30-Year Greenhouse Gas Changes in Megacities Driven by Emission Reduction Policies: A Case Study of Beijing.

This study investigates the spatiotemporal dynamics of urban greenhouse gases amidst energy transition, aiming to assess the emission reduction effectiveness of Beijing's "coal-to-gas" and "coal-to-electricity" policies and to pinpoint the key sources arising from the subsequent structural shifts in the emission profile. We analyzed greenhouse gas concentrations from 1993 to 2024 using gas chromatography. Vertical profile data across the 8-240 m atmospheric layer from 2022 to 2024 were obtained using a pod-based measurement system installed on a 325-m meteorological tower. The results show that the CO2 growth rate initially increased and then decreased during the 1993-2024 period, and the difference in CO2 concentration between the ground and upper air decreased by 20-56% compared to the historical maxima. These trends demonstrate that the energy transition has effectively mitigated the increase in CO2 concentration. A concurrent deceleration in the growth rates of CH4 and N2O was also observed, albeit with less pronounced changes in their vertical gradients. This mitigation is largely linked to the shift from landfill disposal to waste incineration and the promotion of new energy vehicles. In contrast, SF6 emissions increased steadily from 2008 to 2024, primarily driven by growing electricity demand and the expansion of the semiconductor industry. The recent eight-year average growth rate reached 0.52 ppt yr-1. These findings underscore the power and semiconductor sectors as critical domains for targeted SF6 emission control.