Chak Keung Chan

Application of positive matrix factorization in source apportionment of particulate pollutants in Hong Kong

Abstract An advanced algorithm called positive matrix factorization (PMF) in receptor modeling was used to identify the sources of respirable suspended particulates (RSP) in Hong Kong. The compositional data obtained from the Hong Kong Environmental Protection Department from 1992 to 1994 were analyzed. The species analyzed in this study are Al, Ca, Mg, Pb, Na+, V, Cl−, NH4+, SO42−, Br−, Mn, Fe, Ni, Zn, Cd, K+, Ba, Cu, and As. Unlike the conventional receptor modeling algorithm, factor analysis PMF only generates non-negative source profiles. To eliminate sulfate from such factors where it is not physically plausible, special penalty terms were included in the model so that sulfate concentrations could be selectively decreased in specified factors. A 9-factor model containing non-zero sulfate concentrations in three factors gives the most satisfactory source profiles. Ammonium sulfate, chloride depleted marine aerosols and crustal aerosols are the three non-zero sulfate sources. Other factors are marine aerosols, non-ferrous smelters, particulate copper, fuel oil burning, vehicular emission and bromide/road dust. The last two sources can be combined as a single source of vehicle/road dust. The compositional profiles of these factors were also developed. The mass profiles obtained can be improved by further refinement of distribution of sulfate in the sources.

Characterization of chemical species in PM2.5 and PM10 aerosols in Hong Kong

Aerosol samples for PM10 and PM2.5 were collected in wintertime from November 2000 to February 2001 at three different sampling locations in Hong Kong. PM10 and PM2.5 were collected by high-volume (hi-vol.) samplers and the concentrations of major elements, ions, organic and elemental carbons were quantified. The ratios of PM2.5/PM10 were 0.61 and 0.78 at the PolyU campus and Kwun Tong (KT), respectively. These results indicated that the concentrations of PM2.5 contribute the majority of the PM10 fraction. The concentrations of anthropogenic species (e.g. Pb and Cu) in PM10 and PM2.5measured at urban areas were generally higher than at an urban background site (Hok Tsui, HT). The major fractions of sulfate at three monitoring sites are non-sea-salts (nss) sulfates. Although HT is located in coastal areas, the contribution of sea salts to sulfate in fine particles was small, indicating a substantial anthropogenic origin. The OC/EC ratios were less than 2 in PolyU and KT monitoring stations for PM10 and PM2.5. However, the OC/EC ratios were over 3 at HT for both PM10 and PM2.5. This indicates the presence of secondary organic aerosols. Correlations between OC and EC as well as OC and sulfate in HT during both seasons were used to prove that atmospheric transport and transformation of anthropogenic organic species from northeastern area was the dominant source in winter. The chemical composition of the samples was reconstructed from the observed elemental composition. The contribution of the seven components, namely crustal matter, sea salt, ammonium, sulfate, nitrate, elemental carbon and organic matter accounted for 77–84% of the PM10 and PM2.5 mass in the urban area (PolyU and KT) and 74% for Hok Tsui (HT). Sulfate, organic matter and elemental carbon are the major constituents in particles especially in PM2.5 collected at PolyU and KT. The major constituents of PM10 in HT are sea salt and sulfate.

Size distributions of particulate sulfate, nitrate, and ammonium at a coastal site in Hong Kong

Size distributions of particulate sulfate, ammonium, and nitrate were measured at a coastal site in Hong Kong over 22 days using a MOUDI cascade impactor. Sulfate, ammonium, and nitrate were trimodally distributed in the size range of 0.052–10 μm. Sulfate and ammonium were the dominant species in fine particles (Dp<1.8 μm), co-existing in the condensation (0.20 μm) mode and droplet (0.57 μm) modes. The droplet mode dominated the distributions of these two ions. Higher concentrations of droplet mode sulfate were associated with high relative humidity and low clouds. Transport of aerosols from southern China by strong northerly wind also increased the sulfate concentration in Hong Kong. Mode diameters of coarse sulfate and ammonium were about 4–6 μm. Nitrate was mainly distributed in the coarse particles with mode diameter of 3.95±0.69 μm. Coarse mode nitrate was formed by the reaction of gas-phase nitric acid with sea-salt or soil particles. Little fine mode nitrate was found.

The effect of calcination on the microstructural characteristics and photoreactivity of Degussa P-25 TiO2

Changes in the microstructural characteristics of Degussa P-25 titania as a result of calcination have been studied using XRD, BET and TEM. The photocatalytic activities of the samples were also examined using the degradation of phenol as a model reaction. The results indicate firstly that calcination significantly affects both microstructural characteristics and photoactivity and secondly that there is an apparent relationship between photocatalytic activity and certain microstructural characteristics. Over the range of calcination temperatures and durations studied, the sample calcined at 923 K for 3 hours revealed the highest photoreactivity, which can be ascribed to an improvement in crystallinity on calcination. The increase in the rutile content and grain growth caused by the calcination at higher temperatures were observed to decrease the photocatalytic activities of the TiO2 samples.

The water cycles of water-soluble organic salts of atmospheric importance

Abstract In this study, the water cycles of nine water-soluble organic salts of atmospheric interest were studied using an electrodynamic balance (EDB) at 25°C. Sodium formate, sodium acetate, sodium succinate, sodium pyruvate and sodium methanesulfonate (Na-MSA) particles crystallize as the relative humidity (RH) decreases and they deliquesce as the RH increases. Sodium oxalate and ammonium oxalate form supersaturated particles at low RH before crystallization but they do not deliquesce even at RH=90%. Sodium malonate and sodium maleate particles neither crystallize nor deliquesce. These two salts absorb and evaporate water reversibly without hysteresis. In most cases, the solid states of single particles resulting from the crystallization of supersaturated droplets do not form the most thermodynamically stable state found in bulk studies. Sodium formate, sodium oxalate, ammonium oxalate, sodium succinate, sodium pyruvate and Na-MSA form anhydrous particles after crystallization. Sodium acetate forms particles with a water/salt molar ratio of 0.5 after crystallization. In salts with several hydrated states including sodium formate and sodium acetate, the particles deliquesce at the lowest deliquescence relative humidity (DRH) of the hydrates. Except sodium oxalate and ammonium oxalate, all the salts studied here are as hygroscopic as typical inorganic hygroscopic aerosols. The hygroscopic organic salts have a growth factor of 1.76–2.18 from RH=10–90%, comparable to that of typical hygroscopic inorganic salts such as NaCl and (NH4)2SO4. Further study of other atmospheric water-soluble organic compounds and their mixtures with inorganic salts is needed to explain the field observations of the hygroscopic growth of ambient aerosols.

Characteristics of chemical compositions of atmospheric aerosols in Hong Kong: spatial and seasonal distributions

Chemical compositional data of respirable suspended particulate (RSP) measured at 11 stations of the Hong Kong Air Quality Monitoring Network between 1990 and 1994 were analyzed for their seasonal and spatial variations. The concentration of atmospheric aerosols was low in Hong Kong compared to some large cities located in East Asia. RSP and most of the chemical species show seasonal variations which reflect the weather conditions: low concentrations in the rainy season of summer and high concentrations for the rest of the year. Carbonaceous aerosols accounted for approximately half of the mass and dominated the seasonal and spatial variations of RSP concentrations. SO42−, NH4+ and NO3−, accounting for a quarter of RSP mass, were mainly from long distance transport as their spatial distributions throughout Hong Kong were quite even. Marine aerosols, found throughout Hong Kong, accounted for 5.7% of RSP mass. Their seasonal variations in both the marine aerosols and the SO42−, NH4+ and NO3− aerosols were controlled by the East Asian monsoons. The equivalent concentration ratio of Cl−Na+ indicated the presence of the chloride loss reaction and had a minimum ratio in September. Ca, K+, Al and Mn from moving road dusts and construction and reclamation works follow the same seasonal distributions as RSP. Their spatial distributions reflect the presence of a large reclamation project on western Kowloon. The percent mass abundance of C, Zn, Pb, Br− and Ni observed in the roadside station to monitoring vehicle emission pollution were similar to the values in Phoenix, US MW5050 PM2.5 vehicle source profile. Cu was predominantly found in industrial and mixed areas where many small scale printed circuit board factories and ship repairing industries were located.

Performance of a membrane-catalyst for photocatalytic oxidation of volatile organic compounds

The performance of a hybrid Sil-1/nanostructured anatase TiO2 membrane-catalyst and a catalytic titanium silicalite-1 (TS-1) membrane was evaluated for gas-phase photocatalytic oxidation (PCO) of trichloroethylene. The membrane-catalyst outperformed the catalytic plate coated with a similar amount of nanostructured TiO2 catalyst. However, the activity of the catalytic TS-membrane for PCO is low due to the insufficient number of active titanium sites in the TS-1 zeolite.

Water activities of NH4NO3/(NH4)2SO4 solutions

Water activities for mixed ammonium nitrate/ammonium sulfate solutions at relative humidities of 0.35–0.75 were measured using a spherical void electrodynamic balance. The concentrations of singly levitated droplets of nitrate to sulfate mole ratio of n = 1/5, 1/3, 1/2, 1, 2 and 4 in equilibrium with an ambient environment of prescribed relative humidity were measured. To avoid uncertainty in determining the composition of the solid particles, solution properties were determined relative to the known properties at about 80% relative humidity. The concentration of this reference state was estimated by three models of mixed electrolyte solutions, the Zdanovskii-Stokes-Robinson (ZSR), the Kusik and Meissner (KM) and the Pitzer models. The measured total mass fraction of solute of the mixed solutions differed by less than 0.5% when different models were used to calculate the reference state concentration. The water activity data were obtained at ionic strengths as high as 108 molal and used to evaluate predictions from these three models. For n = 1/5 and 1/3, deviations of model predictions from experimental data are within 2%. Generally, predictions of the ZSR model are most consistent with our data. Maximum deviations occur at n=2; 6% for ZSR, 8% for KM and 5% for Pitzer. The deviations can be attributed to binary and ternary solute-solute interactions that the ZSR, KM and elementary version of the Pitzer models do not consider. However, no simple characterization of the interaction parameters is possible; they seem to be strong functions of the fractional ionic strength of the solute and the total ionic strength of the solutions.

Solar photocatalytic thin film cascade reactor for treatment of benzoic acid containing wastewater.

A solar photocatalytic cascade reactor was constructed to study the photocatalytic oxidation of benzoic acid in water under various experimental and weather conditions at HKUST. Nine stainless steel plates coated with TiO(2) catalyst were arranged in a cascade configuration in the reactor. Photolytic degradation and adsorption were confirmed to be insignificant total organic carbon (TOC) removal mechanisms. A turbulent flow pattern and, hence, improved mixing in the liquid film were achieved due to the unique cascade design of the reactor. The photoinduced consumption of oxygen during reactions was demonstrated in a sample experiment. The proposed rate equations provided good fits to 90 data points from 17 experiments. The regression results showed that the TOC removal rates averaged over 30 min intervals did not illustrate significant dependence on TOC(0) and that I(mean) was more important in affecting the photocatalytic process within the ranges of the data examined. The percentage removal of TOC in 7 l of 100 mg/l (or 100 ppm) benzoic acid solutions increased from 30% to 83% by adding 10 ml of hydrogen peroxide solution (30 wt%). Hydrogen peroxide was also shown to enhance the efficiency of the degradation process at elevated temperatures. Ortho-, meta- and para-hydroxybenzoic acids were identified by HPLC analysis as the intermediates of benzoic acid during reactions without the addition of hydrogen peroxide solutions.

The Water Activities of MgCl2, Mg(NO3)2, MgSO4, and Their Mixtures

The water activities of aqueous MgCl 2, Mg(NO3)2, and MgSO solutions and their mixtures - MgCl 2-Mg(NO3)2 (mole ratio 3:1, 1:1, 1:3), MgCl 2-MgSO4 (1:1), Mg(NO3)2-MgSO4 (1:1), and MgCl 2-Mg(NO3)2-MgSO4 (1:1:1 and 1:1:5) - were measured at room temperature from dilute solution to high supersaturation using an electrodynamic balance. The experimental results are compared with the predictions of the Zdanovskii-Stokes-Robinson (ZSR) and the Kusik and Meissner (KM) equations. Both the ZSR and the KM equations can predict the water activities of these mixtures well with an acceptable error of less than +/- 0.02. The ZSR equation is superior to the KM equation as it can give predictions for a larger range of aw where single component bulk data are available. It is also slightly more accurate than the KM equation for this system. The average standard deviations of the difference between the data measured and the predictions of the ZSR and KM equations are 0.004 and 0.01, respectively. The ZSR equation is recommen...