Regionalized Well-to-Wheel Mechanism Diagnosis of Low-Carbon Bus Transition Across Northwest China’s Electricity–Hydrogen Systems

The low-carbon bus transition depends on the electricity and hydrogen pathways that support vehicle operation. This study develops a regionalized WTW mechanism diagnosis for six provincial electricity–hydrogen–bus systems in Northwest China and an adjacent energy-output region. Localized GREET pathway intensities, fleet-level aggregation, Logarithmic Mean Divisia Index (LMDI) decomposition, B0-upstream HFCB exposure tests, local A/B/C perturbations and HFCB intensity-sensitivity checks are combined to evaluate greenhouse gas (GHG) emissions, primary-energy consumption and primary-water burden. The results show that B0–S1 fleet-level GHG reductions range from 9.86% in Shaanxi to 73.81% in Qinghai, while Gansu records the largest absolute decrease, from 126.52 to 41.21 kg CO2-eq/hkm. GHG, primary-energy and primary-water responses diverge: in Qinghai, S1–S2 GHG intensity decreases by 28.76%, while primary-energy consumption and primary-water burden increase by 1.99% and 21.31%, respectively. LMDI results reveal different attribution mechanisms, including dual-driver reduction in Gansu and a counteracting composition effect in Shaanxi. Exposure, perturbation and sensitivity tests indicate that hydrogen-related outcomes depend on pathway intensity, fleet share and break-even margins. The findings support pathway-conditioned screening that coordinates BEB and HFCB expansion with electricity decarbonization, renewable-hydrogen availability and multi-indicator burden assessment.