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Ambient measurements of seventeen 2,3,7,8-polychlorinated dibenzo-p-dioxin/dibenzofuran congeners (2,3,7,8-PCDD/Fs) have been taken in a number of monitoring programs or ad-hoc studies in Hong Kong. The longest monitoring program started at two locations in the territory in July 1997. The other monitoring efforts are ad-hoc studies, varying from a few coordinated sampling events at multiple sites to a year-long monitoring project that targeted suspected local dioxin sources. In this paper, we examined these measurements to understand the ambient levels, temporal and spatial variation, and possible sources of the 2,3,7,8-PCDD/Fs in Hong Kong. The territory-wide annual average concentration of the dioxins was 0.052 pg I-TEQ/m3 measured at the regular monitoring stations in the most recent annual cycle of 2000/2001. This level fell at the lower end of the range of dioxin concentrations measured at other urban locations around the world. The dioxin levels showed a clear seasonality in that elevated concentrations were observed in the winter and lower concentrations in the summer at all monitoring sites with one year or more regular measurements. The measurements indicated that the few known local dioxin sources, including a major chemical waste incinerator facility, landfill sites, and vehicular traffic, are not important contributors to ambient dioxins in Hong Kong. On days of high dioxin concentrations, the 2,3,7,8-PCDD/F congeners were observed to have almost identical compositions with a north-northwest to south-southeast spatial gradient in concentrations at different sampling locations in Hong Kong. This observation, along with other collaborative evidence, established a strong link between high dioxin concentration days in Hong Kong and regional transport of the polluted air masses from the north.

Aerosol mass size distributions of water-soluble organic carbon (WSOC) were studied at a coastal site in Hong Kong. Eleven sets of 72-h samples were collected using an eight-stage impactor sampler over three seasons in 2000–2001. Regardless of air mass origins being mainly marine or continental, WSOC exhibited bimodal size distributions with a dominant fine mode and a minor coarse mode in the size range of >0.43 μm. The two modes had a mass mean aerodynamic diameter (MMAD) of 0.7±0.1 and 4.0±0.3 μm, respectively. The fine WSOC accounted for the major proportion of the total WSOC, ranging from two-thirds to four-fifths. The WSOC mixtures were resolved into 3–5 peaks under controlled progressive heating and combustion conditions. A positive matrix factorization analysis deconvoluted the WSOC mixtures into three component groups of different thermal behaviors. The three groups of WSOC species were postulated to be low, medium, and high molecular weight (MW) polar compounds on the basis of their thermal evolution features. On average, they accounted for 21%, 39%, and 40% of fine WSOC ( 2.1 μm) was largely made of the low MW polar compound group. The low MW group had a bimodal distribution with a dominant coarse mode, likely associated with sea salt aerosols. The medium and the high MW groups had a single mode distribution in the droplet mode, both with an average MMAD of 0.7±0.1 μm. Their droplet mode presence indicated that they were associated with cloud-processed particles. There is evidence to suggest that the high MW group was likely secondary in origin, formed during cloud processing.

We review here measurements of elemental carbon (EC) and organic carbon (OC) in a monitoring network of nine general stations and one roadside station in Hong Kong from 1998 to 2001. The territory-wide mean values for OC and EC in respirable suspended particulate matter (RSP or PM10) during this period are 8.89 and 4.66 μg C/m3, respectively. OC has a clear seasonal pattern with higher concentrations observed in the winter than in the summer at all the monitoring stations. The wintertime OC is two times of or higher than the summertime OC at each of the monitoring stations. Air masses of higher particulate loading enriched with OC transported from northern China are mainly responsible for the higher OC concentrations in the winter. To the contrary, the seasonality of EC is much weaker and variable among the monitoring sites. EC seasonality is found to depend on the distance from and relative location to the city's container port. A cluster of five stations to the immediately north of the container port have higher EC levels in the summer than in the winter as a result of being downwind of ship emissions in the summer, but upwind in the winter. EC concentrations at the three stations further north of the container port does not have discernable seasonality whereas EC at the two stations south of the port have the same seasonality as OC. The variable EC seasonality is a result of two counteracting factors, i.e., stronger influence of marine vessel emissions in the summer versus input of dirtier air masses in the winter. This also reveals that ship emissions at the port are important contributors to EC loadings in the air in Hong Kong. Further efforts are needed to characterize aerosol source profiles of ship emissions for quantifying their relative contributions to the ambient PM10 in the city.

The strong acidity and the concentrations of ammonium, sodium, sulfate, nitrate and chloride in PM2.5 in Hong Kong were measured using a Harvard honeycomb denuder/filter-pack system. The role of atmospheric mixing height in the variation in concentrations of the species in PM2.5 was investigated and the contribution of long-range transport to the ambient aerosols in Hong Kong was quantitatively determined. The average measured strong acidity in the spring, summer, autumn and winter of 2000, and the spring of 2001 was 49(23–92), 27(11–52), 97(71–145), 45(26–57), and 103(78–167) nmol/m3, respectively. Simulations using the aerosol inorganic model (AIM2) revealed that the in situ acidity of PM2.5 was only a minor fraction (23–39%) of the measured strong acidity because of the presence of bisulfate ions. The in situ acidity was correlated with a large difference between the ambient relative humidity and the estimated deliquescence relative humidity of the aerosols. In equivalent concentrations, acidity, ammonium and sulfate accounted for 16% (9–21%), 31% (26–39%) and 42% (40–45%) of the water-soluble inorganic species, respectively. Analysis of air mass trajectories showed that long-range transport of aerosols through the East/South China Sea (marine) and mainland China (continental) was possible. When Hong Kong was under the influence marine air masses, the measured concentrations of sulfate, ammonium and nitrate showed a strong correlation with the atmospheric mixing height. Continental long-range transport of aerosols was found to increase the concentrations of sulfate and ammonium by 49–383% and 33–302%, respectively. In total, about 40% of the sulfate and ammonium of PM2.5 in Hong Kong measured in this study was from continental air masses. However, the influence of the continental air masses on nitrate concentration and acidity was not obvious.

A theory based on the shallow water equation is developed to explain the generation of moving spiral-banded structures in tropical cyclones. The theory proposes that fluctuation of the vorticity distribution in the compact core region can act as a source, generating gravity waves that produce the banded structures. This mechanism is usually called the Lighthill radiation mechanism. A numerical experiment using the Fifth Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) code is used to test this theory. The moving bands in the simulated tropical cyclone show good agreement with the theoretically calculated gravity waves that emerge from a core region with an elliptical distribution of vorticity. Results of the simulated cyclone also illustrate that spiral cloud bands are triggered by the propagating gravity waves.

Aerosol samples collected from July 2007 to March 2008 were used to obtain major aerosol constituents in an urban location in the Pearl River Delta Region (PRD), China. The minimum organic carbon (OC)/elemental carbon (EC) ratio was used to calculate the primary and secondary organic carbon and the extinction effect of the secondary aerosol on visibility was estimated. As indicated in the analysis, the mass of secondary aerosol takes up 50% of the total mass of PM2.5; the OC/EC ratio is larger than 2 and there are significant characteristics of secondary aerosol generation; the levels of secondary OC are comparable with those of sulfate; and there is obvious enrichment of secondary aerosol on more polluted days. In a dry environment, the extinction weight is 59% for the secondary aerosol, while it is as high as 82% if the environment is highly humid (relative humidity [RH] = 95%). The hygroscopic growth of the aerosol can reduce visibility greatly; the secondary aerosol shares much larger quotas on more polluted days. For the Pearl River Delta (PRD), secondary aerosol and carbonaceous aerosol, especially secondary organic carbon (SOC), are a very acute problem; the study of the generating mechanism and sources for secondary aerosol is the key to the effort of controlling visibility in this region. The equation set forth in IMPROVE experiments can only be referenced but is not applicable to evaluate the extinction effect of individual aerosol components on visibility in the PRD region.

[1] A realistic reproduction of planetary boundary layer (PBL) structure and its evolution is critical to numerical simulation of regional meteorology and air quality. Conversely, insufficient realism in the simulated physical properties often leads to degraded meteorological and air quality prognostic skills. This study employed the Weather Research and Forecasting model (WRF) to evaluate model performance and to quantify meteorological prediction differences produced by four widely used PBL schemes. Evaluated were two nonlocal PBL schemes, YSU and ACM2, and two local PBL schemes, MYJ and Boulac. The model grid comprised four nested domains at horizontal resolutions of 27 km, 9 km, 3 km and 1 km respectively. Simulated surface variables 2 m temperature and 10 m wind at 1 km resolution were compared to measurements collected in Hong Kong. A detailed analysis of land-atmosphere energy balance explicates heat flux and temperature variability among the PBL schemes. Differences in vertical profiles of horizontal velocity, potential temperature, bulk Richardson number and water vapor mixing ratio were examined. Diagnosed PBL heights, estimated by scheme specific formulations, exhibited the large intrascheme variance. To eliminate formulation dependence in PBL height estimation, lidar measurements and a unified diagnosis were jointly used to reanalyze PBL heights. The diagnosis showed that local PBL schemes produced shallower PBL heights than those of nonlocal PBL schemes. It is reasonable to infer that WRF, coupled with the ACM2 PBL physics option can be a viable producer of meteorological forcing to regional air quality modeling in the Pearl River Delta (PRD) Region.

The impact of nitrous acid (HONO) chemistry on regional ozone and particulate matter in Pearl River Delta region was investigated using the community multiscale air quality (CMAQ) modeling system and the CB05 mechanism. Model simulations were conducted for a ten-day period in October 2004. Compared with available observed data, the model performance for NOx, SO2, PM10, and sulfate is reasonably good; however, predictions of HONO are an order of magnitude lower than observed data. The CB05 mechanism contains several homogenous reactions related to HONO. To improve the model performance for HONO, direct emissions, two heterogeneous reactions, and two surface photolysis reactions were incorporated into the model. The inclusion of the additional formation pathways significantly improved simulated HONO compared with observed data. The addition of HONO sources enhances daily maximum 8-hour ozone by up to 6 ppbV (8%) and daily mean PM2.5by up to 17 ug/m3(12%). They also affected ozone control strategy in Pearl River Delta region.

The impact of nitrous acid chemistry on regional ozone and particulate matter in Pearl River Delta region was investigated using the Community Mutilscale Air Quality modeling system and the CB05 mechanism. Model simulations were conducted for a ten-day period in October 2004. Compared with available observed data, the model performance for NOx, SO2, PM10, and sulfate is reasonably good; however, predictions of HONO are an order of magnitude lower than observed data. The CB05 mechanism contains several homogenous reactions related to nitrous acid. To improve the model performance for nitrous acid, direct emissions, two heterogeneous reactions, and two surface photolysis reactions were incorporated into the model. The inclusion of the additional formation pathways significantly improved simulated nitrous acid compared with observed data. The addition of nitrous acid sources enhance daily maximum 8-h ozone by up to 6 ppb V (8 %) and daily mean PM2.5 by up to 17 μg m−3 (12 %). They also affected ozone control strategy in Pearl River Delta region.

[1] This study presents Community Multiscale Air Quality (CMAQ) model results for Hong Kong (HK) and the Pearl River Delta region in January, April, July, and October 2004, representing winter, spring, summer, and autumn seasons. Two sets of observations/measurement data are referred to in order to evaluate model performance. The first set consists of hourly observations of criteria pollutants at HK Environmental Protection Department (HKEPD) ambient stations throughout HK, while the second set contains speciated PM measurements, sampled at 6-day intervals, at six of the HKEPD ambient stations. Model-observation agreement is good in PM species sulfate, organic carbon (OC), and elemental carbon (EC). The good performance for OC and EC, when compared to similar evaluation for North America and Europe, demonstrates the importance of correct speciation in an emission sector. Underprediction of NOx and NH3 suggests a desperate need of emission estimation improvement in these species. PM levels are higher in autumn and winter, lower in spring, and lowest in summer, coinciding with northeasterly winds due to continental outflow in autumn and winter, and with southwesterly monsoon in summer. The model is also able to reproduce the seasonal and spatial patterns for PM and O3. In terms of PM composition, the model agrees with the measured fractions of sulfate, OC, and EC. Additionally, CMAQ was rerun with emission sources partially removed, with results suggesting that pollution sources beyond the Pearl River Delta also contribute to the PM and sulfate levels in HK, particularly during the winter season.

[1] The major inorganic constituents and organic tracer compounds in PM2.5 were used in positive matrix factorization (PMF) and chemical mass balance (CMB) models to apportion the primary and secondary source contributions to organic carbon (OC) in Hong Kong during the summer of 2006. Secondary organic aerosol (SOA) tracers of several biogenic and anthropogenic hydrocarbons were included in the PMF analysis. Their inclusion allowed the identification of two components of SOA among seven factors resolved by PMF. One SOA component was mainly associated with secondary sulfate and nitrate. The other SOA component, characterized by biogenic SOA tracers and mixed with biomass burning and vegetative detritus particles, was biomass burning-induced SOA. Secondary OC (SOC) apportioned by PMF (SOCPMF) was on average 6.84 μgC m−3 (65% of PM2.5 OC) on high pollution days under influence of significant regional transport (i.e., regional days) and 0.70 μgC m−3 (25% of PM2.5 OC) on days under the influence of mainly local emissions (i.e., local days). The biomass burning-induced SOA accounted for 20% of the total SOA on the regional days, underlining the importance of biomass burning aerosol source in this region. The average uncertainty for the SOCPMF estimates was ∼20% on the regional days and ∼120% on the local days. SOCPMF was compared with SOC determined by CMB (SOCCMB, i.e., unapportioned OC by CMB analysis) and a tracer-based method (SOCTBM) that apportioned SOC contributions by four hydrocarbon precursors including isoprene, monoterpenes, β-caryophyllene, and toluene. The three estimates of SOC closely tracked with each other among individual samples. The SOCCMB and SOCPMF estimates on the majority of the regional days differed from each other by less than 25%. Good correlations between contributions of SOC and individual primary OC sources apportioned by PMF and CMB further added to the credence to the PMF-derived estimation of secondary and primary OC source contributions by using secondary and primary aerosol organic tracers as the fitting species.

Volatile organic compounds (VOCs) were measured at four stations with different environments in Hong Kong (HK) during two sampling campaigns. Positive matrix factorization was applied to characterize major VOC sources in HK. Nine sources were identified, and the spatial and seasonal variations of their contributions were derived. The most significant local VOC sources are vehicle and marine vessel exhausts or liquefied petroleum gas (LPG) at different stations. Vehicle- and marine vessel-related sources accounted for 2.9-12.7ppbv in 2002-2003 and increased to 4.3-15.2ppbv in 2006-2007. Different from the emission inventory, solvent-related sources only contributed 11- 19% at both sampling campaigns. Therefore, emission control from transport sector should be prioritized to alleviate ambient local VOC levels. Additionally, the contribution of aged VOC, which roughly represents contributions from regional and super-regional transport, also showed moderate increase during the four years, indicating cooperation with environmental authorities in the Pearl River Delta and beyond should be strengthened. All the anthropogenic sources contribute most to Yuen Long and least to Tap Mun. However, Tap Mun exhibited different trends in comparison with the other three stations, especially for sources of vehicle and marine vessel exhausts, LPG and paint solvents. When the local source contributions were incorporated with wind data to derive the directional dependences of sources, we may conclude that the rapid development of Yantian Container Terminal, the associated emissions from marine vessels around the Terminal and the on-site activities were likely responsible for the distinct VOC features at Tap Mun. The current impact from the Terminal is mainly concentrated in the northeastern corner of HK; however, it has the potential threat to other locations if the Terminal continues to expand in such a rapid speed in the coming years. More stringent VOC control measures on activities related to the operation of the Terminal is therefore highly recommended.

[1] The data of a 1-year (2003–2004) simulation with a finest horizontal resolution of 1.5 km, using the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), were analyzed to investigate the seasonal-mean features of the land-sea breeze (LSB) and regional circulation over the Pearl River Delta (PRD) region in southern China. The seasonal-mean diurnal variations reveal the general patterns of the LSB in the four seasons. These small-scale mean flow fields in the region have not been revealed in any previous studies. The results reveal a strong anomalous westerly sea breeze toward the eastern coast of the PRD in the early afternoon that is present in all the four seasons but is particularly strong in autumn and winter and may enhance the low-level convergence in Hong Kong. Furthermore, the condition of the atmosphere in autumn and winter is much more stable when compared with that in spring and summer, which is not favorable for the vertical dispersion of pollutants. The overall effect of these mean meteorological conditions may be an important factor for the generally higher air pollution index observed in Hong Kong during autumn and winter.

[1] This paper applies advance receptor model positive matrix factorization (PMF) source analysis to 1-h resolution VOC data collected at Yufa (rural site) and Peking University, or PKU (urban site), in Beijing. A range of major VOC sources was identified, including vehicle emissions, liquefied petroleum gas, coal combustion, and biogenic emissions. Vehicle activities contributed approximately 62% of VOC loading and 55% of ozone forming potential at the PKU site, compared to 38% of VOC loading and 42% of ozone forming potential at Yufa. These results indicate that the control of vehicle emissions is essential to alleviating VOC pollution, particularly in urban Beijing. We found that VOC emission strengths are relatively consistent throughout the day in the rural area; variation of mixing height therefore is a controlling factor for ground-level VOC concentration. In the urban area, both vehicle activity and variation of mixing height strongly impact VOC levels. Local sources within Beijing appeared to contribute most of the VOCs recorded at both urban and rural areas. However, as some of the VOC species are quite reactive, VOC emitted from distant sources would have been depleted during transportation, concurrent with the formation of secondary pollutants such as ozone and sulfate. Such depletion would mean that this source apportionment analysis would tend to overestimate the ozone forming capacity of local VOC sources compared to emissions from distant sources. Nevertheless, this study illustrates that high-resolution VOC measurements, especially those with a sampling frequency on the scale of less than 1 day, combined with PMF, can make a strong contribution to our understanding of pollutant emissions and transport characteristics and is a useful tool with which to formulate effective pollution control strategies.

The objectives of this study are to combine historical observations and state-of-the-art numerical models (MM5/CALMET system) to map the spatial distribution of wind resources in high resolution, and to help foster a deeper understanding of the wind power potential over southern China (Guangdong). Hourly wind fields were simulated for three entire years (2004-2006). It found that almost 70% of the time, the wind speed along the coast of Guangdong is over 5 m/s, which is deemed a baseline magnitude for typical wind turbines. Spatial plots of the wind speed and power and their variations over Guangdong Province for the three years are also presented.

[1] Aerosol vertical distribution is an important piece of information to improve aerosol retrieval from satellite remote sensing. Aerosol extinction coefficient profile and its integral form, aerosol optical depth (AOD), as well as atmospheric boundary layer (ABL) height and haze layer height can be derived using lidar measurements. In this paper, we used micropulse lidar measurements acquired from May 2003 to June 2004 to illustrate seasonal variations of AOD and ABL height in Hong Kong. On average, about 64% of monthly mean aerosol optical depths were contributed by aerosols within the mixing layer (with a maximum (∼76%) in November and a minimum (∼55%) in September) revealing the existence of large abundance of aerosols above ABL due to regional transport. The characteristics of seasonal averaged aerosol profiles over Hong Kong in the study period are presented to illustrate seasonal phenomena of aerosol transport and associated meteorological conditions. The correlation between AOD and surface extinction coefficient, as found, is generally poor (r2 ∼0.42) since elevated aerosol layers increase columnar aerosol abundance but not extinction at surface. The typical aerosol extinction profile in the ABL can be characterized by a low value near the surface and values increased with altitude reaching the top of ABL. When aerosol vertical profile is assumed, surface extinction coefficient can be derived from AOD using two algorithms, which are discussed in detail in this paper. Preliminary analysis showed that better estimates of the extinction coefficient at the ground level could be obtained using two-layer aerosol extinction profiles (r2 ∼0.78, slope ∼0.82, and intercept ∼0.15) than uniform profiles of extinction with height within the ABL (r2 ∼0.65, slope ∼0.27, and intercept ∼0.03). The improvement in correlation is promising on mapping satellite retrieved AOD to surface aerosol extinction coefficient for urban and regional environmental studies on air quality related issues.

[1] This study investigates the wind energy potential in Hong Kong, a region with a complex terrain, by coupling the prognostic MM5 mesoscale model with the CALMET diagnostic model to produce high-resolution wind fields. Hourly wind fields were simulated for the entire year of 2004. The MM5 simulations were performed on a nested grid from 40.5 km down to 1.5 km horizontal resolution. The CALMET meteorological model was used in a domain that includes the entire Hong Kong region with a high horizontal resolution of 100 m. The MM5 model wind field (1.5 km horizontal resolution) output was input into the CALMET diagnostic meteorological model every hour along with an objective analysis procedure using all available observations. Verification was achieved through two steps. In the first step, the data from three meteorological surface stations that were not assimilated into the CALMET model were compared horizontally with the simulated wind fields. In the second step, the simulated wind fields were compared vertically with the vertical wind profile collected from two upper air sounding stations. The results of this study identified the locations of the highest wind energy potential in HK down to 100 m resolution.

[1] Recent satellite observations show that a layer of haze perpetually hangs over the Pearl River Delta (PRD) region and surface observations show numerous violations of the Hong Kong Air Quality Objective. This layer of haze mostly concentrates in the Pearl River Estuary and a narrow (20 km wide) band along the shoreline, in particular during weak wind situations. Although researchers suspect the land-sea breeze (LSB) circulations “concentrate” or “trap” various pollutants in this region, the physical mechanism of the phenomenon has never been fully explained or quantified. In this paper, a mesoscale atmospheric model (MM5) coupled with the Noah land surface model (LSM), which has bulk urban land use treatments along with a detailed Pearl River Delta land use map, is used to investigate the unique feature and mechanism of this land-sea breeze effect and the temporal evolution. A three-dimensional particle trajectory model is used to understand its associated pollutant transport, trapping and accumulation. A conceptual model is then developed for the perpetual air pollution phenomenon in the region. Further sensitivity experiments are used to illustrate the impact of urbanization and large-scale winds on the pollution processes. Results show that urbanization enhances the pollutant trapping and therefore contributes to the overall poor air quality in the region.

Secondary organic carbon (SOC) is often a significant portion of organic carbon (OC) in ambient particulate matter (PM). The levels and seasonal patterns of SOC in Hong Kong were examined using more than 2000 PM10 measurements made over a 4.5-year period (1998–2002) in a network of ten air quality monitoring stations. The positive matrix factorization (PMF) model was used to analyze this large data set for source identification and apportioning. SOC was subsequently estimated to be the sum of OC present in the secondary sources, i.e., secondary sulfate, secondary nitrate, and secondary organic aerosol. The annual average SOC as estimated by the PMF method was 4.25 µg C/m3 while the summer average was 1.66 µg C/m3 and the winter average was 7.05 µg C/m3. In comparison, the method that uses EC as a tracer for primary carbonaceous aerosol sources to derive SOC overestimated SOC by 70–212% for the summer samples and by 4–43% for the winter samples. The overestimation by the EC tracer method resulted from the inability of obtaining a single OC/EC ratio that represented a mixture of primary sources varying in time and space. We found that SOC and secondary sulfate had synchronous seasonal variation and were correlated in individual seasons, suggesting common factors that control their formation. Considering the well-established fact that both gas phase oxidation and in-cloud processing are important formation pathways for sulfate, the synchronicity of SOC and sulfate suggests that in-cloud pathways are also important for SOC formation. Additionally, the presence of SOC was found to be enhanced more than that of secondary sulfate in the winter. We postulate this to be a combined result of favorable partitioning of semivolatile SOC species in the particle phase and more abundant SOC precursors in the winter.

The Asiatic monsoon is the dominant meteorological feature of the Pearl River Delta (PRD) region in China, and influences the accumulation of locally emitted pollutants, as well as regional transport. To investigate the effect of meteorological characteristics on PM2.5 mass and chemical composition in Hong Kong, a major city of the PRD, 24-h (0000–2400 LST) PM2.5 samples were collected every sixth day from 6 November 2000 to 26 October 2001. The sampling network included a roadside site at Mong Kok (MK), an urban site at Tsuen Wan (TW), and a rural site at Hok Tsui (HT). Air parcel back trajectory and residence time analyses indicate a predominantly northeasterly transport during winter and fall in contrast to a southwesterly-to-southeasterly transport in summer. The highest seasonal PM2.5 concentrations were found in winter, followed by fall, at each of the sampling sites due to elevated organic material (OM) and ammonium nitrate (NH4NO3), consistent with the increased air parcel residence time over potential source regions. Though local mobile sources appeared to dominate the emission of carbonaceous material, a higher organic carbon/elemental carbon (OC/EC) ratio and water-soluble potassium (K+) concentration in winter imply an additional contribution from vegetative burning. Episodic pollution events were identified and investigated with back trajectories and species enrichment factors. Local and regional sources both contributed to the formation of a pollution episode on 28 February 2001. These findings reflect the degree of PM2.5 pollution in Hong Kong and provide valuable insights for planning future monitoring and modeling studies in the PRD region.

[1] A severe air pollution episode occurred over much of the western part of Hong Kong (HK) between 28 and 30 December 1999. In this study, multiple analyses were made to understand the cause of this episode; first, the main emission source of the pollution episode was identified using remote sensing techniques from satellite pictures and was located about 100 km to the northeast of HK. Second, analyses were made on PM10 chemical species characterization in four air quality monitor stations (AQMSs), and a high concentration of potassium ions (K+) on the episode days indicates the significant impact of biomass burning. The ratio of elemental carbon and organic carbon values in two western AQMSs indicated that regional transport, rather than the local vehicle emission and power plant, played an important role in this episode. This result is in contrast with the early finding by H. Liu and J. C. L. Chan that this pollution episode is solely due to local sources. Third, to study the transport, diffusion, and trapping of pollutants during this air pollution episode, two numerical models were coupled in this study: a three-dimensional mesoscale meteorological model MM5 and a three-dimensional Lagrangian particle dispersion model. With the help of the Lagrangian dispersion model we show that this severe air pollution event was associated with transport of pollutants over HK within the boundary layer. These pollutants were subsequently brought down to the surface by the local land-sea breeze circulation and trapped over the western part of HK during the day by the same land-sea breeze.

[1] In this study the synoptic patterns conducive to the occurrence of O 3 episodes in Hong Kong are categorized by an inspection of the weather charts over the period of 1999-2003. The synoptic patterns associated with tropical cyclones originating in the North Pacific Ocean and the South China Sea are found to be the most optimal weather conditions for the occurrence of ozone episodes in Hong Kong. A high-resolution version of the regional, three-dimensional, multiscale photochemical air quality model (Pollutants in the Atmosphere and Their Transport in Hong Kong (PATH)), developed by the Hong Kong Environmental Protection Department in 2000, has been employed to investigate the evolution of one type of ozone episode related to tropical cyclones. A nonhydrostatic meteorological model (MM5) was applied with four-dimensional data assimilation to provide necessary meteorological fields to the air quality model. The performances of both the meteorological and chemical models are evaluated by comparing the simulated results with the available observed data. An integrated process rate analysis is used to examine the relative contributions of individual physical and chemical processes in the formation of ozone episodes for obtaining a better understanding of the mechanisms of photochemical smog events in Hong Kong. Results show that about 30% of the total ozone production is due to local chemical production in the lower atmosphere boundary layer, and about 70% is contributed by interregional transport from southern China into Hong Kong. In addition, four main processes, including horizontal advection, vertical transport, photochemical reactions, and deposition are found to have a significant influence on the ground-level concentration of ozone. The sensitivity experiments indicate that the chemical regime for ozone formation in Hong Kong seems to be limited by volatile organic compounds.

In the run-up to the 2008 Olympic Games in Beijing, Chinese government officials at both the central and municipal levels are keenly aware that they must transform Beijing into a world-class city. According to the Beijing Municipal Environmental Protection Bureau (BJEPB) to improve its air quality some actions are adopting, including taking steps to increase the forested area surrounding the city preventing dust storms, reducing the automotive vehicles, moving polluting factories now inside the fourth ring road ringing the inner city to locations outside of the fourth ring road, and switching the fuel of public buses and taxis from diesel to natural gas, etc. Will they eliminate most serious environmental problems in Beijing? MODIS aerosol products are helping us to answer this kind of questions. A long-term validation has been finished by sun-photometer observations, and the results proved the relative error of MODIS level 2 products was slightly larger than the estimation of Chu et al. (2002) from the results in most AERONET sites. However, the comparison between the products and moisture-corrected air pollution index (API) data, which were daily released to public by EPB, showed a high correlation coefficient. An air pollution episode in 2003 was investigated by the usage of satellite products. Our conclusion for the air pollution control strategy in Beijing is that only reducing the pollution sources from inner city can't fully solve the pollution problems in Beijing and the regional transports from the nearby southern provinces are contributing a lot to the pollution situation in Beijing.

Corrigendum Corrigendum to “Air ventilation impacts of the “wall effect” resulting from the alignment of high-rise buildings” [Atmos. Environ. 43 (2009) 4982e4994] S.H.L. Yim *, J.C.H. Fung , A.K.H. Lau , S.C. Kot c a Institute for the Environment, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Department of Mathematics, The Hong Kong University of Science and Technology, Hong Kong Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong

This paper describes a simple but practical methodology to identify the contribution of primary and secondary air pollutants from the local/regional emission sources to Hong Kong, a highly urbanized city with complex terrain and coastlines. The meteorological model MM5 coupled with a three-dimensional, mutli-particle trajectory model is used to identify salient aspects of regional air pollutant transport characteristics during some typical meteorological conditions over the Pearl River Delta (PRD) region. Several weighting factors are determined for calculating the air mass/pollutant trajectory and are used to evaluate the local and regional contribution of primary pollutants over the PRD to Hong Kong pollution. The relationships between emission inventories, physical paths and chemical transformation rates of the pollutants, and observational measurements are formulated. The local and regional contributions of secondary pollutants are obtained by this conceptual module under different weather scenarios. Our results demonstrate that major pollution sources over Hong Kong come from regional transport. In calm-weather situations, 78% of the respirable suspended particulates (RSP) totals in Hong Kong are contributed by regional transport, and 49% are contributed by the power plants within the PRD. In normal-day situations, 71% of the RSP are contributed by regional transport, and 45% are contributed by the power plants.