Mobile MAX-DOAS observations of tropospheric NO₂ and HCHO during summer over the Three Rivers' Source region in China

Abstract. The tropospheric concentrations of nitrogen dioxide (NO2) and formaldehyde (HCHO) have high spatio-temporal variability, in situ observations these trace gases are still scarce, especially remote background areas. We made four similar circling journeys mobile multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements the Three Rivers' Source region over Tibetan Plateau summer (18–30 July) 2021 for first time. slant column densities (DSCDs) NO2 HCHO were retrieved from measured spectra, with very weak absorptions along driving routes. vertical (VCDs) calculated their DSCDs by geometric approximation method, they further filtered to form reliable data sets eliminating influences sunlight shelters vehicle's vibration bumpiness. observational show that VCDs decreased increasing altitude route, whose levels ± standard deviations 0.40 1.13×1015 molec. cm−2 2.27±1.66×1015 July region. geographical distribution patterns between different journeys, but journeys. elevated routes usually corresponded enhanced transport emissions towns crossed. However, spatial distributions depended significantly on natural meteorological conditions, such as surface temperature. By comparing TROPOMI satellite products MAX-DOAS results, we found large positive offsets atmosphere main area Source. Our study provides valuable information Plateau, benefitting scientific community investigating evolution atmospheric composition at altitudes, validating improving mountain terrains, evaluating model's ability simulate chemistry Plateau.