Major Components and Source Investigation in Suburban Hong Kong : A One Year Monitoring Study

PM2.5 filter sampling was conducted on a daily basis at the HKUST Air Quality Research Supersite (AQRS) for one year from March 2011 to February 2012. Approximately one fifth of the filter samples were subjected to full chemical analysis including major ions, elements, organic carbon (OC), elemental carbon (EC), and non-polar organic compounds (NPOCs). The major ions (sulfate, nitrate, and ammonium) were compared with those measured online by a MARGA system and the two sets of data were found in agreement within 25% or better. The major PM2.5 components (crustal materials, organic matter, soot, ammonium sulfate, ammonium nitrate, and non-crustal trace elements) accounted for 90% of the measured mass with sulfate being the most abundant (32.0%), followed by organic matter (23.5%) and ammonium (11.8%). The monthly variation patterns for different components suggested variable regional/super-regional sources, reflecting variation of transport contribution caused by shifts in synoptic weather conditions. Receptor modeling analysis by Positive Matrix Factorization revealed that secondary sulfate formation process (annual average of 31%), biomass burning (23%), and secondary nitrate formation process (13%) were the three dominant contributing sources to the observed PM2.5 at HKUST AQRS throughout the sampling year. The PM2.5 mass concentrations of all the individual sampling days were within the recently-proposed AQOs standards by the Hong Kong government (35 μg/m for annual average and 75 μg/m for 24-hr average) while approx. 52% of the sampling days were recorded with PM2.5 concentrations exceeding the WHO health 24-hr standards of 25 μg/m. Major composition and source analysis showed that the increased mass concentrations on high PM days were mainly caused by air pollutant transport from the outside-Hong Kong regions. Results from this study indicate the importance of regional/super-regional strategies such as reduction in SO2, NO2 (precursors for secondary inorganic aerosols) and restricting biomass burning for lowering PM2.5 in Hong Kong.


INTRODUCTION
Atmospheric aerosols are a highly variable component of the lower atmosphere and play important roles in environmental issues related to global and regional climate, air quality, visibility degradation, chemistry of the atmosphere, and human health effect.Aerosols affect the climate directly by altering the amount of solar radiation reaching the earth's surface, and indirectly by acting as cloud condensation nuclei or altering cloud particle size and number.Aerosols are also closely related to the visibility problems by scattering and absorption of solar radiation.Furthermore, portions of fine particles have the ability to penetrate deep into the human lungs and cause health problems (Novakov and Penner, 1993;Ramanathan et al., 2001;Kanakidou et al., 2005).
Constituents of atmospheric aerosols can be differentiated as inorganic and organic portions according to their chemical properties.The inorganic ionic species in the aerosol (mainly sulfate, nitrate, and ammonium) has been well studied.Other inorganic species include silicates and metal oxides, which come from soil dust and crustal materials.The organic part of the aerosol, on the other hand, covers a very wide range of organic species.The knowledge of the organic aerosols is far from being complete due to instrumental limitations as well as the wide diversity of the organic materials in their molecular structures, physical and chemical properties.
In the past two decades, it has been recognized that organic material is a major component of atmospheric aerosol, contributing 20-60% to the fine aerosol mass in urban environments (Saxena and Hildemann, 1996;Putaud et al., 2004;Zhang et al., 2007) and as high as 90% in rural and remote areas (Andreae and Crutzen, 1997;Roberts et al., 2001;Zhang et al., 2007).Although the molecular compositions of organic aerosols are not well understood, emerging evidence reveals that organic aerosols play important roles in health effect, direct and indirect aerosol forcing, acid-rain chemistry, and visibility impairment.
Aerosols can also be differentiated as primary or secondary according to their sources and formation pathways.Primary aerosols refer to those that are emitted directly in particulate form from sources and the major sources include incomplete fuel combustion, biomass burning, vehicle exhausts, residential cooking, and bioaerosols (e.g., bacteria, fungi, debris from the plant).Many volatile species emitted into the atmosphere undergo oxidation to yield products which have lower volatility and subsequently partition into particle phase.Such particles are considered as secondary aerosols.Identifying aerosol sources and quantifying the relative contributions of different emission sources and secondary formation processes is an essential initial step of air quality management.The Hong Kong Air Pollutants Emission Inventory for 2011 shows the emission sources for respirable particulate matter (PM 10 ) include navigation (37.1%), road transport (19.0%), public electricity generation (16.0%), non-combustion sources (e.g., quarrying, cooking fumes, dust, 15%), other fuel combustion (e.g., industrial, commercial and domestic applications, 12.0%) and civil aviation (0.93%) (HKEPD, 2013).The major emission sources for PM 2.5 in the PRD region include industrial sources (43.1%),on-road automobiles (36.3%) and power plants (18.7%).There are also some other sources such as navigation (1.8%), biomass burning (1.5%), residential fuel combustion (1.2%) and waste incineration (< 0.1%) (Zheng et al., 2009).While the emission inventory data provide information in identifying major industry sectors contributing to pollution, this information needs to be assessed by source apportionment based on ambient measurements.The latter also provides an estimate of contributions from secondary formation processes.This paper reports results of ambient measurements and subsequent source apportionment.
The ambient concentration and composition of PM 2.5 are highly dependent on multiple factors including emission sources, atmospheric chemistry, and meteorological conditions.An intensive sampling campaign was conducted at an air quality research supersite located in a suburban area of Hong Kong for one year.The supersite can be used as a background site since there are no heavily traveled roads or other major stationary sources within 2 km of the site.Consequently, most of the pollutants observed at this site are believed to be transported from either downtown Hong Kong or more distant urban areas outside the Hong Kong region, making this supersite a suitable place to study the influence of regional/super-regional transport of air pollutants on Hong Kong.The current study aims to characterize and assess the major chemical composition of the collected PM 2.5 samples during a one-year period, discern the underlying chemistry, and investigate the dominant contributing emissions sources.

Sampling
The HKUST Air Quality Research Supersite (AQRS) (http://envr.ust.hk/supersite/) is located in a clean suburban area with little residential and commercial development on the east coast of Sai Kung, Hong Kong (22.33°N,114.27°E).The supersite facility was built on the roof of the seawater pump house on the shorefront (approx.14 m above sea level) on the campus of the Hong Kong University of Science and Technology, facing Port Shelter and Sai Kung.Similar to the other parts of Hong Kong, the sampling site is mainly under the influence of southerly winds during summertime, north-easterlies during winter, while spring and autumn are transitional seasons in between (Hong Kong Observatory, 2013).The AQRS facility has been put in use since 2011 and the first Hong Kong Air Quality Supersite Study was launched in the spring of 2011.The Automatic Weather Station (AWS) conducts continuous measurements of meteorological parameters including wind speed/direction, temperature, relative humidity, sea level pressure, irradiance and rainfall.The PM 2.5 sampling using both high-volume and mid-volume samplers was conducted on a daily basis from March 1, 2011 to February 29, 2012.
High-volume PM 2.5 ambient air samplers (Tisch Environmental Inc., OH, USA) were used to collect 24-hr high-volume PM 2.5 samples on pre-baked 8 × 10' quartz fiber filters at a flow rate of 40 CFM (approx.1.13 m 3 /min).Speciation Air Sampling Systems (SASS, Met One Instrument, Inc., OR, USA) were used to collect 24-hr midvolume PM 2.5 samples at a flow rate of 6.7 LPM.Four channels of the sampler were used to capture the PM 2.5 , one on Teflon filter (47 mm, PALL Life Sciences, MI, USA), one on pre-cleaned Nylon filter (47 mm, PALL Life Sciences, MI, USA) preceded by a MgO-coated denuder, and two on pre-baked quartz fiber filters (47 mm, PALL Life Sciences, MI, USA).No back-up filters were used in this study hence the OC sampling artifacts were not evaluated.Considering the relatively lower levels of both volatile organic compounds (VOCs) and OC in the sampling area, the carbon concentration biases caused by the organic vapor adsorption are not expected to be significant.After the sample collection, all the 47-mm filters were stored in Petri dishes (lined with aluminum foil for quartz fiber filters) and sealed with parafilm.The 8 × 10 inch quartz fiber filters were folded in half and stored in aluminum foil.Filters were refrigerated under 4°C before chemical analysis.
The Teflon filter was used for gravimetric and elemental analysis, Nylon filter for ion analysis, and quartz fiber filter for the analysis of OC and EC.Filter punches from the high-volume samples were utilized for analysis of nonpolar organic compounds (NPOCs) by a thermal desorption-GC/MS technique.Field blanks were collected at the end of each month and were analyzed in parallel to the exposed filter samples as a part of QA/QC procedure (Chow and Watson, 1998).

Gravimetric and Chemical Analyses
PM 2.5 mass concentration was determined gravimetrically by weighing the Teflon filters before and after sampling using a digital balance (Sartorius AG, Model MC 5-0CE, Göttingen, Germany, capacity of 0.1 mg-5.1 g with a sensitivity of ± 1 µg) in the ENVF weighing facility at HKUST.Prior to weighing, filters were equilibrated in a controlled weighing chamber (24-hr mean temperature in the range of 20-23°C and controlled within ± 2°C, 24-hr mean RH in the range of 30-40% and controlled within ± 5%) for at least 24 hours before and after weighing.
Following the sampling schedule of the PM monitoring network in Hong Kong, one sixth of the collected daily samples were chosen for gravimetric and chemical analyses (major ions, OC, EC, elements and NPOCs).Since four intensive sampling campaigns took place in May, August, November 2011 and February 2012 as the first Hong Kong Air Quality Supersite Study, one third of the daily filter samples collected during these four months were analyzed in particular while other filters are archived.
Water soluble ions, Na + , NH 4 2-, and C 2 O 4 2-were extracted from the Nylon filters using double de-ionized water from the Ultrapure Water System (Nanopure Diamond UV/UF, Barnstead, USA).Chemical analyses of the water-soluble ions were carried out using an Ion Chromatograph (IC) (Dionex DX-500, Thermo Fisher Scientific, MA, USA).
OC and EC were analyzed using thermal/optical transmittance (TOT) method on an Aerosol OCEC analyzer (Sunset Laboratory, OR, USA).The analysis protocol follows the ACE-Asia protocol (Schauer et al., 2003), which is a variant of NIOSH protocol (Wu et al., 2012).
Measurements of elements from Sodium to Uranium were carried out on aerosol deposit on Teflon filters using an energy dispersive X-ray fluorescence (ED-XRF, Epsilon 5, PANalytical, The Netherlands).The accuracy of the XRF measurements have been determined in the ENVF lab at HKUST by comparing results with the certified values of the NIST SRM 2783 (Air Particulate on Filter Media).The measured values for 13 elements (Al, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Ti, and Si) are in agreement with the certified values within 12% and 4 elements (V, Zn, Rb, and S) within 25% in agreement with the certified or reference values.
Small portions of high-volume quartz fiber filter samples were used for NPOCs analysis by a thermal-desorption technique on GCMS.Identification was achieved by comparing the mass spectra and retention times of the chromatographic peaks with those of authentic standards.Quantification was based on peak area ratios of select fragment ions characteristic of individual NPOCs to those of internal standards (Ho et al., 2008).

Other Datasets
The HKUST AQRS facility is also equipped with several real-time instruments in addition to the filter-based PM samplers.The real-time instruments for PM measurements include: (1) a SHARP 5030 Monitor (Thermo Scientific Inc., MA, USA), which continuously measures the PM 2.5 mass concentrations; (2) a Model-4 Semi-Continuous OCEC Field Analyzer (RT-OCEC) (Sunset Laboratory, OR, USA), which measures hourly OC and EC concentrations; (3) a MARGA (Metrohm Applikon, The Netherlands), which measures the concentration of inorganic species in PM 2.5 and their related gas-phase components in the ambient air; and (4) an Aerosol Mass Spectrometer (AMS, Aerodyne Research Inc., MA, USA), which quantitatively measures the size and chemical mass loading in real-time for nonrefractory sub-micron aerosol particles.
Datasets obtained from the aforementioned SHARP monitor, RT-OCEC, and MARGA were used for comparisons with the results of filter-based measurements.These inter-comparisons among different independent datasets serve as part of the data validation processes and would provide invaluable information for the future studies on atmospheric chemistry and properties of gaseous pollutants and PM.

Data Validation
Three levels of data validation were performed on the dataset acquired from this study.Level I validation mainly includes flagging measurements for deviations from procedures and identifying invalid values.Level II validation examines the internal consistency among different data and attempts to resolve discrepancies based on known physical relationships between variables.This stage of data validation involves comparing a sum of measured chemical species to mass concentration, comparing relevant measurements from different methods, calculating ammonium and charge balances, reconstructing the mass concentration through major PM composition, and examining time series data to identify and investigate outliers.Level III validation is part of the data interpretation process, mainly focusing on identification of unusual values through parallel consistency tests with other independent datasets.In all cases, a Deming linear least-squares regression was used (Deming, 1943;Cornbleet and Gochman, 1979).Deming regression assumes that the uncertainties in both of the datasets are equal while conventional linear least-squares regression only considers the uncertainties in the dependent variable.
Fig. 1 shows the sum of chemical species plotting against the measured PM 2.5 mass on Teflon filters.The chemical species include those that were quantified on Teflon, Nylon, and quartz fiber filters.To avoid double counting, chloride (Cl -), water-soluble potassium (K + ) and sodium (Na + ), and sulfate (SO 4 2-) are included in the sum while total sulfur (S), chlorine (Cl), sodium (Na), potassium (K), oxalate (C 2 O 4 2-, counted in OC) are excluded.Carbon concentration is represented by the sum of organic carbon (OC) and elemental carbon (EC).Unmeasured ions, metal oxides, or hydrogen and oxygen associated with OC are not counted into the measured concentrations.Therefore, the sum of the individual chemical concentrations determined for PM 2.5 samples should be less than the corresponding mass concentrations obtained from gravimetric measurements.
A strong correlation (R 2 = 0.98) was found between the sum of measured species and the measured mass with a slope of 0.85.The average Y/X ratios indicate that around 80% of the PM 2.5 mass can be explained by the measured chemical species.
SO 4 2-and K + were measured by IC on Nylon filters while total S and total K were measured by XRF on Teflon filters.The ratio of SO 4 2-to S is expected to equal three if all of the sulfur is present as SO 4 2-.The ratio of K + to K is expected to equal or be less than 1.Fig. 2 shows the scatter plots of (a) SO 4 2-versus total S concentrations and (b) K + versus total K concentrations.A good correlation (R 2 = 0.96) was observed for SO 4 2-vs.total S with a slope of 2.75 and the average sulfate to total sulfur ratio was determined to be 2.45 ± 0.43, which meets the validation criteria (SO 4 2-/total S < 3.0).The potassium data comparison showed a bit more scatter (R 2 = 0.94), possibly caused by instrumental and method uncertainties.It is noted that water-soluble potassium was not detected in several samples collected during summer months (e.g., June to August) due to very low PM levels.Hence the ratio of water-soluble potassium to total potassium averages relatively low at 0.73.If these samples were excluded from the dataset, the average ratio of K + /total K would increase to 0.84 suggesting that most of the total potassium is in its soluble ionic form.
For further validation of the ion measurements, calculated versus measured ammonium (NH 4 + ) are compared.NH 4 + is directly measured by IC analysis of Nylon filter extract.NH 4 + is very often found in the chemical forms of NH 4 NO 3 , (NH 4 ) 2 SO 4 , and NH 4 HSO 4 in the PM 2.5 over Hong Kong (Qin et al., 1997;Ho et al., 2003;Louie et al., 2005a, b;Hagler et al., 2006;So et al., 2007).Assuming full neutralization, measured NH 4 + can be compared with the computed NH 4 + .Fig. 3 shows the scatter plot of calculated vs. measured NH 4 + and strong correlations (R 2 = 0.99) can be found for both forms of sulfate while the slopes were quite different (1.05 for ammonium sulfate and 0.62 for ammonium bisulfate).The average ratios of calculated ammonium to measured ammonium suggest that ammonium sulfate is the dominant form for ammonium in the PM 2.5 observed at HKUST AQRS in the year 2011.
The sum of Cl -, NO 3 -, SO 4 2-, and C 2 O 4 2-is compared to the sum of Na + , NH 4 + , K + , Mg 2+ , and Ca 2+ in µeq/m 3 so as to check the charge balance (Fig. 4).The correlation between the anion and cation equivalences was strong (R 2 = 0.99), with a slope of 0.98.The average Y/X ratio is slightly larger than unity since the calculation only accounted for major measured ions and there is a deficiency in cations due to the exclusion of [H + ].Comparisons were conducted between the dataset obtained from the filter samples in this study and those from three real-time instruments, i.e.PM 2.5 mass data from SHARP monitor, OC and EC concentrations from RT-OCEC analyzer, and ion data from MARGA (Fig. 5).The Deming linear regression results are also listed in Table 1.
It can be seen from the Deming regression results that good agreements were achieved between real-time and filter-based measurements for most of the species except for EC, Na + , and K + .The EC concentrations at the HKUST AQRS were generally low throughout the year (0.86 ± 0.53 µg/m 3 ).The very low EC loading, from not detected (< 0.06 µgC/cm 2 ) to 1.75 µgC/cm 2 on the filter in RT-ECOC, caused large uncertainties in instrumental analysis by RT-OCEC, which measured the carbon concentrations on an hourly basis.The more scattered Na + data were likely due to the easy contaminations of sodium in filter materials, during sample delivery and during chemical analysis.It is noted that K + measured by MARGA was only 60% of those from filter analysis together with a higher degree of scattering than NO 3 -, SO 4 2-and NH 4 + .The filter-based measurement results were further compared to the data obtained by the Hong Kong PM 2.5 monitoring network and fairly good slopes (0.69 for Na + , 1.10 for NH 4 + , 1.13 for K + , 1.18 for Cl -, 2.16 for NO 3 -, 0.99 for SO 4 2-, 0.93 for OC and 1.07 for EC, n = 47) and R 2 (0.92 for Na + , 0.96 for NH 4 + , 0.96 for K + , 0.69 for Cl -, 0.94 for NO 3 -, 0.99 for SO 4 2-, 0.98 for OC and 0.85 for EC) were found.This result lends credence to the dataset in this study and further investigation is necessary in detailed inspection of the MARGA measurements of K + .After the Level I and Level II data validations, the valid data percentage of this study is 98.8%.

Reconstruction of PM 2.5 Mass Concentration and the Temporal Variation
An overall view of the PM 2.5 major composition can be obtained by combining various measured species into seven components and looking at the percentage contribution of each component to the total fine PM mass.The seven components include 1) crustal material (fine soil); 2) organic matter, which can be estimated from OC concentration by using a multiplier of 1.4; 3) soot, which can be represented by the EC concentration; 4) ammonium sulfate; 5) ammonium nitrate; 6) non-crustal trace elements; and 7) unidentified material.In particular, the crustal component is estimated by summing the soil-associated elements in their oxide form (Al 2 O 3 , SiO 2 , CaO, K 2 O, FeO, Fe 2 O 3 , and TiO 2 ) (Eldred et al., 1987;Malm et al., 1994;Watson, 2002).The oxide masses were calculated by applying appropriate mass-conversion-factor (mcf) to the measured elements with the following three assumptions: 1) Fe is present as both FeO and Fe 2 O 3 and the two forms are equally abundant in molar concentrations; 2) K is also largely generated by nonsoil sources such as biomass burning.Hence, Fe is used as a surrogate for soil-related K by applying [K] = 0.6 [Fe]; and 3) a few minor elements as Na, Mg, C, and S typically contribute 15% to the sediment mass and thus ignored.Therefore, the formula used to calculate the crustal component mass is as below, (1) The difference between the reconstructed mass consisting of components 1-6 and the measured mass is referred to as unidentified mass.Considering the large uncertainty in Na measurement by XRF, water-soluble sodium is used in calculation instead of total sodium.Thus,  (2) The reconstructed mass is plotting against the measured mass in Fig. 6.A strong correlation (R 2 = 0.98) was observed between the reconstructed mass and measured mass and a slope (0.97) close to unity was obtained.The average Y/X ratio suggests that the reconstructed mass explains approx.90% of the measured PM 2.5 mass.One major uncertainty of the reconstructed mass is from the estimation of OM.An empirical value of 1.4 was used as the [OC]-to-[OM] mass conversion factor in this study and it is worth noting that this factor varies with organic aerosol composition.The [OM]/[OC] ratio of freshly emitted aerosols is expected to be smaller than that of the more aging (oxygenated) aerosols.
The monthly average composition of the PM 2.5 mass is shown in Fig. 7. Sulfate is the most abundant component in the PM 2.5 mass (32.0% in average, ranging from 24.3-41.1%)followed by OM (23.5% in average, ranging from 14.5-32.8%).Nitrate contributions exhibited a very clear seasonal variation with much higher percentages in winter months (approx.9.6%) than those in summertime (approx.4.1%).It is believed that a combination of factors should be responsible for the observed seasonal variation in nitrate concentrations, including the temperature dependence of aerosol nitrate formation (USEPA, 1999) and transport of nitrate from the outside-Hong Kong regions.The lower temperatures in winter months appear to be the driving factor for the higher abundance of nitrate and the elevated influence of transport contribution in winter over summer is consistent with the seasonal change in the prevailing wind directions.
The PM masses contributed by the crustal component were more significant during March and April, 2011.The mass contribution could be as high as 24% of the fine particle mass on individual sampling days during these two months.It is suggested that the elevated crustal-related PM mass was associated with the sandstorms taking place in the northern part of mainland China.
The unidentified mass for PM 2.5 samples at HKUST AQRS accounts for 0-4.9% (monthly average) of the measured mass.Overall, the reconstructed mass agrees with the measured mass within 10%.

PM 2.5 Source Apportionment
Positive Matrix Factorization (PMF) analysis was applied to the valid dataset obtained in this study in order to identify major sources to the measured PM mass and estimate their relative contributions in different seasons.
A total of 21 fitting species were selected including PM 2.5 mass, 8 water-soluble ions (Na + , NH 4 + , K + , Mg ), OC, EC, six elements (Al, Si, Ca, V, Fe, and Ni), two PAHs (indeno[1,2,3-cd]pyrene and benzo[ghi]perylene), and two lumped species characteristic of primary organic aerosol (POA) sources.The two lumped POA species were 1) iso+anteiso, the sum of iso and anteiso-alkanes with carbon number ranging from C 29 to C 33 , representing tracers for cigarette smoke (Rogge et al., 1994;Kavouras et al., 1998); and 2) H+S, the sum of five hopane compounds and five sterane compounds, representing tracers for vehicular exhaust (Schauer et al., 1999(Schauer et al., , 2002)).Several other elements (e.g., Ti, Mn and Cu) were not included since these species can be quantified (concentration > LOQ) less than 80% of the time.The uncertainties for individual species were initially estimated as (s ij + MDL ij /3) (Polissar et al., 1998;Reff et al., 2007), where MDL ij is the method detection limit and s ij is the analytical uncertainty of the corresponding species in the dataset.For data which are below the detection limits, the corresponding uncertainty value was set to ((5/6) × MDL ij ) (Polissar et al., 1998).
Source apportionment modeling was performed using the USEPA PMF 3.0 software (available at http://www.e pa.gov/heasd/products/pmf/pmf.html).This software provides the bootstrap model which is based on the Monta Carlo principle to check the mathematical stability of selected runs (Norris et al., 2008).Each modeling run included 20 base runs and the base run with the minimum Q value was retained as the solution.Solutions for five through ten factors were tested and the eight-factor solution was considered to be the reasonable one.The source profiles of the eight-factor solution are shown in Fig. 8.
The first factor is distinguished by high concentrations of SO 4 2-and NH 4 + , suggesting that it is a source associated with the formation of secondary sulfate aerosols.The OC mass apportioned in this factor can also be considered as secondary OC.
The second and seventh factors both have large amounts of Na + and Mg 2+ , which are signatures of sea salt.The difference between the two factors lies in the abundance of Cl -. "Chloride depletion" occurred during the aerosol aging processes, hence lower chloride level would be expected in aged sea salt particles.
The third factor is distinguished by high concentrations of Al, Si, Ca, and Fe and is suggested to be particles emitted from unpaved roads, construction sites, and windblown soil dust.This PMF-derived crustal soil mass is compared with the crustal component calculated in Section 3.2.The Deming linear regression analysis suggests a very good correlation between the two estimations (R 2 = 0.94) and the crustal soil mass by PMF is averagely 78.8% of that by the empirical formula.In addition to the uncertainties of both estimations, the discrepancy would also be caused by the exclusion of Ti in the PMF analysis.
The fourth factor has a high concentration of NO 3 -and is suggested to be associated with the formation of secondary nitrate aerosols.This factor includes a small amount of OC, which is also considered to be secondary OC.
The fifth factor identified by PMF is distinguished by high concentrations of Ni and V, which are well-known indicators for residual oil combustion (Kowalczyk et al., 1982;Chow and Watson, 2002).Hong Kong has one of the world's busiest container terminals located in Kwai Chung, meanwhile HKUST AQRS is on the shorefront facing the Port Shelter and Sai Kung.The vessel emissions could exert influence on the PM mass in the Hong Kong region.The biomass burning-related sources (the sixth factor) are represented by high concentration of the tracer compound K + .Biomass burning activities are limited in Hong Kong and hence the pollutants generated from these emission sources are postulated to be transported from the outside-Hong Kong regions.

Vehicular Exhaust
The eighth factor is characterized by large amount of H+S, iso/anteiso-alkanes, OC and EC, hence suggesting vehicular emission sources.The cigarette smoke source (with iso/ anteiso-alkanes as tracers) was unable to be separated from the vehicular exhausts, which is not surprising as smoking in indoor public places has been banned by the HKSAR government so that cigarette smoke activities nowadays are more commonly seen at roadside and the emitted species would be readily mixed with the emissions from automobiles.
The resolved source contributions by PMF are provided in Table 2. On an annual basis, the dominant sources contributing to the PM 2.5 levels observed at HKUST AQRS are secondary sulfate formation process (31.4%) and biomass burning (22.8%), followed by secondary nitrate formation process (13.4%).The contribution of vehicular exhaust is the least (3.4%) at this suburban site.
We note that a separate secondary OC factor is not resolved by the PMF due to a lack of secondary organic aerosol (SOA) tracers as input species in the PMF analysis.The OC apportioned into factors 1 (secondary sulfate formation process) and 4 (secondary nitrate formation process) is likely secondary, accounting for an annual average of 17.1% of the total OC.A certain amount of secondary OC would be in factors 6 (biomass burning) and 8 (vehicular exhaust).However, due to the lack of SOA tracers specific to vehicular emissions or biomass burning, it is not yet possible to derive the secondary OC portion in these two sources.This result highlights the necessity of more specific SOA marker measurements in order to determine the relative contributions of primary and secondary OC.
Month-to-month variations of the source contributions are shown in Fig. 9.The secondary sulfate formation process consistently contributed quite a lot during most of the time throughout the year except for June 2011, when the prevailing winds were southerly and local emission sources were dominant.This can be explained by the relatively low SO 2 concentrations in the Hong Kong region, meanwhile suggesting that the majority of the sulfate observed were transported from elsewhere.The sea salt sources (both fresh and aged) are important sources throughout the year at HKUST AQRS which is a coastal site.A significant portion contributed by fresh sea salt was found for June and July when the winds mainly came from the sea.On the other hand, the monthly variation of the contribution by aged sea salt was less evident.The sources of residual oil combustion and vehicular exhaust contributed the most during summer months (June to August) while the contribution by crustal dust was significant during March to June.Larger portion of PM 2.5 was due to the biomass burning-related sources during fall and winter months (~32%) than in the rest of the year, suggesting that these sources are often associated with air pollutants transported from the outside-Hong Kong regions.This result highlights the importance of taking into consideration of biomass burning when modeling regional PM 2.5 and formulating PM control strategies.
It is noted that little PM 2.5 mass in October 2011 and a significant more PM 2.5 mass in November (~2.1 µg/m 3 ) was apportioned to the vehicular exhaust factor.An examination of EC distribution among the PMF-resolved factors reveals that most of the EC in October was associated with factors 1 (secondary sulfate formation process) and 6 (biomass burning).Both of these factors are largely related to the non-local sources.The wind data of October and November (not shown) show that the prevailing winds were easterly in October and northwesterly in November.The northwesterly winds would bring in pollutants related to vehicular emissions from the downtown Hong Kong while the easterly winds were more likely associated with pollutant transport from outside-PRD regions.The change in wind direction provides a possible explanation for the significant difference in vehicular emission contribution in the two months.Another factor that could not be ignored is the inherent shortcoming in PMF modeling rooted in using organic tracers for apportioning sources.Hopanes and steranes (i.e., H+S) are used in PMF to identify and apportion contributions by the vehicular exhaust factor.The atmospheric degradation of organic tracers such as hopanes and steranes are known (Yu et al., 2012) but not taken into account in the current PMF model due to lack of knowledge in degradation kinetics and the age of air mass.This could lead to underestimate of the vehicular emission source (e.g., Subramanian et al, 2006) and the degree of underestimation would be more prominent for more aged air mass, which might be the case for October 2011.
The relative contributions of sources in local areas (operationally defined as within the Hong Kong region) and non-local regions were estimated based on the source category assignment used by Yuan et al. (2012).Sources including vehicular exhaust, residual oil combustion, fresh and aged sea salts are classified as local while secondary sulfate and nitrate formation processes, crustal dust and biomass burning are considered as non-local.Averagely, 34% of the pollutants observed at the HKUST AQRS were generated from the sources within Hong Kong region and 66% were due to the air pollutants transported from the adjacent PRD region or even more distant areas.The contributions from non-local sources maintained at relatively high levels (approx.76.6%) for 3 out of 4 seasons during the year except for summertime (42.6%).This seasonal variation is consistent with the prevailing wind directions which facilitate the pollutant transport during winter (N and NE) and transitional seasons (spring and fall).

Discussions on High and Low PM Days
The PM 2.5 standard recommended by the "Air Quality Guidelines" issued by the World Health Organization in 2005 is 25 µg/m 3 within the 24-hour period of time.By this criterion, 43 out of the 83 sampling days (approx.52%) in this study exceeded the WHO standard (World Health Organization, 2006).The Hong Kong government announced newly-proposed Air Quality Objectives (AQOs) in January, 2012 and the standards for PM 2.5 are an annual average of 35 µg/m 3 and a 24-hour average of 75 µg/m 3 .The PM 2.5 concentration averaged from the sampling days in this study which cover a one-year time period (27.9 µg/m 3 ) did not violate the proposed AQOs annual mean value while none of the individual sampling days recorded the 24-hr average PM 2.5 level exceeding the AQOs standard,  suggesting that the air quality meets the Hong Kong's proposed AQOs for most of the time in the suburban area of Hong Kong.In spite of this, there were several sampling days on which relatively high PM 2.5 concentrations were observed.The average and standard deviation of the PM 2.5 concentrations were calculated from all the individual sampling days.Samples with the daily PM 2.5 concentrations higher than the average value plus one standard deviation (44.0 µg/m 3 ) are defined as high PM days (denoted by "high" hereinafter) while those with daily PM 2.5 concentrations lower than the average value minus one standard deviation (11.7 µg/m 3 ) are referred to low PM days (denoted by "low").
There are 16 high PM days and 18 low PM days by these criteria for PM levels.Fig. 10(a) demonstrates the average reconstructed mass for the high and low PM days defined as above.On high PM days, the increased mass consists of all the major components.This is because that the HKUST AQRS is located in an area where significant sources in the immediate neighborhood of a few kilometers are relatively limited so that majority of the pollutants were very likely transported from distant urban areas.The PMF-derived source contributions were also averaged for these two scenarios (Fig. 10(b)) and it can be seen that local sources of residual oil combustion, fresh sea salt, and vehicular exhaust were more dominant (75.6%) on low PM days.On the other hand, biomass burning, secondary sulfate and nitrate formation processes made significant contributions (85.7%) on high PM days, suggesting that the increased PM mass should be associated with transport of air pollutants from the regions outside Hong Kong.Results from this study indicate the necessity of regional/superregional strategies for reducing PM 2.5 in Hong Kong.

CONCLUSIONS
A PM 2.5 sampling campaign was conducted at the HKUST Air Quality Research Supersite from March 2011 to February 2012.24-hr PM 2.5 filter samples were collected using both high-volume and mid-volume samplers.Samples on a total of 83 sampling days were subjected to chemical analyses including major ions, organic carbon, elemental carbon, elements, and non-polar organic compounds.Three levels of data validation procedures were performed to obtain the valid dataset.

(b)
The sum of crustal materials, organic matter, soluble sodium, ammonium, potassium, sulfate, nitrate, and noncrustal trace elements could explain approx.90% of the measured mass.Overall, sulfate is the most abundant component in the PM 2.5 mass, followed by organic matter and ammonium.As a suburban sampling site with little commercial and residential development, most of the pollutants observed at HKUST AQRS were transported from other areas.Different monthly variation patterns were observed for different components, reflecting variable regional/super-regional sources as a result of variation in synoptic weather conditions.The contribution by crustal materials was greater in March and April than in other months, which is believed to be associated with the dust storms originating from Northern China.Nitrate made more significant contributions in winter months due to its temperature-dependent formation or/and regional/superregional transport of the air pollutants.
PMF analysis was conducted on the valid dataset and an eight-factor solution was adopted.The relative contributions by various sources to the PM 2.5 mass were estimated on a monthly, seasonal, and annual basis.In general, secondary sulfate formation process, biomass burning, and secondary nitrate formation process have been the dominant contributing sources throughout the sampling year.The monthly and seasonal variations of the source contributions suggest that sources including secondary sulfate and nitrate formation processes, biomass burning and crustal dust would mainly come from regions outside Hong Kong (non-local) while fresh sea salt, residual oil combustion, and vehicle exhaust are more likely emission sources within the local Hong Kong region.On an annual basis, non-local sources accounted for 66% of the PM 2.5 concentrations observed at the HKUST AQRS while local sources accounted for the other 34%.
Of the 83 individual sampling days, 43 daily samples were with PM 2.5 mass concentrations exceeding the WHO health standards (25 µg/m 3 ) and none were found exceeding the AQOs standard recently proposed by the Hong Kong government (75 µg/m 3 ).The major chemical composition and the contributing sources were examined for the high and low PM days.The increasing masses of all the major components on high PM days again confirmed that the major portion of the pollutants observed at HKUST AQRS was transported from elsewhere.PMF analysis showed that sources including biomass burning, secondary sulfate and nitrate formation processes, which are all associated with regional/super-regional air pollutant transport from non-local areas, contributed significantly to the PM 2.5 mass on high PM days.

Fig. 1 .
Fig. 1.Scatter plot of sum of measured chemical species versus measured mass on Teflon filter for PM 2.5 sample collected at HKUST AQRS during Mar.2011-Feb.2012.

Fig. 5 .
Fig. 5. Scatter plots of real-time measurements versus filter-based chemical data for PM 2.5 samples collected at HKUST AQRS during Mar.2011-Feb.2012.The x-axis represents the species concentrations obtained from filter samples in the unit of µg/m 3 .The y-axis represents the 24-hr average species concentrations from real-time measurements (SHARP monitor, RT-OCEC, and MARGA) in the unit of µg/m 3 .

Fig. 6 .
Fig. 6.Scatter plot of reconstructed mass versus measured mass on Teflon filters for PM 2.5 samples collected at HKUST AQRS during Mar.2011-Feb.2012.

Fig. 10 .
Fig. 10.(a) Average reconstructed mass and (b) relative source contributions for the high and low PM days at HKUST AQRS during Mar.2011-Feb.2012.

Table 1 .
Results of Deming linear least squares regression of online measurements versus filter-based chemical data including the slope (a), the intercept (b), and the coefficient of determination (R 2 ).

Table 2 .
The relative contributions (%) of different emission sources calculated by month, season, and annually from PMF analysis.