Introducing the geostationary environment monitoring spectrometer

Abstract. To consistently observe deteriorating air quality over East Asia, the National Institute of Environmental Research, Republic of Korea, is planning to launch an environmental observation sensor, the Geostationary Environment Monitoring Spectrometer (GEMS), onboard the GK-2B satellite (a successor to the GeoKOMPSAT-1) in late 2019. GEMS is a hyperspectral spectrometer that covers the ultraviolet–visible range (300 to 500 nm) with full-width at half-maximum of 0.6 nm. It has been designed for the observation of air pollutants and short-lived climate pollutants. GEMS captures images at hourly intervals in the daytime, alternating with the Geostationary Ocean Color Imager-II every 30 min. Over the Seoul Special Metropolitan area, South Korea, the spatial sampling resolution of GEMS is 3.5  ×  8  km (north–south and east–west, respectively). There are 16 baseline products, including aerosol optical depth and the vertical column density of trace gases such as nitrogen dioxide, sulfur dioxide, formaldehyde, and ozone. Research continues into additional applications (e.g., ground-level concentrations and emissions).


Introduction
Nearly three million premature deaths are attributed to air pollution each year; 1 ∼90% of the world's population is in constant danger of exposure to high levels of particulate matter (PM) with an aerodynamic diameter ≤ 2.5 μm (PM2.5). 2 Due to various factors including ongoing industrialization and urbanization with significant changes in climate and geography, especially over East Asia, South Korea had the worst air quality among the Organization for Economic Co-operation and Development (OECD) nations in 2013. Averaged annual exposure to PM2.5 over 3 years (2010 to 2012) was 29.09 μg∕m 3 , significantly greater than the OECD mean of 14.05 μg∕m 3 . 3 In addition, PM10 levels (PM with a diameter ≤ 10 μm), which had been consistently improving since the 1990s, have deteriorated since 2013, while high concentrations of fine particles are becoming more common.
To address this deteriorating air quality, the South Korean government established the "Special Act on the Improvement of Air Quality in Seoul Metropolitan Area" and invested approximately $3 billion during phase 1 (2005 to 2014). 4 Policies regulating air pollutant emissions have resulted in significant reductions in sulfur oxides, nitrogen oxides, volatile organic compounds (VOCs), and PM10 (Table 1). However, the concentrations of nitrogen oxides and PM2.5 remain high when compared with other developed nations. 5,6 Phase 2 is currently underway with a total budget projected to be approximately $4 billion over the next 10 years.
For air quality improvement policies to succeed, the relationships between sources and quantities of air pollutants in the atmosphere need to be more accurately understood. Moreover, optimum reduction quantities and efficient reduction scenarios require determination. Therefore, it is critical to distinguish between the contributions of domestic sources and foreign influxes to direct air quality improvement policies. Additionally, depending on climatic conditions, air pollutants emitted from surrounding nations directly impact the South Korean atmosphere with a lag time of 1 to 2 days. 7 For example, recent increases in air pollution over South Korea have been closely attributed to air pollutant emissions from China. [8][9][10][11] Therefore, multilateral and international efforts have been proposed to determine the effects of long-range air pollutant transport and thus to improve air quality in East Asia. These efforts include the launching of joint research groups between South Korea and China, the operation of long-range transboundary air pollutant (LTP) projects among South Korea, Japan, and China, 12,13 and discussions at the Tripartite Environment Ministers Meeting and Korea-United States Air Quality Campaign. 14,15 The South Korean government is in the process of expanding its ground observation network used for monitoring air quality. It has also been performing episodic aerial monitoring to efficiently establish and enforce air quality policies, and to acquire fundamental scientific data. However, precise assessments are still limited due to deficient data from ground-based observations that do not sufficiently cover contiguous large target areas of interest. The development of advanced remote sensing technologies as well as the consistent monitoring of trace gases in the atmosphere directly contributes to overcoming these limitations, thereby improving our ability to analyze long-range transported air pollutants. 16 Currently operational low Earth orbit (LEO) environmental satellites [16][17][18][19][20][21] are capable of monitoring air quality on a global scale; however, they are temporally limited to observing the same location only every 1 to 3 days. To consistently monitor deteriorating air quality over East Asia, the National Institute of Environmental Research (NIER), Republic of Korea, is planning to launch a satellite-based environmental observation instrument, the Geostationary Environment Monitoring Spectrometer (GEMS), in late 2019. GEMS is expected to perform quantitative analyses on air pollution phenomena in the East Asian region. 22,23 2 GEMS Mission Overview GEMS development was initiated in 2012 along with an advanced ocean color imager, the Geostationary Ocean Color Imager-II (GOCI-II). 24 Both sensors will be launched onboard GeoKOMPSAT-2B (GK-2B) as part of the GK-2 mission, which consists of two satellites (GK-2A/B): 25 the 2A satellite, which carries the Advanced Meteorological Imager (AMI) for weather surveys, and the 2B satellite, which carries GOCI-II and GEMS for ocean and atmospheric environmental monitoring.
The AMI, GOCI-II, and GEMS sensors each have unique spectral and spatial coverage with resolutions optimized for their respective missions. In addition to the major products obtained from each payload, additional products are expected to be utilized by integrating the GK-2A/B observation data based on their different spectral and spatial resolutions. Research to develop potentially new output categories, and to improve the accuracy of existing outputs via the integration of payload data, is currently underway.

Spatial and Spectral Coverage
The region of interest for GEMS covers East Asia, corresponding to longitudes 75°E-145°E and latitudes of 5°S-45°N. Its sampling spatial resolution is 3.5 × 8 km (north-south and east-west, respectively) per pixel at Seoul, with spatial and temporal co-adding depending on product category.
GEMS provides trace gas concentrations through a spectroscopic algorithm that calculates concentrations based on the gas absorption cross section with regards to solar irradiance (Fig. 1). The spectrometer observes ultraviolet-visible wavelengths in the 300-to 500-nm range with a sampling spectral resolution of 0.2 nm, and a full-width at half-maximum (FWHM) of 0.6 nm, which is the sum of all three spectral sampling data sources; thus its operation is limited to the daytime.

Observation Modes
GEMS and GOCI-II will operate alternatively every 30 min to capture hourly images; GEMS covers a wider area, from east of Japan to Indonesia (Fig. 2). Its operational scenario includes a concentrated observation mode that allows enhancement of its signal-to-noise ratio (SNR) by reducing the east-west scan range during the mornings and evenings when solar radiation is weak. GEMS can utilize three observational modes: normal observation, East Asia observation, and local area observation. These observational modes can be utilized to closely monitor corresponding areas during instances of high air pollution ( Table 2).

Overall Design and Performance
In normal mode, GEMS observes its field of view (FOV; Fig. 2) from east to west by moving its scan mirror position a total of 701 times. At a given scan position, 37 images over a period of 63.5 ms are coadded onboard to enhance its SNR; coadding is possible up to 63 times (Fig. 3). However, GEMS is limited to only 30 min of observation per hour, and its allowable FOV decreases for concentrated observations to enhance SNR. Earth radiance values are given as where i is the spectral position and j is the ground position (north-south direction); P1 i;j ðDNÞ is the focal plane subsystem output with 14 bits∕pixel; P2 i;j ðDNÞ is the Earth observation radiance value; CON is the number of Earth observations (generally 37, maximum 63); and DN is the mirror position (generally 1 to 701). The design of GEMS, including the light path, uses optical paths equal to those used in observing the Earth and Sun for spectral calibration (Fig. 4). The grating and reflector spectroscopic parts were designed in the form of an Offner relay to minimize distortion. One scan spatially covers 2048 pixels in a north-south direction (one pixel corresponds to 3.5 km at Seoul) and spectrally covers 1000 pixels (spectral resolution of 0.2 nm); scanned data are sent to ground  control after each observation. Spatial coaddings are performed based on product category to enhance the SNR at the ground station, the Environmental Satellite Center (ESC), which eventually improves the accuracy of the level 2 data.

Spectral Performance
The GEMS bandpass function has been measured at selected wavelengths (Table 3). Using a laser with an accuracy of 0.006 nm, 10 images were captured at each wavelength with a sampling interval of 0.06 nm over the scan range of AE1.8 nm. The collected data were used to determine bandpass function with the wavelengths for the maximum bandpass as well as for the FWHM. The point spread function (PSF) was calculated at four selected wavelengths (300, 370, 430, and 500 nm) and three spatial positions. The PSF was characterized at the spectroscopic level. Signals at both extremes of the bandpass function are hidden by noise, making them difficult to measure. Thus, broadband characterization was performed with multiple laser sources and long pass filters. These data can also be utilized in the assessment of stray light performance with the PSF. Stray light can be also from out-of-band (i.e., wavelengths outside of 300 to  500 nm) and out-of-field, which is spatially affected in the north-south direction. To measure stray light, data from the middle range are collected using a proper laser source intensity (or exposure length) level to ensure that signals at middle wavelengths (370 and 430 nm) are not saturated. Even if the signal in the middle range becomes saturated, the laser source intensity can be set so that data from the two extremes are greater than the noise level and can be subsequently recorded with high SNRs. Similarly, ranges at the extremes can also be analyzed with high SNRs by using a stronger laser source. As shown in Fig. 5, a clean PSF without the influence of noise can be obtained by standardizing collected data to their corresponding laser source intensities. As opposed to methods involving adjustment of light source intensity, GEMS collects data by adjusting the laser exposure time. This method ensures uniformity of the laser source with respect to wavelength and makes standardization easier. In addition, adjusting the laser exposure time is more accurate than adjusting the intensity of the laser source.

Calibration
GEMS covers a wavelength range of 200 nm (between 300 and 500 nm) with a spectral resolution of 0.6 nm; the accuracy of the final output is heavily affected by the slightest perturbations in the observations. Hence, GEMS is expected to validate and calibrate its observational data through methods such as solar observation and light-emitting diode (LED)-assisted linearity analysis. Representative on-orbit calibration activities include solar observations using diffusers of GEMS, dark imaging with the calibration wheel in closed position, utilization of internal LEDs, and the validation of linearity of the results (Table 4).

Working/reference diffuser calibration
GEMS monitors the Sun once a day to validate and calibrate any self-caused changes in the Earth's radiance data. For this reason, GEMS is equipped with a calibration mechanism assembly (CMA), which has an open position for Earth observations, two diffuser positions, and a closed position to prevent incoming debris. GEMS also carries a working diffuser as well as a reference diffuser to correct the degradation of the working diffuser every 6 months. The diffuser is used for solar calibrations before midnight when the incident angle of sunlight on GEMS is ∼30 deg. The diffuser transmission is affected by the solar incidence angle; thus, the solar calibration time during the year is determined at a constant incident angle (Fig. 6).
Observed solar irradiance [F meas ði; jÞ] is used to calibrate the Earth radiance observations from GEMS compared to existing reference values. F meas ði; jÞ is expressed as E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 2 ; 1 1 6 ; 2 4 8 where k r i;j is the pixel radiance calibration coefficient at column i, row j; C i photon ði; jÞ is the measured signal in irradiance mode at column i, row j; BTDF Diffuser ðG i;j Þ is the bidirectional transmission distribution function (BTDF) of the on-board diffuser, which is a function of the goniometric angle; N Coadd is the number of image coadditions; and t int is the single frame integration time.

Dark imaging and on-board LED source linearity calibration
Dark imaging is performed to analyze GEMS noise signals and to offset values when the CMA is in the closed position. The imaging is conducted twice daily (before and after Earth observations) and is used for the current dark correction during the observation. Dark calibration is performed by combining the frames of dark imaging and normal observation, which have the same integration times and numbers of images. In addition, GEMS utilizes its internal LEDs once a week to check the homogeneity of CCDs and the linearity of the image. It collects LED images at 20 different times and interpolation is used in between. Each data element consists of a combination of five frames. The calibration wheel moves to the closed position during LED imaging.
Some GEMS products are provided as vertical column densities (VCD) by retrieving slant column densities and applying the air mass factor (AMF). The acquired data are expected to be used as feedback for the detection of clear sky pixels and calculation of surface reflectance to further contribute to the improvement of accuracy. The accuracy of the products retrieved is needed to be validated after the satellite launched.

Aerosol Optical Depth
Aerosols affect the human body via respiration: they are deposited in the bronchial tract, often reaching the lungs, where they critically endanger health by accumulating in the cardiovascular system. Aerosols can even create economic losses by damaging precision industries. Owing to their detrimental health effects, the World Health Organization (WHO) has classified PM as group 1 carcinogens. 1 Not only does the absorption and scattering of solar energy used by aerosols have a direct and critical effect on global climate change, [26][27][28] indirect meteorological effects are also caused by physical processes related to clouds and precipitation.
While radiative energy in the 300-to 500-nm range is only slightly affected by the ground surface, it is strongly affected by air molecule scattering and the absorption/scattering effects of aerosols, making it useful in aerosol-type determinations or optical depth calculations. The GEMS AOD output algorithm uses a radiative transfer model to calculate a lookup table (LUT) and estimate the AOD by applying an inversion method to compare top-of-atmosphere reflectance and the LUT data. The AOD is very sensitive to clouds; therefore, only clear regions without any clouds are selectively used for calculations. The accuracy of data is determined by the cloud detection accuracy as well as the spectral properties and size distribution of the aerosols used in the LUT. The temporal resolution of the AOD output is identical to the temporal resolution of the general observation and calculations can be performed on a single pixel of the spatial resolution mentioned in Sec. 2.1 (3.5 × 8 km per pixel based on the Seoul area).

Ozone
Ozone is a trace gas composing 0.001% of atmospheric volume. It is a short-lived climate pollutant (SLCP) and its contribution to the greenhouse effect closely follows that of carbon dioxide (CO 2 ) and methane (CH 4 ). 29 Moreover, although stratospheric O 3 absorbs ultraviolet rays from the sun and creates a viable environment on the Earth's surface, tropospheric O 3 is a key secondary pollutant that is detrimental to the human body and various ecosystems.
With recent rapid economic growth in East Asia and the subsequent increase in fossil fuel consumption, a significant volume of air pollutants is emitted and the amount of the secondary pollutant O 3 is increasing, creating concerns for human health and ecological damage. [30][31][32] The GEMS O 3 calculation algorithm outputs total O 3 and a vertical profile, where the wavelength range for the calculation of accumulated tropospheric O 3 concentrations is 300 to 340 nm. Tropospheric O 3 can be calculated with low analytical error using signals in the ultraviolet range at the ground level. The calculation algorithm utilizes a nonlinear optimal estimation to minimize the difference between measured and simulated spectral radiances and the difference between retrieved and a priori state vectors, constrained with measurement and a priori error covariance matrices. 33 The accuracy of the final accumulated O 3 concentration is determined by factors such as the measured satellite signal accuracy and the O 3 sensitivity dependent on vertical height, and is calculated over cloudless, clear sky pixels. The temporal resolution is identical to that of the general observation, and calculations of the accumulated O 3 can be performed by coadding two pixels of GEMS spatial resolution (7 × 8 km).

Nitrogen Dioxide and Sulfur Dioxide
Accumulated NO 2 and SO 2 concentrations are calculated from the GEMS spectral imaging calibration data for 400 to 500 nm and 305 to 320 nm, respectively. Using satellite signals from areas with low amounts of NO 2 or SO 2 as references, the signals of aerosols and air molecules with slow absorption effects are removed. Using the differential optical absorption spectroscopy (DOAS) method, 34 the VCDs are calculated by fitting the gas-specific absorption cross-section data from the database.
The accuracies of the final accumulated NO 2 and SO 2 concentrations are determined from the calculation errors between the AMF and the residual noise signals after the DOAS fitting. The temporal resolution is identical to that of the general observation and calculations of the accumulated NO 2 can be performed by coadding two pixels of GEMS resolution (7 × 8 km). For the sake of improving the precision and SNR of SO 2 , eight pixels of the GEMS resolution are coadded and averaged over up to 3 h.

Formaldehyde
Most of the HCHO in the atmosphere is produced from oxidation of CH 4 . High concentrations of HCHO at the ground level can be observed when it is directly released into the atmosphere (e.g., wildfires); however, the main product is isoprene (C 5 H 8 ). The emission of C 5 H 8 can be derived from the VCD of HCHO, which can be observed via satellites. [35][36][37][38][39][40][41] Satellite observation of HCHO can be a key contributor to the estimation of VOCs over large areas and the reduction of uncertainty.
The accumulated HCHO calculation algorithm of GEMS uses basic optical absorption spectroscopy (BOAS) technology over a wavelength range of 327 to 358 nm. The BOAS technology fits the directly measured satellite radiance signals, numerous variables stored in the library, and the gas-specific absorption cross-section data to calculate the accumulated concentrations of HCHO. The accuracies of the accumulated HCHO concentrations are determined from the calculation errors between the AMF and the residual noise signals after the BOAS fitting.
Calculations for HCHO are performed over clear sky pixels and the temporal resolution is identical to that of the general observation. The calculations of the accumulated HCHO can be performed by coadding eight pixels of the spatial resolution as mentioned in Sec. 2.1 (14 × 16 km∕pixel) and averaged over 3 h.
For the reception, processing, and application of GEMS data, the Korean Ministry of Environment constructed the ESC at the NIER and established a data processing system. The NIER is currently conducting an integrated air quality forecast; 42 a significant improvement in the forecast accuracy is expected with the utilization of GEMS data.
GEMS data are simultaneously received at the Korea Aerospace Research Institute, which controls the satellite bus system, and the Korea Ocean Satellite Center, which is the ground control station for the GOCI-II and holds a mutual data backup relationship with its counterparts (Fig. 7). The raw data observed from GEMS and received via the reception system at the ESC produce the L1B data through the decomposition module, radiation calibration, and geometric calibration. The L2 data are provided in the form of outputs such as AOD ( Fig. 8 and Table 6). To improve the accuracy of the main L2 outputs at the ESC, forecast data from the ocean color satellite GOCI-II and the atmospheric chemistry modeling data are collected. The production of external-data driven value-added products, including mean field and surface concentrations, is in preparation.

Application and Expected Impacts of GEMS
GEMS is expected to provide the hourly distribution of the product variables and simultaneously analyze their emission, transport, and sinks. Unlike those of LEO environmental satellites, the spatial and temporal distribution characteristics of GEMS enable continuous monitoring of O 3 , SO 2 , NO 2 , aerosols, and HCHO from a single location; it thus has a wide range of expected applications, including top-down pollutant emissions and long-range transport.
In particular, it is anticipated that emission calculations and monitoring will be possible through the spatial and temporal observations of tropospheric O 3 , PMs, VOCs, and precursors, 43 and that these will be utilized in the quantitative analysis and transport pathway determination of long-range air pollutants. Furthermore, methods to calculate the surface concentrations of PM could be developed using the satellite-observed AODs, [44][45][46][47] which would be useful in the monitoring of PM distributions, and analyses of their trends and high PM concentration events.
In the field of climate change, GEMS is expected to contribute toward understanding active climate change responses, such as quantitative analyses related to climate change through the consistent monitoring of surface and atmospheric East Asian SLCP (e.g., O 3 , aerosols) concentrations. It is also likely to provide solutions to reduce SLCPs, which, unlike long-lived CO 2 , represent short-term reduction effects.
Furthermore, GEMS is anticipated to be used in the detection of sulfuric gases, as an indicator of volcanic eruptions, for the confirmation, prediction, and analysis of air pollutants (e.g., toxic substances), for understanding transport paths from chemical accidents, and the quantitative estimation of air pollutants from large wildfires.
In particular, the use of GEMS as input data in air quality models is expected to significantly improve forecast accuracies through data assimilation and the amelioration of initial and boundary condition uncertainties. 48,49 GEMS may also bring about quality improvement through the evaluation of air quality prediction models, accuracy enhancement of air pollutant emissions through bottom-up estimations, and through accuracy enhancement and improvement of air quality prediction intervals through assimilation of satellite data and forecast models. [49][50][51] It is also likely to improve the accuracy of air pollution forecasts and warnings through earlier estimation of high air pollutant concentrations. These examples of forecast and warning accuracy improvements will enhance public trust and increase air pollution avoidance rates; thus, improving the health and quality of life of citizens.
Lastly, along with TEMPO 22 and Sentinel-4, 23 GEMS is expected to ensure broad and consistent monitoring of air quality in East Asia, North America, and Europe's concentrated areas. In conjunction with scientific data for air pollutant emissions, transport, and degradation at the global scale, international cooperation among state governments and efficient air quality policy implementation will be achievable. It should be noted that globally spreading air pollution is impossible to mitigate with the efforts of only a few nations. Therefore, the three major geostationary environmental satellites, including GEMS, are anticipated to provide solutions for international cooperation and ensure efficient and effective air quality control.

Conclusions
The South Korean government is developing the first geostationary environment spectrometer (GEMS) and planning its launch in 2019 to consistently monitor deteriorating air quality and establish and enforce efficient and cost-effective air quality policies. GEMS is expected to provide fundamental data on trace gases including NO 2 , SO 2 , HCHO, O 3 , and AOD. In addition, its output can be utilized in air quality prediction models and contribute toward increasing the accuracy of forecasts and warnings that are needed to prevent air pollution, thereby enhancing quality of life. Moreover, GEMS will potentially be able to contribute to international efforts and assist in the development of a clean atmosphere.
Won Jun Choi is a researcher at the National Institute of Environmental Research (NIER). He is working on the overall project for air quality monitoring using GEMS from the beginning to the present, and contributed to making the air quality monitoring plan using space technologies in the environmental field.
Kyung-Jung Moon is a researcher at the NIER. He has been conducting research on the integration and evaluation the GEMS algorithm, and he is working on transforming the integrated algorithms for calculation air quality system using GEMS radiation data.
Jongmin Yoon is a senior researcher at the NIER. He is a middle manager of development for algorithms to calculate the concentration of trace gas using GEMS spectral data in real time, and also of data processing system development.
Ara Cho is a researcher at the NIER. She is developing data processing system of environmental satellite center (ESC). She is studying in the long-range transboundary of air pollutants using satellite data.
Sang-kyun Kim is the director of ESC at the NIER, which oversees the development of GEMS and is responsible for the satellite work in the field of atmospheric environment including GEMS. He is doing the effort to expand the use of environmental satellites, such as the calculation of surface concentration using satellite observations and monitoring of atmospheric sources.
Seounghoon Lee is a senior researcher at Korea Aerospace Research Institute (KARI). He is a principal investigator of development and manufacturing for GEMS hardware which will be loaded on GeoKOMPSAT2B (GK2B). Optical observation equipment mounted on multiple satellites was developed under his responsibility.