Water-soluble Organic Carbon Fraction Research Articles

Variability of organic and elemental carbon, water soluble organic carbon, and isotopes in Hong Kong

Abstract. To determine the levels and variations of carbonaceous aerosol in Hong Kong, PM2.5 and PM10 samples were collected by high volume (Hi-vol) samplers at three monitoring stations (representing middle-scale roadside, urban-, and regional-scale environments) during winter (November 2000 to February 2001) and summer (June 2001 to August 2001) periods. The highest concentrations of organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were found at the middle-scale roadside site with the lowest at the regional-scale site. The percentages of WSOC in total carbon at these sites were inversely correlated with their concentrations (i.e., the highest percentages of WSOC were observed at the regional-scale site). A high WSOC fraction may be associated with aged aerosol because of the secondary formation by photochemical oxidation of organic precursors of anthropogenic pollutants during transport. The annual average of isotope abundances (δ13C) of OC and EC were –26.9±0.5‰ and –25.6±0.1‰, respectively. There were no notable differences for seasonal distributions of carbon isotopic composition, consistent with motor vehicle emissions being the main source contributors of carbonaceous aerosol in Hong Kong. OC 13C abundances at the regional-scale site were higher than those at the middle-scale roadside and urban sites, consistent with secondary organic aerosols of biogenic origin.

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.

Chemical characterization of the ambient organic aerosol soluble in water: 2. Isolation of acid, neutral, and basic fractions by modified size‐exclusion chromatography

A method employing size‐exclusion chromatography (SEC) with Total Organic Carbon detection is developed to isolate and quantify the water‐soluble organic carbon (WSOC) component of ambient aerosol particles by acid, neutral, and basic functional groups. The method provides unique quantitative insights into the characteristics and possible sources of a large fraction of the organic aerosol. The SEC is combined with a XAD‐8 method that separates WSOC into hydrophilic and hydrophobic fractions. Calibrations show the hydrophilic fraction separates into short‐chain aliphatic acids (WSOCxp_a), neutrals (WSOCxp_n, e.g., saccharides, polyols, and short‐chain carbonyls), and organic bases. Recovered hydrophobic fractions are separated into acids (WSOCxrr_a, e.g., aromatic) and neutrals (WSOCxrr_n, e.g., phenols). 13Carbon–nuclear magnetic resonance on ambient samples support the calibration‐based conclusions; however, calibrations only provide a guide to the type of ambient organic compounds expected in each group. Comparisons are made between XAD‐8/SEC results from urban Atlanta summer and winter and biomass burning samples. The largest isolated fraction of Atlanta summer WSOC is WSOCxp_a (29% μg C/μg C), suggesting aliphatic acids of less than C4 or C5 are the dominant secondary organic aerosol product. Combined with WSOCxrr_a, these acid groups are a higher fraction of summer organic carbon (20%) than winter (14%). They are correlated with each other (R2 = 0.74), with WSOCxp_n (R2 = 0.61 and 0.52), and with gaseous 24‐hour averaged volatile organic compounds linked to mobile sources. In biomass burning samples, neutrals (WSOCxp_n and WSOCxrr_n) dominate the WSOC. Atlanta winter samples have characteristics that appear to be a combination of summer and biomass samples.

Characterization of Water-Soluble Organic Carbon in Urban Atmospheric Aerosols Using Solid-State 13C NMR Spectroscopy

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the distribution of carbon functional groups in urban Atlanta aerosol fine (PM2.5) particles. Carbonaceous aerosol particles comprise a significant fraction of the ambient particle mass and are environmentally significant as they may influence radiative and cloud-nucleating properties and can also produce adverse health effects upon inhalation. The water-soluble organic carbon (WSOC) fraction was extracted from multiple 24 h integrated high-volume quartz filter samples and further separated into recovered hydrophobic and hydrophilic fractions using an approach similar to that used to extract humic and fulvic acids from aqueous samples. Solid-state 13C NMR results indicate that WSOC in urban atmospheric aerosol particles is mostly aliphatic in nature (approximately 95% by C mass) with major contributions from alkyl and oxygenated alkyls (approximately 80%), carboxylic acid (approximately 10%), and aromatic functional groups (approximately 4%). The aromatic C is associated with the recovered hydrophobic fraction of WSOC. These spectra have been compared to the 13C NMR results obtained from Suwannee River humic acid and a fractionated biomass burning sample. WSOC, and more importantly, its recovered hydrophobic fraction, is found to be only qualitatively similar to aqueous humic material. The biomass burning sample is significantly different from urban Atlanta WSOC and is composed of substantial amounts of sugar derivatives and phenolic compounds, as expected. The NMR results demonstrate the potential of this technique to investigate aerosol WSOC composition and to study its variations with changes in parameters such aerosol sources.

Seasonal variations of the water soluble organic carbon mass fraction of aerosol in two valleys of the French Alps

Abstract. Concentrations of Water Soluble Organic Carbon (WSOC) and WSOC fraction to Organic Carbon (OC) were measured at two urban sites in valleys of the French Alps during a period of two and a half years. Concentrations were as high as 10–15μg C/m3 in winter, but there is a clear seasonal cycle of the WSOC fraction, with minima occurring during winter. This reflects a marked dependency on temperature, with the average WSOC fraction being stable at 54.8±7.7% and 75.9±6.3% for temperatures in the ranges −10 to +3°C and 12 to 24°C, respectively. Several points are noteworthy in this evolution. First, there are limiting factors that prevent lower mass fractions in the low temperature range and higher mass fractions in the high temperature range. Second, the mass fraction at the lower temperature is rather high, in apparent contradiction with OC being mainly insoluble close to the emission sources. Third, the range of 20% for the change of the WSOC fraction between these extreme conditions is indeed rather narrow when compared to evaluations of the secondary (and supposedly water soluble) OC fraction proposed in the literature, with most of the published values being in the range 40 to 70%. A comparison of the evolution of WSOC concentrations with that of dicarboxylic acids (DCA) clearly indicates the influence of two regimes in the formation of WSOC: one at higher temperatures classically linked with the increase of DCA concentrations and associated with oxidation processes, and another at lower temperatures involving a much lower increase of DCA concentrations. We proposed several hypotheses involving processes that could be responsible for the large concentrations of WSOC in the particulate phase at our sites during winter time.

Carbonaceous and ionic components in wintertime atmospheric aerosols from two New Zealand cities: Implications for solid fuel combustion

Wintertime PM10 samples collected in two New Zealand cities (Christchurch and Auckland) have been analysed for total carbon (TC), elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC), and water-insoluble organic carbon (WIOC) as well as major inorganic ions. Christchurch, which is heavily influenced by residential wood and coal burning in winter, showed significantly higher ambient concentrations of carbonaceous materials than Auckland whose major sources of aerosols are sea-salt and vehicular emissions. In Christchurch, the mean concentrations of TC, EC, WSOC, and WIOC were 21, 6.0, 7.0 and 7.6 μg m−3, respectively. In contrast, those of Auckland were 5.0, 2.0, 1.3 and 1.7 μg m−3, respectively. On average, total carbonaceous materials (EC, water-soluble organic matter and water-insoluble organic matter) accounted for about 70% of PM10 mass in Christchurch but less than 30% in Auckland. Moreover, WSOC/EC and nss-K+/EC mass ratios in Christchurch were found to be substantially higher than those of Auckland. In contrast, nss-SO42−, NO3− and nss-Ca2+ concentrations in Auckland were dramatically enhanced relative to carbonaceous materials. This study suggests that fresh emissions from solid fuel burning are significantly more enriched with WIOC than WSOC, although a significant fraction of WSOC may be a secondary atmospheric product. It is estimated that only one-fifth to one-quarter of the measured OC was attributable to secondary organic aerosols in the two cities in winter.

The chemical composition of inorganic and carbonaceous materials in PM2.5 in Nanjing, China

PM2.5 samples were collected at an urban and a suburban site in Nanjing, China in 2001. They were analyzed for inorganic ions, elemental carbon, organic carbon (OC), water-soluble organic carbon (WSOC), and individual WSOC and nonpolar organic species. Sulfate and organic matter were the two most abundant constituents in these samples. Sulfate accounted for an average of 23% (urban site) and 30% (suburban site) of the identified aerosol mass. Organic matter accounted for an average of 37% (urban) and 28% (suburban) of the identified aerosol mass. WSOC was a significant portion of OC, accounting for about one-third of OC at the urban site and 45% of OC at the suburban site. The suburban–urban gradient in the WSOC/OC ratio also reflected that the aerosol OC was more aged at the suburban location. The correlations of WSOC with sulfate and nitrate suggest that the WSOC fraction was dominated by secondary organics. More than 30 individual WSOC species in the compound classes of organic anions, amino acids, aliphatic amines, and carbohydrates were quantified, accounting for approximately 8% of the WSOC on a carbon mass basis. In addition, 46 individual nonpolar organic compounds in the compound classes of n-alkanes, hopanes, and polycyclic aromatic hydrocarbons were quantified using an in-injection port thermal desorption technique. These nonpolar organic species accounted for less than 7% of the OC on a carbon mass basis. The quantification of individual compounds allowed the identification of major aerosol sources through principal component analysis. Coal combustion, vehicular emissions, secondary inorganic and organic aerosols, and road/sea salt were the major contributing sources to the identified PM2.5 aerosol mass.

Chemical Characterization of Water-Soluble Organic Aerosols at Jeju Island Collected During ACE-Asia

Environmental Context. Atmospheric aerosols — particles suspended in the atmosphere — are responsible for many phenomena, including formation of cloud condensation nuclei and degradation of regional visibility. Water-soluble organic carbon (WSOC) components make up a significant fraction of the aerosols' carbon mass, and have consequently received increasing attention from researchers. The chemical composition of the WSOC fraction, and thus their sources and effects, are not well known. This study focusses on WSOC from samples collected in South Korea as part of ACE-Asia (Asia-Pacific Regional Aerosol Characterization Experiment), a large international collaboration including Asia, the USA, Europe and Australia. Abstract.During the Asia-Pacific Regional Aerosol Characterization Experiment (ACE-Asia) intensive field campaign, aerosol samples of less than 2.5 μm diameter were collected at Jeju Island, South Korea, for chemical characterization of the water-soluble organic carbon (WSOC) fraction. The WSOC fraction had an average mass concentration of roughly half of that of sulfate and accounted for about two-thirds of the organic carbon mass. Thirty individual water-soluble organic compounds, belonging to the classes of mono- and di-carboxylic acids, aliphatic amines, and amino acids, were identified, accounting for 14% of the WSOC on a carbon basis. Oxalic acid was the most abundant single component. An additional 3% of the WSOC was estimated to be monomeric carbohydrates. Thermal analysis of the aerosol’s water extracts indicated that a significant fraction (~50%) of WSOC was thermally recalcitrant, possibly consisting of polymeric materials.

Water‐soluble organic compounds in biomass burning aerosols over Amazonia 2. Apportionment of the chemical composition and importance of the polyacidic fraction

Chemical characterization was performed on carbonaceous aerosols from Rondônia in the Brazilian Amazon region as part of the European contribution to the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA‐EUSTACH). The sampling period (October 1999) included the peak of the burning season as well as the dry‐to‐wet season transition. Characterization of the carbonaceous material was performed by using a thermal combustion method. This enabled determination of aerosol total carbon (TC), black carbon (BC), and organic carbon (OC). A significant fraction of the BC material (on average about 50%) seemed to be highly refractory organic material soluble in water. A more detailed analysis of the water‐soluble organic carbon (WSOC) fraction of the TC was undertaken, involving measurements of WSOC content, high‐performance liquid chromatography (HPLC) separation (with UV detection) of the water‐soluble components, and characterization of individual components by gas chromatography/mass spectrometry (GC/MS). The WSOC fraction accounted for 45−75% of the OC. This high WSOC fraction suggests an aerosol derived mainly from smoldering combustion. Using GC/MS, many different compounds, containing hydroxy, carboxylate, and carbonyl groups, were detected. The fraction of the WSOC identified by GC/MS was about 10%. Three classes of compounds were separated by HPLC/UV: neutral compounds (N), monocarboxylic and dicarboxylic acids (MDA), and polycarboxylic acids (PA). The sum of these three groups accounted for about 70% of the WSOC, with MDA and PA being most abundant (about 50%). Good correlations (r2between 0.84 and 0.99) of BCwater(BC after water extraction) and levoglucosan (both indicators of biomass combustion) with the water‐soluble species (i.e., WSOC, N, MDA, and PA), and their increase in concentrations during the burning period provided strong evidence that biomass burning is a major source of the WSOC. Particularly interesting is that PA and therefore, probably, humic‐like substances (due to their polyacidic nature) are generated in significant amounts during biomass burning. These substances, due to their water solubility and surface tension‐lowering effects, may play an important role in determining the overall cloud condensation nuclei activity of biomass burning aerosols and, consequently, could be important in cloud processes and climate forcing.

Partitioning of the organic aerosol component between fog droplets and interstitial air

Limited information is available on the nature of organic compounds in the tropospheric aerosol and their effect on aerosol hygroscopic properties and cloud condensation nuclei (CCN) ability. Here we analyze samples of liquid droplets and interstitial aerosol, concurrently collected during fog episodes, to determine how the organic compounds are partitioned between the two reservoirs. By comparing the nature and concentration of different organic carbon classes found in the two reservoirs, we find that fog acts as an efficient separator for carbon (C) species on the basis of their chemical properties, with polar water soluble species representing the greater part of total C within fog droplets, and water insoluble C species preferentially found in the interstitial reservoir. Water‐soluble organic species are scavenged by fog droplets to a comparable extent to major inorganic ions and are therefore expected to play an important role in the droplet nucleation process. The main classes of water soluble organic carbon (WSOC) identified in fog water and interstitial aerosol by the techniques traditionally used in aerosol analysis are aliphatic dicarboxylic acids, sugars, aliphatic alcohols, and aliphatic carboxylic acids. However, such species, ∼120 individual compounds, only account for a few percent (<5% on average) of total WSOC. A new class of water soluble macromolecular compounds (MMC), detected in aerosol samples from different areas of the globe, are found to constitute a large fraction (∼40% on average) of WSOC in the fog system (fog droplets plus interstitial aerosol) and represent the main class of water soluble species identified. More than 50% of WSOC still remains undetermined.

The influence of topography on the nature of humic substances in soil organic matter at a site in the Atlantic Coastal Plain of South Carolina

The effect of topography on the nature of humic substances, isolated as water soluble organic carbon (WSOC), fulvic acid (FA), and humic acid (HA) was evaluated by comparing relative proportion and chemical characteristics of these fractions in upland and bottomland Coastal Plain soils in South Carolina. The fractions were characterized by elemental analysis and13C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The majority of humic substances occur as humic acids, with bottomland soils having higher HA/FA ratios when compared to upland soils. We found no significant differences between upland and bottomland humic substances with respect to yields of WSOC and fulvic acids, and in the C and N content of humic and fulvic acids. Carbon-13 CPMAS NMR spectroscopy revealed that the WSOC and fulvic acid fractions were composed largely of O-alkyl-C structures with bottomland soils having higher amounts of these groups. Humic acid C distribution was similar between upland and bottomland soils and was largely composed of aromatic groups. Our results demonstrate that topography influences the formation of humic acid and the structural and chemical properties of the various humic fractions.