Increasing External Effects Negate Local Efforts to Control Ozone Air Pollution: A Case Study of Hong Kong and Implications for Other Chinese Cities

It is challenging to reduce ground-level ozone (O₃) pollution at a given locale, due in part to the contributions of both local and distant sources. We present direct evidence that the increasing regional effects have negated local control efforts for O₃ pollution in Hong Kong over the past decade, by analyzing the daily maximum 8 h average O₃ and O_<i>x</i> (=O₃+NO₂) concentrations observed during the high O₃ season (September–November) at Air Quality Monitoring Stations. The locally produced O_<i>x</i> showed a statistically significant decreasing trend over 2002–2013 in Hong Kong. Analysis by an observation-based model confirms this decline in in situ O_<i>x</i> production, which is attributable to a reduction in aromatic hydrocarbons. However, the regional background O_<i>x</i> transported into Hong Kong has increased more significantly during the same period, reflecting contributions from southern/eastern China. The combined result is a rise in O₃ and a nondecrease in O_<i>x</i>. This study highlights the urgent need for close cross-boundary cooperation to mitigate the O₃ problem in Hong Kong. China’s air pollution control policy applies primarily to its large cities, with little attention to developing areas elsewhere. The experience of Hong Kong suggests that this control policy does not effectively address secondary pollution, and that a coordinated multiregional program is required

Does Ozone Pollution Share the Same Formation Mechanisms in the Bay Areas of China?

As important regions of transition between land and sea, the three bay areas of Bohai Bay (BHB), Hangzhou Bay (HZB), and Pearl River Estuary (PRE) in China often suffer from severe photochemical pollution despite scarce anthropogenic emissions. To understand the causes of high ozone (O3) concentrations, the high O3 episode days associated with special synoptic systems in the three bays were identified via observations and simulated by the weather research and forecasting coupled with community multiscale air quality (WRF-CMAQ) model. It was revealed that the interaction between synoptic winds and mesoscale breezes resulted in slow wind speeds over the HZB and PRE, where air pollutants transported from upwind cities gained a long residence time and subsequently participated in intensive photochemical reactions. The net O3 production rates within the bay areas were even comparable to those in surrounding cities. This finding was also applicable to BHB but with lower net O3 production rates, while high levels of background O3 and the regional transport from farther upwind BHB partially elevated the O3 concentrations. Hence, these three bay areas served as O3 “pools” which caused the accumulation of air pollutants via atmospheric dynamics and subsequent intense photochemical reactions under certain meteorological conditions. The results may be applicable to other similar ecotones around the world

Rapid hydrolysis of NO₂ at High Ionic Strengths of Deliquesced Aerosol Particles

Nitrogen dioxide (NO2) hydrolysis in deliquesced aerosol particles forms nitrous acid and nitrate and thus impacts air quality, climate, and the nitrogen cycle. Traditionally, it is considered to proceed far too slowly in the atmosphere. However, the significance of this process is highly uncertain because kinetic studies have only been made in dilute aqueous solutions but not under high ionic strength conditions of the aerosol particles. Here, we use laboratory experiments, air quality models, and field measurements to examine the effect of the ionic strength on the reaction kinetics of NO2 hydrolysis. We find that high ionic strengths (I) enhance the reaction rate constants (kI) by more than an order of magnitude compared to that at infinite dilution (kI=0), yielding log10(kI/kI=0) = 0.04I or rate enhancement factor = 100.04I. A state-of-the-art air quality model shows that the enhanced NO2 hydrolysis reduces the negative bias in the simulated concentrations of nitrous acid by 28% on average when compared to field observations over the North China Plain. Rapid NO2 hydrolysis also enhances the levels of nitrous acid in other polluted regions such as North India and further promotes atmospheric oxidation capacity. This study highlights the need to evaluate various reaction kinetics of atmospheric aerosols with high ionic strengths

Double-Edged Role of VOCs Reduction in Nitrate Formation: Insights from Observations during the China International Import Expo 2018

Aerosol nitrate (NO3–) constitutes a significant component of fine particles in China. Prioritizing the control of volatile organic compounds (VOCs) is a crucial step toward achieving clean air, yet its impact on NO3– pollution remains inadequately understood. Here, we examined the role of VOCs in NO3– formation by combining comprehensive field measurements conducted during the China International Import Expo (CIIE) in Shanghai (from 10 October to 22 November 2018) and multiphase chemical modeling. Despite a decline in primary pollutants during the CIIE, NO3– levels increased compared to pre-CIIE and post-CIIENO3– concentrations decreased in the daytime (by −10 and −26%) while increasing in the nighttime (by 8 and 30%). Analysis of the observations and backward trajectory indicates that the diurnal variation in NO3– was mainly attributed to local chemistry rather than meteorological conditions. Decreasing VOCs lowered the daytime NO3– production by reducing the hydroxyl radical level, whereas the greater VOCs reduction at night than that in the daytime increased the nitrate radical level, thereby promoting the nocturnal NO3– production. These results reveal the double-edged role of VOCs in NO3– formation, underscoring the need for transferring large VOC-emitting enterprises from the daytime to the nighttime, which should be considered in formulating corresponding policies

Origins of Particulate Organic Acids during High-Altitude Transport over the North China Plain: Results from Mount Tai and a Flight Campaign in Winter 2019

Measurements of atmospheric organic particle composition at higher altitudes are scarce. The present study discusses concentrations and sources of PM and organic constituents based on winter-time observations at Mount Tai and aircraft measurements above the North China Plain (NCP). For PM2.5 at the mountain site, concentrations up to 94 μg m–3 were measured. Correlations with surrounding cities indicated that, despite an observed boundary layer height below the sampling altitude (1534 m asl), polluted air masses from the plain ascended to the mountaintop, possibly due to orographic effects. Organic constituents showed mean concentrations ranging from ∼1 ng m–3 for terpene-derived acids and some branched or unsaturated dicarboxylic acids but could reach up to ∼100 ng m–3 for oxalic acid. A cluster heatmap revealed correlational relationships between the compounds that originate from sources, including traffic, combustion of coal, waste, and biomass as well as secondary formation. Average concentrations of most short-chain organic acids taken in the general upwind direction of Mount Tai at mean flight altitudes of ∼4000 m averaged 10–25% of the mountain ones outside haze periods, suggesting that chemical formation during high-altitude transport could have occurred. One flight sample showed high concentrations of up to 180 ng m–3 for oxalic acid, which is comparable to mountain haze periods, corroborating earlier observations of elevated pollution layers in the area even at such altitudes. CAPRAM multiphase modeling reproduced selected mountain concentrations of malonic and succinic acids reasonably well, while an initial underestimation of oxalic acid could be reduced by including salt formation and thereby enhanced phase partitioning. Overall, this study addresses observational gaps at high altitudes over the NCP and suggests processes and sources that might also be relevant in other regions