Characterizing Aerosols from Spaceborne Infrared Vertical Sounders
Aerosols originate either from natural sources (dust, volcanic, or sea salt aerosols, etc.) or from anthropogenic sources (fires, sulfates, soot...). In its fourth report, the Intergovernmental Panel on Climate Change (IPCC) (Forster et al., 2007) notes that, if aerosol forcings are now better understood than at the time of the Third Assessment Report due to improved measurements and more comprehensive modelling, they remain the dominant uncertainty in radiative forcing, partly because they show a very high spatio-temporal variability. However, most remote sensing studies focus on the solar spectrum, whereas the closure of the terrestrial radiative balance also needs knowledge of the dust effect on terrestrial and atmospheric infrared radiation (Vogelmann et al., 2003). Yet, the dust radiative forcing in the thermal infrared (roughly 3 to 15 µm) cannot be well quantified from measurements in the visible spectrum because refractive index spectra, highly variable, are not reliable enough both in the infrared and in the visible, and because the infrared and visible spectra are not sensitive to the same ranges of particle sizes : the coarse mode (size range >1 µm) is preferentially observed in the infrared, whereas the accumulation mode (0.1-1 µm) is mostly observed in the visible. Remote sensing in the thermal infrared has several other advantages: observations are available both for daytime and nighttime, dust detection is possible over desert (Wald et al., 1998) and, even more important, vertical sounders allow retrieving dust layer mean altitude (Pierangelo et al., 2004a). Measuring dust altitude may be of great importance for the study of dust transport, dust sources and deposit. Moreover, dust-forced cooling/warming of the atmosphere varies as the dust layer ascends or descends and insufficient knowledge of the three dimensional distribution of dust may cause significant errors in the determination of its effect on climate and in global warming prediction (Claquin et al., 1998; Alpert et al., 2004).
Recently, several so-called 2nd generation high spectral resolution infrared sounders or imagers have been launched, among others, AIRS/Aqua, IASI/Metop, and MODIS/Aqua. To this list, may be added two instruments of the AQUA-train: the lidar CALIOP (April 2006) aboard the CALIPSO satellite and the imaging radiometer PARASOL (December 2004). Numerous complementary products may be expected from this synergy of instruments: the very high spectral resolution characterizing AIRS or IASI allows retrieving infrared optical depth of desert dusts (which constitute the largest load in aerosol) together with their mean altitude and size. Also, it might be possible to retrieve the aerosol composition through the identification of specific absorption bands in the infrared. Observations from MODIS, providing high spatial resolution aerosols visible optical depths over sea and land vegetated areas as well as their effective radius, from CALIOP, providing accurate aerosol altitudes, of from PARASOL, allowing discriminating between the aerosol accumulation and coarse modes, together with in situ observations from either the AERONET (sun photometers) or EARLINET (lidars) networks, or from aircraft campaigns, are useful tools for validation purposes.
Dust aerosols characterization from AIRS
Sensitivity studies performed with the high spectral resolution radiative transfer code 4A-OP/DISORT show that dust effect on brightness temperatures may reach several Kelvin for some channels. This effect varies with the AOD, the altitude, and the aerosol model, as well as with the emissivity of the underlying surface.
AIRS 10 µm optical depth and altitude of dust aerosols are retrieved simultaneously. The method developed follows two main steps. First, the observed atmospheric thermodynamic situation is determined as accurately as possible; then, dust properties are retrieved following the method developed by Pierangelo et al. (2004) with some significant improvements added. Both steps use “Look-up-Tables” (LUT) computed for a large selection of atmospheric situations. Seven years of AIRS observations have been processed (January 2003 to December 2009) and compared to MODIS, CALIOP, or PARASOL products. Detailed results may be found in Peyridieu et al. (2010).
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Aerosol optical depth
The analysis of the 7-year timeseries (Peyridieu et al. (2010)) shows that AIRS AODs compare well with MODIS 0.55 μm AOD for three regions of the northern tropical Atlantic (near the West African coastline, downwind of the Sahara to the Caribbean) and of the Indian Ocean (south of the Arabian peninsula). This agreement is particularly satisfactory both during and outside the main dust seasons (summer) and differences between the two products mostly highlight the sensitivity of MODIS to biomass burning (not to speak of pollution or sea-salt aerosols) outside these seasons. Differences are observed far from the sources : the AIRS AOD season lags behind the MODIS AOD season. We proposed two tentative explanations to this phenomenon: (1) a start of the dust season very far from the sources dominated by the fine mode not seen by AIRS, seemingly confirmed by PARASOL-retrieved aerosol optical thickness of the non-spherical mode at 0.55 μm; (2) the use of the MITR aerosol model not obviously adapted to this situation.
Comparing MODIS and AIRS aerosol optical depths also provides a measure of the 10 µm to 0.55 µm extinction coefficient ratio. New products, as, for example, the coarse mode to accumulation mode ratio, could be retrieved by using in synergy AIRS and MODIS observations.
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AIRS-retrieved dust mean layer altitude shows a slow regular decrease from east to west Atlantic already reported in several studies as well as a clear seasonal cycle, with summer transport occurring at higher altitudes than spring transport (Prospero et al., 1981; Chiapello et al., 1995). A north-south positive gradient observed by Pierangelo et al. (2004) is confirmed during the peak of the dust season and could be explained by uplifting at the Intertropical Convergence Zone, located near 10°N in July (Colarco et al., 2003b; Karyampudi et al., 1999).
For the period June 2006-June 2009, the comparison with CALIOP/CALIPSO mean altitude (centroid) of the dust layer shows a good agreement with the AIRS mean altitude although the comparison was limited to cases where the lidar detects only one aerosol layer (about 64% of the cases). The difference seen between the peak-to-trough amplitudes of the two products (smaller for AIRS) is thought to come mainly from the large difference between the spatial resolutions of the two instruments.
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These results illustrate the ability of infrared sounders to retrieve dust layer 10 μm AOD as well as mean dust layer altitude quite accurately.
Robustness and limits of the algorithm
Limits of this approach are mostly due to the intrinsic limit of infrared sounders to sound close to the surface. The still too limited number of accurate refractive index measurements for various dust species in the infrared must also be underlined. We hope that additional measurements (see for example, McConnell et al., 2008) will be conducted, allowing deeper study of the impact of dust composition and opening the way to promising dust mineralogical characterizations from high spectral resolution satellite observations.
Finally, as reported by Liu and Mishchenko (2008), comparison of aerosol datasets from several satellite-based observations is challenging. Each retrieval has its own limitations and shortcomings so large differences may occur. The present algorithm does not make exception and users should be aware of the condition of use and of the quality statements of the product.
The next step is to extend the retrieval of the optical depth and altitude of aerosols to daytime observations and over land, which requires consideration of the infrared surface emissivity at high spectral resolution now available at LMD (Péquignot et al., (2008)). These new developments are presently underway at LMD on the basis of observations made by the new very high resolution Infrared Atmospheric Sounder Interferometer (IASI), offering more and cleaner channels. This will provide a unique global monitoring of dust sources over desert.
Dust aerosol size distribution
A method to constrain dust size distribution over oceans using high spectral resolution infrared AIRS or IASI observations has been developed (Pierangelo et al. (2005)). The main advantage of using infrared radiances is the fact that infrared radiation is only sensitive to coarse mode particles, the effect of the accumulation mode and of the width of the distribution being negligible. The effective radius of the dust coarse mode is of particular interest for evaluating the dust radiative forcing.
Top: Mean dust effective radius of the coarse mode for April-May 2003 (µm), retrieved from AIRS data;
Bottom: Dust mass median diameter for April-May 2003 (µm), simulated with LMDz-INCA.
For the period April to June 2003, the retrieved effective radius of the Saharan dust coarse mode is found to be in good agreement with other measurements (in situ, AERONET). Because of gravitational settling, the retrieved effective radius decreases with transport distance (from 2.4 µm along the African coast to about 2.0 µm over the Caribbean islands). This pattern is consistent with the process of aerosol removal as modelled in LMDz-INCA.
Application to IASI
The method developed for AIRS has recently been extended to observations from the very high spectral resolution sounder Infrared Atmospheric Sounding Interferometer (IASI). Access to a higher spectral sampling, providing more and cleaner channels, makes it possible not only the retrieval of AOD, altitude and dust particles effective radius, but also the retrieval of additional information on dust microphysics (e.g., mineralogical composition or shape).
First results from 2008 IASI observations have been obtained and are currently under validation.
Infrared optical depth of stratospheric volcanic aerosols produced by the eruption of Mount Pinatubo in June 1991 may be retrieved from the observations of the High resolution Infrared Radiation Sounder (HIRS-2) on board the NOAA polar satellites (Pierangelo et al. (2004a)). The method relies on the analysis of the differences between the satellite observations and simulations from an aerosol-free radiative transfer model using collocated radiosonde data as the prime input. Thus aerosol optical depths are retrieved directly without making assumptions about the aerosol size distribution or absorption coefficient.
Evolution of the concentration in time and in space, in particular the migration of the aerosols from the tropics to the Northern and Southern Hemispheres, is found to be consistent with our knowledge of the consequences of this eruption. The aerosol optical depths reached a maximum in August 1991 in the tropical zone (0.055 at 8.3 μm, 0.03 at 4.0 μm, and 0.02 at 11.1 μm). The peak occurred in November 1991 in the southern midlatitudes and in March/April 1992 in the northern midlatitudes. A reanalysis of the almost 25 year archive of NOAA TIROS-N Operational Vertical Sounder ( TOVS) observations holds considerable promise for improved knowledge of the atmosphere loading in volcanic aerosols.