Volatile organic sulfur compounds in a stratified lake

Three volatile organic sulfur compounds (VOSCs), dimethyl sulfide (DMS), carbon disulfide (CS(2)), and dimethyl disulfide (DMDS), were detected in the stratified water column of a lake (Linsley Pond) in Connecticut. The compounds DMS and DMDS appeared in both the oxic and the anoxic portions of the water column, CS(2) was primarily found in anoxic hypolimnion. Algal metabolism and/or bacterial degradation of sulfur-containing amino acids or other organic materials are potential sources of VOSCs in the oxic lake water. Reactions of hydrogen sulfide with organic compounds and microbial degradation of organic matter may be responsible for the production of VOSCs in the anoxic lake water. The vertical distribution patterns of these three VOSCs varied from month to month in the summer, but the daily profiles obtained in one 5-day period in the summer displayed consistency. No clear diurnal pattern for any of the three VOSCs was observed. Based on observation that these VOSCs were not present in surface and near surface waters of Linsley Pond, freshwater inputs of reduced sulfur compounds to the atmosphere may be insignificant.     

Environmental VOSCs--formation and degradation of dimethyl sulfide, methanethiol and related materials

Volatile organic sulfur compounds (VOSCs) play a major role in the global sulfur cycle. Two components, dimethyl sulfide (DMS) and methanethiol (MT) are formed in large amounts by living systems (e.g. algae, bacteria, plants), particularly in marine environments. A major route to DMS is by action of a lyase enzyme on dimethylsulfoniopropionate (DMSP). DMSP has other roles, for instance as an osmoprotectant and cryoprotectant. Demethiolation of DMSP and other materials leads to MT. A major transport process is release of DMS from the oceans to the atmosphere. Oxidation of DMS in the atmosphere by hydroxyl and nitrate radicals produces many degradation products including CO2, COS, dimethyl sulfoxide, dimethyl sulfone, organic oxyacids of sulfur, and sulfate. These materials also have roles in biotic processes and there are complex metabolic interrelationships between some of them. This review emphasizes the chemical reactions of the organic sulfur cycle. For biotic reactions, details of relevant enzymes are provided when possible.     

Emission of volatile organic sulfur compounds (VOSCs) during aerobic decomposition of food wastes

Food wastes collected from typical urban residential communities were investigated for the emission of volatile organic sulfur compounds (VOSCs) during laboratory-controlled aerobic decomposition in an incubator for a period of 41 days. Emission of VOSCs from the food wastes totaled 409.9 mg kg^?1 (dry weight), and dimethyl disulfide (DMDS), dimethyl sulfide (DMS), methyl 2-propenyl disulfide, carbonyl sulfide and methyl 1-propenyl sulfide were the five most abundant VOSCs, with shares of 75.5%, 13.5%, 4.8%, 2.2% and 1.3% in total 15 VOSCs released, respectively. The emission fluxes of major VOSCs were very low at the beginning (day 0). They peaked at days 2–4 and then decreased sharply until they leveled off after 10 days of incubation. For most VOSCs, over 95% of their emission occurred in the first 10 days. The time series of VOSC emission fluxes, as well as their significant correlation with internal food waste temperature (p < 0.05) during incubation, suggested that production of VOSC species was induced mainly by microbial activities during the aerobic decomposition instead of as inherited. Released VOSCs accounted for 5.3% of sulfur content in the food wastes, implying that during aerobic decomposition considerable portion of sulfur in food wastes would be released into the atmosphere as VOSCs, primarily as DMDS, which is very short-lived in the atmosphere and thus usually less considered in the sources and sinks of reduced sulfur gases.     

Wastewater analysis for volatile organic sulfides using purge-and-trap with gas chromatography/mass spectrometry.

This paper presented a modified method for the analysis of volatile organic sulfides (VOS) simultaneously with volatile organic compounds (VOC) in wastewater using purge-and-trap with gas chromatography/ mass spectrometry. Calibration standards were prepared using filtered and nitrogen-purged VOS-free wastewater, acidified to pH 1.4. Samples were also acidified to pH 1.4. This approach minimized the oxidation of methanethiol to dimethyl disulfide (DMDS), which hampered the liquid-phase analysis of VOS. Compounds were concentrated from the liquid phase, and automated analyses were performed without additional equipment, other than that required for routine wastewater VOC analysis. The linear range was 5 to 500 microg/L, with r2 > or = 0.99. The average recovery from replicate analyses of spiked samples was 81 +/- 0.5% for methanethiol, 100 +/- 1.5% fordimethyl sulfide (DMS), and 92 +/- 1.5% for DMDS. Method detection limits were 4.8, 2.8, and 1.2 microg/L for methanethiol, DMS, and DMDS, respectively. The relative percent differences were between 0 and 8%.     

Measurement of biosolids compost odor emissions from a windrow, static pile, and biofilter.

A pilot study was conducted to compare odor emissions from a windrow process and an aerated static pile and to determine the odor reduction efficiency of a pilot two-phase biofilter for odor control of biosolids composting. Chemical compounds identified as responsible for odors from biosolids composting include ammonia, dimethyl disulfide, carbon disulfide, formic acid, acetic acid, and sulfur dioxide (or carbonyl sulfide). Aeration was found to reduce the concentration of ammonia, formic acid, and acetic acid by 72, 57, and 11%, respectively, compared with a nearby windrow, while dimethyl sulfide, carbon disulfide, and sulfur dioxide (or carbonyl sulfide) concentrations were below detection limits. Using dilution-to-threshold olfactometry, aeration followed by biofiltration was found to reduce the odor from biosolids composting by 98%. Biofiltration also altered the character of odor emissions from biosolids composting, producing a less offensive odor with an earthy character. Biofiltration was found to reduce the concentration of ammonia, dimethyl disulfide, carbon disulfide, formic acid, acetic acid, and sulfur dioxide (or carbonyl sulfide) by 99, 90, 32, 100, 34, and 100%, respectively. The concentrations of those odorants were estimated to be 3700, 110000, 26,37,5, and 1.2 times reported human detection limits before the two-phase biofilter, respectively, and 42,9600,18,0,3, and 0 times human detection limits after the biofilter, respectively.     

Chemisorption of hydrogen sulphide and methanethiol by light expanded clay aggregates (Leca)

Removal of volatile sulphur compounds from livestock waste air by biological air filtration may be enhanced by application of packing materials with reactive properties. In this study, light expanded clay aggregates (Leca®) was tested with respect to sorption and potential chemical degradation of H₂S, Methanethiol (MT) and Dimethyl sulphide (DMS). Leca was selected due to its content of minerals, including iron, and due to its high specific surface area. The performance of Leca was evaluated based on breakthrough curves and by comparing the difference between the inlet and outlet gas concentrations. Whereas DMS did not appear to be removed by Leca, both H₂S and MT were removed with variable efficiency depending on the specific conditions. Dimethyl disulphide (DMDS) and dimethyl trisulphide (DMTS) were demonstrated to be produced during the degradation process in relatively high yields. A comparison between ambient air and nitrogen gas conditions showed that the chemisorption of H₂S and MT did not necessarily need oxygen to be present. X-ray analysis of Leca showed an abundance of Fe₂O₃. It is therefore hypothesized that Fe₂O₃ in Leca can remove H₂S and MT by chemisorption. Both air velocity and moisture content clearly affected the capacity of Leca for removal of H₂S and MT. Lower removal is seen at higher air velocities, whereas higher moisture content enhances removal. However, chemisorption of MT by Leca appears to be limited above a threshold moisture level. Potential reaction mechanisms are discussed in relation to the observed effects. The results implicate that Leca can be used as a filter material with reactive properties provided that moisture content is controlled and that an adequate air velocity is used.     

Odorous volatile organic sulfides in wastewater treatment plants in Guangzhou, China

Odorous volatile organic sulfides (VOSs) in headspace atmosphere and wastewaters were identified and quantified synchronously in two municipal wastewater treatment plants (WWTPs) in Guangzhou, China. Dimethyl sulfide (DMS), with concentrations ranging from 0.66 to 5.41 mg/m3, was the major VOS in air samples. Carbonyl sulfide (COS), carbon disulfide (CS2), and dimethyl disulfide (DMDS) ranged from 0.01 to 0.21, 0.03 to 0.44, and 0.06 to 1.61 mg/m3, respectively. Methanethiol was not found in any gas samples. The concentrations of DMS in wastewaters ranged from 23.68 to 308.17 mg/m3 and were also the highest values compared with other VOSs, for all of the treatment processes. Methanethiol was detected in all wastewater samples, except that from the aeration tank, which ranged from 113.08 to 216.82 mg/m3. The COS, carbon disulfide, and DMDS in wastewaters ranged from 0.11 to 2.21, 1.37 to 23.29, and 0.24 to 106.75 mg/m3, respectively. Odors from pollution related to VOS were different in the two plants, and they were strongly associated with the characteristics of incoming wastewater and treatment processes. The VOSs in ambient air samples from nearby residential areas downwind of the plant border were also measured, and the results strongly suggested that control measures are needed for odor pollution in the WWTPs to mitigate malodor in the surrounding neighborhoods.     

Monitoring of atmospheric reduced sulfur compounds and their oxidation in two coastal landfill areas

In this study, the distribution characteristics of reduced sulfur compounds (RSCs) in ambient air were investigated in two coastal landfill (LF) facilities and their surrounding areas. The photochemical conversion of RSCs to sulfur dioxide (SO₂) was also evaluated using a photochemical box model (PCBM). Measurements of RSCs were carried out from both in and around areas of two coastal LFs in Gunsan (G) and Donghae (D) city, Korea during several field campaigns (May through December 2004). The dominant RSCs at the Gunsan landfill (G-LF) were found to be DMS and H₂S, whereas those at the Donghae landfill (D-LF) were H₂S and DMDS. The concentrations of DMS at these study sites were likely to be affected not only by LF processes but also by an oceanic source, while such a pattern was more prominent at the D-LF. The chemical species of RSCs that can exert significant influences on the photochemical production of SO₂ in the LF environment were identified to be H₂S, DMS, or DMDS.     

An on-line analysis of reduced sulfur gases in the ambient air surrounding a large industrial complex

In this study, the concentrations of reduced sulfur compounds (RSC: H₂S, CH₃SH, DMS, and DMDS) were continuously measured from an odor monitoring station over a 4-month period (August–November 2005) using an on-line GC system. The hourly measurement data of RSC, collected along with some major aromatic VOCs (benzene, toluene, xylene, etc.), approached the sum of 1500; the mean for all hourly H₂S was computed to be 295 ppt, while those of the others were seen at 7 (DMS), 1 (CH₃SH), and 0.4 ppt (DMDS). When these RSC data were compared across two seasons and on a 24 h scale basis, the values for either the summer or nighttime periods were generally high relative to their counterparts in the fall and daytime. Analysis of these RSC data generally suggests that most RSCs occur at some ppt concentration ranges and that their values frequently fall below detection limits (DL) values (except for H₂S). If the total number of effective data sets (i.e., above DL values) are compared to each other, the results tend to differ significantly between H₂S and the others: the proportion of effective number was as high as 75% for H₂S, while the others were very low (6% of DMS and even less than that for the others). The distributions of RSC were hence clearly distinguished from those of VOCs in that the determination of the latter was scarcely limited by the instrumental detectability. According to the present study, the H₂S data exhibit strong potential as the malodor tracers, while those of the other RSCs are unlikely due to the limited detectability. The overall results of this study hence suggest that several factors which include the selection of target compounds, the location of the monitoring points, and the scale (or number) of total monitoring points should be considered simultaneously to effectively track down the odor occurrence patterns in areas near strong source processes.     

Role of protein, amino acids, and enzyme activity on odor production from anaerobically digested and dewatered biosolids.

The main objective of this research was to test the hypothesis that bioavailable protein and, more specifically, the sulfur-containing amino acids within the protein, can be degraded by proteolytic enzymes to produce odor-causing compounds--mainly volatile sulfur compounds (VSCs)--during biosolids storage. To achieve these objectives, samples of digester effluent and cake solids were collected at 11 different wastewater treatment plants in North America, and the samples were analyzed for protein and amino acid content and general protein-degrading enzyme activity. At the same time, cake samples were stored using headspace bottles, the concentration of VSCs were measured using gas chromatography, and olfactometry measurements were made by a trained odor panel. The results showed that the bound cake protein content and methionine content was well-correlated with VSC production and the detection threshold measured by the odor panel.     

Sulfuric odorous compounds emitted from pig-feeding operations

The objective of the study was to quantify the concentration and emission levels of sulfuric odorous compounds emitted from pig-feeding operations. Five types of pig-housing rooms were studied: gestation, farrowing, nursery, growing and fattening rooms. The concentration range of sulfuric odorous compounds in these pig-housing rooms were 30–200 ppb for hydrogen sulfide (H₂S), 2.5–20 ppb for methyl mercaptan (CH₃SH), 1.5–12 ppb for dimethyl sulfide (DMS; CH₃SCH₃) and 0.5–7 ppb for dimethyl disulfide (DMDS; CH₃S₂CH₃), respectively. The emission rates of H₂S, CH₃SH, DMS and DMDS were estimated by multiplying the average concentration (mg m⁻³) measured near the air outlet by the mean ventilation rate (m³ h⁻¹) and expressed either per area (mg m⁻² h⁻¹) or animal unit (AU; liveweight of the pig, 500 kg) (mg pig⁻¹ h⁻¹). As a result, the emission rates of H₂S, CH₃SH, DMS and DMDS in the pig-housing rooms were 14–64, 0.8–7.3, 0.4–3.4 and 0.2–1.9 mg m⁻² h⁻¹, respectively, based on pig's activity space and 310–723, 18–80, 9–39 and 5–22 mg AU⁻¹ h⁻¹, respectively, based on pig's liveweight, which indicates that their emission rates were similar, whether based upon the pig's activity space or liveweight. In conclusion, the concentrations and emission rates of H₂S were highest in the fattening room followed by the growing, nursery, farrowing and gestation rooms whereas those of CH₃SH, DMS and DMDS concentrations were largest in the growing room followed by the nursery, gestation and farrowing rooms.     

Photochemical oxidation of reduced sulfur compounds in an urban location based on short time monitoring data

This study examines the oxidation of reduced sulfur compounds (RSCs) in urban ambient air. The photochemical conversions of RSC (such as DMS, CS₂, H₂S, DMDS, and CH₃SH) to a further oxidized form (e.g., SO₂, MSA, and H₂SO₄) were assessed using a photochemical box model. For our model simulation of RSC oxidation, measurements were taken at an urban monitoring station in Seoul, Korea (37.6° N, 127° E) during three separate time periods (e.g., Sept. 17–18, Oct. 23, and Oct. 27–28, 2003). The results indicate that DMS and H₂S were the dominant RSCs with concentrations of 370 ± 140 and 110 ± 60 pptv, respectively. The photochemical conversion of DMDS to SO₂ was found to occur more efficiently than other RSCs. The overall results of our study suggest that photochemical conversion of RSCs accounted for less than 15% of the observed SO₂ during the measurement period. The SO₂ production from DMS oxidation (mainly by the reaction with OH) was found to be affected primarily by the abstraction channel due to high NO_x levels during the experimental conditions.     

Deodorization of dimethyl sulfide using a discharge approach at room temperature

The traditional technologies for odor removal of thiol usually create either secondary pollution for scrubbing, adsorption, and absorption processes, or sulfur (S) poisoning for catalytic incineration. This study applied a laboratory-scale radio-frequency plasma reactor to destructive percentage-grade concentrations of odorous dimethyl sulfide (CH3SCH3, or DMS). Odor was diminished effectively via reforming DMS into mainly carbon disulfide (CS2) or sulfur dioxide (SO2). The removal efficiencies of DMS elevated significantly with a lower feeding concentration of DMS or a higher applied rf power. A greater inlet oxygen (O2)/DMS molar ratio slightly improved the removal efficiency. In an O2-free environment, DMS was converted primarily to CS2, methane (CH4), acetylene (C2H2), ethylene (C2H4), and hydrogen (H2), with traces of hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and dimethyl disulfide. In an O2-containing environment, the species detected were SO2, CS2, carbonyl sulfide, carbon dioxide (CO2), CH4, C2H4, C2H2, H2, formaldehyde, and methanol. Differences in yield of products were functions of the amounts of added O2 and the applied power. This study provided useful information for gaining insight into the reaction pathways for the DMS dissociation and the formation of products in the plasmolysis and conversion processes.     

Anaerobic digestion of raw and thermally hydrolyzed wastewater solids under various operational conditions

In this study, high-solids anaerobic digestion of thermally pretreated wastewater solids (THD) was compared with conventional mesophilic anaerobic digestion (MAD). Operational conditions, such as pretreatment temperature (150 to 170 degrees C), solids retention time (15 to 20 days), and digestion temperature (37 to 42 degrees C), were varied for the seven THD systems operated. Volatile solids reduction (VSR) by THD ranged from 56 to 62%, compared with approximately 50% for MAD. Higher VSR contributed to 24 to 59% increased biogas production (m3/kg VSR-d) from THD relative to MAD. The high-solids conditions of the THD feed resulted in high total ammonia-nitrogen (proportional to solids loading) and total alkalinity concentrations in excess of 14 g/L as calcium carbonate (CaCO3). Increased pH in THD reactors caused 5 to 8 times more un-ionized ammonia to be present than in MAD, and this likely led to inhibition of aceticlastic methanogens, resulting in accumulation of residual volatile fatty acids between 2 and 6 g/L as acetic acid. The THD produced biosolids cake that possessed low organic sulfur-based biosolids odor and dewatered to between 33 and 39% total solids. Dual conditioning with cationic polymer and ferric chloride was shown to be an effective strategy for mitigating dissolved organic nitrogen and UV-quenching compounds in the return stream following centrifugal dewatering of THD biosolids.     

Evaluation of quicklime incorporation in bench-scale and full-scale lime stabilized biosolids using a flat surface pH electrode

Uniform lime incorporation into sewage sludge is critical for biosolid lime stabilization processes. There is no class B biosolids regulation for lime incorporation. The slurry method is currently used to evaluate the pH of limed biosolids, but this method homogenizes the biosolids and potentially masks poor lime mixing. In this study, a flat-surface pH electrode was used in bench-scale and full-scale experiments to measure the pH of lime-stabilized biosolids without creating slurries. The standard deviation of 15 pH measurements at different locations in a biosolid sample was used to assess mixing quality. The bench-scale experimental study showed that well-mixed limed biosolids had consistently high pHs (approximately 12) with low standard deviations (< 0.5 pH units), whereas poorly mixed biosolids had areas with low pH (< 10) and high standard deviations (> 2 pH units). Poorly mixed biosolids exhibited rapid and marked pH reduction, as well as offensive odor generation, whereas well-mixed biosolids resisted pH reduction and offensive odor generation. The full-scale study aimed at improving lime incorporation and biosolids quality confirmed the use of a flat surface pH electrode to capture low pH regions in biosolids that were masked by the current slurry method.     

Evaluation of Quicklime Incorporation in Bench-Scale and Full-Scale Lime Stabilized Biosolids Using a Flat Surface pH Electrode

Abstract

Uniform lime incorporation into sewage sludge is critical for biosolid lime stabilization processes. There is no class B biosolids regulation for lime incorporation. The slurry method is currently used to evaluate the pH of limed biosolids, but this method homogenizes the biosolids and potentially masks poor lime mixing. In this study, a flat-surface pH electrode was used in bench-scale and full-scale experiments to measure the pH of lime-stabilized biosolids without creating slurries. The standard deviation of 15 pH measurements at different locations in a biosolid sample was used to assess mixing quality. The bench-scale experimental study showed that well-mixed limed biosolids had consistently high pHs (~12) with low standard deviations (<0.5 pH units), whereas poorly mixed biosolids had areas with low pH (<10) and high standard deviations (>2 pH units). Poorly mixed biosolids exhibited rapid and marked pH reduction, as well as offensive odor generation, whereas well-mixed biosolids resisted pH reduction and offensive odor generation. The full-scale study aimed at improving lime incorporation and biosolids quality confirmed the use of a flat surface pH electrode to capture low pH regions in biosolids that were masked by the current slurry method.     

A new reliable method for dimethyl sulfoxide analysis in wastewater: dimethyl sulfoxide in Philadelphia's three water pollution control plants.

A simple but reliable procedure was developed to analyze dimethyl sulfoxide (DMSO) in wastewater. The isotope DMSO_d6 was used as the internal standard to ensure accuracy. The DMSO was reduced with stannous chloride and measured as dimethyl sulfide (DMS) with purge-and-trap gas chromatography/mass spectrometry. The method detection limit was at the sub-microgram-per-milliliter level; precision, as measured by standard deviation, was better than +/- 0.5%; and the recoveries were between 95 and 105% at the level of 2 microg/mL. The procedure could use standard analytical instrumentation used for volatile organic compound analysis. A field study was conducted to validate the method and quantify DMSO concentration range in the three water pollution control plants (WPCPs) in the city of Philadelphia, Pennsylvania. Results showed that, when a local chemical facility discharged, DMSO concentration could be as high as 12 mg/L in the influent to a WPCP. This would lead to the formation of a toxic "canned corn" DMS odor during the treatment processes.     

Atmospheric methanethiol emitted from a pulp and paper plant on the shore of Lake Baikal

On-site measurement of methanethiol (CH₃SH) was performed for three years on ships and cars near a pulp and paper plant standing on the shore of Lake Baikal in Siberia, Russia, to investigate the behavior and impact of atmospheric CH₃SH emitted from a point source. Despite its strong odor, there are few reports on atmospheric CH₃SH, while many investigations have been carried out on dimethyl sulfide (DMS). In this work, CH₃SH and DMS were measured every 15 min by a recently developed automated instrument based on single column trapping/separation and chemiluminescence measurement. Hydrogen sulfide, sulfur dioxide and ozone were also measured simultaneously by individual instruments. Of these sulfur compounds, CH₃SH was dominant and its concentration sometimes reached several tens of ppbv. The concentration of CH₃SH was high at night, because of the lack of photodecomposition and local winds from the mountain to the lake. Such time variation was marked in the summer. The CH₃SH level decreased significantly downwind, while decreases in concentrations of other compounds such as DMS and SO₂ were relatively small. From these temporal and spatial variations, the behavior of CH₃SH is described in this paper. The impact of CH₃SH near the Siberian big sources is discussed with the presented data.     

Atmospheric flux of agricultural fumigants from raised-bed,plastic-mulch crop production systems

Atmospheric emission of methyl isothiocyanate (MITC), chloropicrin (CP), 1,3-dichloropropene (1,3-D), and dimethyl disulfide (DMDS) were measured in the field under fumigant application scenarios representative of raised bed–plastic-mulched crop production systems. For three fumigation sites located in Florida, cumulative emissions of 1,3-D, MITC and CP were less than 11%, 6% and 2%, respectively. For three fumigation sites in located in Georgia, cumulative emissions of MITC and CP were <13% and 12%, respectively while DMDS emissions varied from 37% to 95%. In the Florida sites, emission peak flux of CP occurred within the first 6 h after application. Peak emission of 1,3-D and MITC occurred between 100 and 144 h after application. In the Georgia sites where fumigated soil was covered by low density polyethylene (LDPE) plastic, emission peak flux of DMDS and MITC occurred between 12and 48 h after application. Key factors affecting atmospheric emissions were soil moisture, soil tilth and the resistance to fumigant diffusion of the plastic film used to cover soil following application. This study demonstrated reduced atmospheric emissions of agricultural fumigants under commercial production conditions when applied using good agricultural practices including soil water contents above field capacity, uniform soil tilth in the fumigation zone and the use of metalized or virtually impermeable films to further reduce fumigant emissions. The results of this study show a need for regional flux studies due to the various interactions of soil and climate with local agricultural land management practices.     

Photochemical oxidation and dispersion of gaseous sulfur compounds from natural and anthropogenic sources around a coastal location

The photochemical oxidation and dispersion of reduced sulfur compounds (RSCs: H₂S, CH₃SH, DMS, CS₂, and DMDS) emitted from anthropogenic (A) and natural (N) sources were evaluated based on a numerical modeling approach. The anthropogenic emission concentrations of RSCs were measured from several sampling sites at the Donghae landfill (D-LF) (i.e., source type A) in South Korea during a series of field campaigns (May through December 2004). The emissions of natural RSCs in a coastal study area near the D-LF (i.e., source type N) were estimated from sea surface DMS concentrations and transfer velocity during the same study period. These emission data were then used as input to the CALPUFF dispersion model, revised with 34 chemical reactions for RSCs. A significant fraction of sulfur dioxide (SO₂) was produced photochemically during the summer (about 34% of total SO₂ concentrations) followed by fall (21%), spring (15%), and winter (5%). Photochemical production of SO₂ was dominated by H₂S (about 55% of total contributions) and DMS (24%). The largest impact of RSCs from source type A on SO₂ concentrations occurred around the D-LF during summer. The total SO₂ concentrations produced from source type N around the D-LF during the summer (a mean SO₂ concentration of 7.4 ppbv) were significantly higher than those (≤0.3 ppbv) during the other seasons. This may be because of the high RSC and SO₂ emissions and their photochemistry along with the wind convergence.