Short communication Aspect ratio as a function of particle radius: Inversion of extinction and scattering data Miroslav Kocifaj a, b, * , Franti  sek Kundracik a a Faculty of Mathematics, Physics, and Informatics, Comenius University, Mlynsk a dolina, 842 48 Bratislava, Slovak Republic b ICA, Slovak Academy of Sciences, Dúbravsk a Road 9, 845 03 Bratislava, Slovak Republic highlights  A novel method for inversion of scattering and extinction data is developed.  Size distribution and aspect ratio of nonspherical particles can be retrieved.  Aerosol aspect ratio is determined iteratively if refractive index is a-priori known.  The method is effective for large scattering angles including side and back scatter. article info Article history: Received 17 October 2014 Received in revised form 20 January 2015 Accepted 15 February 2015 Available online 6 March 2015 Keywords: Nonspherical particles Aspect ratio Light scattering Multispectral extinction data abstract The sun and sky photometry made concurrently or, alternatively, simultaneous measurements of extinction and scattering data both represent a valuable tool for gathering the information on aerosol particles. Most typically the size distribution and/or refractive index of aerosol particles can be inferred from multispectral and/or multiangle optical data. Extraction of size-dependent aspect ratio of aerosol particles from optical data is a highly non-trivial task since the kernel of the particular integral equation is a non-linear function of the sought solution. The iterative solution to this problem is introduced and demonstrated on synthetically generated data. It is shown that retrieval of size-dependent aspect ratio is possible even for complex morphologies, most typically for irregularly shaped dust particles. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The long-term observations in the Earth's atmosphere have proven that aerosol particles of different origin coexist in diverse multicomponent populations. Such particles interact with solar radiation in a complex way forming the diffuse component of ground reaching radiation. There is no doubt that measuring the parameters of scattered light is a valuable tool in optical charac- terization of atmospheric aerosols because of high information content of spectral sky radiance data (Kokhanovsky, 2009; Dubovik and King, 2000). It is well recognized that aerosol particles of different sizes have different residence times in the atmosphere. For instance, sub- micrometer sized aerosols can remain suspended for up to several weeks in the air (Lata et al., 2003) while other particles can be readily removed by rain. The airborne particles can be of different origin and transported at long distances by wind, but most typically they are produced by local sources of pollution (Swietlicki et al., 1996), like industry, fossil fuel combustion, or other anthro- pogenic activities (Almeida et al., 2005). Basically, urban pollution can be increasingly raised by motor vehicles that emit particles with sizes between 50 and 200 nm which could have a residence time of about 1 week in the atmosphere. Due to processes of for- mation these particles can be non-spherical. In urban areas the dust load is directly related to changes in particle size distribution (Keller and Lamprecht, 1995). Even if the prevailing composition of local particles can be monitored chemi- cally (by analyzing the filtered samples) or inferred indirectly (based on the information on local sources of pollution), the * Corresponding author. Faculty of Mathematics, Physics, and Informatics, Comenius University, Mlynsk a dolina, 842 48 Bratislava, Slovak Republic. E-mail address: kocifaj@savba.sk (M. Kocifaj). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv http://dx.doi.org/10.1016/j.atmosenv.2015.02.078 1352-2310/© 2015 Elsevier Ltd. All rights reserved. Atmospheric Environment 109 (2015) 19e22