Measurements Of Water-soluble Organic Carbon Research Articles

Semi-continuous Measurements of Water-soluble Organic Carbon and Ionic Composition of PM2.5 in Baengnyeong Island during the 2016 KORUS-AQ (Korea-United States Air Quality Study)

This study characterized the water-soluble organic carbon (WSOC) and ionic components of PM2.5 (particles less than 2.5 µm in diameter) from April to June 2016, at Baengnyeong Island The study utilized a particle-into-liquid sampler (PILS) coupled with a total organic carbon (TOC) analyzer and ion chromatography (PILS-TOC-IC). The PM2.5 mean concentration in this study was 23.8±16.7 µg m−3, and the mean of WSOC concentration was 1.6±1.5 µg m−3. Ionic compounds increased by 3.8% in the period in which the air quality standard of PM2.5 (35 µg m−3) was exceeded because the concentration of ionic components increased due to generation by chemical reactions and emissions from a primary source in a nearby urban area. The wind direction and speed were mostly in the northwest and southwest directions and the higher concentration of PM2.5 was also shown in the same directions. In comparison with the previous study, the WSOC/OC ratio was 0.5±0.3 in Baengnyeong Island, which was higher than that at roadside and urban sites and lower than at the background site (Sweden, Aspvreten). It was considered that insoluble OC constituted a major proportion of primary OC that was emitted in urban areas, and the higher WSOC/OC ratios of rural and background sites were affected by inflow. The ratio of nitrate and sulfate in Baengnyeong Island was lower than urban areas in China in other studies, because of emitted aerosols from coal combustion or vehicle emission. The results indicate that Baengnyeong Island was affected by formation of secondary aerosols by chemical reactions and inflow from China as well as domestic emissions. The slope of H+Aersol/SO42− in Baengnyeong Island was 0.079, which was a result of neutralized particle acidity in ambient air. The molar ratio of nitrate and ammonium with sulfate in Baengnyeong Island showed ammoniumrich conditions because ammonium nitrate was increased to homogeneous reaction of ammonia and nitric acid in the gas-phase.

Insights into characteristics of light absorbing carbonaceous aerosols over an urban location in Southeast Asia

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.

Temporal Characteristics of Brown Carbon over the Central Indo-Gangetic Plain.

Recent global models estimate that light absorption by brown carbon (BrC) in several regions of the world is ∼30-70% of that due to black carbon (BC). It is, therefore, important to understand its sources and characteristics on temporal and spatial scales. In this study, we conducted semicontinuous measurements of water-soluble organic carbon (WSOC) and BrC using particle-into-liquid sampler coupled with a liquid waveguide capillary cell and total organic carbon analyzer (PILS-LWCC-TOC) over Kanpur (26.5°N, 80.3°E, 142 m amsl) during a winter season (December 2015 to February 2016). In addition, mass concentrations of organic and inorganic aerosol and BC were also measured. Diurnal variability in the absorption coefficient of BrC at 365 nm (babs_365) showed higher values (35 ± 21 Mm-1) during late evening to early morning hours and was attributed to primary emissions from biomass burning (BB) and fossil fuel burning (FFB). The babs_365 reduced by more than 80% as the day progressed, which was ascribed to photo bleaching/volatilization of BrC and/or due to rising boundary layer height. Further, diurnal variability in the ratios of babs_405/babs_365 and babs_420/babs_365 suggests that the BrC composition was not uniform throughout a day. WSOC exhibited a strong correlation with babs_365 (slope = 1.22 ± 0.007, r2 = 0.70, n = 13 265, intercept = -0.69 ± 0.17), suggesting the presence of a significant but variable fraction of chromophores. Mass absorption efficiency (MAE) values of WSOC ranged from 0.003 to 5.26 m2 g-1 (1.16 ± 0.60) during the study period. Moderate correlation (r2 = 0.50, slope = 1.58 ± 0.019, n = 6471) of babs_365 was observed with the semivolatile oxygenated organic aerosols (SV-OOA) fraction of BB resolved from positive matrix factorization (PMF) analysis of organic mass spectral data obtained from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The low-volatility OOA (LV-OOA) fraction of BB had a similar correlation to babs_365 (r2 = 0.54, slope = 0.38 ± 0.004, n = 6471) but appears to have a smaller contribution to the absorption.

Drying-Induced Evaporation of Secondary Organic Aerosol during Summer.

This study characterized the effect of drying on the concentration of atmospheric secondary organic aerosol (SOA). Simultaneous measurements of water-soluble organic carbon in the gas (WSOCg) and particle (WSOCp) phases were carried out in Baltimore, MD during the summertime. To investigate the effect of drying on SOA, the WSOCp measurement was alternated through an ambient channel (WSOCp) and a "dried" channel (WSOCp,dry) maintained at ∼35% relative humidity (RH). The average mass ratio between WSOCp,dry and WSOCp was 0.85, showing that significant evaporation of the organic aerosol occurred due to drying. The average amount of evaporated water-soluble organic matter (WSOM = WSOC × 1.95) was 0.6 μg m(-3); however, the maximum evaporated WSOM concentration exceeded 5 μg m(-3), demonstrating the importance of this phenomenon. The systematic difference between ambient and dry channels indicates a significant and persistent source of aqueous SOA formed through reversible uptake processes. The wide-ranging implications of the work are discussed, and include: new insight into atmospheric SOA formation; impacts on particle measurement techniques; a newly identified bias in PM2.5 measurements using the EPA's Federal Reference and Equivalent Methods (FRM and FEM); atmospheric model evaluations; and the challenge in relating ground-based measurements to remote sensing of aerosol properties.

Diurnal Variability in Secondary Organic Aerosol Formation over the Indo-Gangetic Plain during Winter Using Online Measurement of Water-Soluble Organic Carbon

Understanding the secondary organic aerosol (SOA) formation is among most important topics in the field of aerosol research because its poor understanding leads to large uncertainty in the assessment of aerosol effects on air quality and climate. This study reports the diurnal and temporal variability in SOA formation over a site (Patiala: 30.2°N, 76.3°E, 249 m amsl) located in the Indo-Gangetic Plain (IGP) during winter using the first online measurements of water-soluble organic carbon (WSOC) over India. Online WSOC, measured with particle-into-liquid sampler (PILS) connected to total organic carbon (TOC) analyzer, ranged from 0.1 to 99 µg m–3 (avg: 15.6, sd: 9.4) with a considerable day-to-day and within the day variability, and attributed to meteorological conditions and regional sources. Diurnal trends of online WSOC suggest significant SOA formation during 7:00 to 22:00 hrs when sources of SOA precursors are active; and loss of SOA occurs during afternoon when ambient air temperature is at its peak. In parallel to online measurements, filter-based particulate matter smaller than 2.5 µm (PM2.5) samples were also collected and analyzed for major cations, anions and carbonaceous aerosols. Filter-based PM2.5 composition suggests that the emissions from biomass burning contribute more to carbonaceous aerosols than those from fossil fuel burning. In spite of this, average primary WSOC was only ~20% whereas secondary WSOC (or SOA) dominated the total WSOC concentration with ~80% contribution. A strong linear relationship between PM1 and WSOC (R2 = 0.83, slope = 0.113, intercept = 4.7), suggests that a significant fraction of fine particles are SOA.

Sources and light absorption of water-soluble organic carbon aerosols in the outflow from northern China

Abstract. High loadings of anthropogenic carbonaceous aerosols in Chinese air influence the air quality for over one billion people and impact the regional climate. A large fraction (17–80%) of this aerosol carbon is water-soluble, promoting cloud formation and thus climate cooling. Recent findings, however, suggest that water-soluble carbonaceous aerosols also absorb sunlight, bringing additional direct and indirect climate warming effects, yet the extent and nature of light absorption by this water-soluble "brown carbon" and its relation to sources is poorly understood. Here, we combine source estimates constrained by dual carbon isotopes with light-absorption measurements of water-soluble organic carbon (WSOC) for a March 2011 campaign at the Korea Climate Observatory at Gosan (KCOG), a receptor station in SE Yellow Sea for the outflow from northern China. The mass absorption cross section at 365 nm (MAC365) of WSOC for air masses from N. China were in general higher (0.8–1.1 m2 g−1), than from other source regions (0.3–0.8 m2 g−1). However, this effect corresponds to only 2–10% of the radiative forcing caused by light absorption by elemental carbon. Radiocarbon constraints show that the WSOC in Chinese outflow had significantly higher fraction fossil sources (30–50%) compared to previous findings in S. Asia, N. America and Europe. Stable carbon (δ13C) measurements were consistent with aging during long-range air mass transport for this large fraction of carbonaceous aerosols.

Atmospheric Aerosol Water-Soluble Organic Carbon Measurement: A Theoretical Analysis

The measurement of Water-Soluble Organic Carbon (WSOC) in atmospheric aerosol is usually carried out by sample collection on filters, extraction in ultrapure water, filtration, and measurement of the total organic carbon. This paper investigates the role of different conditions of sampling and extraction as well as the range of solubilities of the organic compounds that contribute to the WSOC. The sampling and extraction of WSOC can be described by a single parameter, P, expressing the ratio of water used per volume of air sampled on the analyzed filter. Two cases are examined in order to bound the range of interactions of the various organic aerosol components with each other. In the first we assume that the organic species form an ideal solution in the particle and in the second that the extraction of a single compound is independent of the presence of the other organics. The ideal organic solution model predicts that species with water solubility as low as 10(-4) g L(-1) contribute to the measured WSOC. In the other end, the independent compounds model predicts that low-solubility (as low as 10(-7) g L(-1)) compounds are part of the WSOC. Studies of the WSOC composition are consistent with the predictions of the ideal organic solution model. A value of P = 0.1 cm(3) m(-3) is proposed for the extraction of WSOC for typical organic aerosol concentrations (1-10 μg m(-3)). WSOC measurements under high concentration conditions often used during source sampling will tend to give low WSOC values unless higher P values are used.

Spatial and seasonal variations of fine particle water-soluble organic carbon (WSOC) over the southeastern United States: implications for secondary organic aerosol formation

Abstract. Secondary organic aerosol (SOA) in the southeastern US is investigated by analyzing the spatial-temporal distribution of water-soluble organic carbon (WSOC) and other PM2.5 components from 900 archived 24-h Teflon filters collected at 15 urban or rural EPA Federal Reference Method (FRM) network sites throughout 2007. Online measurements of WSOC at an urban/rural-paired site in Georgia in the summer of 2008 are contrasted to the filter data. Based on FRM filters, excluding biomass-burning events (levoglucosan < 50 ng m−3), WSOC and sulfate were highly correlated with PM2.5 mass (r2~0.7). Both components comprised a large mass fraction of PM2.5 (13% and 31%, respectively, or ~25% and 50% for WSOM and ammonium sulfate). Sulfate and WSOC both tracked ambient temperature throughout the year, suggesting the temperature effects were mainly linked to faster photochemistry and/or synoptic meteorology and less due to enhanced biogenic hydrocarbon emissions. FRM WSOC, and to a lesser extent sulfate, were spatially homogeneous throughout the region, yet WSOC was moderately enhanced (27%) in locations of greater predicted isoprene emissions in summer. A Positive Matrix Factorization (PMF) analysis identified two major source types for the summer WSOC; 22% of the WSOC were associated with ammonium sulfate, and 56% of the WSOC were associated with brown carbon and oxalate. A small urban excess of FRM WSOC (10%) was observed in the summer of 2007, however, comparisons of online WSOC measurements at one urban/rural pair (Atlanta/Yorkville) in August 2008 showed substantially greater difference in WSOC (31%) relative to the FRM data, suggesting a low bias for urban filters. The measured Atlanta urban excess, combined with the estimated boundary layer heights, gave an estimated Atlanta daily WSOC production rate in August of 0.55 mgC m−2 h−1 between mid-morning and mid-afternoon. This study characterizes the regional nature of fine particles in the southeastern US, confirming the importance of SOA and the roles of both biogenic and anthropogenic emissions.

Water-soluble organic aerosol in the Los Angeles Basin and outflow regions: Airborne and ground measurements during the 2010 CalNex field campaign

A particle-into-liquid sampler coupled to a total organic carbon analyzer (PILS-TOC) quantified particulate water-soluble organic carbon (WSOC) mass concentrations during the May 2010 deployment of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter in the CalNex field study. WSOC data collected during 16 flights provide the first spatiotemporal maps of WSOC in the San Joaquin Valley, Los Angeles Basin, and outflow regions of the Basin. WSOC was consistently higher in concentration within the Los Angeles Basin, where sea breeze transport and Basin topography strongly influence the spatial distribution of WSOC. The highest WSOC levels were associated with fire plumes, highlighting the importance of both primary and secondary sources for WSOC in the region. Residual pollution layers enriched with WSOC are observed aloft up to an altitude of 3.2 km and the highest WSOC levels for each flight were typically observed above 500 m. Simultaneous ground WSOC measurements during aircraft overpasses in Pasadena and Riverside typically exhibit lower levels, especially when relative humidity (RH) was higher aloft suggestive of the influence of aerosol-phase water. This points to the underestimation of the radiative effects of WSOC when using only surface measurements. Reduced aerosol-phase water in the eastern desert outflow region likely promotes the re-partitioning of WSOC to the gas phase and suppression of processes to produce these species (partitioning, multiphase chemistry, photolytic production); as a result, WSOC is reduced relative to sulfate (but not as much as nitrate) as aerosol is advected from the Basin to the outflows.

XAD 수지에 의한 친수성 및 소수성 수용성 유기탄소의 특성조사

24-hr integrated measurements of water-soluble organic carbon (WSOC) in PM2.5 were made between May 5 and September 25, 2010, on a six-day interval basis, at the Metropolitan Area Air Pollution Monitoring Supersite. A macro-porous XAD7HP resin was used to separate hydrophilic and hydrophobic WSOC. Compounds that penetrate the XAD7HP column are referred to hydrophilic WSOC, while those retained by the column are defined as hydrophobic WSOC. Laboratory calibrations using organic standards suggest that hydrophilic WSOC includes lowmolecular aliphatic dicarboxylic acids and carbonyls with less than 4 or 5 carbons, amines, and saccharides. While the hydrophobic WSOC is composed of compounds of aliphatic dicarboxylic acids with carbon numbers larger than 4~5, phenols, aromatic acids, cyclic acid, and humic-like Suwannee River fulvic acid. Over the entire study period, total WSOC accounted for on average 48% of OC, ranging from 32 to 65%, and hydrophilic WSOC accounted for on average 30.5% (9.3~66.7%) of the total WSOC. Based on the previous results, our measurement result suggests that significant amounts of hydrophobic WSOC during the study period were probably from primary combustion sources. However, on June 9 when 1-hr highest ozone concentration of 130 ppb was observed, WSOC to OC was 0.61, driven by increases in the hydrophilic WSOC. This result also suggests that processes, such as secondary organic aerosol formation, produce significant levels of hydrophilic WSOC compounds that add substantially to the fine particle fraction of the organic aerosol.

Application of high-performance anion-exchange chromatography–pulsed amperometric detection for measuring carbohydrates in routine daily filter samples collected by a national network: 1. Determination of the impact of biomass burning in the upper Midwest

[1] Biomass burning is one of the major sources of organic carbon aerosols. However, there is limited information on the temporal and spatial variability for the impact of biomass burning in most regions of the United States, including the upper Midwest. In an attempt to obtain information on these variabilities, high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) was employed to measure the smoke marker levoglucosan (and various other carbohydrates) on archived daily Federal Reference Monitor (FRM) Teflon filter samples from the PM2.5 NAAQS compliance monitoring network. Levoglucosan data, along with measurements of water-soluble organic carbon (WSOC) and potassium, from the analysis of FRM samples collected at 10 sites in the upper Midwest from March 2004 through February 2005 are presented. Results suggest that WSOC contains a substantial regional component, summer levoglucosan is dependent on both horizontal and vertical transport of fire emissions, and potassium revealed no clear pattern associated with biomass burning impacts. The contribution of organic carbon due to primary biomass burning particle emissions ranged on average from about 5 to 35%, suggesting that for this study in the upper Midwest, >50% of the WSOC is from secondary organic aerosol rather than biomass burning. In a second paper the results from the measurements of the other carbohydrates that HPAEC-PAD analysis can determine are discussed to investigate their sources and trends.

Investigation of possible sources of water-soluble organic carbon particles observed with an online monitoring system

Abstract In this study, an instrument for the continuous measurements of water-soluble organic carbon (WSOC) component in fine particulate matter was developed and its field measurement results were presented. The instrument for collecting atmospheric aerosol particles for the online WSOC analysis uses an aerosol growth technology. Briefly, ambient particles, with aerodynamic diameters less than 2.5 μm, are collected via a cyclone inlet collector by rapidly mixing saturated water vapor with the ambient aerosol. The resulting supersaturated water vapor condenses on all the particles. The resulting liquid is separated from the air stream by an air–liquid separator, filtered, and the carbon content quantified using a total organic carbon analyzer, providing a continuous 4-min integral measurement. An evaluation of the accuracy of the online WSOC measurement system was performed by comparing the results with 24-h filter-based measurement data. The relationship between the 24-h averages of the 4-min data and the 24-h integrated data of the WSOC measurements indicated that the online WSOC concentration was 16% lower than that of the filter-based measurement. The online measurements of WSOC in fine particles were performed in September 2009 at a photochemical network near an industrial complex. Conditional probability functions were used to identify likely local emission source locations of the WSOC observed at the site, indicating that the major WSOC contribution to the site came mostly from the southeasterly (120−150°) and southwesterly (165–210°) directions, which brought air from a coal-fired power plant and traffic on the roads. Based on the temporal evolutions of SO2 and WSOC concentrations, wind direction with respect to the station angle of individual sources, wind speed, and source distance, it was shown that the presence of concurrent transients in SO2 and WSOC concentrations observed for a specific period is likely due to the influence of coal-fired power plants.

Aqueous-phase reactive uptake of dicarbonyls as a source of organic aerosol over eastern North America

We use a global 3-D atmospheric chemistry model (GEOS-Chem) to simulate surface and aircraft measurements of organic carbon (OC) aerosol over eastern North America during summer 2004 (ICARTT aircraft campaign), with the goal of evaluating the potential importance of a new secondary organic aerosol (SOA) formation pathway via irreversible uptake of dicarbonyl gases (glyoxal and methylglyoxal) by aqueous particles. Both dicarbonyls are predominantly produced in the atmosphere by isoprene, with minor contributions from other biogenic and anthropogenic precursors. Dicarbonyl SOA formation is represented by a reactive uptake coefficient γ = 2.9 × 10 −3 and takes place mainly in clouds. Surface measurements of OC aerosol at the IMPROVE network in the eastern U.S. average 2.2 ± 0.7 μg C m −3 for July–August 2004 with little regional structure. The corresponding model concentration is 2.8 ± 0.8 μg C m −3, also with little regional structure due to compensating spatial patterns of biogenic, anthropogenic, and fire contributions. Aircraft measurements of water-soluble organic carbon (WSOC) aerosol average 2.2 ± 1.2 μg C m −3 in the boundary layer (<2 km) and 0.9 ± 0.8 μg C m −3 in the free troposphere (2–6 km), consistent with the model (2.0 ± 1.2 μg C m −3 in the boundary layer and 1.1 ± 1.0 μg C m −3 in the free troposphere). Source attribution for the WSOC aerosol in the model boundary layer is 27% anthropogenic, 18% fire, 28% semi-volatile SOA, and 27% dicarbonyl SOA. In the free troposphere it is 13% anthropogenic, 37% fire, 23% semi-volatile SOA, and 27% dicarbonyl SOA. Inclusion of dicarbonyl SOA doubles the SOA contribution to WSOC aerosol at all altitudes. Observed and simulated correlations of WSOC aerosol with other chemical variables measured aboard the aircraft suggest a major SOA source in the free troposphere compatible with the dicarbonyl mechanism.

CMAQ Model Performance Enhanced When In-Cloud Secondary Organic Aerosol is Included: Comparisons of Organic Carbon Predictions with Measurements

Mounting evidence suggests that low-volatility (particle-phase) organic compounds form in the atmosphere through aqueous phase reactions in clouds and aerosols. Although some models have begun including secondary organic aerosol (SOA) formation through cloud processing, validation studies that compare predictions and measurements are needed. In this work, agreement between modeled organic carbon (OC) and aircraft measurements of water soluble OC improved for all 5 of the compared ICARTT NOAA-P3 flights during August when an in-cloud SOA (SOAcld) formation mechanism was added to CMAQ (a regional-scale atmospheric model). The improvement was most dramatic for the August 14th flight, a flight designed specifically to investigate clouds. During this flight the normalized mean bias for layer-averaged OC was reduced from -64 to -15% and correlation (r) improved from 0.5 to 0.6. Underpredictions of OC aloft by atmospheric models may be explained, in part, by this formation mechanism (SOAcld). OC formation aloft contributes to long-range pollution transport and has implications to radiative forcing, regional air quality and climate.

Simultaneous measurements of particulate and gas-phase water-soluble organic carbon concentrations at remote and urban-influenced locations

[1] The sources, sinks, and overall importance of water-soluble organic carbon (WSOC) in the atmosphere are not well understood. Although the primary historical focus has been on particulate WSOC (WSOCP), here we also present results obtained using a newly developed technique that additionally measures gas-phase water-soluble organic carbon (WSOCG). These first-of-their-kind measurements show that WSOCG can often be more than ten times larger than WSOCP at both urban and remote locations. The average fraction of WSOC residing in the gas phase (fg = WSOCG/(WSOCG + WSOCP)) at five various field sites ranged from 0.64 to 0.93, implying significant differences in WSOC phase partitioning between locations. At Houston, TX, and Summit, Greenland, a repeatable diurnal pattern was observed, with minimum values for fg occurring at night. These trends likely are due, at least in part, to temperature and/or relative humidity related gas-to-particle partitioning. These coincident measurements of WSOC in both the gas and particle phases indicate that a relatively large reservoir of water-soluble organic mass is not taken into account by measurements focused only on WSOCP. In addition, a significant amount of WSOCG is available to form WSOCP or enter cloud droplets depending on the chemical and physical properties of the droplets and/or aerosols present.

Atmospheric water-soluble organic carbon measurements at Summit, Greenland

The recently discovered active photochemistry in the surface layers of polar snow may complicate the interpretation of organic compounds found in ice cores. In order to better understand the transformation and cycling of organic species in Arctic surface snow, measurements of water-soluble organic carbon (WSOC) in the gas (WSOC G) and particle (WSOC P) phases were made during the 2006 summer season at Summit, Greenland. These samples represent the first direct, simultaneous measurements of both WSOC G and WSOC P at Summit. From early June to early July, WSOC G and WSOC P concentrations at 150 cm above the snow averaged 667 and 194 ng C m −3, respectively. This value for WSOC G is very similar in magnitude to the sum of acetic and formic acid gas concentrations measured during previous summers at Summit, suggesting that these two monocarboxylic acids constitute a significant fraction of the mass of measured WSOC G. Firn air measurements of WSOC G revealed concentrations within the snowpack nearly an order of magnitude greater than those in the air just above the snow. During one period, four out of five consecutive nights showed concurrent decreases in WSOC G and increases in WSOC P, likely resulting from temperature-dependent gas-to-particle partitioning, as these episodes occurred during the coldest part of the early morning.

Time‐resolved measurements of water‐soluble organic carbon in Tokyo

Semicontinuous measurements of submicron water‐soluble organic carbon (WSOC) aerosol were made simultaneously with organic carbon (OC) and elemental carbon (EC) in the Tokyo urban area in winter, summer, and fall 2004. The measurements of WSOC and OC/EC were made every 6 min and 1 hour, respectively, using a particle‐into‐liquid sampler (PILS) with a total organic carbon (TOC) analyzer and with an EC‐OC analyzer using a thermal‐optical technique. The PILS and 12‐hour integrated filter measurements of WSOC agreed to within 12%. The WSOC mass concentrations and WSOC/OC ratio showed diurnal variations with peaks at 1200–1400 LT in summer and later in the afternoon in winter. On average, the WSOC/OC ratio was 0.20 and 0.35 μg C/μg C for winter and summer/late fall, respectively. The difference in the winter and summer frequency distributions of the WSOC/OC ratio suggests that the sampled air masses in summer and fall were more photochemically processed than those in winter. Secondary organic carbon (SOC) concentrations were estimated using the EC‐tracer method. The measured WSOC was highly correlated with the derived SOC (r2 = 0.61–0.79), with WSOC/SOC slopes of 0.67 to 0.75 μg C/μg C for each season. These results suggest that the WSOC and SOC were similar in their chemical characteristics in this study. Water‐insoluble organic carbon (WIOC) ( = OC–WSOC) correlated well with EC and CO (r2 = 0.59–0.73). The diurnally averaged WIOC/EC ratios were nearly constant (1.1 ± 0.1 μg C/μg C) throughout the study periods, suggesting that motor vehicle emissions were an important source of WIOC. A dominant portion (about 90% or more) of the POC was water‐insoluble, consistent with previous studies of POC.

Chemical characterization of the ambient organic aerosol soluble in water: 1. Isolation of hydrophobic and hydrophilic fractions with a XAD‐8 resin

Group separation of the aqueous extract of fine particles bearing water‐soluble organic carbon (WSOC) provides unique insights into the sources of organic carbon (OC). XAD‐8 resin coupled with a Total Organic Carbon analyzer allows for direct quantification. We term the fraction of WSOC not retained by a XAD‐8 resin column at pH 2 as hydrophilic WSOC (WSOCxp); this includes saccharides, amines, and carbonyls and aliphatic monocarboxylic/dicarboxylic/oxocarboxylic acids with less than 4 or 5 carbons. The fraction of WSOC retained by XAD‐8, termed the hydrophobic fraction (WSOCxr), include aromatic acids, phenols, organic nitrates, cyclic acids, and carbonyls and monocarboxylic/dicarboxylic acids with greater than 3 or 4 carbons. However, only aromatic compounds (or aromatic‐like compounds with similar properties) can subsequently be extracted from XAD‐8 with high efficiency. By coupling a Particle‐into‐Liquid Sampler with this technique, online measurements of WSOC, WSOCxp, and WSOCxr are possible. Urban measurements from St. Louis and Atlanta, on a carbon mass basis, show an increase in the mean WSOC fraction from winter (51%) to summer (61%), due to increases in the WSOCxp/OC from 0.25 to 0.35. During a summer Atlanta PM event, WSOC to OC was 0.75, driven largely by increases in the WSOCxp. The results are consistent with the view that in the summer, there are increased amounts of oxygenated polar compounds, possibly from secondary organic aerosol production, and that these compounds account for an even larger fraction of OC during stagnation events. A companion paper describes a method to further group speciate these XAD‐8 isolated fractions.

A method for on‐line measurement of water‐soluble organic carbon in ambient aerosol particles: Results from an urban site

An instrument for on‐line continuous measurement of the water‐soluble organic carbon (WSOC) component of aerosol particles is described and results from an urban site in St. Louis are presented. A Particle‐into‐Liquid Sampler impacts ambient particles, grown to large water droplets, onto a plate and then washes them into a flow of purified water. The resulting liquid is filtered and the carbon content quantified by a Total Organic Carbon analyzer providing continuous six‐minute integral measurements with a detection limit of 0.1 μg C/m3. Summer and fall measurements of WSOC and organic carbon (OC) indicated WSOC/OC ratio typically ranged from 0.40 to 0.80. A diurnal variation in WSOC/OC that correlated with ozone was observed over extended periods in June; however, other periods in August had no correlation. The results suggested that WSOC was composed of a complex mixture of compounds that may contain a significant fraction from secondary organic aerosol formation.