Physical explanation of tropospheric aerosol effects on twilight sky color based on photographic observations and radiative transfer simulations

The relationship between the twilight sky and atmospheric state has been studied for the past 50 years, providing a good understanding of the general picture of the radiative transfer process under twilight conditions. However, tropospheric aerosols may strongly affect the spectral characteristics of the twilight glow near the horizon under clear-sky conditions, and the mechanism of this phenomenon is not well understood because of its high complexity. In this study, we observed the twilight sky using a digital camera and calculated the spectral radiance of the twilight sky using a radiative transfer model (RTM). Photographic observations aided by several ground-based radiometric measurements revealed that tropospheric aerosols significantly influenced the twilight sky color. In particular, the sky near the horizon became darker and bluer with increasing aerosol optical depth (AOD), which was consistent with the RTM simulations. The RTM simulations also showed for the first time that bluish multiply scattered light dominated the twilight sky near the horizon and reddish singly scattered light decreased with increasing tropospheric AOD. These simulation results suggest that photographic observations have the potential to characterize tropospheric aerosols under twilight conditions.