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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Apr 2019

Research article | 12 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

A comparison of lognormal and gamma size distributions for characterizing the stratospheric aerosol phase function from OPC measurements

Ernest Nyaku1, Robert Loughman1, Pawan K. Bhartia2, Terry Deshler3, Zhong Chen4, and Peter R. Colarco2 Ernest Nyaku et al.
  • 1Center of Atmospheric Science, Hampton University, Hampton
  • 2NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771, USA
  • 3Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming
  • 4Science Systems and Applications, Inc. (SSAI), 10210 Greenbelt Road, Suite 600, Lanham, Maryland 20706, USA

Abstract. A series of in situ measurements made by optical particle counters (OPC) at Laramie, Wyoming provides size-resolved stratospheric aerosol concentration data for the period of 2008–2017. These data are analyzed in this study for the purpose of assessing the sensitivity of the stratospheric aerosol phase function to the aerosol size distribution (ASD) model used to fit the measurements. The two unimodal ASD models investigated are the uni-modal lognormal (UMLN) and gamma distribution models, with the minimum χ2 method employed to assess how well each ASD fits the measurements. The aerosol phase function (APF) for each ASD is calculated using Mie theory, and is compared to the APF derived from the Community Aerosol and Radiation Model for Atmospheres (CARMA) sectional aerosol microphysics module. Comparing the χ2 values for the fits at altitudes 20 km and 25 km shows that the UMLN distribution better represents the OPC measurements. The importance of data at aerosol radius below 0.1 μm is also demonstrated: When these data are not available from OPC measurements, the gamma distribution provides a more stable derived APF. The gamma distribution also fits the CARMA model results better than the UMLN model, when the CARMA model results are binned to mimic the OPC measurement bins (and therefore measurements between 0.05 and 0.1 μm are excluded).

Ernest Nyaku et al.
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Ernest Nyaku et al.
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Publications Copernicus
Short summary
This paper shows the importance of the nature of aerosol phase function used in the retrieval of the stratospheric aerosol extinction from limb scattering measurements. The phase function is derived either from the parameters a uni-modal log normal or gamma aerosol size distribution. These two distributions were fitted to the same aerosol concentration measurements at two altitudes, and depending on the nature of the measurements, the each distribution shows its strengths.
This paper shows the importance of the nature of aerosol phase function used in the retrieval...