SPEX: the Dutch roadmap towards aerosol measurement from space

SPEX is developed as part of the roadmap for optical instruments of the Netherlands Space Office to support environment and climate research. SPEX is a compact and highly accurate polarimeter to measure atmospheric aerosol based on a novel method for measuring the state of linear polarization: spectral modulation. When operated from space, it can provide global monitoring of aerosols and clouds. This is important for society because these aerosol measurements allow for a more accurate prediction of climate. Furthermore, aerosol pollution in urban areas significantly reduces life expectancy. Developed by a consortium of knowledge institutes and industry under scientific guidance of SRON Netherlands Institute for Space Research, the SPEX instrument has gone through lab, field and airborne testing. The air campaign was carried out on NASA's ER-2 high-altitude airborne science aircraft. These tests have shown that SPEX is accurate enough to determine, not only the well-known aerosol optical thickness (AOT), but also particle specific properties such as the aerosol mean size, the single scattering albedo (SSA) and the complex refractive index. These measurements can be used to discriminate between natural and man-made aerosol. Recently, we have initiated a conceptual design and technology development phase to bring SPEX into low Earth orbit. The very high accuracy of our innovative polarization measurement combined with its spectral resolving power allows for a reduction of the spectral bandwidth compared to existing polarimeter solutions, without loss of performance. By limiting the spectral range, it is possible to incorporate a commercial detector and to include the polarimeter in an optical bench that is currently under development by TNO and Airbus DS NL (which in turn is based on the ESA Sentinel-5 Precursor instrument TROPOMI, to be launched this year), thereby increasing design heritage [8]. The modular approach, the absence of mechanisms, and the manufacturing concept are "designed for scale". This means that small-volume production is possible, bringing the cost of recurring models down without compromising on performance. We therefore call this concept SPEXlite. Opportunities for the application of SPEXlite are for example as supporting payload on the NASA PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission, or as a supporting instrument for future CO2 missions. In addition, scale production could open the way to a range of commercial instruments dedicated to provide climate and air-quality services from ground, air or space. In this contribution we report on first results of the SPEX Airborne flights and the design concept for SPEXlite for a low-earth-orbit satellite platform.