A study of visibility in relation to airborne constituents in Hong Kong

Abstract

A systematic study of airborne respirable suspended particulate (RSP) matter and selected gaseous air pollutants, with special reference to the water-soluble species (WSS), was carried out in two locations in Hong Kong, viz the (i) Kadoorie Agricultural Research Center (KARC) and (ii) City University of Hong Kong (CityU). The primary objectives were: (i) To investigate the size, chemical composition and concentration of water-soluble species, along with meteorological data that are responsible for visibility reduction in both rural and urban areas. (ii) To relate the visibility degradation vis-à-vis light extinction through scattering and absorption. A multivariate model was utilized to achieve this purpose. (iii) To estimate the gaseous and particulate scattering and absorption in the total light extinction budget in Hong Kong. (iv) To estimate the contributions of gaseous and particulate constituents to light scattering and absorption extinctions. (v) To estimate the visibility in Hong Kong using both empirical (statistical) and theoretical (based on Mie theory) approaches for the first time. The salient features of the results obtained are summarized as follows: (i) SO4 represents a non-negligible advected fraction of the fine RSP in the KARC and CityU; NH4 and SO4 exhibit similar mass size distribution patterns with prominent peaks at ~0.3 μm. (ii) Chloride ion exhibits bimodal characteristics with peaks at aerodynamic diameters 0.235 ±0.1 14 μm and 3.029 ± 0.752 μm. (iii) Water-soluble SO4 and NH4 are found to be the predominant particulate light scatterers and are responsible for moderate-to-considerable visibility degradation, sufficient to lower the R(v) down to between 20 km and 10 km. (iv) SO4 can satisfactorily predict the total light extinction. With similar statistical approach, the hourly NO2 and RSP data of the Environmental Protection Department (EPD) network stations was analyzed and the results are compared with the overall findings of the present study. It has concluded that: (i) Potassium apparently acts as a surrogate for carbon, particularly elemental carbon. (ii) The total light extinction may be broken down into ~39% particulate scattering (from nephelometric measurement), 19-29 % particulate absorption, 17 % gaseous absorption (NO2) and an estimated mean Rayleigh scatter of ~4.3%. The unaccounted 16% is due to negative bias assigned to deficiencies in sampling and to various assumptions made. (iii) The theoretically based growth factor for ambient water-soluble species works as well as the relative humidity correction factor in the regression models. (iv) Based on theoretical calculation, ammonium sulphate presumably in the form of (NH4)2SO4 contributes 23-26% particulate scattering extinction, that is within 39% total extinction budget. The characteristics of the common gaseous air pollutants, especially sulfur dioxide (SO2) and nitrogen oxides, NOx (NO-plus-NO2) were also examined with respect to their (i) temporal and site-to-site variability and (ii) relationship with meteorological parameters. Apart from higher NO2 locally at Kwun Tong and Sham Shui Po, these results also indicate that (i) NOx is inversely proportional to [ O3] and to wind speed - in agreement with the largely local (automobile) origin of NOx and (ii) RSP is inversely proportional to precipitation amount (rainfall).

Date of Award	15 Feb 2002
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ronald SEQUEIRA (Supervisor)