Observed CO concentration reveals steep decrease of anthropogenic emissions in winters of 2021 and 2022 in Hangzhou and Yangtze River Delta region, China.

After the initial prevention of COVID-19 in early 2020, China gradually lifted its control measures in the latter half of 2020, leading to a rebound in anthropogenic CO emissions. However, the emergence of a COVID-19 variant in late 2021, particularly in densely populated and economically developed areas, prompted the reimplementation of stringent confinement measures. Furthermore, with the policy shift towards achieving "herd immunity", China fully lifted pandemic control measures in December 2022, resulting in the unrestricted movement of residents and facilitating the widespread transmission of COVID-19 in the ensuing months. But to our knowledge, no studies have yet quantified the relative changes in CO emissions during these successive phases, representing a significant knowledge gap in understanding the impact of varying control measures on anthropogenic CO emissions at both city and regional scales. Consequently, we selected Hangzhou city and the Yangtze River Delta (YRD) region as our study area due to their status as economically developed and densely populated regions in China. In order to mitigate the influence of biological CO flux, we utilized wintertime atmospheric CO observations at two urban and rural sites, along with their gradient, across three years (December 2020-February 2023). We employed two distinct methods with the WRF-STILT model to quantify the relative changes in CO emissions. Our findings indicate that (1) atmospheric CO concentrations at both sites and their gradients in 2022 were significantly lower than those observed in 2020, with modeled simulations using consistent emissions suggesting that changes in emissions were the predominant factor rather than variations in atmospheric transport processes; (2) After applying the source region partition method, anthropogenic CO emissions during the winter of 2021 decreased to 69.8 % ± 1.6 % in Hangzhou city when compared with 2020, while emissions in the YRD region dropped to 92.5 % ± 6.2 %. In winter 2022, emissions in Hangzhou city decreased to 79.9 % ± 1.9 %, and YRD region decreased to 82.0 % ± 7.2 % relative to 2020, highlighting substantial spatial heterogeneity from the city to the regional scale; (3) notably, the observed decreases in CO emissions in both Hangzhou and the YRD were not reflected in prior inventories, which indicated an annual increase of 8 % for 2021 and 2022, suggesting that even the most recent inventories fail to account for the prolonged emission reduction effects occurring over the preceding three years.