Analyses of airborne Be concentrations in Hong Kong using back-trajectories

A total of 71 air samples were collected in Hong Kong area from November 2001 to February 2003 using a highvolume air sampler and a high-volume cascade impactor with five atmospheric pressure stages. The Be radioactivity on each stage was measured using a high-efficiency germanium gamma-ray spectrometer. From the radioactivity of stages, the total airborne Be radioactivity was determined. The activity median aerodynamic diameter (AMAD) of Beassociated atmospheric aerosols was found to be 0.22–1.11 mm and the geometric standard deviation (GSD) was found to be 1.2–10.5. With the assumed mean growth rate (MGR) of atmospheric aerosols of 0.004–0.005mmh 1 and the size of Aitken nuclei of 0.015mm, the residence times of Be-associated atmospheric aerosols were also found from the AMAD. Three-dimensional 4-day back-trajectories were obtained using the HYSPLIT model from NOAA Air Resources Laboratory. These trajectories were used with the measured Be radioactivity to construct regional Be intensity fields for four different altitude levels (less than 1000, 1000–2000, 2000–3000 and above 3000m) with a Geographic Information System (GIS). Low Be intensities were found to have advected from low altitudes (less than 1000m) and oceanic areas. The Be intensities increased for the higher intensity field layers. By comparing the time taken for air masses to come from the Be source to Hong Kong and the residence time determined from the AMAD of Be-associated atmospheric aerosols, good agreement was found if the mean growth rate of 0.005mmh 1 for atmospheric aerosols was used, and the use of back-trajectories was shown to be satisfactory even up to about 6.5 d. By using the residence time with a MGR of 0.005 mmh , the Be source was found to be relatively well confined in the areas of Mongolia and southeastern Siberia, which further supported that the association of Be source with the Siberian anticyclone. r 2004 Elsevier Ltd. All rights reserved.