Atmospheric CH₄ is the second-largest anthropogenic contributor to global warming. However, its emissions, components, spatial–temporal variations and projected changes still remain largely uncertain from city to national scales. CH₄ emissions from waste treatment (including solid waste landfills, solid waste incineration and sewage) account for >50 % of total anthropogenic CH₄ emissions at the city scale, and considering the high temperature sensitivity of CH₄ emission factors (EFs) to biological-process-based sources such as waste treatment, large differences will occur when estimating future CH₄ emissions under different global warming scenarios. Furthermore, the relationships between temperature and waste treatment CH₄ emissions have only been studied in a few site-specific studies and lack representativity for whole cities, which contain various biophysical conditions and show heterogeneous distribution. The above factors cause uncertainty in the evaluation of city-scale CH₄ emissions (especially from waste treatments), and projected changes still remain unexplored. Here we conduct the first tower-based CH₄ observation network with three sites in Hangzhou, which is located in the developed Yangtze River Delta (YRD) area and ranks as one of the largest megacities in China. We found the a priori total annual anthropogenic CH₄ emissions and those from waste treatment were overestimated by 36.0 % and 47.1 % in Hangzhou, respectively. In contrast, the total emissions in the larger region, i.e., Zhejiang Province or the YRD area, were slightly underestimated by 7.0 %. Emissions from waste treatment showed obvious seasonal patterns following air temperature. By using the linear relationship constructed between monthly waste treatment CH₄ emissions and air temperature, we find the waste treatment EFs increase by 38 %–50 % with temperature increases of 10 ^∘C. Together with projected temperature changes from four climate change scenarios, the global-warming-induced EFs in Hangzhou will increase at the rates of 2.2 %, 1.2 %, 0.7 % and 0.5 % per decade for IPCC AR5 (Intergovernmental Panel on Climate Change, the Fifth Assessment Report) RCP (Representative Concentration Pathway) 8.5, RCP6.0, RCP4.5 and RCP2.6 scenarios, respectively. And the EFs will finally increase by 17.6 %, 9.6 %, 5.6 % and 4.0 % at the end of this century. Additionally, the derived relative changes in China also show high heterogeneity and indicate large uncertainty in projecting future national total CH₄ emissions. Hence, we strongly suggest temperature-dependent EFs and the positive feedback between global warming and CH₄ emissions should be considered in future CH₄ emission projections and climate change models.

The Yangtze River Delta (YRD) is the most developed region in China. Influenced by intensive and complex anthropogenic activities, atmospheric pollution in this region is highly variable, and reports are sparse. In this study, a seven-year history of the atmospheric O₃ and NO_x mixing ratios over a typical city, Hangzhou, was presented to enrich the studies on air pollution in the YRD region. Our results revealed that the diurnal variation in NO_x corresponded to traffic rush hours, while O₃ was mainly impacted by photochemical reactions in the daytime. The weekend effect was significant for NO_x, but inapparent for O₃. Two O₃ peaks in May and September were caused by seasonal atmospheric stability and climatic conditions. The lower NO_x and higher O₃ levels observed suggested direct effects from traffic restrictions and large-scale industrial shutdowns during the COVID-19 lockdown in 2020 compared with those in the periods before and after lockdown. The model simulation results showed that O₃ mixing ratios were not only related to regional anthropogenic emissions but were impacted by air mass transportation from surrounding provinces and the China shelf seas. The NO_x mixing ratios showed a decreasing trend, while the O₃ mixing ratios showed the opposite trend from 2015 to 2021, which is indicative of the implementation of the Air Pollution Prevention and Control Acton Plan issued by the Chinese government in 2013.

Atmospheric CH₄ is the second largest anthropogenic contributor to global warming, however its emissions, components, spatiotemporal variations, and projected changes present large uncertainties from city to national scales. CH₄ emissions from waste treatment account for >50 % of total anthropogenic CH₄ emissions at the city scale, and considering the high sensitivity of CH₄ emission factors (EFs) to temperature for biological process-based sources, such as waste treatment, large bias will occur when estimating future CH₄ emissions under different global warming scenarios. Furthermore, the relationships between temperature and waste treatment CH₄ emissions have only been determinized in a few site-specific studies, and these findings lack representativeness for the whole city scale, which contains various biophysical conditions and shows heterogeneous distribution. These factors increase the difficulty of evaluating city-scale CH₄ emissions (especially from waste treatments), and the projected changes remain unexplored. Here, we conduct the first tower-based CH₄ observation network with three sites in Hangzhou city, which is located in the developed Yangtze River Delta (YRD) area and ranks as one of the largest megacities in China. We found that the a priori total annual anthropogenic CH₄ emissions and waste treatment emissions were overestimated by 36.0 % and 47.1 % in Hangzhou city, respectively. However, the total emissions in the larger region of Zhejiang Province or the YRD area was only slightly underestimated by 7.0 %. Emissions from waste treatment showed obvious seasonal patterns according to the air temperature. By using the constructed linear relationship between monthly waste treatment CH₄ emissions and air temperature, we found that the waste treatment EFs increased by 38 %~50 % as the temperature increased by 10 °C. Together with the projected temperature changes from four climate change scenarios, the global warming-induced EFs in Hangzhou city will increase at rates of 2.2 %, 1.2 %, 0.7 % and 0.5 % per decade for RCP8.5, RCP6.0, RCP4.5 and RCP2.6 scenarios, respectively, and by 17.6 %, 9.6 %, 5.6 %, and 4.0 % at the end of this century, respectively. Additionally, the relative changes derived for the whole of China also showed high heterogeneity and indicated large uncertainty in projecting future national total CH₄ emissions. Hence, we strongly suggest the temperature-dependent EFs and positive feedback between global warming and CH₄ emissions should be considered in future CH₄ emission projections and climate change models.