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TROPOSPHERIC AEROSOLS
Aerosols are important components of the Earth-atmosphere-ocean system. They affect climate in various ways:

1. Direct radiative forcing due to scattering or absorption by aerosol
2. Indirect radiative forcing by modification of cloud properties
3. Indirect effect on heterogeneous chemistry


The net effect on the radiative budget depends on aerosol composition and physical properties, the reflectivity of the underlying surface, and on the altitude and thickness of the aerosol layer(s). These properties, as well as aerosol abundance, exhibit large variability in time and space. Satellite remote sensing enables global measurements of atmospheric constituents including aerosols at representative spatial and temporal sampling. Changing trends can therefore be studied at local and global scales.

Types of aerosols include small sulfuric acid droplets (mainly from industry), black soot particles (from fires), desert dust, and sea salt. The size of aerosol particles ranges roughly from 1-100 µm, and although they are often assumed to be spherical, many particles (such as desert dust) are decidedly non-spherical. Due to these differences in size, shape and composition, the different particle types have widely varying optical properties.


Because the size of aerosol particles is on the order of the wavelength of light that we observe (0.2-2 µm), absorption and scattering of light by these particles depends strongly on the observed wavelength. This allows us to deduce the physical properties of aerosol particles (size or size distribution, refractive index) from the spectrum of light reflected by the Earth.


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Our objective is the development of tools for the investigation of aerosol optical parameters and vertical profiles by satellite remote sensing. This is a challenging task because of the diversity of aerosol particles mentioned above. We therefore combine the information from several different aspects of the SCIAMACHY and GOME-2 spectrum:

UV Aerosol Index - UV Aerosol Indices (UVAI) are semi-quantitative measures of aerosols. They are sensitive to aerosol optical properties, mainly single scattering albedo (reflectivity of the aerosol particle) and aerosol optical thickness (concentration of aerosol particles). In addition, the UVAI are sensitive to the height of the aerosol layer. We have recently developed the UV Scattering Index (SCI) to complement the Absorbing Aerosol Index (AAI). Using the SCI, non-absorbing aerosols can be studied.

O2 and O4 Airmassfactors - using differential optical absorption spectroscopy (DOAS) of oxygen (O2) and the oxygen dimer (O4) the length of the sunlight path through the atmosphere can be determined. Because aerosols influence the lightpath length (see above), the absorption of gases like O2 and O4 (of which the concentration is constant) can be used to obtain information about aerosols.

Radiances – The radiances at different wavelengths in the visible wavelength range (from blue to infrared) are also used to gather information on aerosol properties. Ring effect – The amount of inelastic, or Raman, scattering on molecules depends on the presence of aerosols and clouds and on their altitude. An advantage of the Ring effect is that it is relatively insensitive to reflection by the underlying surface.

We combine results from radiative transfer simulations with radiance and DOAS measurements by SCIAMACHY and GOME-2.


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References:
[1] Penning de Vries, M., and T. Wagner: Modelled and measured effects of clouds on UV Aerosol Indices on a local, regional, and global scale, Atmos. Chem. Phys., 11, 12715-12735, 2011.
[2] Wagner, T., Beirle, S., Deutschmann, T., and Penning de Vries, M.: A sensitivity analysis of Ring effect to aerosol properties and comparison to satellite observations, Atmos. Meas. Tech., 3, 1723-1751, 2010.
[3] Penning de Vries, M. J. M., Beirle, S., and Wagner, T.: UV Aerosol Indices from SCIAMACHY: introducing the SCattering Index (SCI), Atmos. Chem. Phys., 9, 9555-9567, 2009.
[4] Wagner, T., B. Dix, C.v. Friedeburg, U. Frieß, S. Sanghavi, R. Sinreich, and U. Platt MAX-DOAS O4 measurements - a new technique to derive information on atmospheric aerosols. (I) Principles and information content, J. Geophys. Res., 109, doi: 10.1029/2004JD004904, 2004.



Results

SCIAMACHY average UV Aerosol Index 2004-2007 The figures below show two seasonal averages of UVAI from SCIAMACHY over the years 2004-2007. The blue colorscale represents absorbing aerosols (AAI), the yellow-red colorscale shows scattering aerosols (SAI).


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A distinct seasonal cycle is observed for AAI, as can be seen in the figures below with the blue colorscale: desert dust emission varies over the year, and also biomass burning is dependent on season. The seasonal cycle of SAI depends largely on cloud cover, but in the Southeast of the U.S.A. a strong SAI signal from secondary organic aerosols (SOA) appears in summer due to plant emissions of volatile organic chemicals. Striping in the figures is due to persisting problems with polarization, work on solving this problem is in progress.

Cloud filter
After application of a cloud filter (cloud fraction <10%, cloud fraction determined by FRESCO+, see www.temis.nl/fresco), the largest sources of “scattering” aerosols (presumably sulfate, nitrate, and secondary organic aerosols), in particular in Southeast Asia and the U.S. East Coast, become even more apparent.

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