Long-term observations of NO₂ using GEMS in China: Validations and regional transport

In 2019, South Korea launched the Geostationary Environment Monitoring Spectrometer (GEMS) to observe trace gases with an hourly temporal resolution. Compared to previous payloads on polar-orbiting satellites, the GEMS payload has significant advantages in detecting the diurnal variation characteristics of NO₂. However, there is still a lack of ground-based validations regarding the overall accuracy of GEMS in the Chinese region. In this study, we conducted a systematic ground validation of GEMS NO₂ data in China for the first time. We validated the accuracy of GEMS NO₂ data in four typical pollution regions in China, namely the Beijing-Tianjin-Hebei region (JJJ), the Yangtze River Delta region (YRD), the Pearl River Delta region (PRD), and the Sichuan Basin region (SCB), based on MAX-DOAS and CNEMC data. The averaged correlations using the two datasets for validation were 0.81 and 0.57, respectively, indicating a high level of accuracy for the data in China. Using the GEMS seasonal averaged NO₂ data, we studied the distribution of NO₂ levels in the four regions. We found that the highest NO₂ in all four regions occurred during winter with concentrations of 1.84 × 10¹⁶ molecules cm^-2, 1.59 × 10¹⁶ molecules cm^-2, 1.58 × 10¹⁶ molecules cm^-2 and 9.47 × 10¹⁵ molecules cm^-2, respectively. The distribution of NO₂ was closely related to the terrain. Additionally, we observed a significant underestimation issue with TROPOMI, exceeding 30 % in many regions. Based on MAX-DOAS, we investigated the vertical distribution of NO₂ in the four regions and found that NO₂ was mainly concentrated below 0.5 km. with the HNU station having the lowest concentration, averaging only 2.12 ppb, which was approximately 41 % of the highest concentration recorded at the CQ station. Furthermore, we conducted a study on regional and cross-regional transport using a combination of MAX-DOAS and GEMS data. We found that the transport flux of NO₂ could increase by over 500 % within 1 h, making a significant contribution to local NO₂ concentrations. The joint observations of GEMS and MAX-DOAS will provide reliable data support for NO₂ research and control in China, making a substantial contribution to environmental protection and sustainable development.