Long-term observations of NO2, SO2, HCHO, and CHOCHO over the Himalayan foothills: Insights from MAX-DOAS, TROPOMI, and GOME-2

Regional air pollution has become one of the utmost environmental concerns in India, especially over economically vibrant and densely populated regions, such as Northern India, including the Indo-Gangetic Plain (IGP). Additionally, the Himalayas adjacent to IGP provide conducive conditions to confine pollutants and transport them to greater horizontal and vertical extents. However, in-situ observations are sparse and limited over the Himalayas, where data retrievals from space-based sensors are fraught with difficulties. In light of this, observations of NO2, 2 , SO2, 2 , HCHO, and CHOCHO are made from a Himalayan foothills site (Pantnagar, 29.03o o N, 79.47o o E and 237 AMSL) utilizing remote sensing observation of Multi-AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS). Data from TROPOspheric Monitoring Instrument (TROPOMI), and Global Ozone Monitoring Experiment-2 (GOME-2) satellite instruments are also presented. We investigate the temporal variations in near-surface mixing ratios, vertical profiles, and tropospheric columns of these trace gases from January 2017 to December 2020. The diurnal variation of NO2 2 and HCHO at different altitude regions (surface to 3 km) were typically urban-type with morning and evening peaks. At the same time, the CHOCHO diurnal variation peaks in the noon hours. We observed annual mean tropospheric NO2 2 vertical column densities (VCDs) of 3.2 x 1015 15 (unit: molecules/cm2), 2 ), tropospheric SO2 2 VCDs of 1.2 x 1016, 16 , tropospheric HCHO VCDs of 1.6 x 1016, 16 , and tropospheric CHOCHO VCDs of 0.6 x 1015 15 from MAX-DOAS observations. MAX-DOAS comparison with the TROPOMI and GOME-2 VCDs shows an underestimation of up to 48% for satellite NO2 2 VCDs, while SO2 2 and HCHO VCDs show nominal biases in the range of 20-30%. The R gf sensitivity calculation shows prominent biogenic sources of VOCs during noon hours, while Rfncalculation f n calculation mostly shows a NOx-limited x-limited ozone production regime over the Himalayan foothill site. The Rfnmonthly f n monthly variations match reasonably well between the MAXDOAS and TROPOMI, while R gf values were higher from satellite observations. This study highlights the factors governing the diurnal and monthly variation of different pollutants over the Himalayan foothill region and asses the space-borne observations for better utilization.