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TROPOSPHERIC OZONE MEASURED WITH IASI
G. Dufour, M. Eremenko, C. Keim, Gilles Foret, J. Orphal, Matthias
Beekmann, M. Höpfner, G. Bergametti, J.-M Flaud
To cite this version:
G. Dufour, M. Eremenko, C. Keim, Gilles Foret, J. Orphal, et al.. TROPOSPHERIC OZONE
MEA-SURED WITH IASI. 2nd EPS / MetOp RAO workshop, May 2009, Barcelone, Spain. �hal-02330351�
TROPOSPHERIC OZONE MEASURED WITH IASI
G. Dufour(1), M. Eremenko(1), C. Keim(1,2), G. Forêt(1), J. Orphal(1,3), M. Beekmann(1), M. Höpfner(3), G. Bergametti(1), J.-M. Flaud(1)
(1)
Laboratoire Inter-universitaire des Systèmes Atmosphériques (LISA), UMR7583, Universités Paris 12 & Paris 7, CNRS, 61 avenue du Général de Gaulle, 94010 Créteil Cedex, France, Email :
Gaelle.Dufour@lisa.univ-paris12.fr
(2)
Now at Astrium GmbH, Germany
(3)
Institut für Meteorologie and Klimaforschung, Forschungszentrum Kalsruhe, Germany
ABSTRACT
In this paper we present tropospheric ozone column amounts (0-6 km) obtained from infrared radiances measured by the IASI instrument aboard the MetOp-A satellite using an altitude-dependent regularization method. As a first demonstration we have focused on the heat wave over Southern Europe in summer 2007 and observed very high values, in agreement with predictions from a chemical transport model of tropospheric photochemistry (CHIMERE) [1]. Our ozone product has been validated by comparing it to ozone profiles from balloon sonde measurements in the northern midlatitudes for the period from July 2007 to August 2008 [2]: A very good agreement was obtained for the scientific products but a significant bias has been identified with the operational products (v4.2). Finally, we show that the seasonal variations of ozone retrieved for different latitude bands are comparable with reference climatologies (mean and variability).
1. INTRODUCTION
Ozone is a key species of tropospheric chemistry affecting the troposphere’s oxidative capacity and is a well-known pollutant with significant impact on health and vegetation [3]. Ozone is also an important greenhouse gas with large radiative forcing in the upper troposphere [4]. Monitoring of tropospheric ozone is essential to quantify its sources, transport, chemical transformation [5] and to evaluate and improve models used for climate and pollution modelling.
Tropospheric ozone concentrations are mainly measured at surface level using national operational networks and vertical information is for the most part provided at selected sites by meteorological balloon sondes or during dedicated aircraft campaigns. In addition to these in situ measurements, satellite observations in the nadir geometry are very promising because of their large spatial coverage. However, tropospheric ozone retrieval from satellite observations is a challenging task because most of the ozone is contained in the stratospheric ozone layer. The first satellite measurements of tropospheric ozone have been obtained using ultraviolet-visible sounders [e.g. 6, 7] but they have some limitations in the mid- and high latitudes. The recent development of infrared nadir sounders allows accurate measurements of tropospheric ozone, with the
advantage that measurements are also possible during the night. The first demonstration of tropospheric ozone column retrieval has been done using the IMG (Interferometric Monitor for Greenhause gases) instrument [8, 9]. More recently, the TES (Tropospheric Emission Spectrometer) instrument has provided measurements of tropospheric ozone [10] with applications to air quality modelling [11] and climate [12].
Here we present tropospheric ozone measured with the IASI instrument over Europe during July 2007 [1] as well as validation results obtained for a larger period (July 2007 to August 2008) using balloon sonde measurements [2].
2. THE IASI INSTRUMENT
The IASI (Infrared Atmospheric Sounding Interferometer) [13] is a nadir viewing spectrometer onboard the MetOp-A satellite and was designed for operational meteorology. The MetOp-A satellite launched in October 2006 flies in a polar sun-synchronous orbit (about 800 km altitude) and crosses the equator at two fixed local solar times 9:30 am (descending mode) and 9:30 pm (ascending mode). The IASI instrument is a Fourier-transform spectrometer with a 2 cm optical path difference covering the 645-2760 cm-1 spectral range. The apodized spectral resolution is 0.5 cm-1 (Full-Width at Half-Maximum). The radiometric accuracy in noise-equivalent radiance temperature at 280 K is 0.28 K at 650 cm-1 and 0.47 K at 2400 cm-1. IASI measures the thermal infrared radiation (TIR) emitted by the Earth’s surface and the atmosphere. The instrument scans the surface perpendicular to the satellite’s flight track with 15 individual views on each side of the track. The distance between two successive overpasses is 25° in longitude (i.e. 2800 km at the equator). For latitudes higher than 45° in latitude, the footprints of two successive overpasses overlap. At the nadir point, the view size is 50 ¯ 50 km2. The view is composed of 4 individual ground pixels with 12 km diameter each. The maximum scan angle of 48.3° from the nadir corresponds to coverage for one swath of about 2200 km in the direction perpendicular to the satellite’s track.
In addition to meteorological products (surface temperature, temperature and humidity profiles, cloud information), the IASI instrument provides distributions of several trace gases (e.g. O3, CO) [1, 14].
_____________________________________________________ 1SPDA4FDPOE&14.FU0Q3"08PSLTIPQ #BSDFMPOB 4QBJO 20–22 May 2009 (ESA SP-675, September 2009)
3. RETRIEVAL METHOD
The ozone products from IASI (mainly partial tropospheric columns) presented in this paper are based on profile retrieval of ozone. The retrievals are performed using the radiative transfer model KOPRA (Karlsruhe Optimised and Precise Radiative transfer Algorithm, [15]) and its inversion module KOPRAFIT, both adapted to the nadir-viewing geometry. A constrained least squares fit method using an analytical altitude-dependent regularization is used. The regularization method applied as well as the error calculations are detailed in [1]. To summarize, the regularization matrix is a combination of zero, first and second order Thikonov [16] constraints with altitude-dependent coefficients. The coefficients are optimized to both maximize the degrees of freedom (DOF) of the retrieval and to minimize the total error on the retrieved profile.
The analysis of IASI data is performed in three steps. First, the effective surface temperature is retrieved from selected windows between 800 and 950 cm-1 considering a blackbody with an emissivity equal to unity. In the second step, the atmospheric temperature profile is retrieved from CO2 lines in the 15 Pm spectral
region and using the ECMWF profiles as a priori. In the third step, the ozone profiles are retrieved from seven spectral windows in the 975-1100 cm-1 region that avoid strong water vapor lines. The spectroscopic parameters of ozone are from the MIPAS database [17] and from HITRAN 2004 for the other species. The a priori profile used during the retrieval is compiled from the climatology of [18] and is the same independently of the time and the location of the observations (in the midlatitudes).
Note that before the retrieval, the IASI spectra are filtered for cloud contamination. Only spectra for clear sky conditions are considered. A quality flag is also applied to the retrieved products to discard unphysical results. The error on the profile ranges between 20 and 40% and the error on the 0-6km columns ranges between 15 and 30% depending on the thermal contrast.
4. TROPOSPHERIC OZONE DURING THE
EUROPEAN HEAT WAVE IN JULY 2007
During summer 2007, one significant ozone pollution episode occurs in Central and Southern Europe between July 14 and July 21 [19]. The occurrence of high ozone concentrations was favoured by persistent anticyclonic conditions in these parts of Europe. On the contrary, Western Europe was under the influence of low pressure system and the surface ozone levels were lower. The ozone episode ended when a low pressure shift came from the Atlantic Ocean to the continent.
Figure 1. Lower tropospheric ozone columns (0-6km) retrieved from IASI (left) and simulated by CHIMERE (right) during two successive days of the 2007 European
heat wave. The cloudy pixels are given in grey. Fig. 1 (left) shows the IASI ozone partial columns retrieved for clear-sky scenes for two days during the heat wave period. Ozone columns larger than 28 DU (Dobson Unit) are observed over Central and Southern Europe in agreement with the regions where surface concentrations exceed the 180 Pg/m3 standard air quality threshold. The analysis of the observations during the heat wave period shows that the high ozone column fields move clockwise from Central to Eastern Europe with a singular structure over Romania (due to the mountains). Relatively low ozone columns are observed over the Western and Northern part of Europe (Fig. 1).
Comparisons with the regional chemistry and transport model CHIMERE [20] have been performed. We used the continental version of CHIMERE (http://euler.lmd.polytechnique.fr/chimere) vertically extended to the entire troposphere [21]. The model runs with a horizontal resolution of 0.5° ¯ 0.5° and with 17 vertical layers from the surface up to 200 hPa. It is driven by the European Center for Medium-Range Weather Forecasts meteorological analyses. LMDz-INCA monthly mean concentrations are used as boundary conditions [22]. The CHIMERE model has been extensively evaluated against surface ozone measurements over Europe showing very good agreement for daily ozone maxima (e.g., averaged bias below 10%, correlation about 80%) [23]. The simulated 0-6km ozone columns are smoothed by the IASI
averaging kernels for the comparison (Fig. 1 right). A statistical comparison [1] over Central Europe shows a mean bias between observed and simulated ozone columns of about - 4%. The root-mean-square (rms) of the differences is of the order of measurement uncertainties and the spatial correlation is significant (0.64). Larger differences, up to 20%, are visible between the simulated and observed columns over Western Europe. These larger differences might be related to the lower boundary layer height in this region. The part of the columns due to free tropospheric ozone is larger in this case and likely less correctly simulated by the model.
5. VALIDATION WITH OZONE SONDE
MEASUREMENTS
In this section, we present validation of the ozone products produced by our team with ozone sonde measurements. All the details of the validation are reported in [2] and the results presented here are based on reference [2]. The validation time period used ranges between June 2007 and August 2008. The spatial and temporal coincidence criteria retained for the comparison with the sonde measurements are ± 110km (square) and 12 hours. The list of the stations used for the comparisons is given in [2]. For the validation, the ozone profiles measured by the balloon sondes are
assumed to be a good estimate of the real state of the atmosphere. Note that the vertical resolution of the sonde measurements is much better than the vertical resolution of IASI products (a maximum of about 1.5 degrees of freedom in the troposphere [1]). The sonde profiles are then smoothed (Eq. 1) by the averaging kernel (A) of the IASI retrievals in order to compare the IASI retrieved profiles with the expected retrieved profiles according to the true state given by the sonde [24].
xsmoothed = xa priori + A(xsonde – xa priori) (1)
However, in the case where the measured signal (by IASI) does not contain enough information about the part of the atmosphere that one is interested in (the troposphere here), the retrieved profile only represents the a priori information that is used for the retrieval. Smoothing the sonde measurements with the corresponding averaging kernels will then lead to replace the true information contained in the sonde measurement by the a priori information. In order to avoid any misleading interpretation of the validation results, the retrieved profiles are compared to both the raw sonde profiles interpolated on the vertical grid of the IASI retrieval (1 km spacing in the troposphere) and the smoothed sonde profiles.
Figure 2. Comparison between mean retrieved IASI profiles, interpolated sonde profiles and smoothed sonde profiles for 3 European stations over summer 2007. The middle and right panels show the absolute and relative differences
Fig. 2 shows the comparison between the retrieved profiles, the interpolated sonde profiles and the smoothed sonde profiles averaged over the summer period 2007 (June to August) for 3 European stations. The agreement between the mean IASI retrieved profile and the mean sonde profile is very good up to 10 km: the difference with the smoothed sonde profile is less than 0.02 ppmv and is consistent with the difference with the interpolated raw sonde profile. To investigate the quality of the individual products, we compared the retrieved individual tropospheric partial columns (0-6km and 0-11km) to the corresponding ozone sonde measurements. The corresponding scatter plots are reported in Fig. 3. The correlation is relatively high in both cases (0.79 and 0.81). The slope of the linear regression is closer to one for the 0-11km columns comparison. This is explained by the larger degrees of freedom of the 0-11 km retrieved columns. These results reveal the ability of IASI to measure precise tropospheric ozone columns even in the lowest part of the troposphere.
Figure 3. Correlation plot between the 6km and 0-11km ozone columns retrieved from IASI and measured by the sonde (both raw and smoothed). Table 1 gives the statistics of the comparisons for the 0-6km partial column. A small mean bias of about 1 Dobson Unit (DU) or 5% is observed over the entire time period considered and the root mean square of the errors is smaller than 3.4 DU or 20% when considering the smoothed sonde columns. The small bias and the variability of the error agree with the total column error that ranges between 3.5 and 4.5 DU depending on the latitude. The bias and the variability are slightly larger than preliminary validation results reported in [1]. However, the results in [1] were obtained for a short period of time (summer 2007) and for only 4 stations. We can conclude that our retrieved product is robust and can be used for scientific studies.
We also performed validation exercise on the operational product. At the time of the study, only the
version v4.2 of the products was available and for a restricted time period (February 2008 to August 2008). The results of the comparison of the 0-6km operational columns with the ozone sonde measurements are summarized in Table 1. Note that the retrieval method applied for the operational treatment of IASI data (neuronal network) does not permit to derive information of the vertical sensitivity of the retrieved product (no averaging kernels are available with this method). Then the sonde measurements used for comparison cannot be smoothed. For comparison, the statistics obtained with our product are also reported for the same time period. The bias between the operational product and the sonde is large (5 DU or 26%), in agreement with a previous validation [25] and remains larger than the target accuracy. The variability of the difference (rmse) is of the same order than those observed with our product (Table 1). Note that the operational product has similar quality than our scientific product when we considered the 0-14 km column [2].
Figure 4. Seasonal variation of the 0-11 km ozone columns for different latitude bands as retrieved from
IASI (monthly mean: red line and individual measurements red squares), measured by the sondes (blue) and provided by the LLM climatology. Dashed
with raw sonde with smoothed sonde products Time period
bias rmse bias rmse our product Jun 07 – Aug 08 1.4 (5.5) 4.7 (24.1) 1.0 (4.8) 3.4 (18.0) our product Feb 08 – Aug 08 0.5 (2.3) 4.8 (22.4) -0.2 (-2.3) 3.4 (16.5) Operational (v4.2) Feb 08 – Aug 08 -5.5 (-26.3) 5.7 (27.1)
Table 1. Statistics of the comparison between the 0-6 km partial ozone columns retrieved from IASI and measured with the ozone sondes. The comparison with the operational product (v4.2) is presented for a restricted time period. The
values are given in Dobson Units and % for those in parenthesis.
As a last check of the robustness of our ozone product (here the tropospheric ozone column), we investigate the ability of reproducing the seasonal cycle of ozone for different latitude bands. The monthly mean columns taken at the ozone sonde stations considered for the validation from both the IASI retrieved product and the coincident ozone sonde measurements are compared in Fig. 4 to the monthly climatological columns derived from the Labow, Logan, McPeters (LLM) climatology [18]. Note that the a priori information considered for the retrieval is independent of the time and then does not influence the seasonal variation retrieved. The climatological values are plotted with their variability (3V). Each individual IASI and sonde measurements considered are plotted to evaluate if the dispersion of the retrieved IASI products is consistent with the climatology. We can note that both the seasonal variation of ozone and the variability are consistent with both the climatology and the coincident sonde measurements.
6. CONCLUSION
The validation results presented in this paper show that the scientific IASI product presents a small bias (< 5%) and the variability of the errors (rmse) is consistent with the errors in the retrieved columns and the ozone measurements used as reference. On the contrary, the operational product (v4.2) is significantly biased (26%).
The study of the heat wave episode during July 2007 and the comparison with the regional air quality model (CHIMERE) show that precise lower tropospheric ozone columns can be retrieved from IASI spectra and can provide information on tropospheric ozone relevant for air quality studies.
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