Capabilities of satellite Geostationary Environment Monitoring Spectrometer (GEMS) NO₂ data for hourly ambient NO₂ exposure modeling

The Geostationary Environment Monitoring Spectrometer (GEMS) is the world's first geostationary instrument that monitors hourly gaseous air pollutant levels, including nitrogen dioxide (NO₂). Using the first-of-its-kind capabilities of GEMS NO₂ data, we examined how well GEMS NO₂ levels can explain the spatiotemporal variabilities in hourly NO₂ concentrations in the Republic of Korea for the year 2022. A correlation analysis between hourly GEMS NO₂ levels and ground NO₂ concentrations showed a higher spatial correlation [Pearson r = 0.56 (SD = 0.20)] than a temporal one [Pearson r = 0.42 (SD = 0.14)], on average. To take advantage of the enhanced spatial predictability of GEMS NO₂ data, we employed a mixed effects model to allow hour-specific relationships between GEMS NO₂ and NO₂ concentrations on a given day in each region and subsequently estimated hourly NO₂ concentrations in all urban and rural areas. The 10-fold cross validation demonstrated R² = 0.72, mean absolute error (MAE) = 3.7 ppb, and root mean squared error (RMSE) = 5.5 ppb. The hourly variations of the relationships were attributed particularly to those of wind speed among meteorological parameters considered in this study. The spatial distributions of hourly estimated NO₂ concentrations were highly correlated between hours [average r = 0.91 (SD = 0.06)]. Nonetheless, they represented the diurnal patterns of urban versus rural NO₂ contrasts during the day [urban/rural NO₂ ratios from 1.22 (5 p.m.) to 1.37 (12 p.m.)]. The newly retrieved GEMS NO₂ data enable temporally as well as spatially resolved NO₂ exposure assessment. In combination with the time-activity patterns of individual subjects, the GEMS NO₂ data can generate 'sub-population' exposure estimates and therefore enhance health effect studies.