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Anthropogenic methane emissions in China increased by 40% in the 2000s, contributing 16% of global anthropogenic emissions. The trend after 2010, however, remains under debate. An improved understanding of major sources and their trends, informed by timely and accurate data, is required to monitor efforts toward climate mitigation goals. Here we update a detailed bottom-up inventory to evaluate recent changes in China’s anthropogenic CH4 emissions. Combining our and other bottom-up inventories and seven global CH4 inversions, we show a slowdown of emission increase after 2010 [0.2 (−0.3 to 0.7) Tg of CH4 year–2] compared to the 2000s [1.2 (0.6–1.8) Tg of CH4 year–2], with a downward trend detected after 2014. Although there are considerable uncertainties, this slowdown is statistically significant (p < 0.001). The slowdown is mainly attributed to stabilized coal production in the 2010s, along with a regional shift of production toward mining areas with low emission factors and increased utilization of coal mine methane. Our results suggest that China’s recent energy policies have helped control coal mine emissions, and further work is needed to narrow down the uncertainty in both bottom-up inventories and top-down inversions.

Two-dimensional (2D) metal halide perovskites are promising tunable semiconductors. Previous studies have focused on Pb-based structures, whereas the multilayered Sn-and Ge-based analogues are largely unexplored, even though they potentially exhibit more diverse structural chemistry and properties associated with the more polarizable ns 2 lone-pair electrons. Herein, we report the synthesis and structures of 2D tin iodide perovskites (BA) 2 (A)Sn 2 I 7 , where BA = n-butylammonium and A = methylammonium, formamidinium, dimethylammonium, guanidinium, or acetamidinium, and those of 2D germanium iodide perovskites (BA) 2 (A)Ge 2 I 7 , where A = methylammonium or formamidinium. By comparing these structures along with their Pb counterparts, we establish correlations between the effect of group IV-cation's lonepair stereochemical activity on the perovskite crystal structures and the resulting semiconducting properties such as bandgaps and carrier−phonon interactions and nonlinear optical properties. We find that the strength of carrier−phonon interaction increases with increasing lone-pair activity, leading to a more prominent photoluminescence tail on the low-energy side. Moreover, (BA) 2 (A)Ge 2 I 7 exhibit strong second harmonic generation with second-order nonlinear coefficients of ∼10 pm V −1 that are at least 10 times those of Sn counterparts and 100 times those of Pb counterparts. We also report the third-order two-photon absorption coefficients of (BA) 2 (A)Sn 2 I 7 to be ∼10 cm MW −1 , which are one order of magnitude larger than those of the Pb counterparts and traditional inorganic semiconductors. These results not only highlight the role of lone-pair activity in linking the compositions and physical properties of 2D halide perovskites but also demonstrate 2D tin and germanium iodide perovskites as promising lead-free alternatives for nonlinear optoelectronic devices.

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