First online observations of greenhouse gases over the Central Himalayas: insights in fluxes, vegetation links, and meteorological controls.

Ground observations with diurnal variability over the Himalayas are crucial to validate climate mitigation efforts, build accurate emissions inventories, and better forecasting. This study bridges the acute scarcity of such measurements by making the first online observations of CO₂ and CH₄ with CO at a mountain site (Nainital, 29.4° N, 79.5° E, 1958 m a.m.s.l.) continuously for 5 years in the Central Himalayas. Observed levels of CO₂ and CH₄ are higher than those at other background sites. The boundary layer evolution and upslope winds determine the diurnal pattern of CH₄ and CO, while CO₂ diurnal and seasonal variations are governed by biospheric uptake. Bivariate analysis shows that higher levels (> 1.98 ppm) of CH₄ are mostly localized and that the distribution of CO₂ is representative of the terrestrial ecosystem. Diurnal variations in CO₂ relate to the meteorology, except during spring when the anthropogenic influence breaks this covariation. Excess and background CO₂ and CH₄ are segregated, and the role of biomass burning in spring and fossil fuel combustion in raising CO₂ is revealed. The role of vegetation is also evaluated with the Normalized Difference Vegetation Index (NDVI) and fluxes from the CarbonTracker model. During autumn, the impact of agricultural and waste emissions on high CH₄ is evidenced. The limited online observations from this region also showed an increasing trend in CO₂ (2.66 ± 0.17 ppm yr^-1) and CH₄ (9.53 ± 0.09 ppb yr^-1), while CO has a decreasing trend (3.15 ± 1.32 ppb yr^-1). The results highlight the complex interplay of anthropogenic emissions, biospheric uptake, and weather patterns with mountain features in shaping CO₂ and CH₄ levels, especially at the diurnal scale.