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84 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.5.13 Trace gas profile retrieval from SCIAMACHY limb measurements Participating scientist Janis Pukite, Sven Kühl, Thomas Wagner, and Walburga Wilms-Grabe Abstract Trace gas profile retrieval of ozone, NO2, BrO and OClO is performed. This can be done in two successive steps. First, slant column densities (SCDs) of trace gas are retrieved by DOAS from SCIAMACHY measurements at different elevations. Second, profile is derived by inverting the SCDs. For that purpose radiative transfer modelling is performed. Figure 2.47: Latitudinal cross-sections of concentrations of ozone, NO2, BrO and OClO according to an orbit on 24.08.2004. Background The SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) on ENVISAT measures solar UV-VIS-NIR radiation transmitted, backscattered and reflected by the atmosphere and the ground. The limb-scanning mode (tangential view) is implemented by the instrument allowing to perform measurements at different elevation angles. In this way the atmosphere is probed at different geometrical configurations that make instrument more or less sensitive to substances located at different altitudes in the atmosphere. To connect the measurements with atmospheric profile of a trace gas the radiative transfer properties of the atmosphere should be applied. Funding See satellite group overview. Methods and results The retrieval of profiles is done in two successive steps. First, SCDs for the trace gases are derived from the SCIAMACHY limb measurements by DOAS method. In the second step the trace gases SCDs are converted into vertical concentration profiles by means of radiative transfer modelling (RTM). For the RTM the full spherical 3D model ”Tracy”, (von Friedeburg, 2003) is used. Inversion is constrained by linear optimal estimation method (Rodgers, 2000). The method demonstrates possibility to retrieve trace gas concentrations for altitudes 15 - 28 km (BrO), 15 - 40 km (ozone and NO2), 15 - 25 km (OClO). There are still uncertainties in retrieval at polar regions probably caused by ozone absorption cross-section dependence from temperature or RTM shortages. Retrieved profiles of NO2 and BrO show good agreement with retrieval done by IUP Bremen and with balloon measurements for NO2. Outlook/Future work An improvement is necessary especially for measurements made at solar zenith angles ≥ 90 ◦ as well as for retrieval of BrO and OClO. Model studies should be performed. Improvement in RTM is required. Main publication Pukite et al. [2005], Butz et al. [2005]
2.5. SATELLITE GROUP 85 2.5.14 Mie theory based characterization and modeling of atmospheric aerosols Participating scientist Suniti Sanghavi Abstract We have developed a Mie model to generate the single scattering properties of a particle given its size and complex refractive index. We put up a set of 24 aerosol scenarios to represent most aerosols in the atmosphere. The Mie theory can be used to simulate these scenarios and thus examine the optical behavior of most atmospheric aerosols. Figure 2.48: Phase functions of different aerosol particles Background Aerosols have gained increasing importance in atmospheric sciences due to the key role they play in regard to, amongst others, the earth’s radiative budget, convective processes and precipitation, and stratospheric chemistry leading, eg., to the formation of the ozone hole. They remain, however, difficult to quantify due to their varied sources, lifetimes and transport mechanisms. Thus they may occur over land or water surfaces and at different heights in the atmosphere. They also display inherent variations in size, shape and chemical composition, leading to different optical properties. We classify aerosols to represent most situations found in the atmosphere and characterize their optical properties using the Mie theory. This can be used to simulate the radiative properties of aerosols occuring in nature. We intend to use the TRACY MC RTM for this purpose. This should especially help quantify the radiative effects of aerosols on the retrieval of trace gas species from the satellite DOAS instrument SCIAMACHY. Work is ongoing for other applications that may include, in conjunction with aerosol climatologies now available at sites such as AERONET, aerosol inversion products from SCIAMACHY radiances. Funding See satellite group overview. Methods and results We have adapted a compilation of 24 distinct aerosol scenarios, consisting of 5 main aerosol types, viz. urban industrial, biomass burning, desert dust, marine and volcanic, each associated with a typical complex refractive index and vertical profile. Each is further subclassified according to coarseness, absorptivity, nonsphericity or height of occurence in the atmosphere(km a.s.l.). We obtain the optical properties, viz. extinction cross section, single scattering albedo and phase function of an aerosol particle of given size and complex refractive index, using the Mie theory (See figure). The Mie theory is a special case of the Maxwell equations with boundary conditions given by the interaction of electromagnetic radiation of given wavelength at the surface of a spherical particle. Using the characteristic complex refractive indices, size distributions and vertical profiles provided by the the scenarios mentioned above, we can use the Mie theory to determine bulk properties, e.g. the Aerosol Optical Thickness (AOT) at a given location. Adding surface reflection, Rayleigh scattering, and molecular absorption allows us to simulate fluctuations in radiances measured by satellite due to variations in the aerosol loading of the atmosphere. Outlook/Future work Implementing aerosols in TRACY RTM (Quantification of the effect of aerosols on SCDs/AMFs in different spectral regions), Aerosol retrieval/inversion from satellite/ground based measurements. Main publication Sanghavi [2003]
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Institut für Umweltphysik und Arbe
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Contents 1 Introduction/Overview of
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CONTENTS 7 3.2.1 Glaciological pilo
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3.2. ICE AND CLIMATE 147 Preunkert,
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