Contribution of Selected Dicarboxylic and ω-Oxocarboxylic Acids in ...

Aerosol Science and Technology, 41:418–437, 2007Copyright c© American Association for Aerosol ResearchISSN: 0278-6826 print / 1521-7388 onlineDOI: 10.1080/02786820701203215

Contribution of Selected Dicarboxylic and ω-OxocarboxylicAcids in Ambient Aerosol to the m/z 44 Signalof an Aerodyne Aerosol Mass Spectrometer

N. Takegawa,1 T. Miyakawa,1 K. Kawamura,2 and Y. Kondo1

1Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan2Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido, Japan

The Aerodyne aerosol mass spectrometer (AMS) employs flashvaporization (600◦C) followed by 70-eV electron impact ioniza-tion (EI) to detect organic and inorganic aerosols. The signal atmass-to-charge ratio (m/z) 44 (mainly CO

+2 ) is considered the

most reliable marker of oxygenated organic aerosol. This studyis the first to evaluate the contribution of selected low molecularweight dicarboxylic acids (diacids) and ω-oxocarboxylic acids (ω-oxoacids) to the particle-phase m/z 44 signal of the AMS mass spec-trum. Ambient measurements were conducted at a surface site inTokyo (35◦39′N, 139◦40′E) during August 3–8, 2003. Diacids andω-oxoacids were measured using a filter sampling followed by ex-traction, derivation, and gas chromatograph-flame ionization de-tector (GC-FID) analysis. The mass concentrations of diacids andω-oxoacids show tight correlation with the m/z 44 signal (r2= 0.85–0.94) during the measurement period. Laboratory experimentswere also performed to determine the fragment patterns of selecteddiacids (C2–C6 diacids and phthalic acids) and ω-oxoacid (glyoxylicacid) in ambient aerosols. Here, we report for the first time that theselected organic acids could account for 14 ± 5% of the observedm/z 44 signal on average during the measurement period. Oxalicacid (C2) is the largest contributor, accounting for 10 ± 4% of theobserved m/z 44 signal. These results would be useful for inter-preting the m/z 44 signals obtained from ambient measurements invarious locations.

1. INTRODUCTIONOrganic compounds are ubiquitous in ambient aerosols and

constitute approximately 10–70% of total dry fine particle massin the troposphere (Turpin et al. 2000, and references therein).

Received 19 August 2006; accepted 8 January 2007.The authors thank M. Koike and K. Asano for their support in the

filter sampling. They also thank J. D. Allan for providing the AMSanalysis software and M. R. Alfarra, J. L. Jimenez, and D. R. Worsnopfor useful information on the fragment patterns of some organic com-pounds. This study was funded by the Japanese Ministry of Education,Culture, Sports, Science and Technology (MEXT) and the JapaneseScience and Technology Agency (JST).

Address correspondence to Nobuyuki Takegawa, Research Cen-ter for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904 Japan. E-mail: [email protected]

The fraction of water-soluble organic carbon (WSOC) is impor-tant in estimating direct and indirect radiative forcing of aerosolsbecause they can significantly alter the hygroscopic propertiesof aerosols (Saxena et al. 1995). Dicarboxylic acids (diacids),which contain two carboxyl groups, are the major constituents ofWSOC and can be found in various locations in the troposphere(Saxena and Hildemann 1996; Kawamura and Ikushima 1993;Kawamura and Yasui 2005; Mochida et al. 2003). Kawamuraand Ikushima (1993) suggested that oxidation of volatile or-ganic compounds (VOCs) is the major source of low molecularweight (C2–C6) diacids observed at an urban site in Tokyo, basedon the seasonal variation of these diacids. More recently, Ervenset al. (2004) proposed that cloud processing could be an im-portant source of low molecular weight diacids, although thereare still many uncertainties in the production pathway of thesecompounds.

The Aerodyne aerosol mass spectrometer (AMS) can providesize-resolved mass concentrations of submicron non-refractory(NR-PM1) aerosols with a time resolution of the order of min-utes (Jayne et al. 2000). The AMS employs flash vaporizationfollowed by electron impact (EI) at ionization energy of 70 eV.There are three kinds of AMS, classified by the type of de-tector employed: quadrupole AMS (Q-AMS), compact time-of-flight AMS (C-ToF-AMS), and high-resolution ToF-AMS(HR-ToF-AMS) (DeCarlo et al. 2006). In this study we usea Q-AMS, which is hereafter referred to simply as the AMS.The mass spectrum of “organics” is defined as the residualfragments after subtracting all the identified signals originat-ing from ambient gas molecules, inorganic compounds, and in-strumental artifacts such as sodium (Na) and potassium (K)ions from surface ionization on the vaporizer (Allan et al.2004). For some organic compounds, the mass spectra of theAMS have been compared to those from the standard refer-ence database provided by the National Institute of Standardsand Technology (NIST) (http://webbook.nist.gov/chemistry/)(Alfarra 2004). Alfarra (2004) has found that the fragmentationpatterns of the AMS are generally shifted to smaller mass-to-charge ratios (m/z) because of its higher vaporization/ionizationtemperature (600◦C) compared to those obtained by gas

418

AMS M/Z 44 VS DIACIDS 419

chromatograph - mass spectrometer (GC-MS) (100◦–200◦C). Inthe AMS, decarboxylation and other thermal degradation oftenoccur prior to ionization.

In spite of this complexity, some m/z peaks can be usedas distinct markers of hydrocarbon-like or oxygenated organicaerosols. The signal at m/z 44, which originates mainly fromCO+

2 , is considered the most reliable marker of oxygenated or-ganic aerosol (Zhang et al. 2005b; and references therein). Pre-vious studies showed that the increase in m/z 44 signal wasoften associated with photochemical processing of air masses(Takegawa et al. 2006a, 2006b; Zhang et al. 2005a), and that them/z 44 signal correlated well with the mass concentrations ofWSOC (Kondo et al. 2007). Diacids are considered the majorcompounds that can contribute significantly to the m/z 44 signal,as well as ω-oxocarboxylic acids (ω-oxoacids) containing car-boxyl and aldehyde groups. However, the contribution of thesecompounds in ambient aerosols to the m/z 44 signals has notbeen evaluated. Further, there has been no published compari-son between the m/z 44 signal and the mass concentrations oforganic acids determined by GC analysis.

The purpose of this paper is to evaluate the contribution of se-lected low molecular weight diacids and ω-oxoacids to the m/z44 signal of AMS mass spectra. Here we use ambient measure-ment data obtained in Tokyo in August 2003 and laboratory ex-periment data obtained using authentic standards of the selectedorganic acids. This study will present fundamental informationthat could be applied to better interpret AMS data obtained invarious locations. This information would also be useful for theanalysis of ToF-AMS data because ToF-AMS employs the samevaporization/ionization scheme as Q-AMS.

2. EXPERIMENTAL

2.1. Ambient Measurements2.1.1. Observation Site

Ambient measurements were conducted at a surface site inTokyo from 21:00LT on August 3 to 20:00LT on August 8, 2003.The observation was made on the top floor of a 5 story build-ing (∼18 m above ground level) on the campus of the ResearchCenter for Advanced Science and Technology (RCAST), Uni-versity of Tokyo (35◦39′N, 139◦40′E). RCAST is located nearthe center of the city and is surrounded by a major highwaysystem (at ∼2 km distance from RCAST). A number of on-lineinstruments including the AMS were installed in the observa-tion room (Takegawa et al. 2006a). The AMS data were recordedevery 10 min.

The mass concentrations of diacids and ω-oxoacids weremeasured using a filter sampling method followed by GC analy-sis. The aerosol samples were collected every 3 h during August4–5, 2003 on a pre-combusted quartz filter using a high-volumeair sampler without any size-cut. After August 5, the aerosolsamples were collected every 12 h. The high-volume samplerwas placed on the rooftop of the observation building.

2.1.2. Quantification Procedure of the AerodyneAMS for Ambient Data

The AMS quantification procedure for ambient data has beendescribed previously (e.g., Jimenez et al. 2003, Alfarra et al.2004). The key equations for organic aerosol are presented herebecause the expression used in this paper is slightly differentfrom those in previous publications. Similar expressions canbe applied to the mass concentrations of inorganic compounds(equations for inorganics are not shown here).

Mm/z = 1

CEorg

1

RIEorg

MWN O3

IENO3

1012

QNASm/z [1]

OA =∑

m/z

Mm/z (m/z = 1−300) [2]

where CEorg and RIEorg represent the average particle collec-tion efficiency (CE) and relative ionization efficiency (RIE) fororganic compounds, respectively. MWNO3 (62 g mol−1) is themolecular weight of nitrate; IEN O3 is the ionization efficiencyfor nitrate. Q (cm3 s−1) is the sample flow rate, and NA is Avo-gadro’s number. The factor of 1012 in Equation (1) is needed forunit conversion. Sm/z (Hz) is the signal (ion count rate) at the m/zoriginating from organic compounds. As mentioned in section 1,the Sm/z is obtained by subtracting the signals from ambient gasmolecules, inorganic compounds, and instrumental artifacts. Forexample, the m/z 44 signal originating from organic compounds(S44) is determined by subtracting the contribution of gas-phaseCO2 (equivalent to 0.09μg m−3 on average). Mm/z (μg m−3) rep-resents the equivalent mass concentration with respect to Sm/z ,and OA (μg m−3) represents the mass concentration of total or-ganic aerosol measured by the AMS. The parameters used inthis study and their abbreviations are summarized in Table 1.

The IENO3 was routinely determined by supplying mono-disperse ammonium nitrate (NH4NO3) particles from a calibra-tion unit. The uncertainty in determining IENO3 was estimated tobe 14% (Takegawa et al. 2005). The uncertainty in the RIE valuesfor major inorganic compounds (sulfate, nitrate, chloride, andammonium) is considered to be small because the mass spectraof these compounds are well defined. The RIE values for organ-ics can vary with functional group: ∼2 for hydrocarbons and∼1.4 for oxygenated organic compounds (Jimenez et al. 2003).We assume RIEorg = 1.4 based on the work of Alfarra et al.(2004). The CE values are assumed to be 0.5 both for inorganicand organic compounds.

For the major inorganic compounds, the combined uncer-tainty (accuracy) due to IENO3, RIEinorg, and CEinorg (inorg =sulfate, nitrate, chloride, and ammonium) has been evaluatedbased on intercomparison with a Particle-Into-Liquid Samplercombined with Ion Chromatography (PILS-IC) (Takegawa et al.2005). The intercomparison was performed on July 23–30 of2003 (prior to the filter sampling period). A PM1 cyclone (URGCorp., USA) was used for the PILS-IC inlet because the size-cut of the AMS (aerodynamic lens) is similar to the commonlyused PM1 cutoff. The linear regression slope of the AMS versus

420 N. TAKEGAWA ET AL.

TABLE 1Definition of parameters used in this study

Parameter Unit Definition

NA mol−1 Avogadro’s numberMWNO3 g mol−1 Molecular weight of nitrate (NO3)Q cm3 s−1 Sample flow rate of the AMSIENO3 None Ionization efficiency of the AMS for nitrateCEorg None Average particle collection efficiency (CE) of the AMS for total organic aerosolRIEorg None Average relative ionization efficiency (RIE) of the AMS for total organic aerosolSm/z Hz Observed ion signal at an m/z peak of the AMS mass spectrum for total organic aerosolMm/z μg m−3 Equivalent mass concentration with respect to Sm/z

OA μg m−3 Mass concentration of total organic aerosol in sample air measured by the AMS(OA = ∑

m/z Mm/z)CEi None CE of the AMS for compound i (i = C2, C3, etc.)RIEi None RIE of the AMS for compound iui

m/z None Relative intensity at an m/z peak of the AMS mass spectrum for compound i ,as determined by the laboratory experiments (

∑m/z ui

m/z = 1)si

m/z Hz Estimated ion signal at an m/z peak of the AMS mass spectrum for compound iRi

m/z None Contribution ratio of compound i to the observed m/z signal (Rim/z = si

m/z /Sm/z)mi μg m−3 Mass concentration of compound i in ambient aerosolεi None Recovery of compound i from the filter samplesfi None PM1 fraction of compound i

PILS-IC ranged from 0.81 to 1.26 for the major inorganic com-pounds. Therefore, the combined uncertainty due to IEN O3,RIEinorg , and CEinorg (and also to the approximation of the AMSsize-cut to PM1) is estimated to be less than 26%. We assumethat this error estimate is also applicable to organic compounds:i.e., the combined uncertainties in Mm/z and OA due to IENO3,RIEorg, and CEorg are both assumed to be 26%. Although we usea constant value for the contribution of gas-phase CO2 at m/z 44(equivalent to 0.09 μg m−3), the uncertainty of M44 due to thevariability in the gas-phase CO2 is relatively minor (∼0.002 μgm−3). The overall validity of the quantification of organic aerosolby the AMS has been evaluated based on intercomparison with aSunset Laboratory semi-continuous elemental and organic car-bon (EC/OC) analyzer (Takegawa et al. 2005). The linear re-gression slope of the AMS OA versus Sunset OC was found tobe 1.8, which is consistent with the expected ratio of organicmatter (OM) to OC in urban air (Turpin and Lim 2001).

2.1.3. Measurements of Dicarboxylic Acidsand ω-Oxocarboxylic Acids in Aerosol Samples

Details of the analytical method have been presented byKawamura and Yasui (2005), and thus only a brief descriptionis given here. The filter samples were extracted with pure waterand concentrated using a rotary evaporator in the laboratory.The concentrates were reacted with 14% BF3/n-butanol at100◦C to derive the carbonyl groups to butyl esters. The butylesters were extracted with n-hexane after adding pure water andthen determined using a gas chromatograph-flame ionizationdetector (GC-FID). Peak identification was performed by com-

parison with the GC retention times of the samples with those ofauthentic standards. Identification of the esters was confirmedby GC-MS analysis. The uncertainty in the measurements isgiven in Kawamura and Yasui (2005). Recoveries of authenticstandards spiked to a pre-combusted quartz filter were 71% foroxalic (C2) acid and better than 80% for malonic (C3), succinic(C4), and adipic (C6) acids. Procedural blank filter samplesshowed small peaks of oxalic and phthalic (Ph) acids in theGC chromatograms, although they were small (<2% for oxalicacid and <5% for phthalic acid). The data used in this analysisare corrected for the procedural blanks but are not corrected forthe recoveries. The analytical errors are within 15% for thesemajor species.

2.2. AMS Laboratory ExperimentsLaboratory experiments were performed between July 2005

and April 2006 to estimate the fragment patterns of selecteddiacids (C2–C6 and Ph) and ω-oxoacid (glyoxylic acid: ωC2)in the AMS vaporization/ionization system. We have selectedthese organic acids because these compounds were the ma-jor constituents of total diacids and ω-oxoacids during themeasurement period (see section 3.1.1). The physical param-eters of the selected organic acids are given in Table 2. Theseparameters were taken from the International Chemical SafetyCards (ICSC) of the International Labour Organization (ILO)(http://www.ilo.org/) and also from Chemical Dictionary (1994).The selected organic acids can be vaporized and/or decomposedat the temperature of the AMS vaporizer (600◦C). The particle


TABLE 2Physical parameters of selected organic acids

Species Melting point, ◦C Boiling point, ◦C

Dicarboxylic acidsOxalic (C2)a 189.5 (Decomposes) N/AMalonic (C3)a 135 (Decomposes) N/ASuccinic (C4)b 185 235 (Dehydrates)Glutaric (C5)a 98 302–304Adipic (C6)a 152 338Phthalic (Ph)b 191 210–211 (Decomposes)

ω-Oxocarboxylic acidsGlyoxylic

(ωC2)b 98 N/A

aThe parameters were taken from the International ChemicalSafety Cards (ICSC) of the International Labour Organization (ILO)(http://www.ilo.org/).

bThe parameters were taken from Chemical Dictionary (1994).

generation system used in this study is basically the same asthat described in Takegawa et al. (2005). The system consistsof a Collison atomizer (Model 3076, TSI, Inc., USA), two dif-fusion dryers, and a scanning mobility particle sizer (SMPS)(Model 3936, TSI, Inc., USA). The organic acid particles weregenerated by atomizing water solutions of authentic standardsof these compounds. Pure oxygen (O2) was used to atomize thewater solutions in order to detect the m/z 28 signal (mainly CO+)originating from the organic acids. The whole system was purged

with O2 for a few hours prior to each experiment to eliminatethe contribution of N+

2 to the m/z 28 signal. The possible depen-dence of the fragment patterns on carrier gas and mass loadingsis discussed in the appendix. We obtained ∼10 mass spectra foreach compound during the laboratory experiment period. Theintegration time for each mass spectrum was generally ∼10 min.

The “best estimate” of the mass spectrum for each compoundis determined using the following procedure (a)–(c): (a) Eachmass spectrum is normalized so that the sum of all m/z peaksis equal to unity. For a compound i (= C2, C3, etc.), the m/zcomponent of each normalized mass spectrum is expressed asui

m/z (∑

m/z uim/z = 1). The ui

m/z value corresponds to the ratioof the m/z signal to the total signal. (b) The average ui

m/z (uim/z)

is calculated for each compound (from ∼10 mass spectra foreach compound). The ui

m/z is referred to as the best-estimatemass spectrum. Note that

∑m/z ui

m/z = 1 holds without addi-tional normalization. (c) The standard deviation of ui

m/z is usedas a measure of the uncertainty. Note that the complete table ofthe best-estimate mass spectra is available as electronic supple-mental material.

3. RESULTS

3.1. Ambient Measurements3.1.1. Mass Concentrations

Table 3 summarizes the average and median values of OA andM44 measured by the AMS and those of selected diacids (C2–C6 and Ph) and ω-oxoacid (ωC2) measured by the GC analysis.

TABLE 3Average mass concentrations of organic compounds on August 3–8, 2003

Speciesa Average 1σ Median Minimum Maximum

AMS organics, μg m−3

Total organics (OA) 9.7 5.7 9.0 2.3 23.8M44 0.79 0.58 0.68 0.08 2.29

Dicarboxylic acids, ng m−3

C2 259.9 244.3 174.0 21.4 937.7C3 98.5 89.0 51.3 7.9 335.7C4 78.2 67.4 52.3 5.7 238.1C5 16.7 16.8 10.2 1.4 62.8C6 11.0 10.0 7.1 1.1 38.4Ph 41.5 30.5 33.9 5.9 119.1Selected diacidsb 505.8 446.6 308.8 47.3 1731.8Total diacidsc 579.6 509.1 357.7 54.5 1971.2

ω-Oxocarboxylic acids, ng m−3

ωC2 37.3 38.3 24.2 2.6 155.7Total ω- oxoacidsc 41.2 41.3 26.2 3.0 168.2

aTotal organics and M44 were measured by AMS. Dicarboxylic acids (diacids) and ω-oxocarboxylic acids (ω-oxoacids) were measured usinga filter sampling followed by GC-FID analysis. The AMS data were averaged over the integration time of the filter sampling.

bSelected diacids represent the sum of the selected diacids listed in this table.cTotal diacids and ω-oxoacids represent the sum of all diacids (C2–C12) and ω-oxoacids (C2–C9), respectively, identified by the GC analysis.

These are listed by Kawamura and Yasui (2005).


TABLE 4Linear regression parameters for selected diacids and

ω-oxoacids versus M44

Species Slope Intercept, μg m−3 r2

Dicarboxylic acidsC2 0.40 ± 0.06 −(0.05 ± 0.06) 0.89C3 0.15 ± 0.02 −(0.02 ± 0.02) 0.94C4 0.11 ± 0.02 −(0.01 ± 0.02) 0.91C5 0.027 ± 0.004 −(0.005 ± 0.004) 0.92C6 0.016 ± 0.003 −(0.002 ± 0.003) 0.89Ph 0.048 ± 0.009 0.003 ± 0.009 0.85Total diacids 0.85 ± 0.09 −(0.10 ± 0.09) 0.94

ω-Oxocarboxylic acidsωC2 0.063 ± 0.009 −(0.01 ± 0.01) 0.91Total ω-oxoacids 0.068 ± 0.009 –(0.01 ± 0.01) 0.91

Selected acids (C2-C6, Ph, and ωC2)Selected acids 0.81 ± 0.09 –(0.10 ± 0.09) 0.94

Total acids (Dicarboxylic acids +ω-Oxocarboxylic acids)Total acids 0.92 ± 0.10 –(0.11 ± 0.10) 0.94

The number of data points is 23. The uncertainties are 95% confi-dence intervals.

The AMS data were averaged according to the integration timeof the filter samples before calculating the average and medianvalues. The total diacids (or ω-oxoacids) represent the sum ofall diacids (or ω-oxoacids) identified by the laboratory analysis.The full list of the identified diacids and ω-oxoacids are givenin Kawamura and Yasui (2005). As described in section 2.2,only major diacids and ω-oxoacid were selected for the presentanalysis. On average, the sum of the selected diacids (C2–C6

FIG. 1. Time series of (a) the mass concentrations of total organics and M44 observed by the AMS and (b) those of oxalic acid (C2: solid), total dicarboxylic acids(total diacids (C2-C12): open), and total ω-oxocarboxylic acids (total ω-oxoacids (C2-C9): shaded) measured by filter sampling followed by laboratory analysis.

and Ph) accounts for ∼87% of the total diacids (C2–C12), andglyoxylic acid accounts for ∼91% of the total ω-oxoacids (C2–C9). Oxalic acid is found as a dominant diacids species. Thisfeature has been observed in various locations in previous studies(Kawamura and Ikushima 1993; Kawamura and Yasui 2005;Mochida et al. 2003). Figure 1 shows the time series of OA andM44 observed by the AMS and the mass concentrations of oxalicacid (C2), total diacids, and total ω-oxoacids measured by thefilter sampling technique. Both M44 and di-/ω-oxoacids show adistinct peak at around noon. Figure 2 shows the correlation plotsof oxalic acid, total diacids, and total ω-oxoacids versus M44.The linear regression parameters (slope, intercept, and r2) forthe selected diacids and ω-oxoacids versus M44 are summarizedin Table 4. The high correlation coefficients (r2 = 0.85–0.94)indicate a strong relationship between M44 and di-/ω-oxo-acids.

The ozone (O3) mixing ratios measured at the same loca-tion also exhibited a distinct peak at around noon during themeasurement period (Takegawa et al. 2006a). Specifically, highconcentrations of O3 exceeding 100 parts per billion by vol-ume were observed at around noon on August 4–6, 2003. Themass concentrations of M44, oxalic acid, total diacids, and to-tal ω-oxoacids showed positive correlations with O3 during thedaytime (06:00–18:00LT), although the r2 values were not veryhigh (0.66, 0.51, 0.59, and 0.62, respectively) (correlation plotsare not shown here). The positive correlation with O3 suggeststhat photochemical production could be an important source ofthese organic acids during the measurement period. This is con-sistent with the findings by Kawamura and Ikushima (1993).

3.1.2. AMS Mass SpectrumFigure 3 represents the average mass spectrum of organ-

ics obtained at 12:00–15:00LT on August 5, 2003. Based on


FIG. 2. Correlation plots of oxalic acid (solid), total diacids (open), and totalω-oxoacids (shaded) versus M44. The lines represent the linear regression results.

laboratory experiments that have previously been performed,the standard AMS analysis procedure assumes that the m/z 18signal (H2O+) from organics is equal to the m/z 44 signal (Allanet al. 2004). This is because the high background level at m/z 18generally makes it difficult to estimate the m/z 18 signal origi-nating from ambient particles and also because H2O moleculesproduced from thermal decomposition of organics are not dis-tinguishable from those originating from particle-phase H2O.On the other hand, the AMS standard analysis procedure doesnot include the m/z 28 signal originating from organics (mainlyCO+) because the observed m/z 28 signal is dominated by ni-trogen (N+

2 ) in air. In this analysis, we assume that the m/z 28signal from organics is also equal to the m/z 44 signal during themeasurement period. The basis for this assumption is describedbelow.

Zhang et al. (2005b) first evaluated the m/z 28 signal fromorganics using particle-time-of-flight (PToF) data. They usedan exponential fit that has the same function form as the one-dimensional Boltzmann velocity distribution functional in order

FIG. 3. Average mass spectrum of organics obtained at 12:00–15:00LT on August 5, 2003.

to represent the N+2 signal in the PToF mode. They have found

that the m/z 28 signal from organics could be ∼30% larger thanm/z 44 during the Pittsburgh Air Quality Study. Here we use asimpler method to estimate the contribution of organics to them/z 28 signal. Figure 4a shows the plots of signal at m/z 28 ver-sus PToF observed during the daytime (high SOA) and nighttime(low SOA). The broad peak at PToF = 0–2 ms corresponds toN+

2 in ambient air. The small difference between the daytimeand nighttime at the tail of the N+

2 signal (PToF = 2–8 ms) cor-responds to the organic fragment. The N+

2 peak height of thehigh SOA data agreed with that of the low SOA data to within0.3%. Figure 4b represents the size distributions of �S28 and�S44, where � denotes the difference between the daytime andnighttime data. The conversion of PToF to vacuum aerodynamicdiameter (dva) is based on polystyrene latex (PSL) calibrations.Although �S28 is very noisy because of the large contribution ofN+

2 to the baseline, it exhibits a very similar size distribution to�S44. The signals were integrated over the size range of dva =30–2000 nm. The ratio of the integrated �S28 to the integrated�S44 is found to be 1.0, suggesting the importance of m/z 28for the analysis of organics. Based on this result, we assume thatthe m/z 28 signal from ambient organic aerosol is equal to m/z44 during the measurement period.

3.2. AMS Laboratory Experiments3.2.1. Mass Spectra of Oxalic Acid and Its Salts

The best-estimate mass spectrum of oxalic acid obtained inthe laboratory is shown in Figure 5. The major peaks of the se-lected organic acids are summarized in Table 5. The mass spec-trum from the NIST standard reference database is also shownfor comparison. In the AMS mass spectrum, m/z 18 (H2O+), 28(CO+), and 44 (CO+

2 ) are the three major peaks. The m/z 44 peakis the largest, accounting for ∼34% of the sum of all m/z peaks.The signals at m/z 16 (O+) and 17 (OH+) originating from ox-alic acid were estimated from the m/z 18 signal, based on thefragment pattern of H2O in the AMS ionizer (Allan et al. 2004).In the NIST mass spectrum, m/z 45 (COOH+) and 46 (CH2O+

2 )are the two major peaks. The similarity between the AMS and


TABLE 5Major mass fragment ions derived from various organic acids

m/z Composition Parent molecules

14 CH+2 C3

18 H2O+ C2, C4, C5, *C6

26 C2H+2 C4

28 CO+ C2, *C4, C5, C6, *Ph, *ωC2

29 CHO+ ωC2

42 CH2CO+, C3H+6 C3, *C5

44 CO+2 *C2, *C3, Ph

45 COOH+ C2

50 C4H+2 Ph

76 C6H+4 Ph

104 C6H4CO+ Ph

Major fragments are defined as those contributing more than 10% ofthe sum of all fragments.

∗Base peak.

NIST mass spectra is the positions of the significant peaks suchas m/z 18, 28, 44, 45, and 46, while the major difference isthe relative intensities of those peaks: the contributions of them/z 18, 28, and 44 peaks in the AMS mass spectrum are muchlarger than those in the NIST spectrum. Thermal decompositionof oxalic acid could result in the formation of H2O (dehydra-tion), CO, CO2 (decarboxylation), and formic acid (HCOOH)(Chemical Dictionary 1994). It is suggested that dehydrationand decarboxylation of oxalic acid could be more significant inAMS spectra than those of NIST due to the higher vaporizertemperature of the AMS (600◦C compared to 100◦–200◦C).

It should be noted that the standard AMS analysis proceduredoes not assign organic fragments for m/z 46 (only nitrate frag-ment) (Allan et al. 2004). Considering the fragment patterns andRIE of nitrate and organics, the m/z 46 signal from oxalic acidcould be comparable to that from nitrate if the mass concentra-tion of oxalic acid is 6–7 times greater than nitrate. Note that theinterference of the m/z 46 signal may not be a problem for thenewly developed HR-ToF-AMS (DeCarlo et al. 2006) becauseit can distinguish NO+

2 from CH2O+2 .

Figure 6 shows the mass spectra of sodium oxalate((COONa)2) and ammonium oxalate ((COONH4)2) obtained inthe laboratory, illustrating the possible dependence of the frag-ment pattern of oxalate on its counter ions. The NIST massspectrum is not available for these compounds. The m/z 45 and46 peaks do not show up in the mass spectrum of (COONa)2,which is expected from its elemental composition. This resultindicates that the mass spectrum of (COONa)2 could be signif-icantly different from (COOH)2. On the other hand, the majorpeaks in the mass spectrum of (COONH4)2 are similar to thoseof (COOH)2. Figure 7a shows the comparison of the individualm/z signals between (COONH4)2 and (COOH)2. Each m/z sig-nal has been normalized by the m/z 44 signal (base peak). Therelative intensities of the m/z 18, 28, 44, 45, and 46 signals for

FIG. 4. (a) Plots of the m/z 28 signal (S28) versus particle-time-of-flight(PToF) observed during the daytime (black) and nighttime (grey). (b) Size dis-tributions of �S28 (black) and �S44 (grey), where � represents the differencebetween the daytime and nighttime data.

(COONH4)2 agree well with those for (COOH)2. However, themass spectrum of (COONH4)2 cannot be explained by the sumof (COOH)2 and NH3: i.e., the observed m/z 16 (NH+

2 ) and 17(NH+

3 ) peaks, which are the major fragments of NH3, appearssmaller by a factor of ∼3 than those expected from the RIEN H3

value for ammonium sulfate/nitrate (RIENH3 = 3.5). There aretwo possible explanations for this feature. The first possibility isthat (COONH4)2 is mainly decomposed to (COOH)2 and NH3

but the RIENH3 value for (COONH4)2 is significantly smallerthan that for ammonium sulfate/nitrate. The second possibil-ity is that (COONH4)2 is not simply decomposed to (COOH)2

and NH3. In fact, (COONH4)2 could be converted to oxalic di-amide, (CONH2)2 by a dehydration process at high temperatures(Chemical Dictionary 1994). In order to test the second possi-bility, we examined the mass spectrum of (CONH2)2 in the lab-oratory. Figure 7b shows the comparison of the individual m/zsignals between (COONH4)2 and (CONH2)2. Again, each m/zsignal has been normalized by the m/z 44 signal. The relative in-tensities of the m/z 18, 28, 44, 45, and 46 signals for (COONH4)2


FIG. 5. Mass spectrum of oxalic acid obtained in the laboratory (solid bars). Mass spectrum from the NIST standard reference database is also shown as shadedbars. Each mass spectrum has been normalized so that the sum of all m/z peaks is equal to unity.

FIG. 6. Mass spectra of sodium oxalate ((COONa)2) and ammonium oxalate ((COONH4)2) obtained in the laboratory.

FIG. 7. (a) Comparison of the individual m/z signals between (COONH4)2 and (COOH)2 and (b) between (COONH4)2 and (CONH2)2. Each m/z signal hasbeen normalized by the m/z 44 signal (base peak). The m/z numbers are shown beside the data points.


FIG. 8. Same as Figure 5 but for the other selected diacids (C3–C6 and Ph) and ω-oxoacid (ωC2).

are significantly different from those for (CONH2)2, indicatingthat the second possibility is unlikely. It is interesting to note thatthe base peak of (CONH2)2 is m/z 44 (CONH+

2 ), which impliesthat amide compounds could be an important source of m/z 44signal in some cases.

The filter sampling/GC analysis detects both (COONH4)2

and (COONa)2 as (COOH)2. In the case of this study, the con-tribution of (COONa)2 in the AMS mass spectrum is likely neg-ligible because of the low concentration of Na+ in the PM1

mode during the measurement period (Takegawa et al. 2005).The contribution of (COONH4)2 may not be negligible becausethere was enough NH3 to fully neutralize sulfate, nitrate, andchloride during this period (Morino et al. 2006). However, wecan use the mass spectrum of (COOH)2 for the present analysisassuming that (COONH4)2 is mainly decomposed to (COOH)2

and NH3 on the AMS vaporizer. Same assumption is applied tothe other selected organic acids. Note that the RIEN H3 issue re-mains uncertain and requires additional laboratory experimentsin future studies.

3.2.2. Mass Spectra of Other Selected Organic AcidsMass spectra of the other selected diacids (C3–C6 and Ph)

and ω-oxoacid (ωC2) obtained in the laboratory are shown inFigure 8. The positions of the significant peaks are similar to theNIST database, although the major fragments of the AMS massspectrum are shifted to smaller m/z numbers as compared toNIST. The difference can be attributed to the higher vaporizationtemperature of the AMS than used in the NIST experiments, asmentioned earlier. These similarities and differences betweenthe NIST and AMS mass spectra have been noted in Alfarra(2004), although m/z 28 is not included in the mass spectra oforganic compounds given in Alfarra (2004).

It should be noted that the m/z 39 signal (mainly C3H+3 ) is

present in the mass spectra of relatively higher molecular weightdiacids (C4–C6 and Ph). The m/z 39 signal is also detected in theNIST mass spectra. The AMS standard analysis procedure doesnot include the m/z 39 signal originating from organics becausethere is an artifact of K+ produced by surface ionization on thevaporizer. The artifact of K+, which varies with each instrument,


TABLE 6Fraction of m/z 44 signal and relationship between m/z 44 and other m/z signals for selected organic acids

Best estimateb

Species Expected mass Numberi Molecular formula fraction of COa

2 ui44 ui

18/ui44 ui

28/ui44 ui

39/ui44 ui

46/ui44 of data

Dicarboxylic acidsC2 HOOC-COOH 0.49 0.34 (8%) 0.45 (39%) 0.67 (8%) N/A 0.12 (40%) 12C3 HOOC-CH2-COOH 0.42 0.21 (13%) 0.41 (72%) 0.35 (10%) N/A 0.007 (14%) 8C4 HOOC-(CH2)2-COOH 0.37 0.069 (27%) 1.8 (35%) 4.9 (45%) 0.023 (80%) 0.057 (54%) 12C5 HOOC-(CH2)3-COOH 0.33 0.049 (22%) 2.4 (32%) 2.5 (15%) 1.1 (21%) 0.026 (19%) 9C6 HOOC-(CH2)4-COOH 0.30 0.071 (3%) 1.6 (18%) 2.3 (8%) 0.69 (6%) 0.017 (10%) 7Ph C6H4(COOH)2 0.27 0.082 (32%) 0.79 (80%) 2.3 (27%) 0.20 (13%) 0.008 (20%) 9

ω-Oxocarboxylic acidsωC2 OHC-COOH 0.60 0.15 (34%) 0.89 (31%) 1.4 (22%) N/A 0.16 (67%) 14

aAssuming that one of the carboxyl groups is detected as the m/z 44 signal.bValues in parenthesis are the relative uncertainties (i.e., ratio of the standard deviation to the average). N/A means that the m/z 39 signal is

not assigned as a fragment of the compound.

depends on the condition of the vaporizer and perhaps on theloadings of ambient aerosol, which generally makes it difficultto extract the m/z 39 signal originating from organic compounds.Figure 9 shows the correlation plots of S39 versus

∑Sm/zfor C2

and C5 diacids. No significant S39 was observed for C2 diacid,which is expected from its elemental composition. On the otherhand, S39 shows tight correlation with

∑Sm/z for C5 diacid

(∼6% of the total), indicating that m/z 39 is one of the significantpeaks for this compound. The m/z 39 signal was also found in themass spectra for C4, C6, and Ph diacids (Figure 8). The r2 valuesof S39 versus

∑Sm/z were 0.88, 0.99, and 0.99 for C4, C6, and

Ph diacids, respectively. The m/z 39 signals from the selectedorganic acids are negligibly small as compared to the signalsfrom total organics during the measurement period, given thelow ambient concentrations of C4–C6 and Ph. However, theremay be some other cases where the m/z 39 signal from organicscould become important. Again, this may not be a problem forthe HR-ToF-AMS because it can distinguish C3H+

3 from K+.Table 6 gives the summary of the m/z 44 fraction and the

relationship between m/z 44 and the other m/z signals (m/z 18,28, 39, and 46) for the selected organic acids. In ambient data, them/z 18, 28, 39, and 46 signals originating from organic aerosolare generally difficult to extract from the observed mass spectra.This information provides the basis for estimating those signalsusing the observed m/z 44 signal.

4. DISCUSSION

4.1. Estimate of m/z 44 Signals from Selected OrganicAcids in Ambient Aerosol

Here we estimate the mass spectra of the selected diacids andω-oxoacid in ambient aerosol in a bottom-up manner. The esti-mate is based on the ambient mass concentrations obtained fromthe filter sampling and the fragment patterns characterized in the

laboratory. The signal at an m/z peak originating from a com-pound i (= C2, C3, etc.) in ambient aerosol, si

m/z , is estimatedas follows (reverse of Equation [1]):

sim/z = 10−12QNA

IENO3

MWNO3RIEi CEi

fi

εimi ui

m/z [3]

where CEi and RIEi represent the CE and RIE values for com-pound i , respectively. mi (μg m−3) is the mass concentrationof compound i in ambient aerosol, εi (≤1) is the recovery ofcompound i from the filter samples based on the GC analysis,and fi (≤1) is the PM1 fraction of compound i .

FIG. 9. Correlation plots of the m/z 39 signal (S39) versus the sum of all m/zsignals (

∑Sm/z) for C2 and C5 diacids. The line represents the linear regression

for the C5 diacid data.


FIG. 10. Mass spectra of total organics (grey), oxalic acid (C2: orange), and the sum of the other selected acids (C3–C6, Ph, and ωC2: blue) in ambient aerosolat 12:00–15:00LT on August 5, 2003. Note that the blue bars are stuck on the orange bars.

From Equations (1) and (3), the contribution ratio of com-pound i to the observed m/z signal, Ri

m/z , is estimated as follows:

Rim/z = si

m/z

Sm/z= CEi

CEorg

RIEi

RI Eorg

fi

εi

mi uim/z

Mm/z[4]

Although we assume CEi /CEorg = 1, RIEi /RIEorg = 1, andfi /εi = 1 in the following discussion, these factors are explicitlyincluded in Equation (4) to evaluate the possible uncertainties(see next section).

Figure 10 shows the estimated mass spectra of oxalic acid(C2) and the sum of the other selected acids (C3–C6, Ph, andωC2) in ambient aerosol at 12:00–15:00LT on August 5, 2003,together with the observed mass spectrum of total organics. Itcan be seen that m/z 28 and m/z 44 are the largest two peaks

FIG. 11. Time series of M44 (3-12-h average: thick shaded line; 10-min average: thin shaded line) and the contribution ratios of oxalic acid (solid circles) andthe sum of the selected acids (open circles) to S44.

from the selected acids and have significant contributions to themass spectrum of total organics. Figure 11 shows the time seriesof RC2

44 , Rselected44 , and M44, illustrating the contribution ratios of

oxalic acid and the selected acids to the m/z 44 signal duringthe measurement period. The 10-min average of M44 is alsoshown for comparison. Both RC2

44 and Rselected44 tend to be higher

during the daytime, suggesting that these ratios were largelycontrolled by photochemical activity. It should be noted thatRselected

44 reaches as high as 24% during the midnight of August 7(8/6/2006 20:00 - 8/7/2006 8:00). The 10-min M44 data indicatesthat there was a significant enhancement of M44 during this timeperiod. Possible explanations for the nighttime peak of SOA (i.e.,formation of SOA by O3 and NO3 and/or shift in gas/particlepartitioning of SOA) were discussed in Takegawa et al. (2006a).Note that the last two data points obtained on August 8 are notused for calculating the RC2

44 and Rselected44 values because the


relative errors of these data points may be large considering thelow concentrations of M44 and possible artifacts in the diacidmeasurements at low concentrations (Ray and McDow 2005).

On average, the selected acids account for 14 ± 5% ofS44. The difference in the integration time of the filter samples(3–12 h) is not taken into account in calculating the average.The error represents the standard deviation and does not in-clude the uncertainties in the ambient measurements and lab-oratory experiments. These uncertainties will be discussed inthe next section. Oxalic acid is the largest contributor, account-ing for 10 ± 4% of S44 on average. It should be noted thatseven low molecular weight compounds alone could explain asubstantial fraction (∼14%) of M44 observed during the mea-surement period. The rest of M44 may have originated frommono-/poly- carboxylic acids and perhaps from peroxides be-cause these compounds could produce CO+

2 in the vaporiza-tion and ionization processes of the AMS. In fact, significantamounts of mono-/poly- carboxylic acids have been found in ur-ban aerosol (e.g., Alves et al. 2002; Fuzzi et al. 2001; Rogge et al.1993).

4.2. Possible UncertaintiesPossible uncertainties in estimating the contributions of ox-

alic acid to the m/z 44 signal are now evaluated using Equa-tion (4). The uncertainty in uC2

44 is estimated to be 8% basedon Table 6. The uncertainty in mC2 is estimated to be 15%, asdescribed earlier. The uncertainty in M44 is assumed to be 26%,as described in section 2.1.2. The uncertainties in RIEC2/RIEorg

and CEC2/C Eorg depend on how the RIE and CE values of ox-alic acid are similar to or different from those of total organicaerosol. Because the assumed RI Eorg (= 1.4) is close to theRIE values for oxygenated organic compounds, the uncertaintyin RIEC2/RIEorg would be small. Alfarra (2004) showed that CEvalues of pure C4 diacid particles generated in the laboratorywere smaller than 0.5. However, it has been demonstrated thatsolid compounds generated in the laboratory (e.g., (NH4)2SO4)generally show lower CE values than those in ambient air. As-suming that oxalic acid in ambient aerosol is internally mixedwith most of the organic compounds in ambient air, the uncer-tainty in CEC2/CEorg would also be small. The uncertainty infC2/εC2 is estimated as follows. The recoveries are estimated tobe ∼0.7 for oxalic acid, as described earlier. During the sum-mer of 2004, we conducted measurements of diacids using ahigh-volume sampler and also using a PM1 low-volume sam-pler at the RCAST observatory. The analysis and evaluation ofthe filter measurements are now being performed (Kawamuraet al., manuscript in preparation). The preliminary results showthat the PM1 diacids account for 40–80% of the high-volumediacids. Based on these results, the fi /εi values are estimated tobe 0.6–1.1 for oxalic acid. Therefore, the uncertainty in fC2/εC2

is estimated to be <40%. In summary, the overall uncertainty inRC2

44 is estimated to be ∼50%.

5. CONCLUSIONSThe relationship between low molecular weight diacids and

ω-oxoacids in ambient aerosol and the AMS m/z signal has, forthe first time, been investigated based on ambient measurementsand laboratory experiments. Oxalic acid was found to be themost abundant compounds of total diacids identified in the filtersamples. The mass concentrations of oxalic acid, total diacids,and total ω-oxoacids correlated well with the m/z 44 signal (r2 =0.85–0.94), indicating a strong link between m/z 44 and thesecompounds.

Detailed analysis of ambient mass spectra and particle time-of-flight data suggests that m/z 28 from organics could be com-parable to m/z 44 during the measurement period, although theAMS standard analysis procedure does not include m/z 28 for themass spectrum of organics. Mass spectra of selected diacids (C2–C6, and Ph) and ω-oxoacid (ωC2) were investigated in the labo-ratory. For the selected compounds, m/z 28 and 44 were foundto be the major fragments. Both ambient and laboratory datasuggest the importance of m/z 28 for the analysis of organics.

Mass spectra of the selected diacids and ω-oxoacid in am-bient aerosol were estimated based on the mass concentrationsof these compounds obtained from the filter sampling and thefragment patterns of these compounds characterized in the labo-ratory. On average, the selected compounds accounted for 14 ±5% of the observed m/z 44 signal during the measurement pe-riod. Oxalic acid was the largest contributor, accounting for 10 ±4% of the observed m/z 44 signal. These results could be use-ful for the interpretation of m/z 44 data obtained from ambientmeasurements.

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APPENDIX. DEPENDENCE OF FRAGMENT PATTERNSON CARRIER GAS AND MASS LOADINGS

In our laboratory experiments, pure O2 was used as the car-rier gas (i.e., compressed gas to atomize the water solutions ofthe selected organic acids) in order to detect the m/z 28 signaloriginating from the organic acids. Synthetic air (N2+ O2) andargon (Ar) were also used for oxalic acid to examine possibleeffects of the carrier gas on the mass spectra. In addition, themass concentrations of oxalic acid were varied by adjusting adilution flow of O2, in order to examine possible dependence ofthe fragment patterns on the mass loadings.

Figure A1 shows the correlation of S44 versus∑

Sm/z foroxalic acid particles generated with O2, synthetic air, and Ar.Note that the S28 signal for synthetic air was estimated basedon the PToF data (see section 3.1.2). Only data obtained withO2 were used for the linear regression analysis. The S44/

∑Sm/z

ratios for the synthetic air and Ar data agreed with the aver-age S44/

∑Sm/z ratio calculated from the O2 data to within

9%, suggesting that the dependence of the fragment patternon the carrier gas is small. The dynamic range of

∑Sm/z

FIG. A1 Correlation of the m/z 44 signal (S44) versus the sum of all m/z signals(∑

Sm/z) for oxalic acid. The solid, open, and shaded circles represent the dataobtained using O2, synthetic air, and Ar, respectively, as the carrier gas of theparticle generation system. The line represents the linear regression for the O2

data.


corresponds to mass concentrations of 3-300 μg m−3 if weassume CEorg = 0.5 and RIEorg = 1.4 in equation [1]. Thecorrelation of S44 versus

∑Sm/z exhibits very good linearity

(r2 = 0.99) over the whole mass concentration range consid-

Best-estimate mass spectra (std: standard deviation)

m/z C2 C2 std C3 C3 std C4 C4 std C5 C5 std

1 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00002 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00003 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00004 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00005 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00006 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00007 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00008 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00009 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

10 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000011 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000012 0.0304 0.0038 0.0356 0.0046 0.0148 0.0026 0.0140 0.002613 0.0007 0.0001 0.0261 0.0038 0.0078 0.0005 0.0095 0.001414 0.0044 0.0018 0.1663 0.0152 0.0188 0.0016 0.0525 0.008315 0.0109 0.0051 0.0335 0.0045 0.0053 0.0009 0.0178 0.001516 0.0060 0.0021 0.0033 0.0020 0.0046 0.0016 0.0044 0.000917 0.0375 0.0131 0.0205 0.0125 0.0288 0.0100 0.0272 0.005418 0.1498 0.0526 0.0821 0.0501 0.1151 0.0401 0.1089 0.021419 0.0002 0.0001 0.0001 0.0001 0.0001 0.0000 0.0001 0.000020 0.0003 0.0001 0.0002 0.0001 0.0002 0.0001 0.0002 0.000021 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000022 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000023 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000024 0.0001 0.0001 0.0056 0.0007 0.0044 0.0006 0.0035 0.000225 0.0000 0.0001 0.0085 0.0009 0.0171 0.0017 0.0098 0.000826 0.0006 0.0003 0.0038 0.0007 0.0933 0.0074 0.0390 0.002827 0.0014 0.0005 0.0011 0.0008 0.0800 0.0084 0.0584 0.004528 0.2279 0.0310 0.0751 0.0089 0.3025 0.0234 0.1191 0.006629 0.0272 0.0066 0.0227 0.0025 0.0313 0.0051 0.0198 0.001230 0.0018 0.0011 0.0028 0.0008 0.0041 0.0006 0.0020 0.000531 0.0005 0.0005 0.0076 0.0013 0.0031 0.0005 0.0051 0.000532 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000033 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000034 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000035 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.000036 0.0000 0.0000 0.0000 0.0000 0.0011 0.0001 0.0019 0.000237 0.0000 0.0000 0.0000 0.0000 0.0025 0.0002 0.0088 0.000538 0.0000 0.0000 0.0000 0.0000 0.0017 0.0002 0.0127 0.000839 0.0000 0.0000 0.0000 0.0000 0.0014 0.0007 0.0522 0.002040 0.0013 0.0007 0.0088 0.0014 0.0016 0.0003 0.0149 0.001541 0.0017 0.0014 0.0339 0.0049 0.0043 0.0004 0.0416 0.003442 0.0013 0.0002 0.1423 0.0171 0.0129 0.0023 0.1405 0.009343 0.0013 0.0007 0.0248 0.0034 0.0040 0.0009 0.0173 0.0008

(Continued on next page)

ered here. Similar results were obtained for m/z 28 (r2 = 0.99for S28 versus

∑Sm/z). This result suggests that the depen-

dence of the fragment pattern on the mass concentration is alsosmall.

TABLE S1 ELECTRONIC SUPPLEMENTAL MATERIAL


Best-estimate mass spectra (std: standard deviation) (Continued)


44 0.3419 0.0267 0.2133 0.0269 0.0691 0.0185 0.0490 0.010645 0.0957 0.0307 0.0372 0.0066 0.0367 0.0066 0.0355 0.001246 0.0401 0.0132 0.0015 0.0002 0.0034 0.0005 0.0012 0.000147 0.0010 0.0006 0.0001 0.0001 0.0011 0.0002 0.0003 0.000148 0.0001 0.0002 0.0001 0.0002 0.0001 0.0001 0.0003 0.000049 0.0000 0.0001 0.0000 0.0000 0.0001 0.0000 0.0015 0.000150 0.0002 0.0002 0.0001 0.0002 0.0002 0.0001 0.0029 0.000151 0.0001 0.0001 0.0000 0.0001 0.0002 0.0001 0.0024 0.000152 0.0000 0.0001 0.0002 0.0001 0.0012 0.0001 0.0029 0.000253 0.0002 0.0002 0.0002 0.0002 0.0052 0.0005 0.0031 0.000354 0.0003 0.0002 0.0000 0.0001 0.0071 0.0013 0.0031 0.000155 0.0013 0.0006 0.0005 0.0005 0.0280 0.0041 0.0277 0.001056 0.0034 0.0017 0.0003 0.0003 0.0418 0.0111 0.0032 0.001257 0.0007 0.0005 0.0002 0.0002 0.0020 0.0005 0.0018 0.000158 0.0005 0.0007 0.0001 0.0001 0.0002 0.0001 0.0077 0.000859 0.0001 0.0003 0.0003 0.0002 0.0001 0.0000 0.0005 0.000160 0.0001 0.0001 0.0281 0.0085 0.0002 0.0001 0.0176 0.002461 0.0000 0.0000 0.0006 0.0003 0.0000 0.0000 0.0004 0.000162 0.0001 0.0001 0.0001 0.0001 0.0000 0.0000 0.0001 0.000163 0.0001 0.0001 0.0000 0.0001 0.0003 0.0000 0.0001 0.000164 0.0002 0.0003 0.0002 0.0002 0.0001 0.0001 0.0001 0.000165 0.0003 0.0005 0.0001 0.0002 0.0000 0.0000 0.0002 0.000166 0.0003 0.0007 0.0002 0.0002 0.0001 0.0001 0.0005 0.000167 0.0003 0.0004 0.0001 0.0001 0.0001 0.0001 0.0002 0.000068 0.0002 0.0002 0.0022 0.0009 0.0002 0.0001 0.0094 0.001569 0.0003 0.0004 0.0024 0.0009 0.0005 0.0002 0.0018 0.000270 0.0002 0.0003 0.0003 0.0002 0.0006 0.0002 0.0063 0.002071 0.0002 0.0002 0.0000 0.0000 0.0006 0.0001 0.0010 0.000172 0.0001 0.0002 0.0000 0.0001 0.0037 0.0004 0.0005 0.000173 0.0010 0.0004 0.0007 0.0003 0.0125 0.0022 0.0062 0.001374 0.0003 0.0004 0.0001 0.0001 0.0116 0.0018 0.0003 0.000175 0.0001 0.0001 0.0000 0.0000 0.0005 0.0001 0.0000 0.000076 0.0000 0.0002 0.0001 0.0001 0.0001 0.0001 0.0000 0.000077 0.0001 0.0002 0.0000 0.0000 0.0001 0.0001 0.0001 0.000178 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0002 0.000179 0.0001 0.0002 0.0000 0.0000 0.0001 0.0001 0.0000 0.000080 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0001 0.000181 0.0002 0.0002 0.0000 0.0000 0.0001 0.0000 0.0001 0.000182 0.0001 0.0002 0.0000 0.0001 0.0004 0.0001 0.0002 0.000183 0.0002 0.0002 0.0000 0.0000 0.0002 0.0001 0.0001 0.000184 0.0000 0.0002 0.0000 0.0000 0.0001 0.0001 0.0001 0.000185 0.0001 0.0001 0.0001 0.0001 0.0000 0.0000 0.0009 0.000386 0.0001 0.0001 0.0011 0.0006 0.0000 0.0000 0.0203 0.004787 0.0000 0.0001 0.0023 0.0009 0.0000 0.0000 0.0026 0.000688 0.0000 0.0001 0.0001 0.0000 0.0000 0.0000 0.0004 0.000289 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0001 0.000090 0.0002 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.000091 0.0001 0.0003 0.0001 0.0001 0.0001 0.0001 0.0001 0.000192 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000




93 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.000094 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0001 0.000095 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0001 0.000196 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0005 0.000297 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0002 0.000098 0.0001 0.0001 0.0000 0.0000 0.0002 0.0001 0.0001 0.000199 0.0001 0.0001 0.0000 0.0000 0.0001 0.0000 0.0000 0.0000

100 0.0001 0.0001 0.0000 0.0000 0.0060 0.0014 0.0000 0.0000101 0.0000 0.0001 0.0000 0.0000 0.0022 0.0005 0.0000 0.0000102 0.0000 0.0001 0.0000 0.0000 0.0001 0.0001 0.0000 0.0000103 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000104 0.0001 0.0001 0.0002 0.0002 0.0000 0.0000 0.0001 0.0002105 0.0001 0.0002 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000106 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000107 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000108 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000109 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000110 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000111 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000112 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000113 0.0001 0.0002 0.0000 0.0000 0.0000 0.0000 0.0001 0.0000114 0.0001 0.0002 0.0000 0.0000 0.0000 0.0000 0.0025 0.0009115 0.0000 0.0002 0.0000 0.0000 0.0000 0.0000 0.0010 0.0004116 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0001 0.0001117 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000118 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000119 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000120 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000121 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000122 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000123 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000124 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000125 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000126 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000127 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000128 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000129 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000130 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000131 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000132 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000133 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000134 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000135 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000136 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000137 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000138 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000139 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000140 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

End of Table S1


TABLE S2

Best-estimate mass spectra (std: standard deviation)

m/z C6 C6 std Ph Ph std ωC2 ωC2 std

1 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000002 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000003 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000004 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000005 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000006 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000007 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000008 0.000000 0.000000 0.000000 0.000000 0.000000 0.0000009 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

10 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000011 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000012 0.011904 0.000384 0.016532 0.003330 0.019254 0.00494613 0.002729 0.000061 0.000908 0.000288 0.007829 0.00096714 0.008574 0.000455 0.002146 0.001843 0.010883 0.00190315 0.005621 0.000560 0.001127 0.000608 0.010848 0.00232016 0.004471 0.000851 0.002019 0.001514 0.004977 0.00127517 0.027946 0.005316 0.012619 0.009461 0.031108 0.00796518 0.111783 0.021268 0.050476 0.037846 0.124431 0.03186019 0.000133 0.000026 0.000056 0.000051 0.000148 0.00003820 0.000224 0.000043 0.000095 0.000085 0.000249 0.00006321 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000022 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000023 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000024 0.001463 0.000054 0.000664 0.000188 0.000984 0.00011725 0.005971 0.000175 0.002530 0.000587 0.001512 0.00029126 0.039705 0.001281 0.009332 0.002580 0.005838 0.00277927 0.072800 0.005255 0.005242 0.002039 0.019387 0.01277228 0.163620 0.009560 0.176104 0.009228 0.202722 0.03294129 0.023896 0.001753 0.005619 0.003019 0.171030 0.05075130 0.001403 0.000058 0.000475 0.000436 0.049330 0.01534631 0.004654 0.000081 0.001309 0.000930 0.006487 0.01478332 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000033 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000034 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000035 0.000000 0.000000 0.000000 0.000000 0.000000 0.00000036 0.000975 0.000062 0.002134 0.000420 0.000000 0.00000037 0.006070 0.000197 0.020434 0.004553 0.000000 0.00000038 0.008829 0.000164 0.024591 0.005105 0.000000 0.00000039 0.048527 0.001980 0.015939 0.002962 0.000000 0.00000040 0.008466 0.000258 0.001388 0.000628 0.003820 0.00117841 0.036503 0.000881 0.001680 0.001236 0.009788 0.00158242 0.025149 0.000788 0.000711 0.000526 0.008297 0.00167643 0.018472 0.001295 0.001541 0.000881 0.004420 0.00176344 0.070780 0.001948 0.082092 0.026012 0.152102 0.05217745 0.027316 0.001881 0.007668 0.001608 0.063991 0.02265446 0.001184 0.000092 0.000644 0.000198 0.019810 0.00683747 0.000393 0.000038 0.000541 0.000069 0.023732 0.00692948 0.000537 0.000045 0.001543 0.000615 0.000685 0.00085249 0.003297 0.000071 0.012402 0.001859 0.000112 0.000092




50 0.011355 0.000320 0.094249 0.013614 0.000313 0.00027351 0.011018 0.000354 0.015456 0.001875 0.000518 0.00011552 0.008112 0.000536 0.006395 0.000617 0.000394 0.00011253 0.018646 0.000791 0.005605 0.000851 0.000642 0.00026554 0.020536 0.001479 0.000437 0.000241 0.000962 0.00025755 0.054086 0.004527 0.000963 0.000546 0.002599 0.00085356 0.013177 0.000292 0.000315 0.000184 0.007997 0.00370857 0.003026 0.000159 0.000470 0.000282 0.001763 0.00075458 0.003002 0.000113 0.000073 0.000067 0.002987 0.00195759 0.001820 0.000214 0.000028 0.000055 0.000410 0.00018160 0.013679 0.001399 0.001057 0.000239 0.000468 0.00024161 0.001418 0.000228 0.003965 0.000308 0.000063 0.00013562 0.001200 0.000153 0.002981 0.000349 0.000083 0.00015063 0.001894 0.000201 0.004828 0.000682 0.000127 0.00015664 0.000809 0.000113 0.003072 0.000831 0.000393 0.00065565 0.003184 0.000364 0.012834 0.001892 0.000250 0.00017866 0.004197 0.000409 0.002307 0.000391 0.000263 0.00021367 0.002490 0.000406 0.000225 0.000339 0.000778 0.00035768 0.005186 0.000780 0.000145 0.000096 0.001081 0.00060069 0.008104 0.000964 0.000154 0.000399 0.001346 0.00037470 0.000693 0.000026 0.000016 0.000124 0.000764 0.00024471 0.000494 0.000078 0.000089 0.000288 0.000377 0.00024472 0.000613 0.000036 0.000656 0.000107 0.000127 0.00020373 0.007990 0.001408 0.009741 0.000754 0.002547 0.00092674 0.001148 0.000153 0.031551 0.002538 0.001079 0.00038775 0.000163 0.000023 0.017086 0.000582 0.001763 0.00069876 0.000128 0.000057 0.110239 0.006922 0.000449 0.00038977 0.000309 0.000021 0.023940 0.000716 0.000307 0.00024078 0.000302 0.000052 0.004087 0.000360 0.000068 0.00021579 0.000411 0.000137 0.000310 0.000258 0.000371 0.00031080 0.003465 0.000346 0.000123 0.000094 0.000225 0.00009981 0.003529 0.000418 0.000121 0.000109 0.000433 0.00034082 0.011635 0.001474 0.000124 0.000100 0.000335 0.00027283 0.002420 0.000269 0.000087 0.000089 0.000499 0.00033384 0.004239 0.000519 0.000087 0.000185 0.000316 0.00017085 0.000869 0.000118 0.000290 0.000127 0.000600 0.00035586 0.000623 0.000087 0.000202 0.000115 0.000380 0.00022287 0.005333 0.001009 0.000089 0.000053 0.000266 0.00017788 0.000269 0.000044 −0.000041 0.000101 0.000118 0.00014089 0.000074 0.000011 0.000086 0.000079 0.000135 0.00020590 0.000022 0.000011 0.000043 0.000092 0.000057 0.00008591 0.000102 0.000040 0.000246 0.000397 0.000259 0.00028492 0.000668 0.000166 0.002739 0.000224 0.000155 0.00016293 0.000077 0.000034 0.004310 0.000598 0.000207 0.00020294 0.000173 0.000042 0.001627 0.000347 0.000144 0.00018295 0.000060 0.000014 0.000177 0.000308 0.000231 0.00022096 0.000111 0.000020 0.000033 0.000138 0.000230 0.00014197 0.000116 0.000027 −0.000036 0.000165 0.000254 0.000159

(Continued on next page)




98 0.000079 0.000027 0.000029 0.000055 0.000266 0.00023099 0.000397 0.000074 0.000020 0.000142 0.000179 0.000221

100 0.016573 0.003489 0.000003 0.000088 0.000169 0.000229101 0.001414 0.000513 0.000189 0.000055 0.000244 0.000300102 0.000273 0.000037 0.000032 0.000093 0.000182 0.000122103 0.000039 0.000018 0.000241 0.000151 0.000129 0.000143104 0.000026 0.000016 0.086466 0.011601 0.000248 0.000199105 0.000035 0.000015 0.042414 0.005802 0.000088 0.000375106 0.000020 0.000009 0.005683 0.002908 0.000066 0.000112107 0.000079 0.000016 0.000379 0.000292 0.000099 0.000150108 0.000126 0.000033 −0.000032 0.000125 0.000058 0.000126109 0.000094 0.000039 −0.000012 0.000154 0.000144 0.000130110 0.002081 0.000389 −0.000003 0.000068 0.000134 0.000113111 0.000488 0.000110 0.000072 0.000078 0.000098 0.000093112 0.000057 0.000031 −0.000055 0.000097 0.000098 0.000231113 0.000045 0.000009 0.000016 0.000199 0.000378 0.000205114 0.000039 0.000015 0.000021 0.000066 0.000171 0.000187115 0.000149 0.000057 0.000070 0.000072 0.000196 0.000189116 0.000050 0.000013 −0.000024 0.000092 0.000125 0.000142117 0.000037 0.000010 −0.000033 0.000157 0.000122 0.000133118 0.000024 0.000009 0.000025 0.000015 0.000098 0.000112119 0.000027 0.000013 0.000187 0.000104 0.000128 0.000140120 0.000012 0.000008 0.000286 0.000121 0.000083 0.000137121 0.000016 0.000008 0.001667 0.000279 0.000092 0.000135122 0.000020 0.000005 0.021228 0.004180 0.000061 0.000165123 0.000008 0.000008 0.005418 0.003269 0.000082 0.000088124 0.000007 0.000005 0.000380 0.000325 0.000061 0.000091125 0.000009 0.000007 0.000056 0.000044 0.000044 0.000109126 0.000021 0.000010 0.000029 0.000043 0.000064 0.000065127 0.000055 0.000007 0.000000 0.000072 0.000076 0.000101128 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000129 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000130 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000131 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000132 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000133 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000134 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000135 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000136 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000137 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000138 0.000002 0.000003 −0.000032 0.000089 0.000073 0.000123139 0.000012 0.000003 0.000043 0.000097 0.000092 0.000115140 0.000005 0.000005 0.000007 0.000072 0.000037 0.000061141 0.000014 0.000014 −0.000039 0.000079 0.000059 0.000098142 0.000012 0.000014 −0.000029 0.000063 0.000076 0.000073143 0.000009 0.000005 −0.000007 0.000057 0.000056 0.000111144 0.000013 0.000009 −0.000021 0.000046 0.000040 0.000100145 0.000010 0.000005 −0.000006 0.000070 0.000036 0.000177146 0.000008 0.000007 0.000006 0.000022 0.000114 0.000212




147 0.000193 0.000039 0.000235 0.000146 0.000613 0.000425148 0.000058 0.000030 0.003720 0.001924 0.000520 0.000514149 0.000000 0.000000 0.004044 0.002606 0.000000 0.000000150 0.000041 0.000053 0.001435 0.001161 0.000083 0.000204151 0.000009 0.000012 0.000231 0.000174 −0.000003 0.000131152 0.000012 0.000005 0.000195 0.000103 0.000033 0.000068153 0.000010 0.000003 0.000090 0.000115 0.000038 0.000065154 0.000013 0.000008 0.000076 0.000094 0.000054 0.000059155 0.000009 0.000007 0.000033 0.000055 0.000042 0.000103156 0.000008 0.000007 −0.000023 0.000074 0.000052 0.000093157 0.000010 0.000005 −0.000003 0.000066 0.000059 0.000136158 0.000008 0.000012 −0.000014 0.000068 0.000047 0.000125159 0.000008 0.000005 −0.000027 0.000056 0.000025 0.000139160 0.000003 0.000003 −0.000036 0.000075 0.000050 0.000085161 0.000004 0.000004 −0.000014 0.000061 0.000105 0.000119162 0.000003 0.000003 −0.000007 0.000042 0.000263 0.000342163 0.000006 0.000005 −0.000016 0.000085 0.000309 0.000410164 0.000005 0.000004 −0.000011 0.000067 0.000067 0.000178165 0.000023 0.000035 0.000005 0.000073 0.000076 0.000070166 0.000009 0.000006 0.000647 0.000347 0.000050 0.000089167 0.000014 0.000010 0.000360 0.000319 0.000068 0.000112168 0.000024 0.000027 0.000152 0.000131 0.000019 0.000119169 0.000012 0.000009 0.000076 0.000092 0.000042 0.000074170 0.000005 0.000005 −0.000008 0.000056 0.000070 0.000075

Contribution of Selected Dicarboxylic and ω-Oxocarboxylic Acids in ...

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