Skylight Color Shifts due to Variations of Urban-Industrial Aerosol Properties: Observer Color Difference Sensitivity Compared to a Digital Camera

Simulations of the sky dome color shifts of a cloudless sky have been performed, assuming three urban-industrial tropospheric aerosol cases. Each of these aerosols is represented by the bimodal lognormal volume distribution (Dubovik et al. 2002 Dubovik, O., Holben, B., Eck, T. F., Smirnov, A., Kaufman, Y. J., King, M. D., Tanré, D. and Slutsker, I. 2002. Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations. J. Atm. Sci., 59: 590–608. [Crossref], [Web of Science ®] , [Google Scholar]). A total of ten parameters are used to control the aerosol characteristics. These ten parameters are perturbed and the resulting sky color shifts are estimated. All simulations have been performed relative to the reference cases that represent the time-averaged state of each aerosol case. We have simulated the sensitivity of the theoretical CIE (Commission Internationale de l′ Eclairage) standard human observer to discriminate sky color changes, due to changes of the aerosol control parameters. A Digital color Camera Model (DCM) has also been incorporated in the simulation scheme. The DCM simulates the performance of a camera-colorimeter created specifically to measure sky color. The DCM has been used to investigate whether a color digital camera can replicate the behavior of the CIE standard observer, with respect to sky color shifts. The standard observer is most sensitive to perturbations of the parameters of the aerosol fine mode, which are particles with diameters less than 0.6 μ m. However his sensitivity is highly variable, depending on the parameter varied, on the aerosol case and on the direction of the observation. The DCM was in excellent agreement with the behavior of the CIE standard observer. The camera simulated sky color measurement accuracy was high. The results show that the camera sensitivity in discriminating color differences is much better than that of the observer.