## 1 Introduction  
Rapid industrialization and urbanization in Asia has raised serious concerns related to air quality and climate change. Persistent emissions of pollutants affect air quality and the regional climate not only for Asia but also for the North Pacific because of long-range transport by westerly wind [1]. Consequently, understanding aerosol optical properties (AOPs) over Asia becomes important to estimate radiative forcing and its effects on human activity. Advances in measurement technology have improved our understanding of the chemical and physical properties of aerosols. Aerosol properties can be measured by ground-based instruments locally and their regional-scale variations can be observed using satellite remote sensing. Geostationary satellite measurements in particular have been widely adopted to monitor the continuous variation of aerosols with high temporal resolution. Over Asia, serial geostationary satellite projects, such as the Geostationary Korea Multi-Purpose Satellite (Geo-KOMPSAT), the Feng-Yun (FY) and the Himawari have been operated by the Korean, Chinese and Japanese governments, respectively and several algorithms to retrieve aerosol information have been developed optimized to each instrument. Since the Geo-KOMPSAT-1 was launched in 2010, its Meteorological Imager (MI) and Geostationary Ocean Color Imager (GOCI) have been used to monitor aerosol properties over northeast Asia. The MI provides a single visible measurement of AOD with 4 km \(\times\) 4 km resolution on a 15 min interval [2] and a multi-visible/near-IR GOCI algorithm retrieves AOD, fine-mode fraction (FMF) and aerosol type with 6 km \(\times\) 6 km resolution on a 1 h interval [3; 4; 5]. The Advanced Himawari Imager (AHI) was launched in 2013 and provides AOD and the Angstrom exponent from multi-visible and near-IR measurements [6]. As part of the Geo-KOMPSAT project, the Geo-KOMPSAT 2A and 2B twin-satellites are planned to be launched in 2018 and 2019, respectively. While the Geo-KOMPSAT1 mission focused solely on the measurement of atmosphere and ocean properties, the Geo-KOMPSAT2 mission is also tasked with understanding the wide variations in air quality over Asia. Towards this goal, a scanning spectrometer, the Geostationary Environmental Monitoring Spectrometer (GEMS), will be launched onboard the GEO-KOMPSAT 2B satellite with GOCI-II. The GEMS will measure the UV-visible spectrum (300-500 nm) with 0.6 nm resolution to retrieve total column ozone (O\({}_{3}\)), aerosol loading and key gas-phase species (NO\({}_{2}\), SO\({}_{2}\), HCHO) concentrations over Asia (5\({}^{\circ}\)S-45\({}^{\circ}\)N, 75\({}^{\circ}\)E-145\({}^{\circ}\)E) with high temporal resolution (1 h). The GEMS will be the first mission to monitor air pollutants over Asia from geostationary orbit. The coverage area of the GEMS instrument includes regions with air pollution from industrial activities and biomass burning as well as from natural sources, such as desert dust. The nominal spatial resolution is 7 km (NS) \(\times\) 8 km (EW) at Seoul (128\({}^{\circ}\)20\({}^{\prime}\)E, 37\({}^{\circ}\)24\({}^{\prime}\)N) and 4.9 km (NS) \(\times\) 8 km (EW) near the equator (128\({}^{\circ}\)20\({}^{\prime}\)E, 5\({}^{\circ}\)00\({}^{\prime}\)S) but that for aerosol properties will be enhanced to 3.5 km (NS) \(\times\) 8 km (EW) at Seoul without pixel binning to avoid possible noise due to subpixel cloud on a pixel. Table 1 lists major specifications of the GEMS. To achieve 0.6 nm spectral resolution, the spectral sampling by one pixel of the focal plane is 0.2 nm. In comparison with low-earth orbit (LEO) environmental satellites, such as OMI (13 km \(\times\) 24 km) and SCHIAMACHY (30 km \(\times\) 60 km), the GEMS has significantly improved spatial and temporal resolution.