Using urban-suburban difference of atmospheric CO₂ to evaluate carbon neutrality capacity in Hangzhou, China

China is the largest emitter of anthropogenic CO₂ globally, with its cities recognized as significant emission hotspots. Consequently, evaluating anthropogenic CO₂ emissions and the carbon neutral capability (CNC) of Chinese cities is critical for climate change mitigation. Despite this importance, no studies to date have assessed recent and future city-scale CNCs using the top-down atmospheric inversion approach, revealing substantial knowledge gaps regarding regional CO₂ budgets. To address these issues, this research focused on Hangzhou, a megacity known for having the highest forest cover among China's provincial capitals, as study region. Year-round atmospheric CO₂ concentration measurements were conducted from December 2020 to November 2021 at two sites: one urban and one suburban. These observations, along with their difference, were utilized to derive city-scale posterior anthropogenic CO₂ emissions and to evaluate recent and future CNCs. Our key findings are as follows: (1) The manufacturing industry, energy industry and oil refineries/transformation industry were identified as the largest contributors to urban-suburban CO₂ difference, accounting for 36.5 %, 21.3 %, and 16.6 %, respectively. Additionally, 82.5 %, 65.2 %, 81.2 % and 86.3 % of total anthropogenic CO₂ enhancements were attributed to emissions within Hangzhou city in winter, spring, summer and autumn, respectively. (2) The posterior annual anthropogenic CO₂ emission for Hangzhou was estimated at 4.65 (±0.72) × 10¹⁰ kg/a, indicating significant biases among different prior CO₂ emission inventories. The annual biological CO₂ sink, derived from multiple products, was estimated at -0.48 (±0.16) × 10¹⁰ kg. (3) The calculated CNC for 2021 was 10.3 % ± 3.4 %, highlighting a substantial gap towards achieving full carbon neutrality. Considering potential increases in ecosystem carbon sinks due to forest age and uncertainties from climate change, it was predicted that at least 65.2 %-82.6 % of anthropogenic CO₂ emissions must be reduced to achieve the goal of full carbon neutrality by year of 2060.