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Study finds external effects negate Hong Kong local efforts to reduce ozone pollution; multiregional policies needed

Researchers from Hong Kong Polytechnic University, Hong Kong’s Environmental Protection Department and UC Irvine present in a paper in the ACS journal Environmental Science & Technology direct evidence that increasing regional effects have negated local control efforts for O3 (ozone) pollution in Hong Kong over the past decade.

The researchers analyzed the daily maximum 8 h average O3 and Ox (=O3+NO2) concentrations observed during the high O3 season (September–November) at Air Quality Monitoring Stations. They found that the locally produced Ox showed a statistically significant decreasing trend over 2002–2013 in Hong Kong. Analysis by an observation-based model confirmed this decline in in situ Ox production, which the team attributed to a reduction in aromatic hydrocarbons.

However, the regional background Ox 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 non-decrease in Ox.

Like big metropolitan areas, Hong Kong has long suffered from serious air-pollution problems, including early reports of photochemical smog in the mid- 1990s. The Hong Kong government has since implemented long-term and step-by-step measures to reduce the emissions of NOx, CO and VOCs from vehicles, power plants, industrial, commercial and residential activities.

While these actions indeed significantly reduced the emissions of O3 precursors, the authors noted, this reduction has not alleviated the O3 pollution problem in Hong Kong.

In this study, we used data obtained from the Air Quality Monitoring Network by the Hong Kong Environmental Protection Department (HKEPD) to analyze the O3 trends over 2002−2013 as well as the local and regional contributions. An observation-based box model (OBM) was then used with the most comprehensive data at a polluted site (Tung Chung) to verify trends in local O3 production and to evaluate the effects of reducing individual O3 precursors. The results of the analysis provide solid evidence that the government’s control efforts have reduced local ozone production in Hong Kong. However, due to increasing contributions from southern/ eastern China, the overall O3 concentration has risen.

—Xue et al.

The team suggested that the rise in regional background O3 levels is due to the increase in O3 levels in the Pearl River Delta (PRD) region, along with the contributions of “super-regional” transport from the upwind regions beyond the PRD—i.e., eastern China.

This increase in “background” O3, due mainly to regional (PRD-originating) and super-regional (eastern China based) contributions, has offset the reduction of local ozone production in Hong Kong, leading to an overall increase in ambient O3 levels. However, without local control efforts would have definitely resulted in a more significant increase in ambient O3 levels in Hong Kong. To successfully control its ozone problem in the future, therefore, Hong Kong must collaborate closely with mainland China, not only the PRD but also the entirety of eastern China.

—Xue et al.

The study also has implications for efforts to reduce secondary pollution in other Chinese cities, the authors suggested.

The long-term data analyzed in the present study in Hong Kong vividly illustrate the importance of dealing with pollution sources over a large geographical area. As many other cities in central and eastern China are closer to regional sources of pollution, it is vital to develop and implement coordinated multiregional policies to mitigate ozone pollution and other secondary problems in these cities.

—Xue et al.

Resources

  • Likun Xue, Tao Wang, Peter K. K. Louie, Connie W. Y. Luk, Donald R. Blak and Zheng Xu (2014) “Increasing External Effects Negate Local Efforts to Control Ozone Air Pollution: A Case Study of Hong Kong and Implications for Other Chinese Cities” Environmental Science & Technology doi: 10.1021/es503278g

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