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.