Narrowing Differences in Urban and Nonurban Surface Ozone in the Northern Hemisphere Over 1990–2020

Surface ozone air pollution is unequally distributed in space and varies over urban and surrounding nonurban areas. Traditionally, urban ozone levels tend to be lower than their nonurban counterparts, resulting from discrepancies in emissions and nonlinearity in photochemistry. However, how the differences in urban vs nonurban ozone evolve over the past decades is uncertain. Here, we construct 6361 pairs of urban and nonurban ozone measurement sites based on available surface monitoring networks to analyze the long-term changes of their ozone differences. We show that urban vs nonurban ozone differences have narrowed substantially in North America, Europe, South Korea, and Japan over the summers of 1990–2020. The hemispheric mean urban vs nonurban ozone differences have decreased by 90% from −5.0 ppbv in the 1990s to −0.5 ppbv in the 2010s. We estimate that the anthropogenic emission reduction of nitrogen oxides is the dominant driver of the narrowing trends. It has suppressed the urban ozone titration and led to closer ozone formation regimes over urban and nonurban areas

Base-Promoted Tandem Synthesis of 2‑Substituted Indoles and <i>N</i>‑Fused Polycyclic Indoles

Herein is developed a base-promoted approach for the synthesis of C2-substituted indoles and N-fused polycyclic indoles via 5-endo-dig cyclization of 2-alkynyl anilines, followed by a 1,3′-acyl migration or a dearomatizing Michael addition process. A range of N–H free indoles and 8,9-dihydropyrido[1,2-a]indol-6(7H)-one scaffolds were synthesized in good to excellent yields with broad scope

Severe Surface Ozone Pollution in China: A Global Perspective

The nationwide extent of surface ozone pollution in China and its comparison to the global ozone distribution have not been recognized because of the scarcity of Chinese monitoring sites before 2012. Here we address this issue by using the latest 5 year (2013–2017) surface ozone measurements from the Chinese monitoring network, combined with the recent Tropospheric Ozone Assessment Report (TOAR) database for other industrialized regions such as Japan, South Korea, Europe, and the United States (JKEU). We use various human health and vegetation exposure metrics. We find that although the median ozone values are comparable between Chinese and JKEU cities, the magnitude and frequency of high-ozone events are much larger in China. The national warm-season (April–September) fourth highest daily maximum 8 h average (4MDA8) ozone level (86.0 ppb) and the number of days with MDA8 values of >70 ppb (NDGT70, 29.7 days) in China are 6.3–30% (range of regional mean differences) and 93–575% higher, respectively, than the JKEU regional averages. Health exposure metrics such as warm-season mean MDA8 and annual SOMO35 (sum of ozone means over 35 ppb) are 6.3–16 and 25–95% higher in China, respectively. We also find an increase in the surface ozone level over China in 2016 and 2017 relative to 2013 and 2014. Our results show that on the regional scale the exposure of humans and vegetation to ozone is greater in China than in other developed regions of the world with comprehensive ozone monitoring

Aerosol Liquid Water Driven by Anthropogenic Inorganic Salts: Implying Its Key Role in Haze Formation over the North China Plain

This study reveals aerosol liquid water content (ALWC) in PM_2.5 ranged from 2% up to 74%, and the associated secondary inorganic fraction rose from 24% to 55%, while ambient relative humidity (RH) increased from 15% to 83% in the atmosphere over Beijing. Unexpectedly, the secondary inorganic fraction in PM_2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry’s law and thermodynamic equilibrium. During haze episodes, simultaneously elevated RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating both nitrate and sulfate salts play key roles in determining ALWC. Coincident with a significant reduction in SO₂ emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving ALWC. Thus, the abundance of ALWC and its effects on the aerosol chemistry and climate should be reconsidered

Multiple Impacts of Aerosols on O₃ Production Are Largely Compensated: A Case Study Shenzhen, China

Tropospheric ozone (O3) is a harmful gas compound to humans and vegetation, and it also serves as a climate change forcer. O3 is formed in the reactions of nitrogen oxides and volatile organic compounds (VOCs) with light. In this study, an O3 pollution episode encountered in Shenzhen, South China in 2018 was investigated to illustrate the influence of aerosols on local O3 production. We used a box model with comprehensive heterogeneous mechanisms and empirical prediction of photolysis rates to reproduce the O3 episode. Results demonstrate that the aerosol light extinction and NO2 heterogeneous reactions showed comparable influence but opposite signs on the O3 production. Hence, the influence of aerosols from different processes is largely counteracted. Sensitivity tests suggest that O3 production increases with further reduction in aerosols in this study, while the continued NOx reduction finally shifts O3 production to an NOx-limited regime with respect to traditional O3–NOx-VOC sensitivity. Our results shed light on the role of NOx reduction on O3 production and highlight further mitigation in NOx not only limiting the production of O3 but also helping to ease particulate nitrate, as a path for cocontrol of O3 and fine particle pollution

High N₂O₅ Concentrations Observed in Urban Beijing: Implications of a Large Nitrate Formation Pathway

The heterogeneous hydrolysis of dinitrogen pentoxide (N₂O₅) is important to understanding the formation of particulate nitrate (pNO₃^–). Measurements of N₂O₅ in the surface layer taken at an urban site in Beijing are presented here. N₂O₅ was observed with large day-to-day variability. High N₂O₅ concentrations were determined during pollution episodes with the co-presence of large aerosol loads. The maximum value was 1.3 ppbv (5 s average), associated with an air mass characterized by a high level of O₃. N₂O₅ uptake coefficients were estimated to be in the range of 0.025–0.072 using the steady-state lifetime method. As a consequence, the nocturnal pNO₃^– formation potential by N₂O₅ heterogeneous uptake was calculated to be 24–85 μg m^–3 per night and, on average, 57 μg m^–3 during days with pollution. This was comparable to or even higher than that formed by the partitioning of HNO₃. The results highlight that N₂O₅ heterogeneous hydrolysis is vital in pNO₃^– formation in Beijing