Assesssement of Trace Gases Variability and Satellite Retrieval Accuracy using Pandora Observations in the Central Himalayas

For the first time, a Pandora spectrometer has been deployed in the central Himalayan region as part of the Pandora Global Network (PGN), at ARIES, Nainital (29.36°N, 79.36°E; 1970 m a.s.l.), a high-altitude remote site in South Asia where Pandora coverage has been negligible; however, pollutant concentrations across South Asia remain very high. Although the site is elevated, it is located adjacent to the Indo-Gangetic Plains (IGP) and is therefore affected by the transport of pollutants from the IGP.

The instrument retrieves column densities of key trace gases, ozone (O₃), nitrogen dioxide (NO₂), and formaldehyde (HCHO), including their total column (TO₃, TNO₂, THCHO) and lower-tropospheric column (LTrNO₂, LTrHCHO) amounts, suitable for validation of satellite observations in this complex mountain topology. Analysis of observations from January 2024 to June 2025 shows clear seasonality, with elevated springtime columns (TNO₂: 4-5 × 10¹⁵ molecules cm⁻²; LTrNO₂: 1 ± 0.1 × 10¹⁵ molecules cm⁻²; LTrHCHO: 0.8 ± 0.1 × 10¹⁶ molecules cm⁻²) and reduced values in the summer–monsoon period (LTrNO₂: ~0.3 ± 0.05 × 10¹⁵ molecules cm⁻²; LTrHCHO: ~0.2 × 10¹⁶ molecules cm⁻²). The seasonal cycle of column NO₂ agrees with surface in-situ NO_y, though their diurnal patterns differ: column NO₂ increasing steadily from morning until the evening period, while surface NO_y peaks around midday and column HCHO shows maximum value during daytime (12-13 hours IST), followed by a decline toward evening.

Pandora observations were also used to evaluate the performance of GEMS and TROPOMI satellite products for O₃ and NO₂ over a complex mountainous environment. For total column ozone, both GEMS and TROPOMI capture the day-to-day variability (R² = 0.98 for both satellites against Pandora). However, GEMS exhibits a systematic underestimation of about 15 ± 5 DU, while TROPOMI shows good agreement during the spring season but overestimates ozone by approximately 10 DU in other seasons. Similarly, both satellites represent the daily variability of TNO₂ reasonably well (R² = 0.71 for GEMS and 0.78 for TROPOMI). However, both instruments generally overestimate TNO₂. In contrast, the performance for LTrNO₂ is considerably poorer, with R² values of only 0.17 (GEMS) and 0.28 (TROPOMI). Thus, showing low sensitivity of satellite retrievals for NO₂ in the lower troposphere.  These results highlight the crucial role of high-quality ground-based measurements such as Pandora in validating satellite retrievals and advancing our understanding of trace gas behaviour in complex terrain.