Calibration and Validation
The project aims at applying FTIR and MAX-DOAS measurements for the validation of air quality and greenhouse gas measurements from the European Copernicus Sentinel-5 Precursor (S5P) and Chinese TanSat satellites. The focus will be on monitoring NO2, O3, HCHO, SO2, CHOCHO, CO, CH4 and CO2 columns and Prof.iles using standardised operation protocols and retrieval methods at the Xianghe site, in Northern China. The FTIR instrument allows measurements of O3, HCHO, CO, CH4 and stratospheric NO2 columns, while the MAX-DOAS instruments can measure NO2, O3, HCHO, SO2 and CHOCHO columns as well as aerosol extinction. The differences between the FTIR and MAX-DOAS common targets, in particular regarding their vertical sensitivity, need to be well understood before combining them together to validate satellite measurements. As FTIR and MAX-DOAS instruments are operated from the same building in Xianghe, we have a good opportunity to compare the FTIR and MAX-DOAS NO2, O3 and HCHO measurements and to combine them together for satellite validation. A strong focus of this project will be the implementation of standardized operation and retrieval protocols for the Xianghe FTIR and MAX-DOAS measurements in order to obtain reliable ground-based measurements that are harmonized with the wider scientific community. To reach this target, we will take full advantage of the ongoing activities in projects such as the European ACTRIS-IMP and ECMWF Copernicus Atmospheric Monitoring Service-related CAMS-27 and the Copernicus Climate Change Service-related C3S_311a_Lot3 (C3S-Baron), which concentrate on the establishment of standards for FTIR and MAX-DOAS operation and data processing. Exchange of knowledge, tools and retrieval software will take place, and access to facilities such as the VCEX processing system will be offered.Validation methodologies will be developed and applied, making use of all available sources of information such as the averaging kernels of the satellite products, cloud information, and the vertical Prof.iles of the various trace gases complemented by aerosol data (which will be derived from MAX-DOAS measurements and AERONET measurements). Comprehensive uncertainty budgets will be derived, addressing accuracy, precision and long-term stability. Based on the obtained validation results, recommendations for satellite product quality improvement can eventually be formulated. The project will contribute to validate the S5P and Chinese CO2 sensors (FY-3H/GAS and TanSat) and its successor measurements during the full duration of the project (2020-2023). In this period, the progressive accumulation of data will allow for improved statistics and refinement of the validation results. This will include analysis of, e.g., seasonal cycle effects and longer-term stability. As Xianghe is located in a sub-urban polluted region with a high and variable aerosol concentration, the ground-based MAX-DOAS and FTIR measurements, together with AERONET aerosol optical depth measurements, are valuable to understand the performance of the satellite measurements under different aerosol conditions. In addition, as Xianghe is about 50 km away from the capital Beijing, representativeness effects will also be investigated especially.The main outcomes of the project will be (1) the collection of the ground-based measurements of standardised FTIR and MAX-DOAS column and Prof.ile measurements of NO2, O3, HCHO, SO2, CHOCHO, CO, CH4 and CO2 columns at Xianghe, Northern China, and (2) an assessment of the corresponding quality of the S5P, FY-3H/GAS and TanSat sensors.
