In 1993 the Hong Kong Polytechnic University (HKPU) established an atmospheric research station at the southeastern tip of Hong Kong in coastal South China (Figure 1). The primary objectives for the studies there are to monitor the composition of the atmosphere, document important trends, and study the processes affecting the fate and transport of air pollution in eastern Asia. Situated on the boundary between the Asian continent and the South China Sea, the Cape D'Aguilar (Hok Tsui) station receives continental outflow in winter and maritime inflow in summer, making it suitable for studying both natural and anthropogenic trace gases and aerosols. Through long-term observation it is possible to study the impact of the increasing air pollution, associated with the rapid Asian economic growth, on the chemistry and climate in eastern Asia, particularly in the subtropical part of eastern Asia.

Figure 1. Location of Hong Kong in eastern Asia.

Measurement activities

Since the establishment of the station, a suite of chemically active trace gases and aerosols have been measured, including O₃ (surface and total column), CO, NO, NO_X, NO_Y, SO₂, C₂-C₆ non-methane hydrocarbons (NMHC), mass and chemical composition of total and suspended particulate (TSP and RSP), and absorbing and scattering coefficients of aerosols. In addition to the efforts carried out at the research station, HKPU scientists have been collaborating with the Hong Kong Observatory to study ozone vertical structure by launching ozonesondes. The station has also accommodated visits of local and overseas scientists for study of acid rain and long-term aerosol trends.

International collaboration

The Cape D'Aguilar research station has been actively involved in several major international atmospheric chemistry studies. Shortly after its establishment, the station took part in the IGAC/NASA PEM-West B study in spring 1994. It has been a part of the IGAC/APARE surface monitoring network, and a regional station in the WMO-GAW network. The station is also involved in the China-MAP project, serving as the southernmost site in the air chemistry network in eastern China; the station's scientists also have been measuring trace gases in the Yangtze Delta region of China since June 1999. The research station will play an active role in the upcoming IGAC/NASA TRACE-P and ACE-Asia studies in spring 2001.

Atmospheric research and some research highlights

Trace gases and gas phase chemistry studies

Measurement of key trace gases has been a major focus of the research program at the station. This is partly because of the very limited investigations that have previously been conducted in Asia on reactive trace gases that control the atmospheric oxidizing capacity. Among the gases under study, ozone and CO have been continuously measured since 1994. Their seasonal trends have been established, showing minima in summer and maxima in winter, or in the case of ozone in late-autumn [Lam et al., 1998] as indicated in Figure 2a for 1994-1996. Typical summertime levels of 10-20 ppbv for O₃ and 90-100 ppbv for CO have been observed (Figure 2), and these levels are considered to represent the background levels in the South China Sea air. A similar summer-low and winter-high pattern has been indicated by C₂–C₆ NMHC measurements using an on-site gas sampling and GC/FID system from October 1995 to March 1997 [Cheung, 1999] and from studies of the chemical composition of aerosols [Cheng et al., 2000]. The Asian monsoonal circulation has been considered to the primary cause of such a seasonal pattern. Monsoon winds transport clean, humid, maritime air to the site in summer and pollutant-laden, dry air masses from the Asian continent in winter. Studies of the summer-time background levels of oxides of nitrogen and sulfur dioxide have been complicated by suspected emissions from ships brought to the site by southwesterly winds.

Figure 2. Monthly median (circle), mean (thick bar), 33 and 67 percentiles (whisker) of surface ozone (a) and CO (b) for 1994-1996.

Reactive nitrogen compounds, NO_X and NO_Y, have been measured, with the application of research-grade instrumentation, to study ozone chemistry and transport processes. During the PEM-West B mission in February-March 1994, NOY and several other gases including O₃, CO, SO₂ and NO were measured to characterize the air masses under the condition of strong continental outflow [Wang et al., 1997]. Later in 1997 and 1999, NO and NO₂ were measured using a set of high-sensitivity and high-selectivity instruments (modified EcoPhysics CLD 700 AL ppt chemiluminescent detector and PLC 760 photolytic converter) during the photochemially active summer and autumn seasons. NO_Y has been measured since September 1999 using catalytic conversion and chemiluminescent detection techniques (TEI 42C-Y Trace Level).

The extensive trace gas (and aerosol) measurements from the HK station have been compared with data collected in limited locations of East Asia as well as other parts of the globe to help characterize the chemical environment of the study site. Autumn is a season of particular interest because the research station is subjected to minimal influence of local emissions but strongly impacted by air masses from the western Pacific and northeastern island of Taiwan. After applying a wind filter to remove the impact of urban plumes from Hong Kong, the levels of the trace gases in that season have been found to be comparable to those of a "polluted" rural atmosphere in the industrialized eastern US or Europe. Figure 3 shows diurnal variations of median concentrations of NOx measured in October and November 1997 for major types of air masses stratified by backtrajectory analysis. Model calculations using observed chemical precursors and meteorological parameters show strong in situ ozone production and abundant radicals in the air masses sampled. The results also suggest that the period from late autumn to early winter is a strategic time for studies of photochemical processes that affect the removal of pollutants emitted in northeast Asia and, further, that the subtropical South China Sea is a suitable place for such studies. Detailed results of ozone chemistry will be included in two forthcoming manuscripts [Wang et al., manuscripts in preparation, 2000].

Figure 3. Diurnal variation of median NOX concentrations for several types of air mass observed in October and November 1997. Type (1): marine western Pacific, (2): weak continental outflow, and (3): strong continental outflow. The large-scale air masses are classified using 10-day back trajectories calculated by Dr. Joyce Harris of the NOAA/CMDL.

Aerosol chemical properties

From May 1993 to March 1996, TSP and RSP aerosol particles were collected with the aim of contributing to the overall understanding of the aerosol levels and their origins in an upwind location of Hong Kong. A high volume air sampling protocol using Whatman EPM–2000 glass microfiber filters and Whatman 41 cellulose filters was used to collect the aerosol for analysis. Inorganic compounds including major ions and various trace elements were measured by ion chromatography, neutron activation analysis, and X-ray fluorescence spectrometry. Detailed results have been reported in Lam et al., 1997 and Cheng et al., 2000.

The mean mass concentrations of TSP and RSP in the Cape D'Aguilar coastal area are 69 and 39 mg m^–3 respectively. The average ratios of Cl–/Na+ in both TSP and RSP are found to be close to the sea water ratio. The Cl–/Na+ concentration ratio for both TSP and RSP slightly deviate from the corresponding ratio for sea water. The annual mean sulfate concentration in TSP was 14 mg m3. The SO₄^2–/NO₃^– ratios are 3.28 for TSP and 4.17 for RSP. The non-sea salt sulfate in RSP accounts for 93% of the total sulfate indicating a substantial anthropogenic origin. Trace elements such as Se, Zn, Pb and As are highly enriched over the concentrations expected from the observed concentrations of crustal material or atmospheric sea salt.

Aerosol optical and sizing measurements

Aerosol optical properties have been monitored from September 1997 to April 1999. The light scattering coefficients have been measured using a M903 integrating nephelometer (Radiance Research, USA) and the light absorption coefficients have been measured using a particle soot absorption photometer (PSAP, Radiance Research). Particle size distributions have been measured using a LASAIR 510 (Particle Measuring Systems, Inc. USA) and an eight-stage Andersen non-viable ambient sizing sampler. The single-scatter albedo, w, was calculated from the measurements and compared with the critical value, w_crit, estimated for the region. During summer, w was found to be less than w_crit, indicating that aerosol population may exert a warming effect in summer. On the other hand, w was found to be larger than w_crit in winter, suggesting a possible cooling effect of aerosols in winter. During the winter, the number concentration of aerosol fine particles with diameters less than 2.5 mm was higher than that of coarse particles by a factor of 10⁴. The variations of the scattering coefficients have been matched with the concurrent variations of particle sizes and it was found that the correlation was best for particle size range of 0.5 to 1.0 mm. Detailed results are being included in a manuscript [Man et al., manuscript in preparation, 2000].

Ozone vertical distributions

Ozonesondes have been launched in Hong Kong in collaboration with the Hong Kong Observatory. Data analyses of the ozone profiles have been performed, with a focus on the seasonal cycle in tropospheric ozone [Chan et al., 1998]. Results show that the tropospheric ozone column has an obvious maximum in spring and a minimum in summer. The former is a common feature at many locations in the Northern Hemisphere. Tropospheric ozone has an especially strong influence on the seasonal cycle of total ozone at Hong Kong. The seasonal cycle of ozone mixing ratio below 2 km is bimodal with ozone peaks in spring and autumn. A frequently observed feature in late autumn and winter is a relative minimum ozone mixing ratio (as low as 30-40 ppbv) in the upper troposphere (from about 9 to 16 km). Trajectory analysis shows this relative minimum in ozone mixing ratios is associated with air masses transported from the tropical region. It is proposed that the East Asia local Hadley circulation is responsible for this feature.

Also, high ozone (80-138 ppbv) events in the lower troposphere (2.5-4.5 km) over Hong Kong have been captured by ozonesonde launches during late winter and spring. Back trajectories showed air masses arriving at the altitude of the ozone peak had passed over continental Southeast Asia, which is where the bulk of biomass burning occurs at this time of the year. With the increased relative humidities accompanying these events, we suggest that the ozone-rich air in the lower troposphere in springtime is the result of photochemical production rather than the result of transport from the upper troposphere of higher latitudes [Liu et al., 1999]. Further analysis of ozonesonde data obtained during the NASA PEM-West B in February 1994 using the NOAA AVHRR satellite image data on global fire strongly suggest that biomass burning in SE Asian countries such as Vietman and Thailand strongly influences the ozone climatology in Southeast Asia and Hong Kong [Chan et al., 2000]. Also, strong ozone enhancements, with mixing ratios up to 100 ppb and an enhancement layer up to 10 km thick, have been observed over Hong Kong during the strong El Niño event in December 1997. Our analysis suggests that these conditions are related to the Indonesian fires in SE Asia at that time.

References

• Chan, L.Y., H.Y. Liu, K.S. Lam, T. Wang, S.J. Oltmans, and J. M. Harris, Analysis of the seasonal behavior of tropospheric ozone at Hong Kong, Atmos. Environ., 31, 159-169, 1998.
• Chan, L.Y., C.Y. Chan, S. Christopher, L.Y. Liu,
• S.J. Oltmans, and J. M. Harris, A case study on biomass burning in Southeast Asia and enhancement of tropospheric ozone over Hong Kong, Geophys. Res. Lett., in press, 2000.
• Cheng Z. L., Lam K.S., L.Y. Chan, T. Wang, and
• K.K. Cheng, Chemical characteristics of aerosol at coastal station in Hong Kong. I. seasonal variation of major ions, halogens and mineral dusts between 1995 to 1996, Atmos. Environ., in press, 2000.
• Cheung, T.F., M. Phil. Thesis, Measurement of atmospheric hydrocarbons and air quality in Hong Kong, The Hong Kong Polytechnic University, 1999.
• Lam, K.S., Z L. Cheng, L.Y. Chan: Aerosol composition at a coastal monitoring site in Hong Kong-Initial results, J. Environ. Sci., 9, 396-410, 1997.
• Lam, K.S., T. Wang, L.Y. Chan, and H.Y. Liu, Observation of surface ozone and carbon monoxide at a coastal site in Hong Kong, In: Proceedings of Quadrennial Ozone Symposium, 1998.
• Liu, H.Y., S.J. Oltmans, L.Y. Chan, J.M. Harris, and
• W. L. Chang, On springtime high ozone events in the lower troposphere from SE Asian biomass burning, Atmos. Environ., 33, 2403-2410, 1999.
• Wang, T., K.S. Lam, L.Y. Chan, A.S.Y. Lee, and
• M.A. Carroll, Trace gas measurements in coastal Hong Kong during the PEM-West B, J. Geophys Res., 102, 28,575-28,588, 1997.