# A Geostationary Instrument Simulator for Aerosol Observing System Simulation Experiments
## 1 Introduction  
Within the next five years, several new instruments will be launched into geostationary orbit with the objective of monitoring air quality. This geostationary constellation will be comprised of TEMPO (Tropospheric Emissions: Monitoring of Pollution) over North America [1], Sentinal-4 over Europe [2], and GEMS (Geostationary Environment Monitoring Spectrometer) over East Asia [3], and will provide an unprecedented level of high-resolution (both spatially and temporally) atmospheric composition data. The three spaceborne platforms will measure ozone (O\({}_{3}\)), nitrogen dioxide (NO\({}_{2}\)), sulfur dioxide (SO\({}_{2}\)), formaldehyde (HCHO), and aerosols from backscattered radiation in the ultraviolet-visiblespectral range (UV-Vis). The large-scale, time-resolved measurements of these pollutants provided by the geostationary constellation are important for managing air quality, and understanding how human activity impacts local, regional, and global distributions of pollution, as well as global climate. Aerosols are one of the most important forcing agents in the climate system. They can be transported around the globe [4], and interact with weather systems [5], scatter and absorb radiation [6; 7; 8], and modify cloud properties [9; 10]. Routine measurements of various aerosol parameters, such as aerosol optical depth (AOD), absorbing aerosol optical depth (AAOD), aerosol index (AI), and single scattering albedo (SSA), are currently available from several sensors in low earth orbit (LEO). These sensors include the twin MODIS (Moderate Resolution Imaging Spectroradiometer) instruments (on-board Terra and Aqua), MISR (Multi-angle Imaging SpectroRadiometer), OMI (Ozone Monitoring Instrument), and POLDER (Polarization and Directionality of the Earth Reflectances) (see Table 1 in [11]). In addition, CALIOP, a spaceborne LiDAR, measures the vertical distribution of aerosol backscattering. Instruments in LEO provide one or two measurements per day, but as aerosol parameters cannot be retrieved under cloudy conditions there can be a large time interval between valid observations. This limits the capability of LEO measurements to track the evolution of aerosol distributions, while a geostationary constellation can provide a continuous view of the areas below the satellites. TEMPO, GEMS, and SENTINEL-4 will provide measurements at a temporal resolution of approximately one hour, enhancing the probability of observing cloud-free scenes. It is expected that the baseline aerosol products for the geostationary constellation, AOD and AI, will be based on the heritage of the OMI and TOMS (Total Ozone Mapping Spectrometer) AOD retrieval algorithms that use reflectance measurements in the UV. However, numerical studies have shown that including coincident measurements from geostationary weather satellites, such as the Geostationary Operational Environmental Satellite-R Series (GOES-R) operated by the U.S. National Oceanic and Atmospheric Administration (NOAA), at complementary wavelengths and viewing geometry can reduce the uncertainty in retrieved AOD as well as enhance the retrieval of fine-mode aerosol fraction [11]. Moreover, higher resolution observations from weather satellites can also improve the geostationary AOD retrieval by providing sub-pixel cloud information. Estimating the usefulness and key characteristics of these planned aerosol products is essential for developing applications for these data. This type of evaluation is referred to as an Observing System Simulation Experiment (OSSE). OSSEs are performed using datasets of synthetic observations developed from a high resolution earth system simulation. The synthetic observations should sample the earth system simulation in such a way as to mimic the observation capabilities of the instrument, including its' viewing geometry and spatial, temporal, and spectral resolution.