The COVID-19 pandemic has put much of the world into lockdown, as one unintended upside to this response, the air quality has been widely reported to have improved worldwide. Existing studies examine the environmental effect of lockdowns at a city- or country-level, few examines it from a global perspective. Using a novel COVID-19 government response tracker dataset, combining the daily air pollution data and weather data across 597 major cities worldwide between January 1, 2020, and July 5, 2020, this study quantifies the causal impacts of 8 types of lockdown measures on changes of a range of individual pollutants based on a difference-in-differences design. The results show that the NO 2 air quality index value falls more precipitously (23–37%) relative to the pre-lockdown period, followed by PM 10 (14–20%), SO 2 (2–20%), PM 2.5 (7–16%), and CO (7–11%), but the O 3 increases 10–27%. Furthermore, intra/intercity travel restrictions have a better performance in curbing air pollution. These results are robust to a set of alternative specifications, including different panel sizes, independent variables, estimation strategies. The heterogeneity analysis in terms of different types of cities shows that the lockdown effects are more remarkable in cities from lower-income, more industrialized, and populous countries. We also do a back-of-the-envelope calculation of the subsequent health benefits following such improvement, and the expected averted premature deaths due to air pollution declines are around 99,270 to 146,649 among 76 countries and regions involved in this study during the COVID-19 lockdown. These findings underscore the importance of continuous air pollution control strategies to protect human health and reduce the associated social welfare loss both during and after the COVID-19 pandemic.

This is a joint publication with the Letter by H. Huang and F. Liu [Phys. Rev. Lett. 121, 126401 (2018)]. In this work, we propose the spin Bott index to identify the quantum spin Hall (QSH) state in both crystalline and non-periodic systems. The applicability of the spin Bott index is confirmed by analyzing the periodic and disorder Kane-Mele models. As an example of non-periodic systems, we systematically investigate the QSH effect in a Penrose-type quasicrystal lattice (QL). We characterized the nontrivial electronic topology of the QL by directly calculating the spin Bott index. In addition, the topological edge states, the localization of wavefunctions and quantized transport signatures are also studied in detail.

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In this note we give a simple proof of the endpoint regularity for the uncentred Hardy–Littlewood maximal function on $\mathbb{R}$. Our proof is based on identities for the local maximum points of the corresponding maximal functions, which are of interest in their own right.

Who lives in the downwind areas of urban ozone plumes? Location theory suggests that the poor should disproportionately populate the downwind area, while the rich are most likely to live outside urban ozone plumes. Dynamically, we would expect that the downwind area would become poorer over time because the rich would "vote with their feet" under the serious impacts of urban ozone plumes. However, the importance of urban ozone pollution could be discounted due to the public's risk perception. According to risk and air quality perception literature, one would expect that the public would likely perceive ozone air pollution as a relatively low risk, compared with other environmental risks and social concerns. Therefore, actual population distribution might not follow what location theory suggests. Who actually does live in the downwind areas of urban ozone plumes? The results from a case study of New York and Philadelphia show that the ozone downwind areas are currently populated by a considerably larger proportion of upper income households and whites than the source areas. Furthermore, the population dynamics data do not provide any evidence for the "vote with their feet" hypothesis. While these findings deviate from the hypotheses prescribed by location theory, they are consistent with what we could expect from theories of risk perception.