Introduction

South Korean Geostationary Environmental Monitoring Spectrometer (GEMS) started novel hourly observations of atmospheric composition over Asia in 2020. GEMS provides the first continuous daytime record of nitrogen dioxide (NO₂) from space. NO₂ is a key driver of atmospheric chemistry and pollution. Before GEMS, only once or twice a day observation was available from the polar-orbiting satellites. In this study, summertime diurnal variations of NO₂ were characterized across major Asian hotspots with aid of the new observations. The source regions include mega cities like Seoul, Beijing, Shanghai (Yangtze River Delta), Guangzhou, Bangkok, and Hanoi, power plants in China and India, and a shipping route (the Strait of Malacca) located between Malaysia and Indonesia. GEMS data reveal consistently elevated NO₂ concentrations in the morning that decline in the afternoon, with distinct regional differences in the timing and amplitude of daily peaks and troughs. These variations reflect the interplay between local nitrogen oxide (NO_x) emission activities and atmospheric chemical processes. A computer model, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) that integrates emission, chemistry, and transport provide insights how much NO_x is emitted for each region and how it varies in the sunlit atmosphere. Diurnal NOₓ emission patterns inferred from GEMS and WRF-Chem vary by region: early morning peaks in Hanoi, Guangzhou, and Bangkok; mid-to-late morning peaks over Seoul and Beijing; and late afternoon peaks in the Yangtze River Delta and the Strait of Malacca. Top-down NOₓ emission estimates derived from hourly GEMS observations—accounting for both temporal variation and chemical loss—yield the most accurate simulations of NO₂ columns.

The Science Behind the Study

GEMS measures columnar gas concentrations, including NO₂, using Differential Optical Absorption Spectroscopy (DOAS) based on hyperspectral radiance measurements across ultraviolet to visible wavelengths from geostationary Earth orbit (GEO), which is typically 40–50 times farther from Earth than low Earth orbit (LEO). The slant path geometry of the observations is converted to vertical column densities at each pixel using vertical profile shapes and scattering weights derived from sophisticated chemical transport and radiative transfer models. Different from LEO satellites, where the solar illumination geometry remains fixed due to the sun-synchronous orbit, GEMS must account for variations in solar geometry throughout the day, in addition to changes in viewing geometry at each pixel.

Why It Matters

NO₂ is a regulated pollutant known to contribute to increased rates of childhood asthma. Together with volatile organic compounds, NO₂ plays a key role in the formation of harmful ground-level ozone through photochemical reactions. Ozone itself is also a regulated pollutant. Due to the nonlinear chemistry involved in ozone production and the influence of multiple factors across varying spatial and temporal scales, many countries continue to face significant challenges in reducing elevated ozone levels. Moreover, NO₂ contributes to the formation of atmospheric aerosols, further exacerbating health risks. GEMS provides high-resolution NO₂ observations across large regions of Asia, offering invaluable data for research in atmospheric chemistry and public health.

The Journey Behind the Research

GEMS was initially conceived to transition LEO satellites, such as OMI and GOME, into GEO in order to observe diurnal variations in air pollutants with high spatial and temporal resolution. The first constellation of the three GEO missions were coordinated to cover the most polluted continents in the Northern Hemisphere. As a key part of Asia’s involvement in a trio of geostationary satellite missions, GEMS monitors air quality over some of the world’s most heavily polluted urban regions. The Tropospheric Emissions: Monitoring Pollution (TEMPO) started hourly observations in 2023 over North America. The upcoming ESA/Copernicus Sentinel-4 geostationary spectrometer is planned for launch in 2025 for hourly observations over Europe and northern Africa. The observational capabilities of the GEO constellation are being extended to the Middle East and Africa through the MEASMA (Middle East and Africa Space-based camera for Monitoring Atmospheric pollution) initiative. This creates a synergistic effect in Earth observations by enabling higher-resolution and more frequent data collection across the continents, thereby improving air pollution research for a substantial portion of the global population.

Future Directions

GEMS measures NO₂ columns at a much finer resolution than the large source box scale used in this study. Its ability to capture fine plume structures enables detailed NOₓ emission estimates at both inter-city and intra-city levels, revealing time-varying emissions linked to daily traffic patterns. As data accumulate and retrieval methods improve, GEMS NO₂ observations will become essential for understanding fine-scale diurnal variations in NOₓ emissions across broad regions of Asia. In addition to nitrogen dioxide, GEMS monitors diurnal variations of ozone, sulfur dioxide, aerosols, volatile organic compounds, ultraviolet index, and other atmospheric factors relevant to human health. The integrated analysis of these multiple measurements offers vital and distinctive insights for diagnosing and forecasting air quality, with significant implications for public health.