HR: 0800h
AN: A41A-0015    [Abstracts]
TI: Retrieving Aerosol Properties by Inverse Methods With Data From Several Instruments
AU: * Fiebig, M
EM: Markus.Fiebig@noaa.gov
AF: National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory, 325 Broadway, Boulder, CO 80305 United States
AU: Ogren, J A
EM: John.A.Ogren@noaa.gov
AF: National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory, 325 Broadway, Boulder, CO 80305 United States
AB: Despite long standing effort, there is still a considerable uncertainty associated with the direct climate forcing of atmospheric aerosol. This is partly due to microphysical aerosol properties which influence the aerosol radiative forcing but cannot be measured directly. Among these properties are the state of mixture of particulate black carbon (BC) and the aerosol asymmetry parameter. The BC state of mixture significantly influences the absorption efficiency of BC and as a result the heating rate of the airmass that contains the BC aerosol. It will be investigated how the approach of combining the information of several instruments using inverse methods can serve to retrieve these microphysical parameters. Many methods for measuring one aerosol property suffer from inter-dependencies to other aerosol properties. For example, when measuring the particle size distribution with an optical particle counter (OPC), assumptions on the shape, morphology, and chemical composition of the aerosol have to be made which determine the optical model and the aerosol refractive index used during data interpretation. The interpretation of nephelometer data in turn needs to make assumptions on the particle size distribution if the truncation of the nephelometer is to be corrected precisely. Using these instruments in parallel on the same aerosol and combining their data into a single retrieval can not only eliminate the named inter-dependencies but additionally yield information on the refractive index and indication on the chemical composition. In this presentation, the method is used to design a more complex combination of mostly optical measurements using existing or modified existing instruments that yields information about the state of mixture of BC. The combination involves measuring the particle size distribution, the bulk chemical composition, the spectral scattering and backscattering coefficients, and the spectral absorption coefficient in a spectral range between 0.3~$\mu$m and 1~$\mu$m wavelength. The optical aerosol properties in the near UV are of particular importance for this application, meaning that existing instruments would need to be modified to implement the measurement strategy. As an example, the method is capable of retrieving the BC state of mixture for continental aerosol with an absorption coefficient of 4~(Mm)$^{-1}$. Deploying this combination of instruments in the field would yield the information on aging of BC which is needed by climate models to improve the assessment of the BC climate effect. Also, it will be shown how the method can be used to retrieve the aerosol asymmetry parameter from a limited set of standard instruments as used by the aerosol monitoring network of the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory. This set consists of a three wavelength scattering and backscattering nephelometer and a single wavelength absorption photometer which are alternately operated with 1~$\mu$m and 10~$\mu$m cut-diameter impactors . Together with the extinction coefficient and single scattering albedo readily calculated from this data-set, the retrieved asymmetry parameter yields a complete set of optical properties for calculating the aerosol direct effect on radiative transfer. With the available instruments becoming more and more accurate, retrieval by inverse methods is a means that promises to help reducing the uncertainty of the direct aerosol climate effect.
DE: 3260 Inverse theory
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting