Dual carbon isotope-based brown carbon aerosol characteristics at a high-altitude site in the northeastern Himalayas: Role of biomass burning

PM2.5 samples (n = 34) were collected from January to April 2017 over Shillong (25.7°N, 91.9°E; 1064 m amsl), a high-altitude site situated in the northeastern Himalaya. The main aim was to understand the sources, characteristics, and optical properties of local vs long-range transported carbonaceous aerosols (CA) using chemical species and dual carbon isotopes (13C and 14C). Percentage biomass burning (BB)/biogenic fraction (fbio, calculated from 14C) varied from 67 to 92 % (78 ± 7) and correlated well with primary BB tracers like f60, and K+, suggesting BB as a considerable source. Rain events are shown to reduce the fbio fraction, indicating majority of BB-derived CA are transported. Further, δ13C (-26.6 ± 0.4) variability was very low over Shillong, suggesting it's limitations in source apportionment over the study region, if used alone. Average ratio of absorption coefficient of methanol-soluble BrC (BrCMS) to water-soluble BrC (BrCWS) at 365 nm was 1.8, indicating a significant part of BrC was water-insoluble. A good positive correlation between fbio and mass absorption efficiency of BrCWS and BrCMS at 365 nm with the higher slope for BrCMS suggests BB derived water-insoluble BrC was more absorbing. Relative radiative forcing (RRF, 300 to 2500 nm) of BrCWS and BrCMS with respect to EC were 11 ± 5 % and 23 ± 16 %, respectively. Further, the RRF of BrCMS was up to 60 %, and that of BrCWS was up to 22 % with respect to EC for the samples with fbio ≥ 0.85 (i.e., dominated by BB), reflecting the importance of BB in BrC RRF estimation.

Year-round observations of stable carbon isotopic composition of carboxylic acids, oxoacids and α-Dicarbonyls in fine aerosols at Tianjin, North China: Implications for origins and aging

To better understand the origins and photochemical processing (aging) of organic aerosols (OA), we studied fine aerosols (PM_2.5) collected at urban (Nankai District (ND)) and suburban (Haihe Education Park (HEP)) Tianjin, North China over a one-year period (2018-2019) for stable carbon isotopic composition (δ¹³C) of water-soluble diacids, oxoacids, α-dicarbonyls and fatty acids. Maleic (M, -18.3 ± 10.9‰ at ND and -23.5 ± 10.2‰ at HEP) and fumaric (F, -22.0 ± 12.1‰ at ND and -22.5 ± 10.5‰ at HEP) acids were found to be most enriched with ¹³C followed by oxalic acid (C₂, -24.7 ± 3.9‰ at ND and -25.9 ± 4.7‰ at HEP) during the campaign. Based on seasonal changes in δ¹³C of selected marker species: C₆ and C₉ diacids, phthalic, glyoxylic and pyruvic acids and glyoxal, and their comparison with the source signatures, we found that water-soluble OA in Tianjin were mainly originated from fossil fuel combustion and biomass burning emissions and were subjected for significant aging. The contribution from fossil fuel combustion including coal combustion was high in autumn and winter, especially at ND. Considering the enrichment of ¹³C in specific species together with linear relations of δ¹³C of selected species with their concentrations, with mass ratios and with the relative abundance of C₂ diacid, we inferred that the photochemical transformations of longer-chain diacids, oxidation of α-dicarbonyls (Gly and mGly), preferably in gas phase, were important in warm period (March-September), whereas the oxidation of Gly, mGly and other precursors in aqueous phase were major in cold period (October-February).

Photodegradation of Molecular Iodine on SiO2 Particles: Influence of Temperature and Relative Humidity

A molecular derivatization method followed by gas chromatographic separation coupled with mass spectrometric detection was used to study photodegradation of molecular I2 adsorbed on solid SiO2 particles. The heterogeneous photodegradation of I2 was studied as a function of temperature and relative humidity in synthetic air to better understand its environmental fate. Two sets of experiments were carried out. In the first set of experiments, the temperature was T = (298 ± 1) K and relative humidity was varied from ≤ 2% to 75%RH under given experimental conditions. In the second set of experiments, the relative humidity within the Pyrex bulb was 40%RH and the temperature was varied from 283 ± 1 ≤ T (K) ≤ 323 ± 1. The obtained results show a considerably enhanced atmospheric lifetime of molecular iodine adsorbed on solid media that does not depend on relative humidity of the environment. The obtained results show that the rate constant for the photolysis of molecular iodine adsorbed on model SiO2 particles depends on temperature and is reported to be J (T)=(1.24 ± 1.4)×10−2×exp[(1482±345)/T]/s over the measured temperature range. The heterogeneous atmospheric residence time () of I2 adsorbed on solid media is calculated to range from 2 to 4.1 h. The experimentally obtained heterogeneous lifetime of I2 is shown to be considerably longer than its destruction by its principal atmospheric sink, photolysis. The observed enhanced atmospheric lifetime of I2 on heterogeneous media will likely have direct consequences on the atmospheric transport of I2 that influences the toxicity or the oxidative capacity of the atmosphere.

Photolytic degradation of molecular iodine adsorbed on model SiO2 particles

A molecular derivatization method followed by gas chromatographic separation coupled with mass spectrometric detection was used to study photolytic degradation of I₂ adsorbed on solid SiO₂ particles. This heterogeneous photodegradation of I₂ is studied at ambient temperature in synthetic air to better understand I₂ atmospheric dispersion and environmental fate. The obtained laboratory results show a considerably enhanced atmospheric lifetime of molecular iodine adsorbed on solid media. The heterogeneous atmospheric residence time (τ) of I₂ is calculated to be τ ≈ 187 min, i.e., τ ≈ 3 h. The obtained heterogeneous lifetime of I₂ is shown to be considerably longer than its destruction by its principal atmospheric sink, namely, photolysis. The observed enhanced atmospheric lifetime of I₂ on heterogeneous media will likely have direct consequences on the atmospheric transport of I₂ that influences the toxicity or the oxidative capacity of the atmosphere.

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in the urban atmosphere of the North China Plain: Implications for aqueous phase formation of SOA during the haze periods

In the past five years, Chinese government has promulgated stringent measures to mitigate air pollution. However, PM_2.5 levels in the China North Plain (NCP), which is one of the regions with the heaviest air pollution in the world, are still far beyond the World Health Organization (WHO) standard. To improve our understanding on the sources and formation mechanisms of haze in the NCP, PM_2.5 samples were collected during the winter of 2017 on a day/night basis at the urban site of Liaocheng, which is one of the most polluted cities in the NCP. The samples were determined for molecular distributions and stable carbon isotope compositions of dicarboxylic acids and their precursors (ketocarboxylic acids and α-dicarbonyls), levoglucosan, elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC). Our results showed that oxalic acid (C₂) is the dominant dicarboxylic acid, followed by succinic acid (C₄) and malonic acid (C₃), and glyoxylic acid (ωC₂) is the most abundant ketocarboxylic acids. Concentrations of C₂, glyoxal (Gly) and methylglyoxal (mGly) presented robust correlations with levoglucosan, suggesting that biomass burning is a significant source of PM_2.5 in the NCP. Moreover, C₂ and Gly and mGly linearly correlated with SO₄^2-, relative humidity (RH), aerosol liquid water content (LWC) as well as particle in-situ pH (pH_is), indicating that aqueous-phase oxidation is the major formation pathway of these SOA, and is driven by acid-catalyzed oxidation. Concentrations and relative abundances of secondary species including SNA (SO₄^2-, NO₃^- and NH₄⁺), dicarboxylic acids, and aerosol LWC in PM_2.5 are much higher in the haze periods than in the clean periods, suggesting that aqueous reaction is a vital role in the haze formation. In comparison with those in the clean periods, stable carbon isotopic compositions (δ¹³C) of major dicarboxylic acids and related SOA and the mass ratios of C₂/diacids, C₂/Gly and C₂/mGly are higher in the haze periods, indicating that haze particles were more aged and enriched in secondary species.

Enhanced aqueous-phase formation of secondary organic aerosols due to the regional biomass burning over North China Plain

This study reveals the impact of biomass burning (BB) on secondary organic aerosols (SOA) formation in the North China Plain (NCP). Filter samples were analyzed for secondary inorganic aerosols (SIA), oxalic acid (C₂) and related aqueous-phase SOA compounds (aqSOA), stable carbon isotope composition of C₂ (δ¹³C(C₂)) and aerosol liquid water content (ALWC). Based on the PM_2.5 loadings, BB tracer concentrations, wildfire spots and air-mass back trajectories, we distinguished two episodes from the whole campaign, Episode I and Episode II, which were characteristic of regional and local BB, respectively. The abundances of PM_2.5 and organic matter in the two events were comparable, but concentrations and fractions of SIA, aqSOA during Episode I were much higher than those during Episode II, along with heavier δ¹³C(C₂), suggesting an enhanced aqSOA formation in the earlier period. We found that the enhancement of aqSOA formation during Episode I was caused by an increased ALWC, which was mainly driven by SIA during the regional BB event. Our work showed that intensive burning of crop residue in East Asia can sharply enhance aqSOA production on a large scale, which may have a significant impact on the regional climate and human health.

Determination of the stable carbon isotopic compositions of 2-methyltetrols for four forest areas in Southwest China: The implications for the δ13C values of atmospheric isoprene and C3/C4 vegetation distribution

Isoprene is the most abundant non-methane hydrocarbons (NMHCs) in the troposphere and is released predominantly by vegetation. The δ¹³C values of atmospheric isoprene vary with different plant types (e.g. C₃ and C₄ plants). In this work, aerosol samples were collected in four forest areas in Sichuan Province, China, i.e., the Baima Spring Scenic Area (BM), the Panzhihua Cycas Nature Reserve (PZ), the Gongga Mountain National Nature Reserve (GG) and the Wolong National Nature Reserve (WL) during the summers of 2010-2012. The stable carbon isotopic compositions of 2-methyltetrols, the stable products of isoprene oxidation by OH, were measured using a GC/C/IRMS (gas chromatography/combustion/isotopic ratio mass spectrometry) with methylboronic acid derivatization. The stable carbon isotopic fractionation coefficient of isoprene oxidized by OH (^OHε_i) was derived in laboratory. With the δ¹³C values of 2-methyltetrols, ^OHε_i and meteorological parameters, the δ¹³C values of atmospheric isoprene were calculated. The results show that forests can remarkably change the δ¹³C values of isoprene in the regional scales, making significant contributions to isoprene emissions. Moreover, C₃/C₄ proportions of shrubs and grasses depend on altitudes. The average δ¹³C values of atmospheric isoprene are -24.18 ± 1.72‰, -25.81 ± 1.36‰, -24.96 ± 0.94‰, -25.89 ± 1.35‰ for BM, PZ, GG and WL, respectively. The average δ¹³C value of atmospheric isoprene in SW China and the surrounding areas was -25.23 ± 1.44‰. C₄ plants emitted 26.9 ± 10.3% of isoprene in the research atmosphere.

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of Abiotic Degradation of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies

Methyl bromide (CH₃Br) and methyl chloride (CH₃Cl) significantly contribute to stratospheric ozone depletion. The atmospheric budgets of both compounds are unbalanced with known degradation processes outweighing known emissions. Stable isotope analysis may be capable to identify and quantify emissions and to achieve a balanced budget. Degradation processes do, however, cause isotope fractionation in methyl halides after emission and hence knowledge about these processes is a crucial prerequisite for any isotopic mass balance approach. In the current study, triple-element isotope analysis (²H, ¹³C, ³⁷Cl/⁸¹Br) was applied to investigate the two main abiotic degradation processes of methyl halides (CH₃X) in fresh and seawater: hydrolysis and halide exchange. For CH₃Br, nucleophilic attack by both H₂O and Cl^- caused significant primary carbon and bromine isotope effects accompanied by a secondary inverse hydrogen isotope effect. For CH₃Cl only nucleophilic substitution by H₂O was observed at significant rates causing large primary carbon and chlorine isotope effects and a secondary inverse hydrogen isotope effect. Observed dual-element isotope ratios differed slightly from literature values for microbial degradation in water and hugely from radical reactions in the troposphere. This bodes well for successfully distinguishing and quantifying degradation processes in atmospheric methyl halides using triple-element isotope analysis.

Sources, compositions, and optical properties of humic-like substances in Beijing during the 2014 APEC summit: Results from dual carbon isotope and Fourier-transform ion cyclotron resonance mass spectrometry analyses

Humic-like substances (HULIS) are a class of high molecular weight, light-absorbing compounds that are highly related to brown carbon (BrC). In this study, the sources and compositions of HULIS isolated from fine particles collected in Beijing, China during the 2014 Asia-Pacific Economic Cooperation (APEC) summit were characterized based on carbon isotope (¹³C and ¹⁴C) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses, respectively. HULIS were the main light-absorbing components of water-soluble organic carbon (WSOC), accounting for 80.2 ± 6.1% of the WSOC absorption capacity at 365 nm. The carbon isotope data showed that HULIS had a lower non-fossil contribution (53 ± 4%) and were less enriched with ¹³C (-24.2 ± 0.6‰) relative to non-HULIS (62 ± 8% and -20.8 ± 0.3‰, respectively). The higher relative intensity fraction of sulfur-containing compounds in HULIS before and after APEC was attributed to higher sulfur dioxide levels emitted from fossil fuel combustion, whereas the higher fraction of nitrogen-containing compounds during APEC may have been due to the relatively greater contribution of non-fossil compounds or the influence of nitrate radical chemistry. The results of investigating the relationships among the sources, elemental compositions, and optical properties of HULIS demonstrated that the light absorption of HULIS appeared to increase with increasing unsaturation degree, but decrease with increasing oxidation level. The unsaturation of HULIS was affected by both sources and aging level.

Enrichment of 13C in diacids and related compounds during photochemical processing of aqueous aerosols: New proxy for organic aerosols aging

To investigate the applicability of compound specific stable carbon isotope ratios (δ¹³C) of organics in assessment of their photochemical aging in the atmosphere, batch UV irradiation experiments were conducted on two ambient (anthropogenic and biogenic) aerosol samples in aqueous phase for 0.5–120 h. The irradiated samples were analyzed for δ¹³C of diacids, glyoxylic acid (ωC₂) and glyoxal. δ¹³C of diacids and related compounds became larger with irradiation time (i.e., aging), except for few cases. In general, δ¹³C of C₂-C₄ diacids showed an increasing trend with decreasing chain length. Based on δ¹³C of diacids and related compounds and their relations to their concentrations, we found that C₂ and C₃ are enriched with ¹³C during the photochemical decomposition and production from their higher homologues and oxoacids. Photochemical breakdown of higher (≥C₃) to lower diacids is also important in the enrichment of ¹³C in C₃-C₉ diacids whereas their production from primary precursors causes depletion of ¹³C. In case of ωC₂ and glyoxal, their photochemical production and further oxidation to highly oxygenated compounds both cause the enrichment of ¹³C. This study reveals that δ¹³C of diacids and related compounds can be used as a proxy to trace the aging of organic aerosols during long-range atmospheric transport.

Analyzing sites of OH radical attack (ring vs. side chain) in oxidation of substituted benzenes via dual stable isotope analysis (δ(13)C and δ(2)H)

OH radicals generated by the photolysis of H2O2 can degrade aromatic contaminants by either attacking the aromatic ring to form phenolic products or oxidizing the substituent. We characterized these competing pathways by analyzing the carbon and hydrogen isotope fractionation (εC and εH) of various substituted benzenes. For benzene and halobenzenes that only undergo ring addition, low values of εC (-0.7‰ to -1.0‰) were observed compared with theoretical values (-7.2‰ to -8‰), possibly owing to masking effect caused by pre-equilibrium between the substrate and OH radical preceding the rate-limiting step. In contrast, the addition of OH radicals to nitrobenzene ring showed a higher εC (-3.9‰), probably due to the lower reactivity. Xylene isomers, anisole, aniline, N,N-dimethylaniline, and benzonitrile yielded normal εH values (-2.8‰ to -29‰) indicating the occurrence of side-chain reactions, in contrast to the inverse εH (11.7‰ to 30‰) observed for ring addition due to an sp(2) to sp(3) hybridization change at the reacting carbon. Inverse εH values for toluene (14‰) and ethylbenzene (30‰) were observed despite the formation of side-chain oxidation products, suggesting that ring addition has a larger contribution to isotope fractionation. Dual element isotope slopes (∆δ(2)H/∆δ(13)C) therefore allow identification of significant degradation pathways of aromatic compounds by photochemically induced OH radicals. Issues that should be addressed in future studies include quantitative determination of the contribution of each competing pathway to the observed isotope fractionation and characterization of physical processes preceding the reaction that could affect isotope fractionation.

Stepwise Oxidation of Aqueous Dicarboxylic Acids by Gas-Phase OH Radicals

A leading source of uncertainty in predicting the climate and health effects of secondary organic aerosol (SOA) is how its composition changes over their atmospheric lifetimes. Because dicarboxylic acid (DCA) homologues are widespread in SOA, their distribution provides an ideal probe of both aerosol age and the oxidative power of the atmosphere along its trajectory. Here we report, for the first time, on the oxidation of DCA(aq) by ·OH(g) at the air-water interface. We found that exposure of aqueous HOOC-Rn-COOH (Rn = C2H4, C3H6, C4H8, C5H10, and C6H12) microjets to ∼10 ns ·OH(g) pulses from the 266 nm laser photolysis of O3(g)/O2(g)/H2O(g) mixtures yields the corresponding (n-1) species O═C(H)-Rn-1-COO(-)/HOOC-Rn-1-COO(-), in addition to an array of closed-shell HOOC-Rn(-H)(OOH)-COO(-), HOOC-Rn(-2H)(═O)-COO(-), HOOC-Rn(-H)(OH)-COO(-), and radical HOOC-Rn(-H)(OO·)-COO(-) species. Oxalic and malonic acids, which are shown to be significantly less hydrophobic and reactive than their higher homologues, will predictably accumulate in SOA, in accordance with field observations.

New insights to the use of ethanol in automotive fuels: a stable isotopic tracer for fossil- and bio-fuel combustion inputs to the atmosphere

Ethanol is currently receiving increased attention because of its use as a biofuel or fuel additive and because of its influence on air quality. We used stable isotopic ratio measurements of (13)C/(12)C in ethanol emitted from vehicles and a small group of tropical plants to establish ethanol's δ(13)C end-member signatures. Ethanol emitted in exhaust is distinctly different from that emitted by tropical plants and can serve as a unique stable isotopic tracer for transportation-related inputs to the atmosphere. Ethanol's unique isotopic signature in fuel is related to corn, a C4 plant and the primary source of ethanol in the U.S. We estimated a kinetic isotope effect (KIE) for ethanol's oxidative loss in the atmosphere and used previous assumptions with respect to the fractionation that may occur during wet and dry deposition. A small number of interpretive model calculations were used for source apportionment of ethanol and to understand the associated effects resulting from atmospheric removal. The models incorporated our end-member signatures and ambient measurements of ethanol, known or estimated source strengths and removal magnitudes, and estimated KIEs associated with atmospheric removal processes for ethanol. We compared transportation-related ethanol signatures to those from biogenic sources and used a set of ambient measurements to apportion each source contribution in Miami, Florida-a moderately polluted, but well ventilated urban location.

Isotopic composition of formaldehyde in urban air

The isotopic composition of atmospheric formaldehyde was measured in air samples collected in urban Seattle, Washington. A recently developed gas chromatography-isotope ratio mass spectrometry analytical technique was used to extract formaldehyde directly from whole air, separate it from other volatile organic compounds, and measure its (13)C/(12)C and D/H ratio. Measurements of formaldehyde concentration were also made concomitant with isotope ratio. Results of the analysis of nine discrete air samples for delta(13)C-HCHO have a relatively small range in isotopic composition (-31 to -25 per thousand versus VPDB [+/-1.3 per thousand]) over a considerable concentration range (0.8-4.4 ppb [+/-15%]). In contrast, analyses of 17 air samples for deltaD-HCHO show a large range (-296 to +210 per thousand versus VSMOW [+/-50 per thousand]) over the concentrations measured (0.5-2.9 ppb). Observations of deltaD are weakly anticorrelated with concentration. Isotopic data are interpreted using both source- and sink-based approaches. Results of delta(13)C-HCHO are similar to those observed previously for a number of nonmethane hydrocarbons in urban environments and variability can be reconciled with a simple sink-based model. The large variability observed in deltaD-HCHO favors a source-based interpretation with HCHO depleted in deuterium from primary sources of HCHO (i.e., combustion) and HCHO enriched in deuterium from secondary photochemical sources (i.e., hydrocarbon oxidation).

Measurements of the 12C/13C Kinetic Isotope Effects in the Gas-Phase Reactions of Light Alkanes with Chlorine Atoms

The carbon kinetic isotope effects (KIEs) of the reactions of several light non-methane hydrocarbons (NMHC) with Cl atoms were determined at room temperature and ambient pressure. All measured KIEs, defined as the ratio of the Cl reaction rate constants of the light isotopologue over that of the heavy isotopologue (Clk12/Clk13) are greater than unity or normal KIEs. For simplicity, measured KIEs are reported in per mil according to Clepsilon=(Clk12/Clk13 -1)x1000 per thousand unless noted otherwise. The following average KIEs were obtained (all in per thousand): 10.73+/-0.20 (ethane), 6.44+/-0.14 (propane), 6.18+/-0.18 (methylpropane), 3.94+/-0.01 (n-butane), 1.79+/-0.42 (methylbutane), 3.22+/-0.17 (n-pentane), 2.02+/-0.40 (n-hexane), 2.06+/-0.19 (n-heptane), 1.54+/-0.15 (n-octane), 3.04+/-0.09 (cyclopentane), 2.30+/-0.09 (cyclohexane), and 2.56+/-0.25 (methylcyclopentane). Measurements of the 12C/13C KIEs for the Cl atom reactions of the C2-C8 n-alkanes were also made at 348 K, and no significant temperature dependence was observed. To our knowledge, these 12C/13C KIE measurements for alkanes+Cl reactions are the first of their kind. Simultaneous to the KIE measurement, the rate constant for the reaction of each alkane with Cl atoms was measured using a relative rate method. Our measurements agree with published values within+/-20%. The measured rate constant for methylcyclopentane, for which no literature value is available, is (2.83+/-0.11)x10-10 cm3 molecule-1 s-1, 1sigma standard error. The Clepsilon values presented here for the C2-C8 alkanes are an order of magnitude smaller than reported methane Clepsilon values (Geophys. Res. Lett., 2000, 27, 1715), in contrast to reported OHepsilon values for methane (J. Geophys. Res. (Atmos.), 2001, 106, 23, 127) and C2-C8 alkanes (J. Phys. Chem. A, 2004, 108, 11537), which are all smaller than 10 per thousand. This has important implications for atmospheric modeling of saturated NMHC stable carbon isotope ratios. 13C-structure reactivity relationship values (13C-SRR) for alkane-Cl reactions have been determined and are similar to previously reported values for alkane-OH reactions.

Determination of stable carbon isotopes of organic acids and carbonaceous aerosols in the atmosphere

A wet oxidation method for the compound-specific determination of stable carbon isotopes (delta(13)C) of organic acids in the gas and aerosol phase, as well as of water-soluble organic carbon (WSOC), is presented. Sampling of the organic acids was done using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to an ion chromatography (IC) system. The method allows for compound-specific stable carbon isotope analysis by collecting different fractions of organic acids at the end of the IC system using a fraction collector. delta(13)C analyses of organic acids were conducted by oxidizing the organic acids with sodium persulfate at a temperature of 100 degrees C and determining the delta(13)C value of the resulting carbon dioxide (CO(2)) with an isotope ratio mass spectrometer. In addition, analysis of delta(13)C of the WSOC was performed for particulate carbon collected on aerosol filters. The WSOC was extracted from the filters using ultrapure water (MQ water), and the dissolved organic carbon was oxidized to CO(2) using the oxidation method. The wet oxidation method has an accuracy of 0.5 per thousand with a precision of +/-0.4 per thousand and provides a quantitative result for organic carbon with a detection limit of 150 ng of carbon.

Development of two-dimensional gas chromatography/isotope ratio mass spectrometry for the stable carbon isotopic analysis of C(2)-C(5) non-methane hydrocarbons emitted from biomass burning

A two-dimensional gas chromatography/combustion/isotope ratio mass spectrometry (2D-GC/C/IRMS) system was developed for stable carbon isotopic measurements of C(2)-C(5) non-methane hydrocarbons (NMHCs) in biomass burning smoke. The 2D-GC/C/IRMS system successfully improved the accuracy and precision for the measurements of C(4) and C(5) saturated compounds in a smoke sample by selective injection of target compounds into a combustion furnace and consequently allowed us to provide complete baseline separation for all individual NMHCs. The analytical precision of the delta(13)C of each compound was better than 0.5 per thousand for more than 500 pmolC injections and 2.1 per thousand for 30 pmolC injections, which was estimated from replicate analysis of standard gases. This system was applied to the analysis of NMHCs in smoke samples collected from laboratory biomass burning experiments. From the combustion of three fuel materials (rice straw, pine wood, and maize), we found that the isotopic fractionation between fuel material and individual NMHCs is almost independent of the fuel material and thus the delta(13)C values of the fuel materials are reflected in delta(13)C values of most of NMHCs. However, only i-butane emitted from maize combustion showed anomalous (13)C-depletion of -11.6 per thousand relative to the delta(13)C value of maize. Such a large (13)C depletion suggests the specific isotopic fractionation process which is attributed to the maize combustion itself or the chemical properties of i-butane during production from a radical recombination reaction.

A new concept linking observable stable isotope fractionation to transformation pathways of organic pollutants

Measuring stable isotope fractionation of carbon, hydrogen, and other elements by Compound Specific Isotope Analysis (CSIA) is a new, innovative approach to assess organic pollutant degradation in the environment. Central to this concept is the Rayleigh equation which relates degradation-induced decreases in concentrations directly to concomitant changes in bulk (= average over the whole compound) isotope ratios. The extent of in situ transformation may therefore be inferred from measured isotope ratios in field samples, provided that an appropriate enrichment factor (epsilonbulk) is known. This epsilonbulk value, however, is usually only valid for a specific compound and for specific degradation conditions. Therefore, a direct comparison of epsilonbulk values for different compounds and for different types of reactions has in general not been feasible. In addition, it is often uncertain how robust and reproducible epsilonbulk values are and how confidently they can be used to quantify contaminant degradation in the field. To improve this situation and to achieve a more in-depth understanding, this critical review aims to relate fundamental insight about kinetic isotope effects (KIE) found in the physico(bio)chemical literature to apparent kinetic isotope effects (AKIE) derived from epsilonbulk values reported in environmentally oriented studies. Starting from basic rate laws, a quite general derivation of the Rayleigh equation is given, resulting in a novel set of simple equations that take into account the effects of (1) nonreacting positions and (2) intramolecular competition and that lead to position-specific AKIE values rather than bulk enrichment factors. Reevaluation of existing epsilonbulk literature values result in consistent ranges of AKIE values that generally are in good agreement with previously published data in the (bio)-chemical literature and are typical of certain degradation reactions (subscripts C and H indicate values for carbon and hydrogen): AKIEc = 1.01-1.03 and AKIEH = 2-23 for oxidation of C-H bonds; AKIEc = 1.03-1.07 for SN2-reactions; AKIEc = 1.02-1.03 for reductive cleavage of C-Cl bonds; AKIEc = 1.00-1.01 for C=C bond epoxidation; AKIEc = 1.02-1.03 for C=C bond oxidation by permanganate. Hence, the evaluation scheme presented bridges a gap between basic and environmental (bio)chemistry and provides insight into factors that control the magnitude of bulk isotope fractionation factors. It also serves as a basis to identify degradation pathways using isotope data. It is shown how such an analysis may be even possible in complex field situations and/or in cases where AKIE values are smaller than intrinsic KIE values, provided that isotope fractionation is measured for two elements simultaneously ("two-dimensional isotope analysis"). Finally, the procedure is used (1) to point outthe possibility of estimating approximate epsilonbulk values for new compounds and (2) to discuss the moderate, but non-negligible variability that may quite generally be associated with epsilonbulk values. Future research is suggested to better understand and take into account the various factors that may cause such variability.