K.S. Lam

Chemical characteristics of aerosols at coastal station in Hong Kong. I. Seasonal variation of major ions, halogens and mineral dusts between 1995 and 1996

Abstract Aerosols samples (total suspended particulate “TSP” and PM10 particulate) were collected at coastal monitoring station in Hong Kong between 1995 and 1996. They were analyzed to investigate the seasonal cycle among major ions (Na+, NH4+–N, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−), halogen elements (Br, I) and mineral dusts. The concentration of major ions showed a summer minimum and a winter maximum in a coastal region of Hong Kong. Halogen (Br, I) and dust concentrations exhibited a very similar variation. From the molar equivalence of Na+ and Cl−, it was found that the Cl− in TSP or PM10 was deviated from sea-salt (NaCl) component. Bromine (Br) in aerosols evidently originated from marine source but iodine (I) in aerosols may have been generated from natural and anthropogenic sources including possible biomass emission. The ratios of nonsea-salt sulfate (nss-SO42−) to nitrate (NO3−) and trace elements (As, Sb, Se, Pb, V and Zn) were used to explain the different pollution emissions.

A nonurban ozone air pollution episode over eastern China: Observations and model simulations

Air quality data gathered from five nonurban sites in China over a 12-month period from August 1994 to August 1995, along with meteorological observations from the same region and period, are used to identify and characterize a nonurban ozone (O3) pollution episode in China. Because of the influence of the Asian Monsoonal Circulation, high O3 concentrations were not observed at the nonurban sites during the summer months. However, enhanced O3 concentrations were observed during the other seasons, especially the fall and early winter. A more detailed inspection of the O3 data during the period from October 15, 1994, to January 15, 1995, indicated the occurrence of a multipleday episode in late October/early November when high O3 concentrations were observed at all four monitoring sites located in eastern China. Meteorological conditions during the episode were characterized by the presence of a strong and stationary high-pressure ridge over eastern China; synoptic conditions quite similar to those observed during regional O3 pollution episodes over the United States, Canada, and Europe. An updated version of the Regional Acid Deposition Model (RADM) driven by meteorological fields derived from the Regional Climate Model (RegCM) and spatially disaggregated anthropogenic emissions prepared by the Chinese Academy of Meteorological Sciences is used to simulate 3 months of the observed O3 data from China. Comparisons between observations and model calculations indicate that the model is able to reproduce some of the key features of the O3 distribution and its relationship to the concentration of one primary pollutant (i.e., sulfur dioxide) provided the comparison is made using averaging times of several days or more. However, simulation of day-to-day variations in O3 at a given site was poorly correlated with observations. Model simulations suggest that peak O3 concentrations during this episode would respond to changes in NOx and VOC emissions in a spatially inhomogeneous manner. In general, rural areas in southern China tend to be NOx-limited, but rural areas in northern China tend to be VOC-limited. The Yangtze Delta region, where the highest O3 concentrations were observed and predicted to occur, was found to be transitional between VOC and NOx limitation.

A study of surface ozone and the relation to complex wind flow in Hong Kong

Abstract Ozone measurements made from 5 sites in Hong Kong have been analyzed, including those from one upwind, one downwind, and three urban locales. The data are analyzed in terms of the seasonal and diurnal trends. A subset of data in autumn is further analyzed to study the relationship between the ozone spatial pattern and wind flow as well as other meteorological parameters. The results show that averaged ozone levels at most sites exhibit maxima in autumn, which appears to be a unique feature for eastern Asia. On average the daily maximum 1-h concentrations are found to be higher in the western (normally downwind) site than those on the eastern side and in urban areas. Examination of surface wind patterns and other meteorological parameters suggest that elevated ozone concentrations on the western side occur during the days with intense solar radiation, light winds, and in the presence of a unique wind circulation. The wind reversal in the western parts under the “convergence” flow is believed to be an important cause of the high-ozone events observed there. Such wind flow may re-circulate/transport nearby urban plumes (in this case the Hong Kong–Shenzhen urban complex). Examination of chemical data from the western site has shown that averaged afternoon SO2 to NOx ratios on days with wind reversal are larger than those of typical urban Hong Kong and that a significant SO2 enhancement was clearly indicated on several occasions. The SO2 enhancement may be interpreted as being the evidence to suggest the contribution of regional sources and/or Hong Kong’s power plants (both containing high SO2). A case study has shown that when moderately strong northwesterly wind prevails, elevated ozone and SO2 can be transported to western Hong Kong from the inner Pearl Delta region. This study has also indicated that under the impact of ENE winds the eastern side of Hong Kong is not frequently affected by the re-circulating ozone plumes present in the western side.

Analysis of the seasonal behavior of tropospheric ozone at Hong Kong

Abstract Ozonesondes have been launched at Hong Kong (22.2°N, 114.3°E) since 1993. The results of data analysis of the ozone profiles are presented, with a focus on the seasonal cycle in tropospheric ozone. The 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. The summer minimum is attributed to the onset of the summer monsoon when air flow from the Asian continent is replaced by air from the South China Sea or the tropical Pacific. The tropospheric ozone has an important 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 of 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 of ozone is associated with air masses coming from the tropical region. It is proposed that the East Asia local Hadley circulation is responsible for this feature.

Meteorological and Chemical Characteristics of the Photochemical Ozone Episodes Observed at Cape D’Aguilar in Hong Kong

As in many metropolitan areas around the world, air pollution in Hong Kong is an increasing concern. In this paper the authors present the observations of ozone (O 3) pollution episodes made at a nonurban coastal location in Hong Kong. Four O3 episodes were observed in 1994, during which hourly averaged O3 concentrations exceeded 100 ppbv and in one case reached 162 ppbv. Recirculation of urban air caused by the reversal of surface winds was found to be an important mechanism for transporting the ‘‘aged’’ urban plumes to the monitoring site. Concurrent measurements of CO, SO2, NO, and O3 provided an insight to the chemical characteristics of the air masses, and the chemical data appeared to suggest that the high levels of O 3 during the episodes were produced in the urban plumes that were mainly characteristic of vehicle emissions. The relationship between O3 and CO in two of the episodes may be represented by a linear approximation, and a nonlinear relationship between O3 and CO was found in another. Ozone levels observed at the nonurban site were higher than those at two urban locations.

A modeling study on acid rain and recommended emission control strategies in China

This paper presents a brief description of the sources and characteristics of air pollution in China, documenting acid rain aggravation and its regional distribution in the past years. Simulation of SO2 ground-level concentration and sulfur deposition in 1995 was performed with the Nanjing University developed acid deposition model system (NJUADMS) and compared with the national observations and the model output of the RAINS-ASIA. Furthermore, the acid rain control policy and its countermeasures adopted for the country are presented.

The characteristics of ozone and related compounds in the boundary layer of the South China coast: temporal and vertical variations during autumn season

Abstract We present measurements of several trace gases made at a subtropical coastal site in Hong Kong in October and November 1997. The gases include O3, CO, SO2, and NOx. The surface measurement data are compared with those from an aircraft study [Kok et al. J. Geophys. Res. 102 (D15) (1997) 19043–19057], and a subset of the latter is used to show the vertical distribution of the trace gases in the boundary layer. During the study period, averaged concentrations at the surface site for O3, CO, NOx, and SO2 were 50, 298, 2.75, and 1.65 ppbv, respectively. Their atmospheric abundance and diurnal pattern are similar to those found in the “polluted” rural areas in North America. The measured trace gases are fairly well mixed in the coastal boundary layer in the warm South China region. Large variability is indicated from the data. Examination of 10-day, isentropic back trajectories shows that the measured trace gases are influenced by maritime air masses, outflow of pollution-laden continental air, and the mixing of the two. The trajectories capture the contrasting chemical features of the large-scale air masses impacting on the study area. CO, NOx and SO2 all show higher concentrations in the strong outflow of continental air, as expected, than those in the marine category. Compared with previously reported values for the western Pacific, the much higher levels found in the marine trajectories in our study suggest the impacts of regional and/or sub-regional emissions on the measured trace gases at the study site. The presence of abundant O3 and other chemically active trace gases in the autumn season, coupled with high solar radiation and warm weather, suggests that the South China Sea is a photochemically active region important for studying the chemical transformation of pollutants emitted from the Asian continent.

Ground-based measurements of total ozone and UV radiation by the Brewer spectrophotometer #115 at Hong Kong

Ground-based total ozone (O3) and spectral solar ultraviolet (UV) radiation measurements recorded by a Brewer #115 at Hong Kong (Cape D’Aguilar, 22113 0 N, 114115 0 E, 60 m mean sea level) are presented. The ground-based data show good agreement with the satellite data (the correlation coefficient r is 0.942) but there is a systematic difference between the two data sets. In general, the TOMS ozone is about 3% higher than the Brewer ozone. The seasonal variations of total ozone and UV radiation from January 1995 to April 2001 are discussed. Total ozone has a seasonal cycle, it reaches a maximum in May and minimum in January. A quasi-biennial oscillation signal is clearly identified in the total ozone time series. Surface UV radiation also exhibits seasonal cycles that have maxima in July and minima in January. The solar zenith angle (SZA) is confirmed as a crucial factor governing both the seasonal change and magnitude of the surface UV radiation. Clouds and total ozone are also important. The relationship between total ozone and surface UV is discussed. The variability of surface UV due to SZA and cloud are eliminated by selecting UV measurements taken at 451 SZA and under a clear sky. The filtered UV-B shows an anti-correlation with total ozone. It is estimated that 1% increase of total column ozone can cause a 1% decrease of surface UV-B. The analysis has shown that within the period January 1995–April 2001, the total ozone over Hong Kong increased and surface UV decreased. r 2002 Elsevier Science Ltd. All rights reserved.