Visibility degradation across Hong Kong: its components and their relative contributions

Daily average levels of nitrogen dioxide (NO2) and the time-matched, relative humidity-corrected airborne respirable suspended particulate (RSP*) data from four air quality monitoring stations of the Environmental Protection Department (EPD) Network were subjected to analysis employing a linear multivariate model for visibility in Hong Kong. The respective NO2 and RSP* contributions to the extinction coefficient for Hong Kong Observatory, σext(HKO) are estimated to be between 6–31% (mean: 17%) and 58–68% (mean: 62.5%), with a remainder of 11–30% (mean: 21%). The latter includes a small contribution due to Rayleigh scatter. The observed σext agrees reasonably well with model prediction. During the July–October 1999 period, the particle scattering coefficient, σsp was determined at City University (CityU) in Kowloon with the aid of an integrated nepthelometer employing 3, 1-h sampling periods. This daily average value of σsp(CityU) is well correlated with the corresponding σext(HKO). It is provisionally estimated that the fractional contribution by particulate absorption to σext(HKO) is likely to be less than that by particulate scattering. A reasonable apportionment for σext(HKO) is 39% particulate scattering, 19–29% particulate absorption, 17% gaseous absorption, and an estimated mean Rayleigh scatter of 4.3%. The unaccounted 16% is due to a negative bias assigned to deficiencies in sampling and in the various assumptions made. Results of continuous, 8-h daytime nephelometric measurements of σsp at the CityU, and the water-soluble ambient RSP constituents sampled in tandem at the same site during the February–April 2000 period suggest that SO4*, NH4* and K are apparently associated with light scattering at CityU. However, only the SO4* at this sampling site is related to time-matched σext(HKO). Based on these and other results from a previous study, it is concluded that (i) σext(HKO) is proportional to σsp, measured elsewhere in Hong Kong at distances between several to 10 km from the HKO headquarters and (ii) most of the fine SO4 aerosol in Hong Kong is apparently advected into the territory.