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, σ_ext(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 σ_ext agrees reasonably well with model prediction. During the July–October 1999 period, the particle scattering coefficient, σ_sp 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 σ_sp(CityU) is well correlated with the corresponding σ_ext(HKO). It is provisionally estimated that the fractional contribution by particulate absorption to σ_ext(HKO) is likely to be less than that by particulate scattering. A reasonable apportionment for σ_ext(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 σ_sp 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 σ_ext(HKO). Based on these and other results from a previous study, it is concluded that (i) σ_ext(HKO) is proportional to σ_sp, 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.