The impact of atmospheric circulation on the air quality of Hong Kong in summer and winter

Abstract

In this study, the characteristics of meteorological patterns associated with the variation of air quality in Hong Kong are investigated. Highly similar daily API variation among stations suggests that they are driven by the same factors and can be represented by averaging the API of the stations. Wavelet analysis shows that strong coherence signals with a 256–512-day oscillation are found between the API and local meteorological factors but weak coherence is found in the high-frequency domain. Monthly correlation and moving correlation illustrate large coefficient fluctuations in different seasons. Examination of the relationship between the monsoon index, local meteorological patterns, and air quality illustrates a strong relationship between the monsoon index and meteorological factors but a weak relationship with air quality in summer, which suggests that summer and winter are controlled by different factors. Significant correlation coefficients between the air quality in Hong Kong and synoptic meteorological factors suggest that air pollution in Hong Kong during summer is controlled not just by local meteorological factors. Composite analysis shows that obvious westward-moving patterns including cyclonic flow and geopotential height are highly associated with the API variation. A correlation between the API, the subtropical high, and meteorological patterns suggests that the subtropical high is one possible factor controlling the air quality in Hong Kong. On the other hand, a backward clustering trajectory illustrates that cluster trajectories associated with high pollution are associated with continental air mass transport from the northwest, north, and northeast. Cyclonic flows to the west of the WNP and Taiwan are observed in those cluster trajectory days. Except for the subtropical high, tropical cyclones are another parameter affecting the air quality in Hong Kong. TCs that enter Taiwan, east of Hong Kong, and west of Hong Kong are accompanied by high, medium, and low air pollution, respectively. Backward trajectories illustrate three different trajectories corresponding to three TC activity regions. Multiple regression is employed to analyze the impact of local meteorological patterns for each region. Zonal wind and meridional wind always have a strong impact on the air pollution in each group, which shows that the API increases with the northerly and westerly wind components. Air pollution occurs in winter more frequently than in summer. Correlation illustrates that not only basic parameters such as wind speed, precipitation, and relative humidity influence the air quality; a significant dipole correlation pattern of temperature is observed in central China and the SCS associated with the variation of air pollution. Cold air movement is suggested to be related to air pollution. An increase in wind speed and precipitation is observed during the onset of a cold surge, which rapidly reduces air pollution levels. On the other hand, the decline of a cold surge occurs during high air pollution, suggesting that the temperature increases in the north will induce a westerly wind anomaly, which reduces the wind speed and lowlevel wind shear. Other than the API, variation in visibility is also associated with air quality. Examinations of different environmental parameters suggest that the intraseasonal component dominates the overall circulation anomalies in both summer and winter.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wen ZHOU (Supervisor)