Light scattering and absorption properties of aerosols in the Hong Kong region

Abstract

Aerosol problem is one of the major sources of air pollution Hong Kong. In order to obtain more information on aerosols in this region, an air monitoring station has been set up at Cape D'Aguilar, a coastal site located at the southeastern tip of Hong Kong, to collect long-term air quality data. In this study, the light scattering and absorption properties of aerosols were measured from the end of September 1997 to April 1999. It was found that aerosol levels, as reflected by the scattering and absorption coefficients of aerosols, showed seasonal variations in Hong Kong region. High aerosol level was observed in winter, with mean extinction coefficient (sum of scattering and absorption coefficients) of 105 Mm-1. Low aerosol level was found during the summer months, with mean extinction coefficient of 15 Mm-1. The transition periods were observed in spring and autumn. Diurnal variations were also observed, with maximum values occurring in the morning, around noon time, when human activities and photochemical processes increased. There was good correlation (r = 0.85) of the light extinction coefficient with the mass concentration of particulate matter below 10um (PM10). The mass extinction efficiency and the mass scattering efficiency were found be 1.54 m2g-1 and 0.98 m2g-1, respectively, lower than those found elsewhere. Visual range (VR) was deduced from the measured scattering and absorption coefficients. The correlation between the calculated and observed VR varied with ambient relative humidity (ARH). The best correlation coefficient was 0.72 for ARH < 60%, with only the contributions from aerosols considered in the calculated VR. The empirical constant c appropriate in the Hong Kong situation was 1.56 at ARH < 60%, comparable to those found elsewhere. The critical value of the single-scattering albedo Wcris has been estimated to range from 0.59 to 0.79. The average of single-scattering albedo W was found to be 0.72 during the period of study. In particular, the mean value obtained during February and March was 0.66, a drop from the value of 0.85 measured in the same two months of 1994. This may imply that aerosols had the trend of warming the climate over the past years. Using back trajectory model air masses reaching Hong Kong could roughly be classified into 7 types. Aerosol level was the highest for type LC (local circulation), followed by continental types and marine types. Local wind directions had also some effects on the aerosol levels, with winds from the northwesterly and northeasterly directions higher than that from the southeasterly and southwesterly. Based on some prescribed criteria 44 episodes had been identified during the period of measurements. They could be roughly grouped into 5 different categories according to the synoptic patterns influencing their occurrence. It was found that there were slightly more episodes occurring in autumn and winter than in spring and summer.