiently accurate for the rest of the troposphere.

**3.3 Application to IASI data for CO**<sup>2</sup> **estimation**

corresponding to the IASI overpass.

range 300 to 450 ppmv.

coefficients of Eq. 15.

the estimate at each channel.

as follow,

As for the case of CO2, CO has well defined and known rotation transitions, which yield an absorption band, centered in between the atmospheric window at 4.67 *μ*m (2142 cm−1). Because CO is a linear molecule, its strongest absorption features are regularly spaced and yield a periodic pattern, whose period is <sup>≈</sup> 4 cm−<sup>1</sup> (see Fig. 9). For this reason, the

Fig. 9. The figure shows (a) two synthetic IASI spectra in the spectral region of CO absorption; one of the spectra has been calculated with zero load of CO. (b) The difference between the two evidences the sinusoidal appearance of the CO absorption features. Panel (c) shows the CO reference profile used for radiative transfer calculations.

interferogram has to show a characteristic CO-beating at *x* = 1/4cm = 0.25 cm. We consider a partial interferogram extending from 0.21 to 0.31 cm for a width Δ*x* = 0.1 cm. With the choice Δ*x* = 0.1, the factor of noise reduction of Eq. 11 is approximately 4.45. In addition, since the CO band at 4.67 *μ*m is completely covered by IASI band 3 (this extends from 2000 to 2760 cm−1), the interferogram has been built up for band 3 alone. Finally, the reference CO profile

Observations. Methodological Aspects and Application to IASI 17

<sup>263</sup> Fourier Transform Spectroscopy with Partially Scanned Interferograms as a Tool to Retrieve

on the atmospheric model. However, as in the case of CO2 the standardization parameters, *μ*<sup>1</sup> and *s*<sup>1</sup> do depend on the state vector. In case of noise-free radiances the quadratic fit of Eq. 19

qCO (ppbv)

Fig. 11. Quadratic best fit of *qCO* vs *D*<sup>1</sup> for (a) a tropical model of atmopshere and (b) a High

provides estimates for *qCO* within ±0.50 ppbv. In case of noisy radiances, the accuracy can be

For a tropical model of atmosphere the typical standard deviation of *qCO* estimated by Eq. 19 is ≈ 16 ppbv (around 15% of its present climatological value). This figure increases to about 25 ppbv in a case of a High Latitude Winter air mass. These figures hold for one single IASI observation. The noise can be halved by considering an average over the 2× 2 pixel mask of

As done for CO2, the sensitivity of the *d*1-channel to CO variations has been computed with the help of Eq. 12 and it is compared to that of the equivalent IASI spectrum-channel, *r*(*σ*1) ≡ *r*<sup>1</sup> in Fig. 12. It is seen that the difference spectrum improves the sensitivity of a factor two and more. The figure also allows us to get insight into understanding the atmospheric pressure-range at which the retrieval approach is sensitive. As for the case of CO2, it is seen that the sensitivity extends to a broad range in between 900 to 100 hPa, therefore extending form the Planetary Boundary Layer to the upper troposphere. In contrast to the results of Fig. 12, the sensitivity analysis shows that the *d*<sup>1</sup> channel at hand is poorly sensitive to other

The mechanism of the procedure to estimate CO columnar amount from IASI observations is the same as that illustrated for CO2 in section 3.3. During the JAIVEx experiment the CO profile was recorded by airborne in situ profiles (JAIVEx, 2007) recorded with the commercial gas instrument AL 5002 VUV Fast Fluorescence CO Analyser (produced by Aerolaser GmbH). The analyser employs the measurement of the fluorescence of CO when exposed to UV light at a wavelength of 150 nm, which is proportional to the concentration of CO. The measurements covered the lower-middle troposphere (1000 to 400 hPa) and extended to the upper part of the

atmospheric parameters. This analysis is not shown here for the sake of brevity.

−2 <sup>0</sup> <sup>2</sup> <sup>4</sup> <sup>0</sup>

a)

−2 <sup>0</sup> <sup>2</sup> <sup>4</sup> <sup>0</sup>

b)

HLW Model of atmosphere

Best Fit Data Points

D1

<sup>2</sup>

var(*d*1) (20)

D1

computed by the usual rule of variance propagation directly from Eq. 19, we have

2 *a*1 *s*2 1 *d*<sup>1</sup> + *b*1 *s*1

var(*qCO*) =

Tropical model of atmosphere

Best Fit Data Points

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

qCO (ppbv)

Latitude Winter (HLW) model of atmopshere.

the IASI FOV geometry.

**4.1 Application to IASI data**

atmosphere based on climatology.

is assumed from climatology and is shown in Fig. 9(c). The columnar amount is 109.12 ppbv (parts per billion per volume).

Figure 10 shows a detail of the interferogram in the optical path difference range 0.21 to 0.31 cm. The interferogram has been computed for a tropical model of atmosphere with the CO reference profile, shown in 9(c). A comparison with a case of zero CO load is also provided in the same figure. The corresponding difference spectra are shown in Fig. 10(b).

Unlike the case for CO2, we have that in the range 0.21 to 0.31 cm, the interferogram gets much signal from the other emission atmospheric sources, which means that the CO signal may be masked from other emitting gases. Moreover, IASI band 3 is that of the three bands with lower signal-to-noise ratio.

However if we look at the difference spectrum, we see that around 2150 cm−<sup>1</sup> there is a spectral region where the signal is almost determined by the CO signal alone. Once again this behaviour stresses that the difference spectrum can isolate emission feature of a given gas. Needless to say a search for *d*-channels which are mostly sensitive to CO shows that they

Fig. 10. (a) Inteferogram in the range 0.21 to 0.32 cm for a case of a tropical model of atmosphere; (b) the corresponding *d*-spectrum for the IASI band 3.

are in the range 2150 to 2200 cm−1. Three channels in this range, namely at 2191.25, 2193, 2195 cm−1, provide estimates of the CO columnar amount, *qCO* whose accuracy is in between 16 to 19 ppbv. Considering that the present average valus of atmospheric CO is around 109 ppbv, they allow an estimate with an accuracy within 15 to 20%. However, the channels are strongly correlated, therefore a methodology as that shown for CO2 is not possible in this case. For this reason, we have focused on one single channel, that at 2195 cm−1, which achieves the better retrieval performance for CO.

In a case of noise-free radiances the dependence of *qCO* on *<sup>d</sup>*(*σ*1) <sup>≡</sup> *<sup>d</sup>*1, with *<sup>σ</sup>*<sup>1</sup> <sup>=</sup> 2195 cm−<sup>1</sup> is parabolic on the full range, which spans from *qCO* = 0 to *qCO* = 328 ppbv. In terms of standardized quantities, the quadratic relation is

$$q\_{CO} = a\_1 D\_1^2 + b\_1 D\_1 + c\_1 \tag{19}$$

where the regression coefficients can be computed through simulation of synthetic radiances belonging to different values of CO columnar amount, *qCO*. Doing so, we have found the result shown in Fig. 11, where we summarize the quadratic best fit for the two reference models of atmosphere: tropical and High Latitude Winter. As in the case of CO2, we see that despite the large difference in the state vector, the quadratic fit is accurate for both models. In case we use the standardized difference-radiance, *D*<sup>1</sup> the regression coefficients do not depend on the atmospheric model. However, as in the case of CO2 the standardization parameters, *μ*<sup>1</sup> and *s*<sup>1</sup> do depend on the state vector. In case of noise-free radiances the quadratic fit of Eq. 19

Fig. 11. Quadratic best fit of *qCO* vs *D*<sup>1</sup> for (a) a tropical model of atmopshere and (b) a High Latitude Winter (HLW) model of atmopshere.

provides estimates for *qCO* within ±0.50 ppbv. In case of noisy radiances, the accuracy can be computed by the usual rule of variance propagation directly from Eq. 19, we have

$$\operatorname{var}(q\_{CO}) = \left(2\frac{a\_1}{s\_1^2}d\_1 + \frac{b\_1}{s\_1}\right)^2 \operatorname{var}(d\_1) \tag{20}$$

For a tropical model of atmosphere the typical standard deviation of *qCO* estimated by Eq. 19 is ≈ 16 ppbv (around 15% of its present climatological value). This figure increases to about 25 ppbv in a case of a High Latitude Winter air mass. These figures hold for one single IASI observation. The noise can be halved by considering an average over the 2× 2 pixel mask of the IASI FOV geometry.

As done for CO2, the sensitivity of the *d*1-channel to CO variations has been computed with the help of Eq. 12 and it is compared to that of the equivalent IASI spectrum-channel, *r*(*σ*1) ≡ *r*<sup>1</sup> in Fig. 12. It is seen that the difference spectrum improves the sensitivity of a factor two and more. The figure also allows us to get insight into understanding the atmospheric pressure-range at which the retrieval approach is sensitive. As for the case of CO2, it is seen that the sensitivity extends to a broad range in between 900 to 100 hPa, therefore extending form the Planetary Boundary Layer to the upper troposphere. In contrast to the results of Fig. 12, the sensitivity analysis shows that the *d*<sup>1</sup> channel at hand is poorly sensitive to other atmospheric parameters. This analysis is not shown here for the sake of brevity.

#### **4.1 Application to IASI data**

16 Will-be-set-by-IN-TECH

is assumed from climatology and is shown in Fig. 9(c). The columnar amount is 109.12 ppbv

Figure 10 shows a detail of the interferogram in the optical path difference range 0.21 to 0.31 cm. The interferogram has been computed for a tropical model of atmosphere with the CO reference profile, shown in 9(c). A comparison with a case of zero CO load is also provided in

Unlike the case for CO2, we have that in the range 0.21 to 0.31 cm, the interferogram gets much signal from the other emission atmospheric sources, which means that the CO signal may be masked from other emitting gases. Moreover, IASI band 3 is that of the three bands

However if we look at the difference spectrum, we see that around 2150 cm−<sup>1</sup> there is a spectral region where the signal is almost determined by the CO signal alone. Once again this behaviour stresses that the difference spectrum can isolate emission feature of a given gas. Needless to say a search for *d*-channels which are mostly sensitive to CO shows that they

0.22 0.24 0.26 0.28 0.3 −0.2

optical path difference, x (cm)

with CO zero CO

> with CO zero CO

<sup>2000</sup> <sup>2100</sup> <sup>2200</sup> <sup>2300</sup> <sup>2400</sup> <sup>2500</sup> <sup>2600</sup> <sup>2700</sup> −2

Fig. 10. (a) Inteferogram in the range 0.21 to 0.32 cm for a case of a tropical model of

wave number, σ (cm−1)

are in the range 2150 to 2200 cm−1. Three channels in this range, namely at 2191.25, 2193, 2195 cm−1, provide estimates of the CO columnar amount, *qCO* whose accuracy is in between 16 to 19 ppbv. Considering that the present average valus of atmospheric CO is around 109 ppbv, they allow an estimate with an accuracy within 15 to 20%. However, the channels are strongly correlated, therefore a methodology as that shown for CO2 is not possible in this case. For this reason, we have focused on one single channel, that at 2195 cm−1, which achieves the better

In a case of noise-free radiances the dependence of *qCO* on *<sup>d</sup>*(*σ*1) <sup>≡</sup> *<sup>d</sup>*1, with *<sup>σ</sup>*<sup>1</sup> <sup>=</sup> 2195 cm−<sup>1</sup> is parabolic on the full range, which spans from *qCO* = 0 to *qCO* = 328 ppbv. In terms of

where the regression coefficients can be computed through simulation of synthetic radiances belonging to different values of CO columnar amount, *qCO*. Doing so, we have found the result shown in Fig. 11, where we summarize the quadratic best fit for the two reference models of atmosphere: tropical and High Latitude Winter. As in the case of CO2, we see that despite the large difference in the state vector, the quadratic fit is accurate for both models. In case we use the standardized difference-radiance, *D*<sup>1</sup> the regression coefficients do not depend

*qCO* = *a*1*D*<sup>2</sup>

(a)

(b)

<sup>1</sup> + *b*1*D*<sup>1</sup> + *c*<sup>1</sup> (19)

0 0.2

0

atmosphere; (b) the corresponding *d*-spectrum for the IASI band 3.

d(σ), W m−2 (cm−1)−1 sr−1

<sup>2</sup> x 10−3

I(x), W m−2 sr−1

the same figure. The corresponding difference spectra are shown in Fig. 10(b).

(parts per billion per volume).

with lower signal-to-noise ratio.

retrieval performance for CO.

standardized quantities, the quadratic relation is

The mechanism of the procedure to estimate CO columnar amount from IASI observations is the same as that illustrated for CO2 in section 3.3. During the JAIVEx experiment the CO profile was recorded by airborne in situ profiles (JAIVEx, 2007) recorded with the commercial gas instrument AL 5002 VUV Fast Fluorescence CO Analyser (produced by Aerolaser GmbH). The analyser employs the measurement of the fluorescence of CO when exposed to UV light at a wavelength of 150 nm, which is proportional to the concentration of CO. The measurements covered the lower-middle troposphere (1000 to 400 hPa) and extended to the upper part of the atmosphere based on climatology.

Observations. Methodological Aspects and Application to IASI 19

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

<sup>265</sup> Fourier Transform Spectroscopy with Partially Scanned Interferograms as a Tool to Retrieve

in situ estimates (2 for 29 April 2007, 4 for 30 April 2007, and 3 for 04 May 2007) have been averaged and these three average values are shown as flat lines in Fig. 14(a). The comparison

Fig. 14. (a)- CO integrated amount estimated from IASI (this work) and in situ observations.

provided in Fig. 14 shows that IASI agrees with in situ observations in evidencing a slight decrease of the CO load for the target area on 30 April 2007 in comparison to those over passed by IASI on 29 April and 04 May, respectively. This is also evidenced if we compare the average values for the day 29 April 2007 and 30 April 2007. For 29 April 2007 we have a mean value of (119.6 ± 7.0) ppbv for IASI against a value of 123.7 ppbv estimated from in situ observations. For the day 30 April 2007, both IASI and in situ observations show a lower

Finally, as done for CO2, for illustrative purposes, Fig. 14(b) shows a monthly map of CO

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

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

Average CO concentration for July 2010 (ppbv)

(b) Mediterranean case study

<sup>0</sup> <sup>5</sup> <sup>10</sup> <sup>15</sup> <sup>20</sup> <sup>25</sup> <sup>50</sup>

Number of IASI sounding

(a) JAIVEx case study

In situ, 29 April 2007 In situ, 30 April 2007 In situ, 04 May 2007 IASI

(b)- IASI CO for July 2010 over the Mediterranean area.

value for the CO load, IASI (114.2 ± 4.2) ppbv, in situ 111.0 ppbv.

computed over the Mediterranean area for the month of July 2010.

1220 cm−<sup>1</sup> exhibit the poorest sensitivity to the state vector, but methane.

100

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

150

qCO (ppbv)

200

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 computed for the corresponding channel of the difference-spectrum.

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

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.

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 estimate of the CO profile.

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 in situ estimates (2 for 29 April 2007, 4 for 30 April 2007, and 3 for 04 May 2007) have been averaged and these three average values are shown as flat lines in Fig. 14(a). The comparison

Fig. 14. (a)- CO integrated amount estimated from IASI (this work) and in situ observations. (b)- IASI CO for July 2010 over the Mediterranean area.

provided in Fig. 14 shows that IASI agrees with in situ observations in evidencing a slight decrease of the CO load for the target area on 30 April 2007 in comparison to those over passed by IASI on 29 April and 04 May, respectively. This is also evidenced if we compare the average values for the day 29 April 2007 and 30 April 2007. For 29 April 2007 we have a mean value of (119.6 ± 7.0) ppbv for IASI against a value of 123.7 ppbv estimated from in situ observations. For the day 30 April 2007, both IASI and in situ observations show a lower value for the CO load, IASI (114.2 ± 4.2) ppbv, in situ 111.0 ppbv.

Finally, as done for CO2, for illustrative purposes, Fig. 14(b) shows a monthly map of CO computed over the Mediterranean area for the month of July 2010.
