The Spectro-polarimeter for Planetary Exploration one (SPEXone) instrument aboard NASA's Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) platform is a multi-angle polarimeter. It measures the intensity, Degree of Linear Polarization (DoLP), and Angle of Linear Polarization (AoLP) of sunlight reflected from Earth’s atmosphere, land surface, and ocean. The focus of SPEXone is to achieve highly accurate DoLP measurements, which facilitate accurate characterization of aerosols in the atmosphere.

Aerosols are small solid or liquid particles suspended in the air that affect climate directly through interaction with solar radiation. Aerosols affect climate indirectly by changing the micro- and macro-physical properties of clouds. According to the Intergovernmental Panel on Climate Change, aerosols are the largest source of error in quantifying the radiative forcing of climate change. SPEXone will enable measurements of optical and micro-physical properties of aerosols with unprecedented detail and accuracy.