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Emissions of fine particulate matter (PM 2.5) from on-road traffic and their influence on air quality and human health are of major concern in urban areas. Exposure to traffic-related PM 2.5 indoors has received considerable attention as people spend about 80% of their time in indoor environments, but little information is currently available on the assessment and mitigation of this exposure. A systematic field study was conducted with the key objective to assess and mitigate indoor human exposure to traffic-related PM 2.5 in a typical naturally ventilated residential apartment. Results indicated that traffic-related PM 2.5 levels indoors exceeded the air quality guidelines (12 μg/m3), and the PM 2.5 levels decreased significantly (74%) while using an indoor air cleaner. The human health risk assessment based on the bioavailable fraction of toxic trace elements revealed a substantial reduction in potential health risk while using the air cleaner. Overall, the major outcomes of this study would help develop effective air pollution control strategies to reduce indoor human exposure to PM 2.5 and potential human health risk caused by vehicular pollution in urban areas. Image 1 • Assessment and mitigation of traffic-related indoor PM 2.5 was studied. • The lowest levels of PM 2.5 indoors were observed while using the air cleaner. • Potential carcinogenic health risk was reduced while using the air cleaner. • Keeping windows closed only provided partial protection to PM 2.5 exposure indoors. • More effective mitigation of exposure to PM 2.5 indoors is needed in Singapore. [ABSTRACT FROM AUTHOR]

Abstract: Bacterial pathogens in airborne particulate matter (PM) and in rainwater (RW) were detected using a robust and sensitive Real-Time PCR method. Both RW and PM were collected simultaneously in the tropical atmosphere of Singapore, which were then subjected to analysis for the presence of selected bacterial pathogens and potential pathogen of health concern (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Aeromonas hydrophila). These pathogens were found to be prevalent in both PM and RW samples with E. coli being the most prevalent potential pathogen in both types of samples. The temporal distribution of these pathogens in PM and RW was found to be similar to each other. Using the proposed microbiological technique, the atmospheric deposition (dry and wet deposition) of bacterial pathogens to lakes and reservoirs can be studied in view of growing concerns about the outbreak of waterborne diseases. [ABSTRACT FROM AUTHOR]

Abstract: Atmospheric nutrients have recently gained increased attention as significant additional sources of new nitrogen (N) and phosphorus (P) loading to aquatic ecosystems. The levels of nutrients (NO3 −–N, NH4 +–N, ON, DIP and OP) in aerosols and rainwater were measured and their respective deposition fluxes were estimated in the Marina catchment area, Singapore, from April 2007 to March 2008. During the study period, the mean atmospheric dry particulate fluxes (mgm−2 month−1) were 21.64±13.56, 2.95±1.73, 24.21±9.59, 4.48±1.74 and 7.19±2.83 for NO3 −–N, NH4 +–N, ON, DIP and OP, respectively. In addition, the mean wet deposition fluxes (mgm−2 month−1) were 59.60±45.29, 30.39±12.99, 39.03±20.70, 1.79±1.03 and 2.46±1.29 for NO3 −–N, NH4 +–N, ON, DIP and OP, respectively. Wet deposition made a larger contribution to total fluxes than dry deposition for N species especially NO3 − and NH4 +, but P species showed the opposite trend. Both TN and TP showed different seasonal variations with peak wet deposition fluxes in Dec 2007 and Jan 2008 as compared to dry deposition fluxes which peaked during the inter-monsoon (IM) period. Other than the local emissions, air mass backward trajectory analysis indicated that the atmospheric nutrient concentration may be affected by transboundary transport of nutrients from regional sources during different monsoon seasons. Statistical correlation analysis showed that the deposition fluxes were associated with meteorological factors. In addition, the N:P ratio in annual total (dry particulate+wet) atmospheric deposition flux in this study was 10.5, implying that the aquatic ecosystems in Singapore, fed principally by atmospheric nutrients, may tend toward N-limitation. [Copyright &y& Elsevier]

Abstract: The levels of a range of SVOCs (PAHs, OCPs, and PCBs) in atmospheric particulate and gaseous phases and rainwater samples were studied in Singapore from June 2007 to May 2008. Yearly average concentrations of PAHs, OCPs and PCBs in air (particle+gas) were 136.5ngm−3, 695.8pgm−3 and 27pgm−3, respectively while those of PAHs and OCPs in rainwater (particulate+dissolved) were 1218.1 and 114.2ngL−1, respectively. The lower molecular weight (LMW) PAHs were more abundant than those higher molecular weight (HMW) PAHs in rainwater. Among OCPs, HCHs were found in rainwater in relatively larger amounts than DDTs. Similar distributions patterns were also found in air samples. Statistical correlation analysis indicated that the atmospheric occurrence of SVOCs had positive correlation with ambient temperature except for particulate PAHs and negative correlations with relative humidity and wind speed; incoming solar radiation (SR) was negatively associated with PAHs, but there was almost no correlation between SR and the occurrence of OCPs and PCBs. ANOVA analysis revealed that the temporal variations were statistically significant for PAHs and PCBs in air samples and for OCPs in rainwater. In addition, local traffic emissions, trans-boundary transport of pesticides from regional sources and Aroclors 1248 and 1254 were identified as probable sources for PAHs, OCPs and PCBs in the atmosphere, respectively, based on molecular diagnostic ratios and principal component analysis (PCA) together with the assistance of air mass backward trajectories. [Copyright &y& Elsevier]

An accelerated solvent extraction (ASE) method has been developed for the determination of polycyclic aromatic hydrocarbons (PAHs) present in both atmospheric particulate and gaseous phases in this study. Extraction parameters such as the combination of solvents, extraction temperature, and static extraction time were investigated and optimized. Effective extraction was achieved using a 3:1 mixture of n-hexane and acetone as extraction solvents at 100°C in 30 min for all the compounds studied. The optimized extraction method was compared with conventional extraction methods and validated using National Institute of Standards and Technology (NIST)-certified standard reference material (SRM) 1649a. The recoveries obtained for certified 12 PAHs were in the range of 82-126% with relative standard deviation (RSD) between 6 and 28%. The validated ASE technique was used followed by gas chromatography-mass spectrometry (GC-MS) for the determination of PAHs distributed between gaseous and particulate phases in the atmosphere of Singapore. Total average concentrations of PAHs in air samples were 33.54 ± 19.32 ng m-3, with 4.72 ± 2.80 ng m-3 in particulate phase and 28.82 ± 16.92 ng m-3 in gaseous phase, respectively. The results obtained from this study are compared to those reported from other areas of the world. [ABSTRACT FROM AUTHOR]

Accelerated solvent extraction (ASE) has been applied to the quantitative extraction of organochlorine compounds (OCs), including organochlorine pesticides (HCHs, DDXs) and polychlorinated biphenyls (PCBs) present in both atmospheric particulate and gaseous phase. Extraction parameters such as the combination of solvents, extraction temperature, and static extraction time were investigated and optimized. Effective extraction can be carried out using a 3:1 mixture of n-hexane and acetone as extraction solvents at 100°C in 20min for all the compounds studied. The entire analytical procedure developed in this study proves to be reliable as evident from the analysis of specific surrogate standards with the mean recoveries per sample being greater than 82%. The optimized method was validated using NIST-certified SRM 1649a. Semi-volatile OCs, in the atmosphere of Singapore were quantified using the optimized ASE method together with GC–MS. Total average concentrations of ΣHCHs, ΣDDXs, and ΣPCBs in air samples were 244.9±88.5pgm−3, 7.7±4.1pgm−3, and 34.1±19.7pgm−3, respectively. The distribution of these compounds between the gas and particulate phase is discussed. Possible sources of atmospheric OCs are evaluated based on the molecular ratio of specific compounds and backward air trajectory analysis. [Copyright &y& Elsevier]

As part of a major study to investigate the indoor air quality in residential houses in Singapore, intensive aerosol measurements were made in an apartment in a multistory building for several consecutive days in 2004. The purpose of this work was to identify the major indoor sources of fine airborne particles and to assess their impact on indoor air quality for a typical residential home in an urban area in a densely populated country. Particle number and mass concentrations were measured in three rooms of the home using a real-time particle counter and a low-volume particulate sampler, respectively. Particle number concentrations were found to be elevated on several occasions during the measurements. All of the events of elevated particle concentrations were linked to indoor activities based on house occupant log entries. This enabled identification of the indoor sources that contributed to indoor particle concentrations. Activities such as cooking elevated particle number concentrations ≤2.05 x 105 particles/cm³. The fine particles collected on Teflon filter substrates were analyzed for selected ions, trace elements, and metals, as well as elemental and organic carbon (OC) contents. To compare the quality of air between the indoors of the home and the outdoors, measurements were also made outside the home to obtain outdoor samples. The chemical composition of both outdoor and indoor particles was determined. Indoor/outdoor (I/O) ratios suggest that certain chemical constituents of indoor particles, such as chloride, sodium, aluminum, cobalt, copper, iron, manganese, titanium, vanadium, zinc, and elemental carbon, were derived through migration of outdoor particles (I/O <1 or ≈ 1), whereas the levels of others, such as nitrite, nitrate, sulfate, ammonium, cadmium, chromium, nickel, lead, and OC, were largely influenced by the presence of indoor sources (I/O > 1). [ABSTRACT FROM AUTHOR]

• Personal exposure to PM 2.5 and BC in Danang(Vietnam) and Singapore was assessed. • Exposure to PM 2.5 and BC by commuters was much higher in Danang than Singapore. • Exposure to PM 2.5 and BC in homes should also be considered. • Heavy-duty vehicles and motorcycles are the major contributors to PM in both cities. • Health effects of motorized transport can be avoided by switching to active mobility. The present study investigated the personal exposure (PE) of commuters to fine particles (PM 2.5) and black carbon (BC) in two cities: Singapore and Danang (Vietnam) with different socio-economic conditions. The PE measurement was conducted during a multimodal round trip from Singapore to Danang and back with various on-road transport modes in urban micro-environments within each city by using portable devices. Real-time assessment of PE revealed that more than 60% of the integrated inhaled dose of PM 2.5 by commuters to occurred at airport concourses and transit micro-environments to/from the aeroplane by apron bus. Whereas for BC, the transit by apron bus made a major contribution (> 50%) to the total integrated exposure. The PE to PM 2.5 and BC in transport micro-environments was an order of magnitude higher in Danang compared to Singapore while using various on-road modes of transport in each city. Elevated concentrations of airborne particles in Singapore and Danang were significantly contributed by heavy-duty diesel vehicles and motorcycles, respectively. Moreover, a reduction in years of life expectancy is likely to occur among urban commuters while using motorized transport compared to active mobility (cycling). [ABSTRACT FROM AUTHOR]

Light absorbing carbonaceous aerosols (LACA) consisting of black carbon (BC) and brown carbon (BrC) have received considerable attention because of their climate and health implications, but their sources, characteristics and fates remain unclear in Southeast Asia (SEA). In this study, we investigated spatio-temporal characteristics of LACA, their radiative properties and potential sources in Singapore under different weather conditions. Hourly BC concentrations, measured from May 2017 to March 2018, ranged from 0.31 μg/m3 to 14.37 μg/m3 with the mean value being 2.44 ± 1.51 μg/m3. High mass concentrations of BC were observed during the south-west monsoon (SWM, 2.60 ± 1.56 μg/m3) while relatively low mass concentrations were recorded during the north-east monsoon (NEM, 1.68 ± 0.96 μg/m3). There was a shift in the Absorption Ångström exponent (AAE) from 1.1 to 1.4 when the origin of LACA changed from fossil fuel (FF) to biomass burning (BB) combustion. This shift is attributed to the presence of secondary BrC in LACA, derived from transboundary BB emissions during the SWM. Lower AAE values were observed when local traffic emissions were dominant during the NEM. This explanation is supported by measurements of water-soluble organic carbon (WSOC) in LACA and the corresponding AAE values determined at 365 nm using a UV–vis spectrophotometer. The AAE values, indicative of the presence of brown carbon (BrC), showed that photochemically aged LACA contribute to an enhancement in the light absorption of aerosols. In addition, spatio-temporal characteristics of BC in the intra-urban environment of Singapore were investigated across diverse outdoor and indoor microenvironments. High variability of BC was evident across these microenvironments. Several air pollution hotspots with elevated BC concentrations were identified. Overall, the results stress a need to control anthropogenic emissions of BC and BrC in order to mitigate near-term climate change impacts and provide health benefits. Image 1 • Black carbon showed diurnal and seasonal variations due to changes in its sources. • Presence of both primary and secondary brown carbon was observed. • Light absorption by brown carbon is linked with the composition of organics. • Contributions from biomass burning to BC and BrC dominated during the dry season. • Key sources of BC in Singapore were construction sites, temples and bus stops. This paper provides insights into the concentration levels, diurnal variations, sources and radiative properties of carbonaceous aerosols over an urban environment in Southeast Asia. [ABSTRACT FROM AUTHOR]

Abstract: Submicron particle number concentration (PNC) and particle size distribution (PSD) in the size range of 5.6–560 nm were investigated in Singapore from 27 June 2009 through 6 September 2009. Slightly hazy conditions lasted in Singapore from 6 to 10 August. Backward air trajectories indicated that the haze was due to the transport of biomass burning impacted air masses originating from wild forest and peat fires in Sumatra, Indonesia. Three distinct peaks in the morning (08:00–10:00), afternoon (13:00–15:00) and evening (16:00–20:00) were observed on a typical normal day. However, during the haze period no distinct morning and afternoon peaks were observed and the PNC (39,775 ± 3741 cm−3) increased by 1.5 times when compared to that during non-haze periods (26,462 ± 6017). The morning and afternoon peaks on the normal day were associated with the local rush hour traffic while the afternoon peak was induced by new particle formation (NPF). Diurnal profiles of PNCs and PSDs showed that primary particle peak diameters were large during the haze (60 nm) period when compared to that during the non-haze period (45.3 nm). NPF events observed in the afternoon period on normal days were suppressed during the haze periods due to heavy particle loading in atmosphere caused by biomass burning impacted air masses. [Copyright &y& Elsevier]

Abstract: Street dust serves as an important archive for environmental contamination in industrialized countries. Heavy metals which are found in street dust such as Cd, Cu, Ni, Pb, and Zn cause pollution in different environmental media. This study was carried out to characterize the composition of different elements embedded in street dusts and to investigate their leaching behavior in the presence of different aqueous media. Samples of street dusts were collected on a weekly basis for 6 months from three different locations in Singapore, viz. residential, commercial and industrial areas, and processed in the laboratory to determine the concentration of 13 elements (Al, As, Co, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Ti, V, and Zn). Concentrations showed considerable variations between sites, and within the same site over a period of time. Dust samples collected from the industrial area were of serious concern as they comprise elevated concentrations of most of the potentially toxic metals such as Cd, Cr, Cu, Pb, and Zn. The surface morphology and presence of different elements in street dust samples were confirmed using SEM/EDX analysis. The enrichment factor, used to describe the chemical characteristics of street dusts, revealed that most of the elements have anthropogenic origin. Of the different media used in dissolution of elements from street dusts, the river water and acidified deionized (DI) water (0.01M HNO3) were found to promote significant leaching of most of the elements. With the aid of dissolution kinetic data, the rate constants of dissolution of various elements were determined. [Copyright &y& Elsevier]

Black carbon (BC), an important indicator of traffic-related air pollution (TRAP) in urban environments, is receiving increased attention because of its adverse health effects. Personal exposure (PE) of adults to BC has been widely studied, but little is known about the exposure of young children (toddlers) to BC in cities. We carried out a pilot study to investigate the integrated daily PE of toddlers to BC in a city-state with a high population density (Singapore). We studied the impact of urban traffic on the PE of toddlers to BC by comparing and contrasting on-road traffic flow (i.e., volume and composition) in Singapore in 2019 (before the COVID-19 pandemic) and in 2020 (during the COVID-19 pandemic). Our observations indicate that the daily BC exposure levels and inhaled doses increased by about 25% in 2020 (2.9 ± 0.3 μg m−3 and 35.5 μg day−1) compared to that in 2019 (2.3 ± 0.4 μg m−3 and 28.5 μg day−1 for exposure concentration and inhaled dose, respectively). The increased BC levels were associated with the increased traffic volume on both weekdays and weekends in 2020 compared to the same time period in 2019. Specifically, we observed an increase in the number of trucks as well as cars/taxis and motorcycles (private transport) and a decline in the number of buses (public transport) in 2020. The implementation of lockdown measures in 2020 resulted in significant changes in the time, place and duration of PE of toddlers to BC. The recorded daily time-activity patterns indicated that toddlers spent almost all the time in indoor environments during the measurement period in 2020. When we compared different ventilation options (natural ventilation (NV), air conditioning (AC), and portable air cleaner (PAC)) for mitigation of PE to BC in the home environment, we found a significant decrease (>30%) in daily BC exposure levels while using the PAC compared to the NV scenario. Our case study shows that the PE of toddlers to BC is of health concern in indoor environments in 2020 because of the migration of the increased TRAP into naturally ventilated residential homes and more time spent indoors than outdoors. Since toddlers' immune system is weak, technological intervention is necessary to protect their health against inhalation exposure to air pollutants. [Display omitted] • Our case study shows daily toddler's exposure to BC is higher in 2020 than in 2019. • An increase in the number of private transports was observed. • We also found a decline in the number of public transport. • The portable air cleaner is most effective at reducing indoor BC. [ABSTRACT FROM AUTHOR]

• Elevated indoor levels of biomass burning derived-PM were observed in Singapore. • Air cleaner is most effective at reducing indoor PM with minimal energy consumption. • The operational cost of IAQ control while using air cleaner is low. • Thermal comfort provided by air cleaner is within the acceptable range. • Health risk associated with inhalation exposure is reduced following PM removal. During the 2019 smoke haze episode in Singapore, elevated levels of fine particulate matter (PM 2.5) were observed, deteriorating both ambient and indoor air quality (IAQ). We investigated the mitigation of indoor human exposure to PM 2.5 of outdoor origin under diverse exposure scenarios with and without filtration of PM 2.5 during both hazy and non-hazy days. The key objective of our study was to make a comparative evaluation of the effectiveness of portable air cleaners (PACs) and air conditioning (AC) systems equipped with particle filters in improving IAQ and to assess related long-term carcinogenic and non-carcinogenic health risks. We conducted real-time measurements of PM 2.5 , black carbon mass concentrations and particle number concentrations in both indoor and outdoor areas, quantified the relative concentrations of the water-soluble fraction of toxic trace elements in PM 2.5 for health risk assessment, and estimated the levels of thermal comfort. In addition, we calculated the total estimated cost of indoor air pollution control. Our findings suggest that indoor air cleaners are more effective at mitigating human exposure to airborne particles and reducing health risk with less consumption of electricity and better cost-effectiveness compared to AC. This information would be beneficial for public health interventions during major air pollution events. [ABSTRACT FROM AUTHOR]

As part of the 7 SouthEast Asian Studies (7SEAS) program, a solar radiation and chemistry sampling site (“supersite”) was developed at the National University of Singapore (NUS) to monitor regional air quality. The first intensive operations period for this site occurred between August and October 2009, a period that coincided with a moderate El Nino event and enhanced tropical burning, particularly in peatlands. We use data from this period to analyze the transport of biomass burning emissions in the Maritime Continent (MC) to the NUS supersite. An overview of the aerosol environment is provided for Singapore, followed by more detailed discussion of four aerosol events. The 2009 burning season was similar to those described in previous analyses, which showed that fire activity begins in the western half of the MC in Sumatra and propagates eastward in time. Similarly, agricultural burning occurs first, generally followed by deforestation and peatland fires. Some of the biomass burning emissions make their way into the free troposphere, where they are transported regionally by the prevailing wind patterns. Our analyses show that the seasonal winds at 850 hPa (∼1500 m) shift transport patterns from source regions to the southwest of Singapore, to regions to the southeast over the course of the summer monsoon, patterns that allow Singapore to be impacted by peak burning regions in the MC. In contrast, winds at the surface are more typically from the south and southeast, demonstrating the prevalence of vertical wind shear over the region. As a result of the variable source regions influencing different levels of the atmosphere over Singapore, in-situ surface observations of aerosol mass concentrations are not always consistent with inferences of the presence of enhanced aerosol concentration from column optical depth. Our findings confirm the complexity of aerosol sources and transport over the MC, and the key role that biomass burning emissions play in influencing column aerosol optical depth and total particulate mass concentrations at the surface. The sea-level altitude of the NUS supersite means that non-local pollution transported above the boundary layer cannot be reliably sampled and characterized, but the combined effects of local emissions and downward-mixed, non-local pollutants in Singapore were consistently measured. [ABSTRACT FROM AUTHOR]

• Influence of air quality on bikeability index was evaluated. • The proposed air quality sub-index involves cyclists' exposure to PM 2.5 and BC. • Open-source data, land-use regression models, and deep neural network were utilized. • Cyclists' exposure to TRAP is a significant component of the bikeability index. • The proposed framework is useful for recommending cycling routes in cities. Previous studies on bikeability/cycling index have explored factors that influence cycling in cities, and developed indicators to characterize a bicycle-friendly environment. However, despite its strong influence on cycling behavior, cyclists' exposure to traffic-related air pollution has been often disregarded. To close this knowledge gap, we propose a comprehensive bikeability index that comprises four sub-indices: accessibility, suitability, perceptibility, and prevailing air quality in the vicinity of cycling routes. We evaluate cyclists' exposure to fine particulate matter and black carbon, and used open-source data, land-use regression models, deep neural networks and spatial analysis. The application of the proposed bikeability framework reveals that the inclusion of air quality makes a significant difference when calculating bikeability index in Singapore and hence it merits serious consideration. We believe that the newly developed framework will convince city planners to consider the importance of assessing cyclists' exposure to airborne particles when planning cycling infrastructure. [ABSTRACT FROM AUTHOR]

The characterization of aerosol acidity has received increased attention in recent years due to its influence on atmospheric visibility, climate change and human health. Distribution of water soluble inorganic (WSI) ions in 12 different size fractions of aerosols was investigated under two different atmospheric conditions (smoke-haze and non-haze periods) in 2012 using the Micro-Orifice Uniform Deposit Impactor (MOUDI) and nano-MOUDI for the first time in Singapore. To estimate the in situ acidity ([H(+)]Ins) and in situ aerosol pH (pHIS), the Aerosol Inorganic Model version-IV under deliquescent mode of airborne particles was used at prevailing ambient temperature and relative humidity. The study revealed an increase in the levels of airborne particulate matter (PM) mass and concentrations of WSI ions for all size fractions during the smoke-haze period, which was caused by the trans-boundary transport of biomass burning-impacted air masses from Indonesia. A bimodal distribution was observed for concentrations of SO4(2-), NO3(-), Cl(-), K(+) and Na(+), whereas concentrations of NH4(+), Ca(2+) and Mg(2+) showed a single mode distribution. The concentration of WSI ions in PM1.8 during the smoke-haze period increased by 3.8 (for SO4(2-)) to 10.5 (for K(+)) times more than those observed during the non-haze period. The pHIS were observed to be lower during the smoke-haze period than that during the non-haze period for all size fractions of PM, indicating that atmospheric aerosols were more acidic due to the influence of biomass burning emissions. The particles in the accumulation mode were more acidic than those in the coarse mode.

Recurrent smoke-haze episodes that occur in Southeast Asia (SEA) are of much concern because of their environmental and health impacts. These haze episodes are mainly caused by uncontrolled biomass and peat burning in Indonesia. Airborne particulate matter (PM) samples were collected in the southwest coast of Singapore from 16 August to 9 November in 2009 to assess the impact of smoke-haze episodes on the air quality due to the long-range transport of biomass and peat burning emissions. The physical and chemical characteristics of PM were investigated during pre-haze, smoke-haze, and post-haze periods. Days with PM2.5 mass concentrations of ≥35 μg m(-3) were considered as smoke-haze events. Using this criterion, out of the total 82 sampling days, nine smoke-haze events were identified. The origin of air masses during smoke-haze episodes was studied on the basis of HYSPLIT backward air trajectory analysis for 4 days. In terms of the physical properties of PM, higher particle surface area concentrations and particle gravimetric mass concentrations were observed during the smoke-haze period, but there was no consistent pattern for particle number concentrations during the haze period as compared to the non-haze period except that there was a significant increase at about 08:00, which could be attributed to the entrainment of PM from aloft after the breakdown of the nocturnal inversion layer. As for the chemical characteristics of PM, among the six key inorganic water-soluble ions (Cl(-), NO3(-), nss-SO4(2-), Na(+), NH4(+), and nss-K(+)) measured in this study, NO3(-), nss-SO4(2-), and NH4(+) showed a significant increase in their concentrations during the smoke-haze period together with nss-K(+). These observations suggest that the increased atmospheric loading of PM with higher surface area and increased concentrations of optically active secondary inorganic aerosols [(NH4)2SO4 or NH4HSO4 and NH4NO3] resulted in the atmospheric visibility reduction in SEA due to the advection of biomass and peat burning emissions.

The impact of rainwater on the microbial quality of a tropical freshwater reservoir through atmospheric wet deposition of microorganisms was studied for the first time. Reservoir water samples were collected at four different sampling points and rainwater samples were collected in the immediate vicinity of the reservoir sites for a period of four months (January to April, 2012) during the Northeast monsoon period. Microbial quality of all fresh rainwater and reservoir water samples was assessed based on the counts for the microbial indicators: Escherichia coli (E. coli), total coliforms, and Enterococci along with total heterotrophic plate counts (HPC). The taxonomic richness and phylogenetic relationship of the freshwater reservoir with those of the fresh rainwater were also assessed using 16 S rRNA gene clone library construction. The levels of E. coli were found to be in the range of 0 CFU/100 mL-75 CFU/100 mL for the rainwater, and were 10-94 CFU/100 mL for the reservoir water. The sampling sites that were influenced by highway traffic emissions showed the maximum counts for all the bacterial indicators assessed. There was no significant increase in the bacterial abundances observed in the reservoir water immediately following rainfall. However, the composite fresh rainwater and reservoir water samples exhibited broad phylogenetic diversity, including sequences representing Betaproteobacteria, Alphaproteobacteria, Gammaproteobacteria, Actinobacteria, Lentisphaerae and Bacteriodetes. Members of the Betaproteobacteria group were the most dominant in both fresh rainwater and reservoir water, followed by Alphaproteobacteria, Sphingobacteria, Actinobacteria and Gammaproteobacteria.

Graphene oxide (GO) was employed in the present study for removal of two commonly occurring algal toxins, microcystin-LR (MC-LR) and microcystin-RR (MC-RR), from water. The adsorption performance of GO was compared to that of commercially available activated carbon. Further, adsorption experiments were conducted in the presence of other environmental pollutants to understand the matrix effects of contaminated water on the selective adsorption of MC-LR and MC-RR onto GO. The environmental pollutants addressed in this study included different anions (nitrate NO3-, nitrite NO2-, sulphate SO4(2-), chloride (Cl(-)), phosphate PO4(3-) and fluoride (F(-))) and cations (sodium (Na(+)), potassium (K(+)), magnesium (Mg(2+)) and calcium (Ca(2+))). GO showed very a high adsorption capacity of 1700 μg/g for removal of MC-LR and 1878 μg/g for MC-RR while the maximum adsorption capacity obtained with the commercial activated carbon was 1481.7 μg/g and 1034.1 μg/g for MC-LR and MC-RR, respectively. The sorption kinetic experiments revealed that more than 90% removal of both MC-LR/RR was achieved within 5 min for all the doses studied (500, 700 and 900 μg/L). GO could be reused as an adsorbent following ten cycles of adsorption/desorption with no significant loss in its adsorption capacity., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Aerosol acidity is one of the most important parameters that can influence atmospheric visibility, climate change and human health. Based on continuous field measurements of inorganic aerosol species and their thermodynamic modeling on a time resolution of 1h, this study has investigated the acidic properties of PM2.5 and their relation with the formation of secondary inorganic aerosols (SIA). The study was conducted by taking into account the prevailing ambient temperature (T) and relative humidity (RH) in a tropical urban atmosphere. The in-situ aerosol pH (pH(IS)) on a 12h basis ranged from -0.20 to 1.46 during daytime with an average value of 0.48 and 0.23 to 1.53 during nighttime with an average value of 0.72. These diurnal variations suggest that the daytime aerosol was more acidic than that caused by the nighttime aerosol. The hourly values of pH(IS) showed a reverse trend as compared to that of in-situ aerosol acidity ([H(+)]Ins). The pH(IS) had its maximum values at 3:00 and at 20:00 and its minimum during 11:00 to 12:00. Correlation analyses revealed that the molar concentration ratio of ammonium to sulfate (R(N/S)), equivalent concentration ratio of cations to anions (RC/A), T and RH can be used as independent variables for prediction of pH(IS). A multi-linear regression model consisting of RN/S, RC/A, T and RH was developed to estimate aerosol pH(IS)., (Copyright © 2013 Elsevier B.V. All rights reserved.)

An improved ion chromatographic (IC) method has been developed for the separation of nitrate in filter extracts in the presence of high concentrations of nitrite. This analytical method was successfully used for an indirect measurement of ozone (O3) in outdoor and indoor air, following its collection using a nitrite-impregnated passive sampler. The limit of detection and the limit of quantification, using the modified IC method, were 6 microg l(-1) (3sigma) and 20 microg l(-1) (10sigma), respectively. Improved detection limits and low baseline noise were obtained with the use of eluent generator and high-capacity ion exchange column. The optimized method was used for assessing O3 concentration in both indoor and outdoor environments of 28 child care centers (CCCs) located in different parts of Singapore. The O3 concentrations ranged from 0.1 to 11.95 parts per billion (ppb) in indoor and from 3.2 to 21.7 ppb in outdoor environments during the study period. It was found that, among the CCCs investigated in this study, air-conditioned CCCs and those located in close proximity to traffic emissions had significantly lower O3 concentrations indoors.

Toxicological studies have implicated trace metals adsorbed onto airborne particles as possible contributors to respiratory and/or cardiovascular inflammation. In particular, the water-soluble metal content is considered to be a harmful component of airborne particulate matter. In this work, the trace metal characteristics of airborne particulate matter, PM2.5, collected in Singapore from February to March 2005 were investigated with specific reference to their bioavailability. PM2.5 mass concentrations varied between 20.9 mug/m3 and 46.3 microg/m3 with an average mass of 32.8 microg/m3. During the sampling period, there were several bushfires in Singapore that contributed to sporadic increases in the particulate air pollution, accompanied by an acrid smell and asthma-related allergies. The aerosol samples were subjected to analysis of trace elements for determining their total concentrations as well as their water soluble fractions. Our results showed an increase in concentration of several water-soluble trace metals during bushfires compared to their urban background levels in Singapore. In order to measure the human exposure to particulate air pollution, the daily respiratory uptake (DRU) of several trace metals was calculated and compared between haze and nonhaze periods. The DRU values were significantly higher for several metals, including Zn, Cu, and Fe, during bushfires. Electron paramagnetic resonance (EPR) measurements showed that the particulate samples collected during bush fires generate more toxic hydroxyl radicals (OH.) than those in the background air, due to the presence of more soluble iron ions.