Despite growing research on greenhouse gas (GHG) emissions from inland waters, few systematic efforts have been made to assess the regional-scale GHG emissions from Asian rivers under increasing anthropogenic stress. We examined factors controlling longitudinal and seasonal variations in the partial pressure of CO₂ (pCO₂), and CH₄ and N₂O concentrations in the Ganges, Mekong, and Yellow River (Huang He) by simultaneously measuring gas concentrations and stable C isotopes, and optical properties of dissolved organic matter (DOM) from 2016 to 2019. The levels of pCO₂ and CH₄ were distinctively higher in polluted tributaries and affected reaches of the Ganges and Mekong than in the Yellow River. The highest levels of N₂O were found in the Ganges, followed by Yellow River and Mekong. Across these basins, dry-season mean concentrations of CO₂, CH₄, and N₂O were 1.6, 2, and 7 times higher than those measured in the monsoon season, respectively. This seasonality was consistent with that of δ¹³C-CO₂, while δ¹³C-CH₄ showed an opposite pattern. The overall results suggest that neglecting localized pollution impacts on GHG emissions from increasingly urbanized river basins can result in inaccurate estimates of global riverine GHG emissions.

The air-water gas flux (F) of CO₂, CH₄, and N₂O was calculated according to the equation F = k ΔC, where k is the gas transfer velocity and ΔC is the air-water gas concentration gradient (Lauerwald et al., 2015). To estimate k, we used a range of k normalized to a Schmidt number of 600 (k600) estimated for global small and large rivers by Lauerwald et al. (2015): 6.61-8.64 m/day for the tributaries and wastewater drains; 3.49-4.41 m/day for the river mainstems.The actual gas transfer velocity kactual was estimated from k600 and the measured water temperature (Raymond et al., 2012). The fluxes of CO₂, CH₄, and N₂O (mg/m²day) were calculated from the estimated kactual and measured ΔC values.FCO₂-min, FCH₄-min, and FN2O-min were calculated using the lower range of k valuesk = 3.49 for mainstem river (i.e., upper, middle, and lower reaches)k = 6.61 for tributary and wastewaterFCO₂-max, FCH₄-max, and FN₂O-max were calculated with upper range of k valuesk = 4.41 for mainstem river (i.e., upper, middle, and lower reaches)k = 8.64 for tributary and wastewaterThe three fluorescent dissolved organic matter (FDOM) components from parallel factor (PARAFAC) analysis with 126 samples are defined as C1 (humic like), C2 (microbial humic like), and C3 (protein like).