Unexplained and Unresolved Mass

1. Introduction

Hundreds of individual organic compounds have been identified in the organic atmospheric aerosol so far (e.g. Saxena and Hildemann, 1996), however, together they constitute less than 10% of the organic carbon (OC) of urban and rural aerosol (e.g. Rogge et al., 1993a; Puxbaum et al., 2000). The main analytical method used so far for separating and identifying organic individual species was Gas-Chromatography coupled to Mass Spectrometry (GC/MS). While aerosol extracts of non polar and weakly polar species were directly accessible to GC/MS analysis, for polar species such as organic acids, carbonyls, and multi functional compounds various derivatisation reactions had to be employed to increase the range of species to be identified. One of the recent results of the use of new derivatizing reagents was the observation of levoglucosan and related anhydrosugars in aerosol samples and their use as tracers for biomass combustion (Simoneit et al., 1999); as well as of novel di- and tricarboxylic acids in fine tropical aerosols (Zdrahal et al., 2001). Very recently Claeys et al. (2004) identified tetrols as major oxidation products of isoprene in aerosols from the Amazonian region.

The analysis of extractable organic fractions so far concentrated on molecules accessible to Gas Chromatography – with carbon–atoms generally less than 40. From considerations of the solubility, the behavior in the thermoanalytical methods as well as from observations with microscopical techniques we conclude that a large and until recently unaccounted fraction of the continental organic aerosol consists of polymeric or oligomeric substances.

Rogge et al. (1993a) and Zappoli et al. (1999) have shown, that a considerable part of the organic aerosol is not soluble in water and organic solvents, which points to larger molecular sizes of the insoluble compounds. Matthias-Maser and Jaenicke (1995) have demonstrated that up to 40% of the number of particles > 0.2 µm AD over a continental site were considered of "biogenic origin". The high contribution of biogenic material to the particle number concentration points to biopolymers as a main source for the insoluble organic constituents in the atmospheric aerosol. Bauer et al. (2002a) introduced a quantification method for calculating the contribution of spores to OC in aerosols based on spore counts, and found considerable amounts of carbon from spores in background aerosol in Austria (Bauer et al. 2002b). But, to make the story still more complicated, the main constituents of the organic aerosol according to current knowledge are the “Humic Like Substances” (HULIS), occurring in the aerosol as water soluble as well as water insoluble fractions (Havers et al., 1998). HULIS are present ubiquitously in continental aerosol samples at concentrations (HULIS-carbon) from 7-24 % of the OC (Havers et al., 1998; Zappoli et al., 1999; Facchini et al., 1999). HULIS are definitely macromolecular substances, possibly with manifold origin (e.g. from biomass burning – Facchini et al., 1999; or from secondary reactions in the atmosphere – Jang and Kamens, 2001; Gelencser et al., 2002 & 2003; Limbeck et al., 2003; Iinuma et al., 2004, Kalberer et al., 2004).

Here we compile available data to investigate which species in the organic aerosol  contribute significantly to the so far “unaccounted organic carbon”, and which part is still unaccounted..

2. Unexplained and Unresolved Organic Mass

2.1. Indications from Group-Specific Methods

Solubility

In different reports it was made clear that a larger fraction (e.g. up to 50%) of the organic carbon in urban aerosol was not soluble in solvents of different polarities (e.g. Rogge et al. 1993°; Zappoli et al. 1999). Insolubility in water and organic solvents is a property observed widely in natural systems, in particular cell walls of plant and animals. Such bio-polymeric material is based on cellulose, hemicelluloses and lignin in plants, and on different forms of proteins in animals. Also anthropogenic polymeric materials are generally insoluble: different types of plastics, fibers and synthetic rubber. The high fraction of insoluble material in continental aerosols is indicative for the polymeric state of the compounds, potentially of natural as well as anthropogenic origin.

According to Weissenbök et al. (2000) 70% of the insoluble filterable particulate carbon from snow collected in Austria at 3100 m elevation accounted of “modern” carbon, which is taken as evidence, that biogenic material dominates the insoluble aerosol fraction – even in the mid troposphere.

Microscopical Detection of Bio-Particles

The use of microscopical techniques for identifying the nature of individual particles dates back a long time. A large collection of individual particles identified with light microscopy as well as electron microscopy was edited by McCrone and Delly 1973 (“Particle Atlas”).

Matthias-Maser and Jaenicke (1995) introduced a combination of light and electron microscopic techniques allowing a quantitative assay of the fraction of “bio-particles” related the total number of particles in an air sample. Their method for the larger particles is based on staining the protein-containing particles and observing blue colored vs. black or non-colored particles. For the smaller particles REM investigation, combined with EDS spectra was applied. Bio particles were identified according to their shape and content of bio-tracer elements (e.g. Phosphorus and Potassium).

Bauer et al. (2002a) investigated the carbon content of different species of airborne spores and used these numbers for determining the contribution of  bacteria and fungal spores to the organic carbon content of cloud water, precipitation and aerosols. Fungal spores were found in the size fraction of 2.15-10 µm of organic background aerosol at a mountain site forming on the average 6 % of the organic carbon (OC) of the “coarse” size fraction (Bauer et al., 2002b).

Pollen was considered of less importance for PM10 or PM2.5 aerosol size fractions. However, Schäppi et al. (1997) demonstrated, that some types of pollen grains expel upon influence of rain water much smaller particles, in the case of birch pollen of allergenic property. Thus, pollen may also be a source of fine particles.

In a larger cooperative project taking place during the non-burning season in Amazonia a range of techniques was applied to quantify the contribution of natural sources to OC, based on microscopic and advanced chromatographic techniques (Blaszo et al., 2003; Graham et al., 2003).

Thermographic Techniques

Analysts applying thermographic techniques for determining OC were for long aware, that a “refractory” organic carbon fraction is omni present in atmospheric aerosols. Puxbaum (1979) described cracking of biopolymers as source of overlapping peaks with the black carbon peak. He investigated the thermal behavior of dried leaves, wood, pollen, natural rubber and lignin. All these substances formed double peaks during linear temperature programmed heating in oxygen, with partial overlap of the black carbon peak. The group of compounds showing the double peaks was referred to as organic debris. Also Ellis and Novakov, 1982 identified in thermograms of rural aerosol samples poorly resolved peaks (marked with a, b, g, d), corresponding to volatilization and/or oxidation of carbon species of increasing thermal/oxidation stability. After normalization to the source thermograms the excess peaks b and g— were hypothesized to be high molecular weight polymeric material, however, suggested to be a first-order measure of secondary organic carbon.

 

Figure 1: Thermograms (linear heating in O₂) of two aerosol PM10 samples from an urban industrialized (Liesing) and a suburban (Schafberg) site. The highest peak at 440°C is BC, the last peak at 600°C is from carbonates. The “refractory OC” peak at 380°C is present in the background air and does not increase from urban activities.

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Chromatographic  Techniques

Coupling of  Liquid Chromatographic set ups with Mass Spectrometers applying Electrospray Ionisation (“ESI-MS”) allowed to determine the range of the molecular weights of organic compounds from aerosol extracts assigned to the HULIS-Fraction (e.g. Kiss et al., 2003). Similarly, formation of polymers from smog chamber experiments was demonstrated using capillary electrophoresis – ESI-MS (Iinuma et al., 2004) or laser desorption ionization mass spectroscopy (Kalberer et al., 2004).

3. Classification of Atmospheric  “Bio-Aerosols”

“Bio-Aerosols” in the atmosphere may be viable or “dead”, detritus or debris of living matter, plants or animals. Such particles we will classify as “primary biogenic” particles. However, more recently, results from laboratory study indicate, that aerosol might also form on secondary pathways in the atmosphere from biogenic emissions.

Table 1: Classification of Bio-Particles or related aerosol constituents

PRIMARY
VIABLE	Bacteria Fungal Spores Algea Pollen	Sattler et al. 2001 Bauer et al. 2002a, b Schäppi et al. 1997
DEBRIS/DETRITUS	Cellulose Lignin Amino Acids	Kunit and Puxbaum 1996 Blazso et al. 2003 Zhang and Anastasio 2003
“TRACER”	Hydrocarbons Fatty Alcohols Fatty Acids Etc.	Simoneit 1980
SECONDARY
“SMALLER MOLECULES” (< 300 Da)	Terpene-Oxidation Prod.1) Tetrols (Isoprene-Oxidation Products)	Kavouras et al. 1998 Claeys et al. 2004
“MACROMOLECULES” (> 300 Da)	HULIS2)	Havers et al. 1998

¹⁾  Aerosol formation via absorption in pre-existing organic aerosol

²⁾  HULIS are not exclusively of biogenic origin

4. Humic-Like Substances (“HULIS”) in the Atmosphere

OBSERVATIONAL STUDIES OF PHYSICAL AND CHEMICAL PROPERTIES OF HULIS

In mass closure studies on organic aerosol “humic-like substances (HULIS)”, or “organic macromolecules”, or “polycarboxylic acids”, are generally claimed to resolve a large fraction of unexplained mass of particulate organic carbon. Strictly speaking, however, HULIS are not and can never be speciated, their presence can only be inferred from various bulk aerosol measurements. Although by now there is conclusive evidence that such compounds are ubiquitous in the water-soluble organic aerosol, their quantitative determination is still subject to great conceptual and experimental uncertainties.

Humic acids were first reported in aeolian dust by Simoneit (1977), and these findings were confirmed by Simoneit and Mazurek (1982) who found that humic acids constituted a major fraction of rural aerosol. Based on observed H/C ratio and d¹³C values, a mixed origin from soil and lacustrine mud was inferred by the authors. Therefore the occurence of HULIS in soil-derived aerosol seemed to be well-understood.

At about the same time, another line of research was started which later turned out to be more relevant to HULIS as we see it now. These studies were the very first to establish the presence of polymeric matter in fine aerosol and postulated its secondary origin. In the thermograms of rural aerosol samples poorly resolved peaks were distinguished by Puxbaum 1979, and postulated to origin from biogenic polymeric substances. Similarly, Ellis and Novakov (1982) observed such peaks, marked them with a, b, g, d, corresponding to increasing thermal/oxidation stability, respectively. After normalization to the source thermograms the excess peaks b and g— were suggested to be a first-order measure of secondary organic carbon— and were hypothesized to be of high molecular weight polymeric material, judged solely on the basis of their thermal/optical properties. This idea was far too premature at that time to gain widespread recognition. It took another 15 years for the first field observations to rediscover atmospheric polymers (Havers et al., 1998; Zappoli et al., 1999).

Combination of the thermal technique with water extraction revealed additional features of the bulk organic matter. For example, about half of the refractory aerosol component appeared to be soluble in water (Gelencsér et al., 2000a). The thermal properties of this refractory carbon differed markedly from those of the coarse aerosol, but seemed to resemble those of a reference humic acid on pre-baked quartz filters.

The bulk characterization of organic carbon, in particular of its water-soluble fraction led to the surprising conclusion that almost all observed properties resembled closely those of natural humic substances. The first observations of this kind by UV-VIS spectrophotometry and proton nuclear magnetic resonance spectrometry (HNMR) were made on urban particulate matter (Havers et al., 1998). These authors were the first to introduce the term “humic-like substances”, HULIS, which has become widely accepted in the literature.

At about the same time, a comprehensive study was published on the bulk properties of water-soluble organic matter in aerosol from a polluted, rural and marine environment (Zappoli et al., 1999). Their major finding was that WSOC made up a significant fraction of fine aerosol carbon, and its detailed analytical characterization revealed a stunning resemblance to a reference humic acid. In spite of the high degree of similarity, the authors refrained from using the term “humic-like” for this major class of compounds. Instead, they termed this fraction “macromolecular”, though none of their analytical methods yielded direct evidence that these species were indeed of high molecular weight. They suggested biomass burning to be most likely source, and postulated direct condensation of high-molecular weight burning products as a possible formation mechanism. Their hypothesis assigned primary anthropogenic origin to this compound class, and was able to account for its observed abundance in the fine particle size range.

These pioneering works induced further studies on the occurrence and properties of HULIS in atmospheric aerosol. New analytical techniques were used to reveal the properties of the bulk WSOC, and to compare them to those of natural humic matter. It was shown that the electrochemical properties and metal-complexing ability of bulk organic matter in polluted fog water were nearly the same as those of a reference humic acid (Gelencsér et al., 2000b). Other studies applied various separation methods to characterize such compounds, most of which were based on their acid-base properties. These efforts unambiguously proved that a traditional speciation will not be possible: the chromatograms or electropherograms showed a few poorly resolved broad peaks and/or an unresolved “hump”. Capillary zone electrophoresis of polluted fog water and aqueous extract of rural fine aerosol suggested a broad distribution of charge-to-size ratios of HULIS (Krivácsy et al., 2000). The observed pH-dependence implied that most acidic groups were found to be weaker acids than acetic acid.

Decesari et al. separated the WSOC of the fine aerosol collected at a polluted site and the organic fraction was divided into there generic classes by preparative ion-exchange chromatography (2000). These were neutral/basic compounds, mono- and dicarboxylic acids, and polycarboxylic acids (with at least 3 negative charges per molecule). They determined the chemical structure of these broad compound classes by ¹HNMR spectrometry. The spectra of the neutral/basic compounds revealed the presence of mainly hydroxylated/alkoxylated aliphatic species, with indications for the presence of polyols. Mono- and dicarboxylic acids were shown to be predominantly aliphatic carboxylic acids and hydroxy carboxylic acids, whereas polycarboxylic acids had a more pronounced unsaturated character, with an aromatic core having aliphatic chains with −COOH, −CH₂OH, −COCH₃ or −CH₃ terminal groups. The observed features of the polyacidic compounds closely resembled those of terrestrial and aquatic humic matter. In general, the polycarboxylic acids were the most abundant class of WSOC throughout the year, except in summer, when mono- and diacids were predominant (Decesari et al., 2001). It should be noted that a large fration of the WSOC was UV-absorbing, and the specific UV-absorptivity was highest for the class of polycarboxylic acids. On the basis of group separation and HNMR measurements of the WSOC fraction, a representative mixture of individual compounds was suggested to simulate the physical and chemical properties of aerosol WSOC in model calculations (Fuzzi et al., 2001). The selection of the model compounds should be regarded as a conceptual approach which would serve as a basis for further research on organic aerosol.

            Varga et al. developed another preparative-scale separation method for the isolation of HULIS from the aqueous extracts of aerosol (2001). They carefully optimized their method to isolate the fraction of WSOC that retained the key spectral properties also characteristic of reference humic and fulvic acids. Their separation was based on molecular interactions with the non-dissociated species, and is therefore conceptually different from the method based on the separation of ions, since polycarboxylic acids can be separated both in their ionic and molecular forms. Therefore both methods are believed to target broadly the same generic class of compounds, i.e. the terms “polycarboxylic acids” and “HULIS” possibly refer largely to the same fraction of organic aerosol. Since, however, there has been no intercomparison between these methods this statement is merely based on an assumption derived from the observed chemical properties of the isolated compounds, as well as on the fundamental principles of the separation.

            There is, however, an important conceptual difference between the two methods. The method by Varga et al. (2001) allows the isolation of HULIS from inorganic compounds too, which is not possible in method by Decesari et al. (2000). This fact allows analytical determinations to be performed which otherwise would not be feasible in the presence of interfering inorganic species. For example, the elemental composition of HULIS isolated from rural fine aerosol was found to be remarkably constant throughout the year, corresponding to an average molar ratio of C:H:N:O of 24:34:1:14 (Kiss et al., 2002).

            An important step towards the understanding of the origin of HULIS in rural aerosol collected in summer was the determination of their molecular weight distribution by ultrafiltration, liquid chromatography-atmospheric pressure ionization mass spectrometry, and vapor pressure osmometry (Kiss et al., 2003). The most interesting finding of this study was that virtually all WSOC passed through an ultrafiltration membrane having a 500 Da nominal molecular weight cut-off. The isolated HULIS—which made up of more than half of WSOC by mass—was further characterized to determine their ion mass distribution which was found to be continuous between about 100 and 500 Dalton, with maxima in the range of 200-300 Da. These conclusions were confirmed by vapor pressure osmometry which provided direct estimates for the average MW of HULIS. The average molecular weight was found to be markedly lower than those of reference aquatic humic and fulvic acids under the same conditions. These observations are among the very few that pointed to important differences between HULIS and natural humic substances, and imply distinct mechanisms of their formation.

Yu et al. have recently evaluated the mass size distributions of WSOC in marine and continental aerosol (Yu et al., 2004). Regardless of their origin, WSOC in aerosol exhibited a bimodal size distribution, with a dominant fine mode and a minor coarse mode having mass mean aerodynamic diameters of 0.7± 0.1 and 4.0 ± 0.3 mm, respectively. The mass in the fine mode ranged from two-thirds to four-fifths of that of the total WSOC. Both modes were further deconvoluted to low, medium, and high molecular weight polar compounds based on their thermal evolution features. While the low MW species had a bimodal distribution with a dominant coarse mode, the medium and high MW compounds exhibited a single peak in the droplet mode. This was interpreted as evidence that these latter species—which might also be humic-like substances—likely form during cloud-processing of aerosol. This finding would support the possible formation of HULIS in cloud processes (Gelencsér et al., 2003).

            In fact, very few analytical techniques are capable of providing chemical information directly on the carbonaceous component of the bulk aerosol collected on filter substrates or impactor plates. One of these methods is pyrolysis-gas chromatography-mass spectrometry which allows organic structure elucidation directly from aerosol filters. Taking into account the fact that HULIS contain functional groups (e.g. carboxylates) which yield non-specific thermal decomposition products (e.g. carbon dioxide) upon conventional analytical pyrolysis, in its very first application in aerosol chemistry a derivatization technique was introduced (Gelencsér et al., 2000c). The thermally assisted hydrolysis-methylation allowed labile functional groups to be converted into their respective esters, thus preventing decarboxylation upon pyrolysis and yielding more specific pyrolysis products.         The analysis of rural fine aerosol by this method revealed overall structural similarities to those of natural humic substances. The predominant pyrolysis degradation products both in aerosol and terrestrial humic acids were n-alkanoic acids, a,w-dicarboxylic acids (in the carbon number range of C₄–C₉), and benzenedicarboxylic acids. The apparent structural similarity to terrestrial humic substances made the authors suggest the term “atmospheric humic matter” in place of HULIS. The rationale behind this suggestion was that HULIS in aerosol were thought to be chemically indistinguishable from the wide variety of natural humic substances present in other reservoirs.

Using the same method Subbalakshmi et al. found similar compounds in urban aerosol, except that there higher substituted lignin pyrolysis products were also observed (2001). The pyrograms of biomass burning aerosol from Brazil, however, revealed some differences with respect to those of rural fine aerosol (Blazsó et al., 2003). Most importantly, in biomass burning aerosol there were several higher substituted aromatic compounds which were absent from rural aerosol. These species—which are typical lignin degradation products—were also shown to be present in the pyrogram of soil humic and fulvic acids (Martin et al., 1994). This finding made Gelencsér et al. reconsider their previous results on HULIS in rural fine aerosol (Gelencsér et al., 2002). Their conclusion was that in spite of all apparent similarities, these differences unambiguously prove the disparate origin of HULIS, namely their atmospheric formation in heterogeneous or multiphase processes.

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Laboratory evidence for the formation of HULIS in heterogeneous and multiphase processes

            Several hypothesis that heterogeneous or multiphase reactions can lead to the formation of humic-like substances (HULIS) as major SOA components were put forward by groups in Europe and the US. Gelencsér et al. (2002) observed HULIS formation in liquid phase from organic acids which are ubiquitous in the atmosphere. Jang et al. (2002) have presented laboratory evidence on acid-catalyzed heterogeneous carbonyl chemistry on aerosol particles, including various acid-catalyzed reactions, such as hydration, hemiacetal and acetal formation, aldol condensation, and polymerization in the aerosol phase. In terms of possible HULIS formation it is important that equilibrium between an aldehyde and its hydrate favors the hydrate form and reacts further with carbonyls to yield dimers, trimers, and polymers. However, it is important to point out that these reactions, even polymerization are thought to be reversible.

            Limbeck et al. (2003) have then presented laboratory evidence for the irreversible formation of HULIS in heterogeneous reactions for the case of dienes like isoprene in the presence of sulfuric acid. The reactions yielded colored polymeric products whose humic-like character was evidenced by UV-spectrometry, thermal analysis, and FTIR diffuse reflectance spectroscopy. The authors hypothesized that isoprene—whose SOA formation was thought to be negligible (Pandis et al., 1991, Griffin et al., 1999)—is processed to humic-like polymers (HULIS) on highly acidic atmospheric sulfate clusters. It should be noted that these experiments were performed in bulk.

In the same year, Gelencsér et al. (2003) presented evidence about the irreversible formation of HULIS in atmospheric multiphase reactions in the laboratory. The precursors were aromatic hydroxy acids which are abundant lignin pyrolysis products in biomass burning aerosol. They showed that even a single representative compound can react with OH radicals yielding colored products under typical conditions prevalent in cloud water. The time-scale of the reactions was found to be hours, implying that the process does have atmospheric significance. The reactions proceed by radical dimerization and oligomerization to yield higher molecular weight products. A follow-up study on the molecular weights of the reaction products has revealed a continuous distribution well below 1000 Dalton, which confirms the high degree of similarity to atmospheric HULIS (Hoffer et al., 2004). In addition, this study has proven conclusively that the process is oligomerization rather than polymerization. In the same study, the results implied that HULIS consist of condensed and partially oxidized (e.g. quinone-like) phenolic structures crosslinked with short-chain aliphatic bridges which form by the oxidative cleavage of the phenolic ring.

Two smog-chamber studies on the formation of HULIS or oligomers have been reported until now in 2004.  

Iinuma et al., (2004) investigated the ozonolysis of alpha-pinene in the presence of acidic particles. A thermographic method for the determination of TOC showed an increase of particle phase organics by 40% for the experiments with higher acidity. CE-ESI-MS analysis showed a large increase in the concentration of compounds with M_w>300 from the experiments with sulfuric acid seed particles. Although in this paper the term HULIS is not used, the apparent similarity of the oligomers to HULIS (or vice versa) in their molecular weight distribution may entitle us to use these terms interchangeably.

Kalberer et al. (2004) showed recently, that atmospheric polymers may form also from light initiated photochemistry of a trimethylbenzene – NO_x mixture.

Experimental and/or observational evidences back up all the hypothesis presented here, and so far there have been three studies in smog chambers which all support the oligomer formation. It is possible or even likely that to a certain extent all mechanisms could be operative — this would explain the ubiquitous nature and abundance of HULIS in continental fine aerosol. What is known for certainty is that biomass burning is a source of HULIS—however its secondary origin from biogenic as well as anthropogenic sources, and in particular the aerosol formation rates, is still to debated.

 

Figure 1: Pathways of HULIS formation

5. Organic Carbon Mass Balance

A remarkable set of new organic species of oxygenated organic compounds as well as of the group parameter “humic like substances” have been identified in the last years in the atmospheric aerosol. However, still there is a lack of quantitative data for different environments. Available data  expressed as % of “compound-carbon” from OC are compiled in Table 2.  The largest contribution of an individual group of compounds stems from humic like substances, which occur in the atmospheric aerosol in water soluble and water insoluble forms.  The group of “saccharides” include levoglucosan, which is a tracer for wood or other biomass combustion. Based on test fires with different types of log wood a relation of 100+/-40 mg of Levoglucosan per gram of fine particle emitted OC was derived (Fine et al. 2001). Thus, from Levoglucosan levels in the aerosol the biomass-OC fraction can be estimated from Biomass-OC = 10* Levoglucosan (Eq. 1). Organic aerosol levels as derived from Levoglucosan data range from 5-50% at European as well as Tropical sites. OC from biomass combustion, however contains humic like substances. Therefore it is not possible at present to sum up the group contributions from biomass burning as derived from equation 1 and the group of HULIS. 

Table 2: Potential for contribution of different organic species or groups to % of OC observed at US or European Continental sites

	%C of OC		Reference
Alkanes - C	1
Unresolved Complex Mixture-C	5
Mono- and dicarboxylic acids-C	5-10
Tricarboxylic acids	?	Amazonia 6%	Zdrahal et al. 2001
Tetrols - C	?	Amazonia 10-20%	Claeys et al. 2004
Plant Debris (Cellulose)-C	1-5	Vienna	Puxbaum & Tenze-Kunit 2003
Bacteria & Spores - C	1-5	Austria/Alpine	Bauer et al. 2003b
“Saccharides” - C	0,3-2	Ghent Up to 5% Rhondonia	Zdrahal et al. 2002
HULIS Water soluble-C	7-24
HULIS Water insoluble-C	5-20
Sum	20-80

 

6. Recommendations

 

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