Transport of dusts from East Asian and non-East Asian sources to Hong Kong during dust storm related events 1996-2007

摘要

Over a twelve year period from 1996 to 2007, 76 dust storm related events (as days) in Hong Kong were selected for study, based on Aluminium and Calcium concentrations in PM_(10). Four of the 76 events reach episodic levels with exceedances of the Hong Kong air quality standards. The purpose of the study is to identify and characterize dust sources impacting Hong Kong. Global distribution of aerosols in NASA's daily aerosol index images from TOMS and OMI, are compared to plots generated by NRL(US)'s Navy Aerosol Analysis and Prediction System. Possible source areas are assigned by computing air parcel backward trajectories to Hong Kong using the NOAA HYSPLIT model. PM_(10) and elemental data are analyzed for crustal mass concentrations and element mass ratios. Our analysis reveals that 73 out of the 76 dust events (96%) involve non-East Asian sources-the Thar, Central/West Asian, Arabian and Sahara deserts (Saharan influence is found in 63 events), which are previously not known to affect Hong Kong. The Gobi desert is the most frequent origin of dust, affecting 68 dust events while the Taklamakan desert impacts only 30 of the dust events. The impact of the Gobi desert in March and December is apparently associated with the northeast monsoon in East Asia. Our results also show a seasonal pattern in dust impact from both East Asian and more remote sources, with a maximum in March. Dust event occurrences are conspicuously absent from summer. Dust transport to Hong Kong is commonly associated with the passage of frontal low-pressure systems. The coarse size fraction of PM_(10) concentrations were, as indicated by Al, Ca and Fe concentrations, about 4-8 times higher during dust events. The mean Ca/Al ratios of sources involving the Taklamakan desert are notably higher than those for non-East Asian sources owing to a higher Ca content of most of the East Asian deserts. The Fe/AI ratios follow a similar trend. Contributions from the desert sources are grossly estimated where possible, by using the average Al abundance of 8% in the upper continental crust to convert the Al mass in the PM_(10) to dust concentrations. This is done for the six events identified with air mass purely of non-East Asian origin and the two events related only to the Thar/Arabian/Sahara deserts. Results reveal that the average contribution from the non-East Asian sources (including C/W Asia) is approximately 10% and, that from the Thar/Arabian/ Sahara deserts is about 8%.