Deriving Aerosol Parameters from in–situ Spectrometer Measurements for Validation of Remote Sensing Products Sebastian Riedel *a,b , Joanna Janas b , Peter Gege a , Natascha Oppelt a a Kiel University Department of Geography, Ludewig–Meyn–Str. 14, 24098 Kiel, Germany; b German Aerospace Center (DLR), Remote Sensing Technology Institute, 82234 Weßling, Germany ABSTRACT Uncertainties of aerosol parameters are the limiting factor for atmospheric correction over inland and coastal waters. For validating remote sensing products from these optically complex and spatially inhomogeneous waters the spatial resolution of automated sun photometer networks like AERONET is too coarse and additional measurements on the test site are required. We have developed a method which allows the derivation of aerosol parameters from measurements with any spectrometer with suitable spectral range and resolution. This method uses a pair of downwelling irradiance and sky radiance measurements for the extraction of the turbidity coefficient and aerosol Ångström exponent. The data can be acquired fast and reliable at almost any place during a wide range of weather conditions. A comparison to aerosol parameters measured with a Cimel sun photometer provided by AERONET shows a reasonable agreement for the Ångström exponent. The turbidity coefficient did not agree well with AERONET values due to fit ambiguities, indicating that future research should focus on methods to handle parameter correlations within the underlying model. Keywords: Aerosol optical thickness; turbidity coefficient; Ångström exponent; spectrometer measurements; atmospheric correction; sun photometer; inland waters; field measurements. 1. INTRODUCTION Atmospheric correction is a prerequisite for the application of quantitative evaluation methods to remote sensing data in optically complex coastal and inland waters. In particular when applying physically based bio–optical models, atmospheric correction is the limiting factor in the results accuracy [1]. The most critical parameter for atmospheric correction is the aerosol, especially for turbid and spatially inhomogeneous coastal and inland waters [2]. For validation of atmospheric correction accurate aerosol parameters, measured close to a number of matchup locations are required. Therefore it is often valuable to measure aerosol parameters during field campaigns on a boat. Aerosol parameters are usually derived from stationary instruments (self–aligning sun photometers [3] or rotating shadowband radiometers [4]) at fixed locations or from mobile handheld sun photometers [5] at a number of test sites during field campaigns. The spatial coverage of ground based sun photometers is often insufficient for validation purposes, and targeting the sun with the required accuracy below 1° with a handheld device on a boat can be challenging [6]. To be more flexible during field campaigns, we have developed a method which allows the derivation of aerosol parameters from measurements with a field spectrometer. While sun photometers measure the radiance from the direction of the sun, this method uses downwelling irradiance and sky radiance measurements. Since no accurate alignment is necessary, the data can be acquired fast and reliable at almost any place during a wide range of weather conditions. In contrast to sun photometers and shadowband radiometers, spectrometers are commonly used by researchers in aquatic optics or limnology. Easy–to– conduct field spectrometer measurements would therefore be a viable way to improve atmospheric correction procedures over inland and coastal waters. 2. MATERIALS AND METHODS 2.1 Measurement Setup All measurements used in this paper were conducted with a Freedom VIS FSV–305 spectrometer from Ibsen Photonics [7]. The FSV–305 has a usable spectral range of 350–830 nm with a spectral resolution of approximately 1.6 nm/FWHM and numerical aperture of approximately 0.16. The sensor has no temperature stabilization or temperature measurement, thus the dark current is measured frequently during data acquisition and subtracted from the measurements. The * riedel@geographie.uni-kiel.de; phone +49 (0) 8153 28 1969 Remote Sensing of Clouds and the Atmosphere XXII, edited by Adolfo Comerón, Evgueni I. Kassianov, Klaus Schäfer, Richard H. Picard, Konradin Weber, Proc. of SPIE Vol. 10424, 1042404 © 2017 SPIE · CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2280290 Proc. of SPIE Vol. 10424 1042404-1