Studies on properties and source region of airborne 7Be in Hong Kong

Abstract

A total of 71 particulate samples were collected during the period from Nov 2001 to Feb 2003 in Hong Kong by a high volume air sampler and a cascade impactor with five atmospheric pressure stages. The collection time ranged from 1 to 4 days, and all of the readings were subsequently normalized to 48-hour values. After the air sampling, the samples were measured using a high efficiency germanium gamma spectrometer for the gamma-ray photons with energy Eγ = 477 keV emitted by 7Be. The two properties, viz., total activity (in counts) and the activity median aerodynamic diameter (AMAD) of 7Be were then determined. In our experiments, the total intensity for 7Be ranged from 260 to 1230 counts (with an average of 722 counts). Lower 7Be intensities were found in summer times, during which frequent rain always occurred. On the other hand, passage of cold fronts, which usually occurred in winter, brought atmospheric air masses with higher 7Be intensities to Hong Kong. The activity median aerodynamic diameter (AMAD) and the geometric standard deviation (σg) of the 7Be-attached aerosol particles ranged from 0.218 to 1.114 μm (with an average of 0.643 μm) and from 1.217 to 10.511 (with an average of 2.411), respectively. With the assumed mean growth rate (MGR) of atmospheric aerosols from 0.004 to 0.005 μm h-1 and the size of Aitken nuclei of 0.015 μm, the residence times of 7Be-associated atmospheric aerosols were also found from the AMAD with an average value of 5.87 days. Correlations between 7Be properties and different meteorological parameters (e.g., relative humidity) were also investigated. The 7Be intensity was found to be anti-correlated to both the relative humidity and the mean cloud cover with high confidence. On the other hand, both the relative humidity and the mean cloud cover were found to be positively correlated to AMAD. The back-trajectory method was also employed with an attempt to identify the source region of 7Be coming to Hong Kong. The HYSPLIT model from NOAA Air Resources Laboratory was applied. By using a Geographic Information System (GIS), the 4-day endpoints given by the HYSPLIT model were used to construct intensity fields of 7Be at different altitudes. Generally speaking, low 7Be intensities were found to advect from low altitudes (less than 1000 m) and oceanic areas, while higher 7Be intensities were found in higher intensity-field layers. The 7Be source region was found to be probably located in the Mongolian region, which was associated with the Siberian anticyclone in winter months. The last part of the study was focused on the comparison between the residence time of 7Be-associated atmospheric aerosols calculated from the AMAD values and the time taken by the corresponding air masses from the source region of 7Be to come to Hong Kong determined from back trajectory analyses. In this part of the study, the source region of 7Be was first assumed to be in the area of Mongolia with altitudes higher than 3000 m. It was found that, with the mean growth rate (MGR) of 0.005 μm h-1 for atmospheric aerosols, the determined residence time from the AMAD values were generally consistent with the travel time of the corresponding air masses to Hong Kong. Along with this, the use of back-trajectories were shown to be satisfactory even up to about 6.5 d. On the other hand, by using the residence time with a MGR of 0.005 μm h-1 as the travel time in the back trajectory analysis, the 7Be source was found to be relatively well confined in the areas of Mongolia and south-eastern Siberia, which further supported that the 7Be source was associated with the Siberian anticyclone in the winter months.

Date of Award	4 Oct 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwan Ngok Peter YU (Supervisor)