Evaluation of the Stability of EPIC Oxygen Channels with Observations andRadiative Transfer Model Simulations Over the South Pole

The Earth Polychromatic Imaging Camera (EPIC) is a 10-channel instrument aboard the Deep Space Climate Observatory (DSCOVR) satellite orbiting the Sun at the Lagrange-1 point, L1, about 1.5 million kilometers away from Earth. EPIC's 10 spectral channels include 4 in ultraviolet (UV) and 6 in visible/near-infrared (Vis/NIR) spectral regions. Special to EPIC are two pairs of O2 bands with the absorption bands at 688 nm (B-band) and 764 nm (A-band) and the nearby non-absorbing bands at 680nm and 780nm, respectively. The EPIC observations provide retrievals of total column ozone, sulfur dioxide, aerosol, cloud, and vegetation, etc. for the sun-lit side of Earth every 1~2 hrs. The O2 bands are used extensively in the retrieval of EPIC cloud products. The retrieval of cloud effective pressure and height are essentially based on O2 band ratios. In addition, the O2 band ratios are used innovatively in the cloud mask over ice/snow surface and ocean glint regions.

In order to monitor the performance of the O2 A- and B-band channels and ensure the absolute calibration of these channels, the EPIC cloud team has been monitoring the EPIC measurements of these channels over the Amundsen-Scott South Pole Station. The South Pole location is chosen because the surface is bright and signal is dominated by surface reflectance. However, even clear sky measurements taken at around the same time of the year, i.e., December of each year, small fluctuations within the months, and small, but significant trends in the ratios of A-band and B-band are observed. To understand these fluctuations and trends, we performed radiative transfer model (RTM) simulations with input from radiosonde profiles and column ozone observation for selected clear sky times in the months of December and January from 2015 to 2022. We also applied wavelength-dependent spectral albedos observed over South Pole snow surface. While it is quite challenging to match model simulations with the observations for all spectral channels at all times, the RTM simulation is able to capture systematic variations with the geometry angles. In particular, the O2 band ratios can be well predicted by the total airmass. The small orbital change of the satellite over the years has led to changes of geometric angles over South Pole and is responsible for the observed trends in the O2 band ratios. When model simulated contributions from angle variations and trends are deducted from the observations, the residual O2 band ratios are found to be stable since 2015.