Seasonal and Diurnal Variations of Tropospheric Formaldehyde over Lhasa on the Qinghai–Xizang Plateau

Rapid environmental changes over the Qinghai–Xizang Plateau (QXP) demand improved knowledge of formaldehyde (HCHO) distributions, but observational constraints have left their spatiotemporal patterns largely unexplored. This study performed ground-based remote sensing measurements of HCHO using the multi-axis differential optical absorption spectroscopy (MAX-DOAS) technology from August 2021 to March 2023 in Lhasa (29.66°N, 91.14°E; 3552.5 m above sea level). The tropospheric HCHO vertical column densities (VCDs) were retrieved, their temporal variations and relationship with surface wind, air temperature, and local Sang offering activity were investigated, and the corresponding Tropospheric Monitoring Instrument (TROPOMI) satellite product was validated. It is found that the seasonal variation patterns of tropospheric HCHO VCDs presented two peaks, one in summer and the other in winter. The diurnal variation patterns of tropospheric HCHO VCDs during the daytime presented a “W” shape in spring and summer, and a “U” shape in autumn and winter. No significant changes in the levels and variation patterns of tropospheric HCHO VCDs were found during the Coronavirus Disease 2019 (COVID-19) lockdown period compared to those in a normal year. The dominant transport path for HCHO in Lhasa was along the river valley. The trends of HCHO variations with air temperature were opposite between cold and warm conditions. On average, the local Sang offering activities enhanced the HCHO levels in Lhasa. TROPOMI did not capture the significant seasonal variations of tropospheric HCHO VCDs observed by multi-axis differential optical absorption spectroscopy (MAX-DOAS). Overall, these results are beneficial to air pollution control and satellite validation over the high and complex QXP.