ARTICLE pubs.acs.org/est
Oxidation of Atmospheric Humic Like Substances by Ozone: A Kinetic and Structural Analysis Approach Christine Baduel,†,|| Maria E. Monge,‡,|| Didier Voisin,*,† Jean-Luc Jaffrezo,† Christian George,‡ Imad El Haddad,§,^ Nicolas Marchand,§ and Barbara D’Anna‡ †
UJF-Grenoble 1/CNRS, Laboratoire de Glaciologie et Geophysique de l’Environnement (LGGE), UMR 5183, Grenoble, F-38041, France ‡ Universite de Lyon 1/CNRS, UMR5256, IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Villeurbanne, France § quipe Instrumentation et Reactivite Atmospherique, Universite d’Aix-Marseille/CNRS, UMR 6264, Laboratoire Chimie Provence, E Marseille, F-13333, France
bS Supporting Information ABSTRACT: This work explores the heterogeneous reaction between HUmicLIke Substances (so-called HULIS) and ozone. Genuine atmospheric HULIS were extracted from aerosol samples collected in Chamonix (France) in winter and used in coated flow tube experiments to evaluate heterogeneous uptake of O3 on such mixtures. The uptake coefficient (γ) was investigated as a function of pH (from 2.5 to 10), O3 concentration (from 8 to 33 1011 molecules cm3), relative humidity (20 to 65%) and photon flux (from 0 to 1.66 1015 photons cm2 s1). Reactive uptake was found to increase in the irradiated experiment with pH, humidity and photon flux. The extract was characterized before and after exposure to O3 and/or UV light in the attempt to elucidate the effect of the photochemical aging. Carbon content measurements, UVvis spectroscopy and functional groups analysis revealed a decrease of the UV absorbance as well as of the carbon mass content, while the functionalization rate (COOH and CdO) and therefore the polarity increased during the simulated photochemical exposure.
’ INTRODUCTION The term HULIS (humic like substances) is used to define the organic fraction found in aerosols and hydrometeors that exhibits similar chemical properties (UVvis absorbance, fluorescence) to the ubiquitous humic substances present in aquatic and terrestrial systems. HULIS-type material has been found in marine, soil dust, biomass-burning, biogenic, and urban fine aerosols,16 representing a major part of the organic fraction. HULIS are involved in several atmospheric processes including light absorption7 and cloud droplets formation.8 Based on seasonal cycles of HULIS concentrations all over Europe, it was hypothesized1 that the main sources are biomass burning in winter, and secondary production in summer. This has been recently confirmed in urban settings through tracer studies, and has been shown to be associated to distinct variations in specific absorbance of HULIS per unit mass of carbon.2 Whereas primary biomass burning emissions of HULIS have been well characterized,9,10 secondary pathways leading to their formation and evolution are still poorly characterized. These include photochemical production in clouds11 as well as heterogeneous production from various mechanisms.1214 Yet, further photochemical processing of HULIS within the atmosphere has not been widely explored, though it might impact optical and r 2011 American Chemical Society
hygroscopic properties,15 as their ability to activate cloud droplets. These poorly recognized processes involve direct photolysis of HULIS, heterogeneous reactions with gas phase oxidants such as O3 or OH radicals, and/or photosensitized reactions with oxidants such as O3 or NO2. Most studies in the atmosphere have focused on the watersoluble fraction of humic like substances (HULISWS), although this is only a portion of the total humic like fraction (HULIST). The latter, extracted in alkali solution, has been shown to roughly double the amount of the water-soluble fraction of humic like substances present in the aerosol 1,17,16 and has a higher absorption coefficient compared to the water-soluble counterpart, especially under 350 nm, making HULIST more prone to be involved in photochemistry.16 The reactivity toward O3 of fulvic and humic acids, as proxies of HULIS, has been investigated under simulated atmospheric conditions;18,19 the heterogeneous loss of ozone on humic and fulvic acids films and aerosol was Received: February 20, 2011 Accepted: May 11, 2011 Revised: May 10, 2011 Published: May 16, 2011 5238
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Environmental Science & Technology important and strongly photoenhanced.19 However, no real structural analysis of HULIST has ever been done, and HULISWS have been shown to differ from humic substances in terms of aromaticity, acidity and molecular weight. As the reactive O3 loss with organic compounds depend on their molecular arrangement and functionality, humic substances reactivity toward O3 cannot be directly used to infer HULIST (photo)chemical reactivity and aging.20 The aim of this work was to study the reactivity of genuine HULIST aerosol extracts toward O3, by investigating the associated kinetics in the dark and under irradiation as a function of humidity, temperature, photon flux, and solution pH. The genuine HULIST extract was characterized before and after ozonation under irradiation to understand the structural modifications from photochemical aging.
’ EXPERIMENTAL METHODS Aerosol Sampling and HULIST Extraction. Details on the sampling site and procedures can be found elsewhere.16 Briefly, the sampling site was located in the suburban area of the Chamonix town (6520 1600 E; 45160 3400 N; altitude 1038 m a.s.l.), surrounded by tall mountains culminating with the Mont Blanc (4810 m a.s.l.). Previous studies 2,21 have shown that at this site, residential wood burning was a significant source of organic aerosol in winter. PM10 sampling took place from 10th to 19th December 2007 and was based on a day/night pattern, with 12 h duration. The 16 samples used here for HULIST extraction have also been extensively used for PM10 source apportionment and chemical characterization studies,2,21 and a list of available complementary measurements is given in Supporting Information (SI) Table S1. The extraction of HULIST was adapted from our previous work16 to accommodate larger amounts of carbon (see detailed protocol in the SI). Briefly, a fraction (360 cm2) of each filter was extracted by sonication in a 0.1 M NaOH solution, then filtered and extracted first on anion exchange columns (GE Healthcare, HiTrap FF), then on C18 SPE column (IST, 2210010-H) to eliminate chloride ions from the first extraction step, as those may interfere in the photo-oxidation experiments.22 The 16 resulting extracts were pooled together, evaporated to dryness, and the dried HULIST were redissolved in 36 mL of ultrapure water. HULIST Initial Characterization and Aging Experiment. The total carbon content of HULIST was determined using a Thermo-Optical Transmission (TOT) method on a Sunset Lab analyzer, following the newly developed EUSAAR2 protocol.23 Light absorption spectra of the HULIST sample before and after the aging experiment were measured with a Dionex UVD340U diode array UVvis spectrophotometer. Functional group analysis was performed by atmospheric pressure chemical ionization (APCI)-tandem mass spectrometry (Varian 1200 L). This technique is based on the ability of the carboxylic, carbonyl, and nitro functional groups to lose a specific neutral fragment or to produce a characteristic ion in the collision cell. It enables the quantitative determination of functional groups concentration in the aerosol extract. When divided by the carbon content in the extract, this analysis gives access to the functionalization rate, expressed as percent of carbon atoms bearing the functional group of interest.21 Photochemical aging was simulated for 1.5 mL of the same HULIST solution used in the kinetic experiment (12.5 mg C L1,
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Table 1. Correlations Observed in Chamonix in Winter Between HULIST (y, μgC m3) and Other Aerosol Components (x, μg m3) (15 Points) correlation equation
correlation coefficient
x = HULISWS
y = (2.15 ( 0.09) x 0.08 ( 0.15 r2 = 0.96; p < 0.001
x = EC
y = (0.55 ( 0.13) x þ 0.71 ( 0.65 r2 = 0.46; p = 0.006
x = levoglucosan y = (3.0 ( 0.4) x þ 0.16 ( 0.8 x = OC
r2 = 0.83; p < 0.001
y = (0.28 ( 0.03) x 0.05 ( 0.29 r2 = 0.92; p < 0.001
pH 5.8) by irradiating the solution in pyrex vials during 3 h with a Xe lamp (75W) (1.8 1017 photons cm2 s1), simulating solar irradiance. During irradiation 45 ppbv of O3 was bubbled in the solution using a flow of 60 μL min1. Control experiments were performed by irradiating the solution with a flow of air or by bubbling O3 in the dark for 3 h. O3 Uptake Coefficient Measurements. The uptake coefficients of O3 on HULIST films are measured in an atmospheric pressure coated wall flow tube.19 Further details are provided in the SI. The thermostatted (288 K) Pyrex reactor is surrounded by seven fluorescent lamps (Philips, CLEO 20W) having a continuous emission in the 300420 nm range and a stable total irradiance of 1.66 1015 photons cm2 s1.24 The organic film is deposited on the inner surface of the Pyrex tube by drying 0.5 mL of the HULIST extract (12.5 mg C L1). The total flow rate inside the reactor is controlled at 335 ( 2 mL min1, ensuring a laminar regime. Ozone is generated by flowing pure air through a UV light source (UVP-ozone generator) and fed into the flow tube through a movable injector at five different positions corresponding to contact times from 0.7 to 2.7 s between O3 and the HULIST film. The ozone concentration, temperature, and relative humidity are monitored at the exit of the flow tube. Experiments were conducted at 20, 40, and 65% RH. The initial solutions’ pH was adjusted by adding a few microliters of NaOH or H2SO4, (50 mM). Ozone photolysis was shown to be negligible in all experiments, and no photoenhanced uptake was observed on photochemically inactive films (such as those made of NaCl) Uptake coefficients were calculated by fitting a pseudo first order kinetics18 to the observed time dependent ozone loss rate. This gives a pseudo first order observed loss rate constant kobs that is related to the observed geometric uptake coefficient (γobs) by kobs ¼ γobs 3
Æcæ d
ð1Þ
where d is the inner diameter of the tube, and Æcæ = (8RT/πM)0.5 is the ozone mean thermal velocity. The observed uptake coefficient was then corrected for gas phase diffusion limitation using the Cooney Kim Davis method,25 with diffusion coefficients calculated from Fuller et al.26 The applied correction did not exceed 15%, for uptake coefficients approaching 105.
’ RESULTS AND DISCUSSION HULIST Sources and Initial Characterization. During the sampling period, local NO emissions resulted in high concentrations (86 ( 69 ppbv), titrating O3 (2 ( 1 ppbv). HULIST are strongly correlated with Levoglucosan a tracer of biomass burning (Table 1), and have a weaker correlation with elementary carbon (EC), which is mainly emitted by fossil fuel combustion 5239
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Table 2. Functionalisation Rates for Composite HULIST Sample and for Bulk OC on Individual Filters21 R—COOH
R—CO—R’
R—NO2
composite HULIST
11.4 ( 2.5%
0.62 ( 0.13%
1.3 ( 0.3%
extract average (and range)
2.1 (1.6 3.1)
0.5 (0.3 0.6)
0.1 (0.1 0.2)
for bulk OC
(road transportation and residential heating). This suggests that our HULIST originate mainly from fresh wood burning emissions, as already evidenced for HULISWS.2 The measured functionalization rates (Table 2) show that there is 510 times more acidic functions per carbon atom in the bulk HULIST extract than in the general OC fraction, whereas the content of carbonyl functions is very similar. This clearly shows that the acid functions present in the aerosol in Chamonix in winter belong to the HULIST fraction. Uptake Coefficients. Figure 1 shows the uptake coefficient as a function of photon flux, O3 concentration, pH, and relative humidity under dark (solid circles) and light (open circles) conditions. Default conditions for the experiments are 20% RH, 30 ppbv of ozone, pH 5.8 for the starting HULIST solution, and a total irradiance of 1.66 1015 photons cm2 s1 (300420 nm). Figure 1 clearly shows that for all experimental conditions, the measured uptake is higher under near-UV irradiation than in the dark, indicating a light-induced process for ozone destruction at the surface of the coating. The presence of light may lead to complex radical photochemistry.27 The uptake coefficient increases linearly from 1.6 106 to 1.3 10 5 when the photon flux increases from 0 to 1.66 1015 photons cm2 3 s1 (Figure 1.A.). This linear dependence indicates that the number of reactive sites at the irradiated surface is proportional to the number of photoactivated species. Extrapolating our measurements to typical atmospheric irradiance over the same wavelength range (1.9 1016 photons cm2 3 s1) would lead to an estimated ozone uptake coefficient of ∼1.3 104 under standard experimental conditions; this is twice higher than the estimated uptake coefficient for humic acid thin coatings under similar conditions,19 and does not take into account the eventual effect of irradiation above 420 nm. As illustrated in Figure 1B, the uptake coefficient (γ) dependence on O3 concentration is non linear; under near-UV irradiation the uptake decreases from 1.3 105 to 3.5 106 for O3 concentration increasing from 30 to 130 ppbv. Such non linear dependence has been previously observed for O3 on humic and fulvic acids standards,18,19 as well as for NO2 heterogeneous photoconversion to HONO on humic acids.28,29 The potential reasons for the inverse dependence of the uptake coefficient on O3 concentration are a limitation either by saturation of the adsorbed precursor or by the photochemical activation rate. That is, as O3 gas phase concentration increases, the surface of the HULIS coating becomes saturated with adsorbed O3 molecules. Hence, the total number of collisions increases but effective collisions that lead to O3 loss become practically constant; leading to the decrease of the uptake coefficient. On the other hand, O3 is suggested to be reduced by photochemically activated electron donors present in the substrate; the inverse dependence can also be explained if the overall reaction becomes controlled by the availability of photoreduced species.28
Figure 1C shows the ozone uptake coefficient as a function of the pH of the solution used to prepare the coatings. Under dark conditions (solid circles), γ increases slightly from 1.3 106 to 1.9 106. The increase is more significant under irradiation, from 5.3 106 (pH 3.8) to 21 106 (pH 11). Carboxylic acids (COOH, pH > 45) and phenol (Φ-O, pH > 89) present in HULIST deprotonate successively as pH increases. Conjugated carboxylate (especially with R-ketocarboxylate) and phenolate will enhance HULIST absorbance (see SI Figure S1) and the availability of photochemically activated electron-donor groups. Hence, pH increase will facilitate electrophilic attack by O3 under irradiation, accelerating the electron transfer reaction (second step of Stemmler’s mechanism 28). The O3 loss on HULIST coatings increases with relative humidity (Figure 1D), both under dark and irradiation. Humic substances are an aggregated structure with polar and non polar groups, which can be affected together with its reactivity in presence of water.3032 When dry, these polar groups are unavailable for gas interaction due to strong interactions between them through hydrogen bonding. Moreover, the carboncarbon double bonds may be sterically shielded by the polar groups and their high degree of cross-linking. The latter may hinder the electrophilic attack by ozone. Then, the observed O3 loss increase at higher humidity could be attributed to the hydration of polar groups which uncoils the HULIS molecules, leading to an expansion of the structure and making them more accessible for O3 reaction.30 An increase in water content of the film also favors deprotonation of HULIS photoreactive moieties, which could lead to easier electron transfer to O3 on humid film and explain our observations. Such a mechanism also works on humic or fulvic films. Yet, ozone uptake on such films was found to decrease with RH,19 which was explained by quenching of photoexcited species in the film by water molecules. Our observation would then suggest that photoexcited species produced from HULIS are not as quenchable, as supported by the lower production rates of triplet states organic matter and singlet oxygen upon irradiation by rainwater HULIS than by surface water.20 Photochemical Aging of HULIST. The structural changes upon photochemical aging were followed by total organic carbon analysis, functional analysis and UVvis spectroscopy. Aliquots of the initial HULIST solution were exposed to ozone and/or UV irradiation for 3 h. The aged solutions were compared to the untreated HULIST solution. Oxidation (eventually photoinduced) in an organic mixture is expected to proceed via oxygen addition and via carboncarbon bonds cleavage. The first pathway leads to an increase in carbonyl and acidic functions and a slight increase in molecular weight. Carboncarbon bond cleavage leads to a decrease in molecular weight with subsequent emission of volatile species,19 including CO and CO2,33 which translates into a loss of carbon mass. All these processes have been proved for different humic acids, with varying relative importance depending on the specific oxidation process: photolytic,3436 chemical,37,33 or a mixture of both.19 The present work on HULIST is in agreement with this behavior. Carbon concentration decreases during each aging experiment (Figure 2.A), being more pronounced in the ozonation experiment under irradiation (29% TOC decrease) compared to either photolysis (17% TOC decrease) or ozonation (12% TOC decrease). Functional analysis (Figure 2) indicates that carbonyl functions are formed for all conditions. Carboxylic acids are formed under ozonation and slightly destroyed during 5240
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Figure 1. Effect of various experimental parameters on the measured uptake coefficient of ozone on HULIST films, under irradiation (open circles) and in the dark (solid circles). Default experimental conditions are 30 ppbv of O3, pH 5.8, 20% RH and 1.66 1015 photons cm2 s1 (300420 nm). Error bars are derived from the uncertainties associated to the calculation of the uptake coefficients and are 1σ precision. On panel (A); dotted lines are the 95% confidence interval for the linear fit to the data.
photolysis, which is consistent with previous observations.3436 This decrease in COOH concentration corresponds to net decarboxylation. When converted from molCOOH/L into molCarbon/L, this decrease gives the carbon loss from decarboxylation. This net carbon loss is 0.50 ( 0.14 mmol L1 while the total carbon loss is 2.3 ( 1.3 mmol L1; this implies that photochemical decarboxylation of HULIST represents 21 ( 14% of the total observed carbon loss. Therefore, the remaining carbon loss has to be explained by VOCs or CO emissions. This hypothesis agrees with the measured gas phase emission of formaldehyde, methanol, acetone (or propanal) and acetaldehyde observed upon photolysis under weak near-UV conditions of humic acid films using PTR MS.19 However, it is in contrast with previous studies on humic acids reactivity in water, where decarboxylation was suggested as the major carbon loss pathway.38,39 Nitro functions (RNO2) show a behavior similar to acidic functions, as their carbon contribution drops from 1.3 to 0.5%, whereas the contribution of carbonyl functions only slightly increases. After ozonation in the dark, all functional moieties increase, moderately for nitro- and acidic- functions, and nearly doubling for carbonyl functions. This is expected from the classic cleavage of alkene double bonds present in HULIST reacting with ozone. Similarly, oxidation of humic substances, showed an increase of the O/C ratio and formation of aliphatic and aromatic carboxylic acids.40,41 This functionalization pattern would render HULIST more hydrophilic. Under combined exposure of light and ozone the acidic functions largely increase, suggesting that formation of acidic functionalities upon ozonation is more efficient than their destruction by
photoinduced decarboxylation. The opposite is observed for nitro functions: their generation by ozonation seems less effective than their destruction by photodegradation, as their contribution decreases in the combined experiment. In general, the combination of ozonation and near UV-irradiation highly affect the structure of groups. HULIS, forming more COOH and R(CdO)R Figure 3 shows the absorbance of HULIST for each treatment. The decrease in absorbance is significant upon exposure to light, and to a lesser extent for the combined exposure to light and ozone. Photo bleaching is a well-known process.15 It has been proposed that the equilibrated hydration reaction of alkenes double bonds could be displaced toward the alcohol by photons, thereby reducing the absorbance of the mixture.15 For ozonation in the dark, the decrease in absorbance can be related to O3 attack to double bonds, which is compensated by the formation of absorbing carbonyl functions. Then, for irradiated ozonation, our observation suggests that the formation of absorbing acids and carbonyl functions does only partially compensate for the photoenhanced hydration of double bonds. The spectral slope ratio, SR, of 275295 nm and 350400 nm has been proposed as a proxy for DOM average molecular weight in surface waters.42 When applied to our aging experiment (SI Figure S4), we observe an increase in SR after exposure to UV, and a further increase when exposed to UV and ozone, consistent with decreased molecular weight during aging. Atmospheric Implications. Photoenhanced heterogeneous uptake of ozone by HULIS is found to be significantly faster than by any humic or fulvic acid standard used as a proxy so far. Using 5241
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Figure 2. HULIST evolution during aging experiments. Functionalization rates (B, C, D) are defined as functional groups molar concentration divided by the carbon molar concentration, and thus represent the fraction of HULIST carbon atoms bearing the specified function. Uncertainties include analytical standard deviation for carbon measurement and statistical uncertainty on functional group calibration for functionalization rates.
Figure 3. Evolution of absorptivity (ε) of the HULIST fraction during the aging experiment, where ε derives from Beer’s Law: A = ε C L, with A the measured Absorbance, L the optical path, and C the HULIS concentration in units of molC 3 L1 (ε in L 3 mol1 3 cm1, right axis) or in gC 3 mL1 (ε in cm2 3 gC1, left axis).
our extrapolated value for atmospheric irradiances (γ = 1.3 104) and a typical aerosol surface concentration ((S/V) = 500 μm2/cm3) leads to a first order loss of ozone k(I) = 6 106 s1. Although such an extrapolation is debatable, as HULIS on
natural aerosol differ certainly from the aerosol films used here, it emphasizes the possible contribution of HULIS to the ozone budget, for example in biomass burning plumes, where HULIS is a major contributor to the aerosol. 5242
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Environmental Science & Technology As importantly, this ozone uptake will fuel intense photochemical aging of the aerosol. We show here that this aging will proceed by building up oxygenated functions and decreasing the average molecular size of the HULIS fraction, making it more hydrophilic. This will participate in the now well established transformation of primary, more hydrocarbon like aerosol into more oxygenated, better cloud condensation nuclei aerosol. Of particular importance to this transformation is the notable enrichment of the HULIST fraction in carboxyl groups, as their presence will modulate HULIST solubility in water, and increase their surfactant properties, again influencing their CCN ability. It will also modulate the HULIST capacity to adsorb smaller organic molecules and complex heavy metals, therefore potentially having an influence on pollutant transport. Ozone reactivity toward organic material is rather selective, so that the mechanisms exposed here should be completed with mechanisms from the interaction with other atmospheric oxidants such as H2O2 or OH.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed description of analysis protocols and uptake coefficient calculations; Table S1: complementary measurements made on the same filters during the sampling period; Figure S1: spectral irradiance of the light source; Figure S2: compared uptake on HULIS and NaCl film; Figure S3: absorptivity spectrum of HULIST at different pH; Figure S4: Evolution of spectral slopes during aging of HULIS. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ33 476 51 44 41; fax: þ33 476 82 42 01; e-mail:
[email protected]. Present Addresses
^ Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, 5232 Villigen PSI, Switzerland. )
Author Contributions
These authors contributed equally to this work.
’ ACKNOWLEDGMENT This work is supported by ADEME, INSU, LEFE CHAT and the French Ministere de l’Ecologie, de l’Energie, du Developpement durable et de l’Amenagement du Territoire and the Agence National de le Recherche Scientifique (ANR) for the Grant Photoaero. ’ REFERENCES (1) Feczko, T.; Puxbaum, H.; Kasper-Giebl, A.; Handler, M.; Limbeck, A.; Gelencser, A.; Pio, C.; Preunkert, S.; Legrand, M. Determination of water and alkaline extractable atmospheric humic-like substances with the TU Vienna HULIS analyzer in samples from six background sites in Europe. J. Geophys. Res. 2007, 112, D23S10. (2) Baduel, C.; Voisin, D.; Jaffrezo, J. Seasonal variations of concentrations and optical properties of water soluble HULIS collected in urban environments. Atmos. Chem. Phys. 2010, 10, 4085–4095. (3) Cavalli, F.; Facchini, M. C.; Decesari, S.; Mircea, M.; Emblico, L.; Fuzzi, S.; Ceburnis, D.; Yoon, Y. J.; O’Dowd, C. D.; Putaud, J. P.;
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Environmental Science & Technology
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dx.doi.org/10.1021/es200587z |Environ. Sci. Technol. 2011, 45, 5238–5244
