Water-soluble aerosol and visibility degradation in Hong Kong during autumn and early winter, 1998

Abstract

During the October–December 1998 period, 30 daily samples of size-separated airborne respirable suspended particulates (RSP) were collected at the quasi-rural Kadoorie Agricultural Research Centre (KARC) in central New Territories (NT), Hong Kong, Special Administrative Region (SAR). Results of analysis indicate that sulphate is the predominant water-soluble species, and that sulphate, nitrate and ammonium together contribute to most of the total water-soluble fine aerosol mass. An interesting result obtained through principal component analysis (PCA) following varimax rotation of the bivariate correlation matrix for water-soluble species is that the first component (PC1) is made up exclusively of SO 4 and NH 4 ions. The stoichiometric ratio and correlation coefficient between the two ions suggest that ammoniated sulphate compounds are the probable species responsible for the PC1. Further, the use of a linear multivariate visibility model which accommodates the effect of relative humidity (RH) shows that SO 4 and NH 4 are the only anions important in visibility degradation. It is found that SO 4 in aerosol at the KARC can be used to predict the visual range (or extinction coefficient) recorded from Kings Park, Kowloon, ∼ 10 km away. This result suggests that SO 4 (and possibly NH 4) is, generally, likely to be of regional rather than of local origin. Further observations suggest that the model is most applicable to a moderate visual range, 10 km<R(v)⩽20 km under a rather broad range of ambient relative humidity, 40%<RH⩽80. However, this inference does not preclude the contributions to visibility degradation — mostly by absorption — by some of the water-insoluble aerosol constituents, including carbon, or the pollutant gas, NO 2.