All-Sky Imaging of Skylight Polarization With Wildfire Smoke

Abstract:

We developed an all-sky imaging system for measuring skylight polarization in the visible and near-infrared spectral range. This instrument is used together with a multi-channel solar radiometer, an aerosol/cloud dual-polarization lidar, in-situ aerosol sensors, and a wide-angle thermal infrared cloud imager to study skylight polarization and its variation with changing aerosols, clouds, and underlying surface reflectance. All of these instruments except the lidar operate continuously outdoors. During August 2012, the continuously deployed instruments recorded the transition from relatively clean air to thick wildfire smoke. This presentation will summarize the temporal evolution of the aerosols and the resulting skylight polarization change as the smoke plume expanded and dissipated. During this event, the 500-nm zenith aerosol optical depth determined along the path to the Sun increased from 0.05 to 0.8 and the ground-level aerosol extinction coefficient at 530 nm wavelength increased from near 10 Mm^-1 to 490 Mm^-1. During the same time period, the maximum skylight polarization at 450 nm wavelength decreased from 0.7 to near 0.2. The skylight degree of linear polarization in an arc 90-degrees from the Sun was observed to transition from a nearly constant value across the arc to a highly spatially variable pattern because of the spatially nonuniform smoke plume.