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Abstract

The tropospheric concentrations of nitrogen dioxide (NO2) and formaldehyde (HCHO) have high spatio-temporal variability, and in situ observations of these trace gases are still scarce especially in remote background areas. We made four similar circling journeys of mobile MAX-DOAS measurements in the Three Rivers’ Source region over the Tibetan Plateau in summer (18–30 July) 2021 for the first time. The differential slant column densities (DSCDs) of NO2 and HCHO were retrieved from the measured spectra with very weak absorptions along the driving routes. The tropospheric NO2 and HCHO vertical column densities (VCDs) were calculated from their DSCDs by the geometric approximation method, and they were further filtered to form reliable data sets by eliminating the influences of sunlight shelters and vehicle’s vibration and bumpiness. The observational data show that the tropospheric NO2 and HCHO VCDs decreased with the increasing altitude of the driving route, whose background levels were 0.40 ×1015 molec·cm−2 for NO2 and 2.27 ×1015 molec·cm−2 for HCHO in July 2021 over the Three Rivers’ Source region. The NO2 VCDs show similar geographical distribution patterns between the different circling journeys, but the levels of the HCHO VCDs are different between the different circling journeys. The NO2 VCDs tended to have peak values in the early morning and late afternoon, while the diurnal variation pattern of HCHO VCDs changed with the driving route. The elevated NO2 VCDs along the driving routes were usually corresponding to enhanced transport emissions from the towns crossed. However, the spatial distributions of the HCHO VCDs depended significantly on natural and meteorological conditions, such as surface temperature. By comparing TROPOMI satellite products and mobile MAX-DOAS results, we found that TROPOMI is still unable to identify the fine-scale spatial variability in tropospheric NO2 and HCHO VCDs in the background atmosphere over the Tibetan Plateau. Our study provides valuable data sets and information of NO2 and HCHO over the Tibetan Plateau, benefitting the scientific community in investigating the evolution of atmospheric composition with high spatio-temporal resolution in the background atmosphere at high altitudes, validating and improving the satellite products over mountain terrains, and evaluating the model’s ability in simulating atmospheric chemistry over the Tibetan Plateau.