Introduction
Organic aerosols (OA) account for a substantial fraction of
ambient submicron aerosol mass in urban and rural/remote environments, with
important impacts ranging from human health to climate forcing (IPCC, 2013;
Pope and Dockery, 2006). In recent years the Aerodyne aerosol mass
spectrometers (AMS; Canagaratna et al., 2007) have seen wide use for
characterizing the composition, the elemental ratios (H : C, O : C,
N : C, S : C, OM : OC) (Aiken et al., 2007, 2008) and the approximate
carbon oxidation state (OS‾C≈2× O : C-H : C) of OA (Kroll et al., 2011). This information
provides key constraints for understanding aerosol sources, processes,
impacts, and fate, and for experimentally constraining and developing
predictive aerosol models on local, regional, and global scales.
Organic aerosol elemental ratios can be measured with a number of analytical
techniques besides the AMS. These include combustion analysis
(O'Brien et al., 1975; Krivacsy et al., 2001; Kiss et al., 2002),
electrospray ionization coupled to ultra-high-resolution mass spectrometry
with (ESI)
(Nguyen and
Schug, 2008; Altieri et al., 2009; Bateman et al., 2009; Kroll et al.,
2011; Mazzoleni et al., 2010), nuclear magnetic resonance (NMR) spectroscopy
(Fuzzi et al., 2001), Fourier transform infrared
spectroscopy (FTIR) (Gilardoni et al., 2009; Mysak et al., 2011), and X-ray photoelectron spectroscopy (XPS)
(Mysak et al., 2011). Gas chromatography–mass
spectrometry (GC-MS) (Williams et al., 2006) and chemical ionization mass
spectrometry (CIMS) with aerosol collection interface have also recently
been coupled to a high-resolution time-of-flight mass spectrometer to allow
for determination of elemental ratios (i.e., O : C and H : C) of organic aerosols
(Lopez-Hilfiker et al., 2014; Yatavelli and Thornton, 2010; Williams et al., 2014). Each of these techniques has its own strengths and weaknesses. AMS
measurements of bulk aerosol elemental composition are obtained directly
from the average elemental compositions of the individual fragment ions
observed in high-resolution AMS spectra. One strength of the AMS approach is
that it offers the capability of online, sensitive detection of aerosol
elemental composition. A weakness is its use of empirical corrections that
can affect the accuracy of the calculated elemental ratios. This manuscript
evaluates the accuracy of the AMS elemental analysis approach over a wider
range of OA species than has been studied before.
In the AMS, aerosol particles are focused into a beam in a high-vacuum
chamber and typically flash-vaporized on a tungsten vaporizer at a
temperature of 600 ∘C before constituents are detected with electron
ionization (EI) mass spectrometry. Thus, the elemental composition obtained
from AMS mass spectra can be potentially biased by two sources: vaporization
and ion fragmentation. Organic molecules, particularly oxidized organic
species comprising oxidized organic aerosol (OOA), can decompose during the
AMS vaporization process to form stable molecules with elemental
compositions that differ from the original parent molecule. Carboxylic acids
and alcohols, for example, are known to undergo thermally induced
dehydration and decarboxylation as follows (Moldoveanu, 2009):
RCOOH⟶ΔCO2+H2O+CO+R′
RCOH⟶ΔH2O+R′
The decomposition products are all ionized and detected by the AMS. The loss
of neutral CO2, CO, and H2O from the parent carboxylic acid and
alcohol molecules results in the formation of organic ions in EI (R′+
and their fragments) that differ significantly from their parents in
chemical identity and elemental composition. The accuracy with which the
parent elemental ratios are calculated from AMS measurements will depend on
the accuracy with which the C, H, and O masses in all of the decomposition
fragments are measured or accounted for. Mass spectral interferences from
gas and particle species further complicate accurate determinations of
H2O+ and CO+ intensities for OA sampled in air
(Aiken et al., 2008).
Previous work by Aiken et al. (2007, 2008) showed
that O : C and H : C ratios of laboratory standard molecules can be estimated to
within 31 and 10 % (average absolute value of the relative error,
respectively) with the AMS. The “Aiken-Explicit” (A-E) method
averages the elemental composition of all measured fragment ions observed
in high-resolution mass spectra and uses H : C and O : C calibration factors
derived from laboratory measurements of standard organic molecules. The
calibration factors account for differences between the elemental
compositions of the detected fragment ions and their parent molecules, e.g.,
due to the tendency of more electronegative fragments with high O content to
end up as neutrals rather than as positive ions during the ion fragmentation
process. The “Aiken-Ambient” (A-A) method is similar; however, it uses
empirically estimated H2O+ and CO+ intensities for OA sampled
in air. The Aiken-Ambient method is widely used for elemental analysis of
ambient and chamber OA because the intensities of H2O+ and
CO+ originating from OA are difficult to separate from those
originating from other background species in air.
In this study we extend the Aiken et al. (2007, 2008) elemental analysis
calibrations to a wider range of OA species. The Aiken et al. (2007, 2008)
calibration data set used consisted of reduced primary OA (POA)-like organic
species and a few OOA surrogates such as dicarboxylic, fulvic, and amino
acids. The species chosen for the present study contain multi-functional
oxygenated moieties and have high O : C values that are more representative of
ambient OOA species. We investigate the extent to which thermal
decomposition of these species (cf. Reactions R1 and R2) bias elemental
ratio measurements obtained with the AMS. AMS data from the laboratory
standard molecules are used to re-evaluate the Aiken-Explicit and
Aiken-Ambient methods for calculating elemental ratios. An
“Improved-Ambient” (I-A) method (for AMS measurements performed in air) is
determined as part of this study; the changes caused by application of the
Improved-Ambient method to previously published ambient and chamber data are
discussed. Empirical relationships used to determine O : C and H : C ratios from
unit mass resolution AMS spectra are also updated to reflect the improved
calibrations.
Methods
Aerosol standards
A list of the aerosol standards used in this study is given in Table 1. This
list includes alcohols, diacids, polyacids, esters, and other species with
multiple functionalities such as keto and hydroxy acids. All of the
standards were purchased from Sigma-Aldrich (purity ranges > 96 %) except for three synthesized standards including a racemic mixture
of δ-isoprene epoxydiol (IEPOX) diastereomers known to be
intermediates in isoprene oxidation chemistry, as well as known
isoprene-derived SOA constituents cis- and
trans-3-methyl-3,4-dihydroxytetrahydrofurans (Lin et al.,
2012; Zhang et al., 2012).
Aerosol particles were generated by dissolving small amounts of each
standard in about 100 mL of distilled water, followed by atomization. The
standards were atomized with argon carrier gas instead if nitrogen, since
gaseous nitrogen in air produces a very large peak at m/z 28 that make CO+
aerosol signals very difficult to separate and quantify (even at
high-resolution). Detection of CO+ is of great interest since this ion
is a likely thermal decomposition fragment of acids and potentially other
species in OOA. The resulting polydisperse aerosol was then dried (with two
silica gel diffusion dryers in series) in order to remove any remaining
water from the atomization process and sampled directly into the AMS. The
humidity of the flow after drying was spot checked for several experiments
and was found to reproducibly be < 4 %. Any H2O that was not
removed from the particles after exposure to these conditions is likely to
have been further lost by evaporation when the particles encounter the 2 mbar sampling conditions of the AMS aerodynamic lens. Taken together it is
likely that the aerosol H2O was negligible in these experiments and
uncertainties due to the presence of aerosol H2O should have been
small. The atomization setup was thoroughly cleaned between standards and
blank water runs were carried out in between standards to ensure that
cleaning between each set of standards was successful.
AMS operation and data analysis
The HR-ToF-AMS instrument and its data analysis procedures have been
described in detail in previous publications
(Canagaratna et al.,
2007; DeCarlo et al., 2006). The HR-ToF-AMS can be usually operated in two
ion optical modes (V or W) with differing spectral resolutions. For these
experiments the AMS was operated in the more sensitive V-mode. The
resolution of this mode (resolving power of ∼ 3000) was high
enough to resolve the key isobaric fragments observed from the standards
studied here. The higher signal levels observed in the V-mode also allowed
for the use of low-concentration samples in the atomizer, thereby minimizing
cross-contamination between standards and avoiding signal saturation of the
AMS detector or acquisition card. High-resolution ions up to the molecular
weight of each standard were fitted in order to account for all of its ion
fragments. The AMS data analysis software packages SQUIRREL (version 1.51H)
and PIKA (version 1.10H) were used for the analysis of the high-resolution
mass spectra. This software allows for ready calculation of elemental ratios
from both A-A and A-E methods. The A-A calculation uses the default organic
fragmentation wave proposed by Aiken et al. (2008) and the A-E method uses a
copy of the default organic fragmentation wave in which the entries for
m/z 28, 18, 17, and 16 are replaced to use measured ion intensities rather than
estimated values. The I-A elemental ratios discussed below use A-A values
and marker ion relative intensities calculated from normalized organic mass
spectra output by the PIKA software.
A list of standards analyzed in this study and their molecular
O : C and H : C ratios. Standards are categorized according to their functionality into
broad groups. All standards were studied with EI AMS, while standards also studied with
VUV-AMS are noted in the last column.
Name
Formula
O : C
H : C
VUV-AMS
Multifunctional
Cis-Pinonic Acid
C10H14O3
0.3
1.4
X
2-Oxooctanoic Acid
C8H14O3
0.37
1.75
Acetylsalicylic Acid
C9H8O4
0.44
0.89
X
Homovanillic Acid
C9H10O4
0.44
1.11
X
4-Acetylbutyric Acid
C6H10O3
0.5
1.67
5-Oxoazaleic Acid
C9H14O5
0.55
1.56
X
Levulinic Acid
C5H8O3
0.6
1.6
Gamma Ketopimelic Acid
C7H10O5
0.71
1.43
X
3-Hydroxybutyric Acid
C4H8O3
0.75
2
2-Ketobutyric Acid
C4H6O3
0.75
1.5
3-Hydroxy-3-Methylglutaric Acid
C6H10O5
0.83
1.67
1,3-Acetonedicarboxylic Acid
C5H6O5
1
1.2
?-Ketoglutaric Acid
C5H6O5
1
1.2
X
Lactic Acid
C3H6O3
1
1.67
Pyruvic Acid
C3H4O3
1
1.33
Citric Acid
C6H8O7
1.16
1.33
X
Diglycolic Acid
C4H6O5
1.25
1.5
Malic Acid
C4H6O5
1.25
1.5
X
Oxaloacetic Acid
C4H4O5
1.25
1
Glycolic Acid
C2H4O3
1.5
2
Tartaric Acid
C4H6O6
1.5
1.5
X
Alcohols
Cis-3-methyl-3,4-dihydroxytetrahydrofuran
C5H10O3
0.6
2
Racemic mixture of δ-Isoprene Epoxydiols
C5H10O3
0.6
2
Trans-3-methyl-3,4-dihydroxytetrahydrofuran
C5H10O3
0.6
2
Mannitol
C6H14O6
1
2.33
Mannose
C6H12O6
1
2
X
Sucrose
C11H23O11
1
2.09
X
Xylitol
C5H12O5
1
2.4
X
Diacids
Sebacic Acid
C10H18O4
0.4
1.8
Azelaic Acid
C9H16O4
0.44
1.78
Pimelic Acid
C7H12O4
0.57
1.71
X
Adipic Acid
C6H10O4
0.66
1.67
X
Glutaric Acid
C5H8O4
0.8
1.6
X
Maleic Acid
C4H4O4
1
1
X
Succinic Acid
C4H6O4
1
1.5
X
Malonic Acid
C3H4O4
1.33
1.33
X
Oxalic Acid
C2H2O4
2
1
Polyacids
1,3,5-Cyclohexanetricarboxylic Acid
C6H9O6
1
1.5
X
Tricarballylic Acid
C6H8O6
1
1.33
X
1,2,4,5-Benzenetetracarboxylic Acid
C6H6O8
1.33
1
X
Esters
Dibutyl Oxalate
C8H18O4
0.5
2.25
Gamma Ketopimelic Acid Dilactone
C6H8O4
0.57
1.14
X
Ethyl Pyruvate
C5H8O3
0.6
1.6
Dimethyl 1,3-Acetonedicarboxylate
C7H10O5
0.71
1.43
Data collection occurred over several months and some standards were
repeatedly measured at different points in time with the same instrument.
Fig. S1a in the Supplement
shows the standard deviations in O : C and H : C values (calculated using
Aiken-Ambient method) obtained during these measurements. As can be seen, for
most standards O : C and H : C values obtained on a given instrument are
reproducible to < 5 and < 3 %, respectively. Figure S1b
and c compare O : C and H : C values obtained for different standards on
three AMS instruments. The values compare well across instruments (O : C
within 4 %, H : C within 7 %).
Scatterplots between known elemental compositions and AMS elemental
ratios obtained with the Aiken-Explicit (A-E; panels a and
b), Aiken-Ambient (A-A; panels c and d), and
Improved-Ambient methods (I-A; panels e and f). A 1 : 1
line is shown for reference in all plots. The standards examined in this
study are colored according to their chemical functionality. Also shown are
previously published standard molecule data from Aiken et al. (2007).
For most of the experiments the AMS vaporizer was operated at a power
corresponding to 600 ∘C. The thermocouple readout from the vaporizer is
sensitive to its exact placement on the vaporizer and can sometimes differ
from instrument to instrument or vary with instrument use. Thus, the
measurements were standardized by varying the vaporizer power to minimize
the width of a monodisperse 350 nm NaNO3 aerosol size distribution
measured by the AMS. The time-of-flight traces of the NO+ ion (m/z 30) from NaNO3 were monitored as a function of vaporizer ion current.
The optimum AMS vaporizer current is obtained by subtracting 0.1 amps from
the vaporizer current at which the narrowest NO+ ion time-of-flight
traces are observed from NaNO3. Typically this optimum AMS vaporizer
current is near 1 amp. In most cases the thermocouple readout at the optimum
heater power setting read temperatures in the range 590–600 ∘C,
indicating that the thermocouples in these instruments were providing a
reasonably accurate measure of the actual heater temperature. In addition to
the standard 600 ∘C operation, a few experiments were also performed at
200 ∘C (about the lowest temperature at which the AMS vaporizer can be
operated continuously) in order to investigate how the amount of thermal
decomposition and ion fragmentation changed with temperature. In both of
these cases, the typical vaporization timescale for particles was measured to
be on the order of 100 μs.
VUV ionization
Northway et al. (2007) described the adaptation of an HR-ToF-AMS to the
vacuum ultraviolet (VUV) beam at the Advanced Light Source (Lawrence
Berkeley Laboratory). We performed a similar adaptation in this study and
generated and analyzed selected standards (see Table 1) using the same
procedures discussed above. Previous work has shown that compared to 70 eV
EI-AMS spectra, VUV-AMS spectra are typically less complex, with reduced ion
fragmentation and increased molecular ion intensity
(Canagaratna et al.,
2007; Northway et al., 2007). Molecular ions observed in VUV-AMS spectra of
unoxidized and slightly oxidized squalane have been successfully used to
obtain chemical and mechanistic insight into the squalane oxidation reaction
(Smith et al., 2009). Moreover, the tunability of the
VUV light can be used to investigate the chemical identity of species by
measuring their threshold ionization energy (Leone et al., 2010).
The threshold ionization energy of most organic molecules is 10.5 eV and
those of H2O, CO2, and CO molecules are 12.62, 13.77, and
14.01 eV, respectively (NIST Chemistry WebBook:
http://webbook.nist.gov/chemistry/). Thus, in this experiment the 8 to
14.5 eV VUV range was used.
Elemental analysis (EA) methods
The procedure for obtaining elemental ratios (O : C, H : C) from AMS spectra was
first developed by Aiken et al. (2007, 2008). The atomic O : C and H : C ratios
are obtained in terms of the relative mass concentrations of O (MO) and
C (MC) and H (MH) as follows:
O:C=αO:C×(MO/MC)×(MWC/MWO)H:C=αH:C×(MH/MC)×(MWC/MWH)
MWC, MWO, and MWH are the atomic weights of C, O, and H, respectively.
Since AMS ion intensities are proportional to the mass of the original
molecules present (Jimenez et al., 2003), MC, MO, and MH are
obtained as a sum of the appropriate ion intensities across the complete
organic spectrum (including H2O+, CO+, and CO2+) as
follows:
MC=∑j=m/zminm/zmaxIjFc,MO=∑j=m/zminm/zmaxIjFO,MH=∑j=m/zminm/zmaxIjFH,
where Ij is the ion intensity of the jth ion in the spectrum and
FC, FO, FH are the relative carbon, oxygen, and hydrogen mass
fractions for that ion. Calibration parameters (αO:C and
αH:C) account for preferential losses of some atoms to
neutral fragments rather than ion fragments during the fragmentation processes. The tendency
of hydrocarbon fragments to form positive ions more readily than those
containing the more electronegative O atom, for example, can result in such
a detection bias. Aiken et al. (2008) obtained slopes of 0.75 and 0.91 (i.e.,
αO:C=1/0.75 and αH:C=1/0.91), respectively,
by comparing measured and known O : C and H : C values for a range of organic
standards according to Eqs. (1) and (2).
In AMS elemental analysis, Eqs. (1) and (2) are applied in two different
ways which we refer to here as the Aiken-Explicit and Aiken-Ambient methods
(Aiken et al., 2008). The
Aiken-Explicit method is used when organic signals at H2O+ and
CO+ can be directly measured. Laboratory measurements performed in an
atmosphere of dry argon, for example, do not contain the interfering
H2O or N2 species and allow for direct measurement of the
organic signals at CO+ and H2O+. The organic signals at
CO+ have also been obtained under ambient conditions from AMS size
distributions and by monitoring changes in the m/z 28 intensities
(Zhang et al., 2005; Takegawa et al.,
2007). Calibrations have also been carried out in laboratory chamber
experiments under controlled relative humidity to determine the interference
signals and obtain the organic signals at CO+ and H2O+ by subtraction (Chen et al., 2011; Nakao et
al., 2013).
The Aiken-Ambient method is used for measurements performed in air where the
interferences from gaseous N2 and H2O are difficult to
estimate. Since most field measurements and laboratory chamber measurements
are performed under the latter conditions, this method has in practice been
the most widely used method of obtaining elemental ratios from AMS
measurements. In the Aiken-Ambient method, the organic H2O+ and
CO+ intensities used in Eqs. (3)–(5) are empirically estimated rather
than directly measured. The H2O+ / CO2+ and
CO+ / CO2+ ratios recommended by Aiken et al. (2008) were
empirically estimated from limited ambient OA measurements available at the
time to be 0.225 and 1, respectively. The CO+ / CO2+ ratio was
determined from AMS size distribution measurements where the gas-phase
signal from N2 can be separated from the particle phase CO signal
intensities (Zhang et al., 2005; Takegawa
et al., 2007). The H2O+ / CO2+ mass ratio was empirically
estimated to conserve OA mass concentrations that resulted from the new
CO+ / CO2+ ratio. This H2O+ / CO2+ empirical
mass ratio corresponds to a raw ion signal ratio of either 0.225, assuming
H2O+ and CO2+ were each formed with a relative
ionization efficiency (RIE) of 1.4 or 0.321, using a recently determined
RIE of 2.0 for the formation of H2O+ (Mensah et al.,
2011).
Fractional AMS ion intensities (relative to the total ion signal for
each standard) measured for CO2+, CO+, and H2O+ from
each of the laboratory aerosol standards studied here. The standards are
classified according to functionality and then arranged according to
increasing O : C. Repeat measurements were performed for some of the
standards to investigate the consistency of measured mass spectra between
different HR-ToF-AMS instruments. Repeat measurements performed for the same
standard are arranged together and denoted by red horizontal bars on the
bottom axis of the graph.
Results and discussion
Evaluation of Aiken-Explicit and Aiken-Ambient methods
We evaluated the performance of both Aiken-Explicit and Aiken-Ambient
methods over a large range of species, including those with higher O : C and
more multifunctional moieties than originally studied by Aiken et al. (2008). Panels a and b in Fig. 1 show elemental ratios obtained with the
Aiken-Explicit method for the laboratory standards studied here. The
Aiken-Explicit method results reproduce actual O : C and H : C ratios for all
the standard molecules with an average absolute value of the relative error
(referred to as “error” in the rest of this manuscript) of 20 and 12 %,
respectively. This is consistent with the accuracies reported by Aiken et al. (2008) and confirms that the Aiken-Explicit method can be used for a
wide range of OA species.
Figures 1c and 1d show Aiken-Ambient results for the laboratory standards.
In general the Aiken-Ambient O : C values are biased low for all the
standards, and observed errors are dependent on the functional groups contained in the
different standard molecules. The Aiken-Ambient values for multifunctional
standard molecules are biased low by 28 % and those for diacids and
alcohols are biased low by 46 %. The error in Aiken-Ambient H : C values for
all standards is smaller, but alcohols and diacids are still biased low
compared to multifunctional species.
Measurements of H2O+, CO+ and CO2+ signal
intensities with Electron Ionization (EI)
The only difference between the Aiken-Explicit and Aiken-Ambient methods is
the measured vs. estimated H2O+ and CO+ ion intensities.
Since these ion intensities are estimated based on assumed H2O+
and CO+ ratios to CO2+, we investigate trends in the relative
signal intensities of these three key ions in the observed standard mass
spectra. Figure 2 shows the fractional AMS ion intensities (relative to the
total ion signal for each standard) measured for these key thermal
decomposition products in the spectra of the different laboratory standards.
The standards are separated according to functionality, and they are
arranged according to increasing molecular O : C. Measurements of the same
standard on different instruments are shown as separate bars on the graph.
The general agreement between different instruments supports the
reproducibility and transferability of the results obtained here to other
AMS instruments. The relative intensities of the three ions vary according
to specific differences in the decomposition mechanisms including those
shown in Reactions (R1) and (R2) above. Spectra from carboxylic acids,
esters, polyacids, and multifunctional acids have higher fCO2+ (defined
as the intensity of CO2+ divided by the total ion intensity) and
fCO+ than alcohols, indicative of decarboxylation. On the other hand,
spectra from alcohols have negligible fCO2+ and significant
fH2O+, indicative of dehydration (Reaction R2).
Figure 3a shows the fCO+ vs. fCO2+ scatterplot for all the
standards in this study. For most multifunctional systems, the fCO+/fCO2+
ratio is relatively consistent with the assumed value of 1 from Aiken et al. (2008). The measured fCO+/fCO2+ ratios for alcohols and most diacids
are ≥ 2 which likely contributes to the additional underestimation in
O : C that is observed for these species with the Aiken-Ambient method. These
measurements are generally consistent with previous studies that have shown
that most laboratory SOA (thought to contain a mixture of multifunctional
species) yield fCO+/fCO2+ values around
1 (Chhabra et al., 2010; Chen et al., 2011)
with exceptions of SOA produced by isoprene photooxidation (2.63; Chen et
al., 2011) and glyoxal uptake under dark, humid conditions (5.0; Chhabra
et al., 2010), both of which contain products that are rich in hydroxyl
functional groups but poor in carboxyl groups
(Hastings et al., 2005; Lin et al., 2012). Ambient
estimates are also in the similar range of 0.9–1.3
(Takegawa et al., 2007; Zhang et
al., 2005). The fCO+/fCO2+ ratios discussed above are summarized in
Table 2.
Summary of fragment ion ratios observed for standard molecules, chamber
SOA, and ambient SOA. The entry * denotes the use of Aiken assumptions for the ratio in
the fragment table.
Obs H2O+ /
Obs CO+ /
Frag Wave H2O+ / CO2+
Frag Wave H2O+ / CO2+
Literature
CO2+
CO2+
RIE H2O=1.4
RIE H2O=2
Reference
AMS Frag Table
Aiken Assumptions
0.32
1
0.32
0.225
Aiken et al. (2008)
OA Standards
Multifunctional
0.5–1.5
1–2
0.5–1.5
0.35–1.05
This Study
Polyacids
1
1–2
1
0.7
Diacids
2
2
2
1.4
Esters
0.5–1
1
0.5–1
0.35–0.7
Alcohols
> 10
> 4
> 10
> 7
Ambient Aerosol
Pittsburgh, USA
*
1.3
*
*
Zhang et al. (2005)
Tokyo, Japan
*
1
*
*
Takegawa et al. (2007)
Whistler Mtn, Canada
1
*
1
0.7
Sun et al. (2009)
Chamber SOA
Isoprene Photooxidation (Low NOx)
3.9
1.03–2.6
3.9
2.7
Chhabra et al. (2010),
Chen et al. (2011)
Isoprene Photooxidation (NOx)
0.3
*
0.3
0.2
Nakao et al. (2013)
α-pinene+O3
0.8–1
1–1.1
0.8–1
0.6–0.7
Chhabra et al. (2010),
Chen et al. (2011),
Nakao et al. (2013)
β-caryophyllene+O3
0.7–1.3
1.2
0.7–1.3
0.5–0.9
Chen et al. (2011),
Nakao et al. (2013)
Toluene Photooxidation (NOx)
1.8
1
1.8
1.3
Hildebrandt Ruiz et al. (2014)
Aromatics Photooxidation (NOx, Low NOx)
0.3–1.3
*
0.3–1.3
0.2–0.9
Nakao et al. (2013)
Naphthalene Photooxidation (Low NOx)
*
1.2
*
*
Chhabra et al. (2011)
The relationships between fH2O+ and fCO2+
for the standard spectra are shown in Fig. 3b and Table 2. The observed
signal intensity ratios in the spectra are larger than those calculated from
the empirical mass ratios of Aiken et al. (2008). The measured
fH2O+/fCO2+ ratio of multifunctional species
varies from near 0 to over 2, and many diacids are between 1 and 2 (although
some are substantially lower than 1). Polyols and alcohol spectra have even
higher ratios, mainly due to their lack of CO2+. As shown in Table 2,
similar departures from the assumed fH2O+/fCO2+ ratios were originally observed for chamber SO0A by
Chen et al. (2011) (0.84–3.91) and more recently by Nakao et al. (2013)
(0.33–1.23). We note that in mixed ambient aerosols the fH2O+/fCO2+ ratios would be moderated by the presence of species
other than alcohols. However, high values for this ratio (1.0) were also
reported for ambient measurements from Whistler Mountain (Sun et al., 2009).
It is clear from Fig. 3 that the biases in the elemental ratios obtained with
the Aiken-Ambient method are due to underestimations of the assumed
fH2O+/fCO2+ and fCO+/fCO2+ values. The H2O+ and CO+ intensities
observed for alcohols, in particular, are severely underestimated in the
current assumptions since the estimates are tied to CO2+, an ion that
is not produced in any significant intensity in spectra of species that do
not contain -C(O)OR moieties (e.g., alcohols).
Scatterplots between AMS fractional ion intensities for CO+
and CO2+ (panel a) and H2O+ and CO2+
(panel b). The empirical ratios used for each of these relationships
in the Aiken-Ambient calculations are shown as solid lines with the
appropriate slopes. In panel (b), two solid lines are shown to
reflect the measured ratios that correspond to possible H2O RIE values
ranging from 1.4 to 2 (see Sect. 2 for more information). Dashed lines in
both panels are included for reference to visualize the range of slope
values.
In the Aiken-Ambient method, the intensities of the OH+ and O+
fragments of H2O+ are estimated according to the ratios measured
for gas-phase H2O. Figure S4 shows the scatterplots
of measured fOH+ vs. fH2O+ and fO+ vs. fH2O+ for all the
laboratory standards. The empirical estimate used in the default AMS
fragmentation table (Allan et al., 2004) for the
OH+ / H2O+ ratio is very consistent with the observed relative
intensities, indicating that the OH+ ion indeed arises from the
fragmentation of molecular water from thermal decomposition of the
standards. The consistency in these fragmentation patterns also holds for
various chamber SOA (Chen et al., 2011). The O+ / H2O+ ratio,
in contrast, shows substantial scatter as the dominant source of the
O+ for our standards appears to be fragmentation of CO2+ rather than H2O+ (Fig. S4c). Alcohols, which do not produce
CO2+, are an exception with O+ / H2O+ ratios
that are much closer to the empirical estimates. Fragmentation of
CO2+ to yield O+ (O+ / CO2+ ∼ 6 %) is currently not accounted for in the AMS elemental ratio analysis
and will contribute to the underestimation observed in AMS O : C values.
Measurements of H2O+, CO+ and CO2+ with VUV
ionization
The H2O+, CO2+, and CO+ signals observed in the
AMS are produced by dehydration and decarboxylation processes that take
place before ionization (i.e., on the vaporizer surface or in the gas-phase
after evaporation) and/or after 70 eV electron-impact ionization (i.e.,
fragmentation of thermally excited ions). VUV-AMS measurements were used to
examine the production mechanisms of these ions in more detail. VUV-AMS data
were obtained for many standards with the AMS vaporizer set to both
200 and 600 ∘C (see Table 1). All experiments were carried out
under an argon atmosphere. A VUV-AMS spectrum of glutaric acid is shown in
Fig. 4a as an example. This spectrum was observed with the AMS vaporizer at
200 ∘C and a VUV photon energy of 10.5 eV. Since VUV is a
“softer”
ionization method than EI, this spectrum would be expected to contain only
the glutaric acid molecular ion if thermal decomposition on the vaporizer
was negligible. However, even at this lower vaporizer temperature, the
molecular ion of glutaric acid (m/z 132) has very low intensity and organic ion
fragments corresponding to loss of neutral H2O, CO, and CO2 from
glutaric acid are observed instead. CO2+, CO+, and
H2O+ are negligible in this spectrum at this VUV photon energy.
Figure 4b shows the CO2+, CO+, and H2O+ signals
observed from glutaric acid as a function of VUV energy. The onsets of
CO2+, CO+, and H2O+ signals are observed to occur
at VUV energies that correspond to the ionization energies of neutral
H2O, CO2, and CO molecules (12.62, 13.77, and 14.01 eV,
respectively), rather than the 10.5 eV ionization energies of the observed
organic ions. This indicates that these ions are formed by VUV ionization of
neutral CO2, CO, and H2O molecules rather than by
dissociative ionization of glutaric acid. Neutral CO2, CO, and
H2O fragments formed upon photoionization of glutaric acid could
further ionize to give rise to these signals. This process requires the
absorption of two photons in the ionization region, however, and is
therefore unlikely. Instead, the most likely source of CO2+,
CO+, and H2O+ signals is direct VUV ionization of neutral
CO2, CO, and H2O molecules formed from thermal
decomposition of organic species on the AMS vaporizer. VUV-AMS measurements
of the other organic standards also show a lack of parent ions and fragments
corresponding to loss of CO2, CO, or H2O moieties (see Fig. S2),
indicating that a wide range of oxidized organic species undergo dehydration
and decarboxylation upon heating to temperatures greater than 200 ∘C.
VUV-AMS spectrum of glutaric acid obtained under an argon
atmosphere. The spectrum was obtained at a VUV energy of 10.5 eV and a
vaporizer temperature of 200 ∘C. Ions corresponding to loss of
CO2, CO, and H2O moieties from the parent ion (M+) are
observed. (b) Glutaric acid VUV-AMS signals as a function of VUV
monochromatic photon energy. The signal intensity of
C5H6O3+, which corresponds to the [M-H2O]+ ion,
and the signal intensities of CO2+, CO+, H2O+ are
shown. The gas phase ionization energies (IE) for neutral CO2, CO, and
H2O molecules are shown as colored vertical lines.
Effect of vaporizer temperature on H2
O+, CO+, and CO2+
Thermal denuder measurements have shown that ambient OA needs to be heated to
a minimum temperature of ∼ 225 ∘C for several seconds in order
to insure quantitative vaporization of a significant fraction of ambient
oxidized OA (Huffman et al., 2009). Figure S3 compares the trends in
fH2O+, fCO+, and fCO2+
observed with the AMS (using EI) at vaporizer temperatures of 600 and
200 ∘C. The total CO2+, CO+, H2O+
decomposition fragment intensities observed for both temperatures is
remarkably similar across the standards. In most cases, fH2O+
is slightly higher at 200 ∘C compared to 600 ∘C, while
fCO+ follows the opposite trend and fCO2+
changes little between the two temperatures. This indicates that dehydration
is facile for these acids and alcohols even at 200 ∘C, which is also
consistent with the VUV results shown in Fig. 4. The extent of thermal
decomposition observed in the AMS is likely influenced by its specific
vaporization conditions (i.e., porous tungsten hot surface and high-vacuum
conditions). For example, Lloyd and Johnston (2009) reported that in
laser-desorption–electron-ionization analysis of SOA, the signal due to
CO2+ was much lower than in AMS spectra of the same aerosol type, and
attributed the difference to differences in the vaporization conditions. Our
measurements suggest that thermally induced decomposition could affect the
interpretation of organic measurements from other aerosol chemistry
measurement techniques that utilize thermal desorption on surfaces, even if
temperatures of only 200 ∘C are reached. Such techniques include
aerosol gas chromatography–mass spectrometry (GC-MS) and thermal-desorption
chemical ionization mass spectrometry (CIMS) (e.g., (Lopez-Hilfiker et al.,
2014; Williams et al., 2006; Yatavelli and Thornton, 2010; Holzinger et al.,
2013). Proton transfer reaction – mass spectrometry (PTR-MS) measurements of
heated ambient filters by Holzinger et al. (2010), for example, show low
molecular weight fragments at higher thermal desorption temperatures,
consistent with this finding. The specific impact of the surface materials,
vaporization temperatures, and pressure conditions on the decomposition
reactions of OA should be the focus of future studies.
Improved-Ambient method
It is clear from Fig. 3 that the relationships among fCO2+,fH2O+, and fCO+ are variable and cannot be
well prescribed with a single empirical relationship. Furthermore, as
discussed in Sects. 3.2 and 3.3, O : C and H : C calculated with the
Aiken-Ambient method are biased low because the empirical estimates used in
this method often underestimate the intensities of the H2O+ and/or
CO+ fragments. Acidic species are observed to be a large source for
CO+ and H2O+ fragments while alcohols are a significant
source of H2O+ fragments.
A correction that is dependent on both acid and alcohol content of the OA is
needed to address this composition dependence in the OA fragmentation.
Previous AMS measurements have shown that fCO2+ can be
used as a surrogate for acid content (Duplissy et al., 2011; Takegawa et al.,
2007). An AMS surrogate for alcohol moieties has not been identified before,
but spectra obtained during this study indicate that fCHO+,(m/z29) can be used as a surrogate for alcohol content. As shown in
Fig. S5, spectra of standard species with no alcohol content have minimal
fCHO+ < 0.05, while those with non-zero alcohol
content show fCHO+ values ranging from 0.05 to 0.15. High
fCHO+ values are found for polyols as well as multifunctional
species with non-acid OH groups. Some esters are also observed to yield
fCHO similar to species with non-acid OH groups. The cleavage of
aldehydes to give CHO+ is not generally observed to be important
(McLafferty and Turecek, 1993). Previous studies have shown that CHO+ is
also an atmospherically significant ion and a key oxygen-containing ion in
many types of ambient and chamber aerosol (Ng et al., 2010a). The
f29(fCHO+) fragment has also been used to monitor
photooxidation of glyoxal and related species in the aqueous phase (Lee et
al., 2011). Based on these results, a composition-dependent correction factor
with a linear dependence on fCO2+ and fCHO+
was examined. While the CHO+ fragment is easily resolved from the
isobaric C2H5+ organic fragment in high-resolution AMS spectra,
it overlaps with 15N14N+ fragments from N2 in air. Thus,
a background correction must be used in order to obtain an accurate value of
fCHO+. AMS data acquired while sampling through a particle
filter can be used to obtain the information needed for such a background
correction (as is already necessary for the accurate determination of
fCO2+).
We performed a multiple linear regression between the known elemental ratios
of the OA standards and those determined from Eqs. (1) and (2) to obtain the
best-fit constants and coefficients as follows:
O:CI-A=O:CA-A×[1.26-0.623×fCO2++2.28×fCHO+]
H:CI-A=H:CA-A×[1.07+1.07×fCHO+]
In the equations above, the Improved-Ambient (I-A) elemental ratios are
expressed as a product of Aiken-Ambient (A-A) elemental ratios and a
composition-dependent correction factor. This allows for simple
recalculation of the Improved-Ambient elemental ratios from Aiken-Ambient
values without the need for performing a re-analysis of the raw mass spectra
and can be easily applied to already published AMS results.
Figures 1e and 1f show the O : C and H : C values obtained for the standards
after using the Improved-Ambient method in Eqs. (6) and (7). The corrections
remove the systematic O : C underestimation seen in Fig. 1c for the
multi-functional species. The diacids and alcohols are still biased low in
O : C, but the bias has been reduced. The errors in the O : C and H : C elemental
ratios calculated for the standards with the Improved-Ambient method are
28 and 13 %, respectively. Smaller errors (8 %) are observed for the
values of OM : OC using the Improved-Ambient method (see Fig. S6).
Figure 5a compares the approximate carbon oxidation state values calculated
from the Improved-Ambient elemental ratios and the known standard elemental
ratios of the organic standards. The two sets of values agree within a
standard deviation of 0.5 OS‾C units. The
largest deviations are observed for the laboratory standards whose
Improved-Ambient O : C values are biased lower than the known values. Figure 5b shows a comparison between the OS‾C values
calculated for the standard molecule data using the Aiken-Ambient and
Improved-Ambient methods. The error bars on the standard data indicate the
estimated uncertainty in the calculated Aiken-Ambient OS‾C values
(using propagation of O : C (H : C) errors of 28 %
(14 %) observed for multifunctional species with Aiken-Ambient method). In
general, the agreement between the Aiken-Ambient OS‾C values
and the Improved-Ambient OS‾C values is much better than the propagated errors. This
indicates that the oxidation state values derived from AMS data (and
potentially from other techniques using thermal desorption MS) are more
robust and less variable than measured values of H : C and O : C.
(a) Scatterplot of Improved-Ambient
OS‾C values (2× O : C – H : C)
of the organic standards vs. their known molecular
OS‾C values. The Improved-Ambient method was
applied with the default AMS organic fragmentation wave (colored solid
circles) as well as with the Hildebrandt Ruiz et al. (2014) changes to the
organic fragmentation wave. (b) Scatterplot of Aiken-Ambient
OS‾C values of the organic standards vs. the
corresponding Improved-Ambient method values. The error bars denote the
propagated uncertainty in the Aiken-Ambient
OS‾C values due to the uncertainties in the
Aiken-Ambient O : C and H : C values. The solid line shows the 1:1
relationship.
The robustness of the OS‾C parameter is largely
due to its invariance with respect to dehydration (and hydration) processes
(Kroll et al., 2011). The OS‾C value that is currently calculated from AMS spectra is not
strictly invariant with respect to hydration and dehydration because the AMS
fragmentation table neglects small amounts of H+ formed from
fragmentation of H2O+ ions (Hildebrandt Ruiz et al., 2014). Other
fragments, such as OH+, and O+ are properly accounted for as
discussed above. Figure 5a shows the effect of calculating Improved-Ambient
OS‾C values with a fragmentation table that
includes the H+ fragment (see Supplement for details).
The Hildebrandt Ruiz et al. (2014) correction results in slightly smaller
OS‾C values than obtained with the default AMS
fragmentation table due to a small (< 3 %) increase in the
Improved-Ambient H : C.
Estimated accuracy of the Improved-Ambient method for mixtures
The errors observed in the Improved-Ambient elemental ratios of individual
OA standards are expected to be upper limits for the corresponding errors in
mixtures, where inaccuracies in individual molecule predictions can
compensate for each other. The expected improvement in accuracy for mixtures
is investigated here for randomly generated theoretical mixtures of the OA
standard molecules.
Theoretical standard mixtures were generated by combining equimolar fractions
of up to 25 different individual OA standards. Each mixture of individual
standards is expressed as ∑i niCxiHyiOzi,
where ni is the mole fraction of standard i in the mixture, and xi,
yi, and zi are the number of C, H, and O atoms within a molecule of
standard i. The O : C ratio for any given mixture is calculated as
follows:
O:Cmix(I-A)=∑i(O:Ci(I-A)×nixi)/∑i(nixi)
O:Cmix(Molecular)=∑i(O:Ci(molecular)×nixi)/∑i(nixi)
H : C ratios are calculated analogously with the appropriate substitutions in
Eqs. (8) and (9). For each type of mixture, 1000 different randomly
generated versions were examined and the average absolute value of relative
errors in the calculated Improved-Ambient elemental ratios over all 1000
variants is calculated for each mixture. For the 1000 mixtures made of 25
standards, the O : C ratios ranged from 0.3 to 0.83 and the H : C ratios ranged
from 1.36 to 1.92. The mixtures made of 10 standards covered a wider range
of O : C ratios (0.18–1.02) and H : C ratios (1.15–2.02). For comparison, the
average Improved-Ambient O : C (H : C) values of LV-OOA are 0.84 (1.43) and of
SV-OOA are 0.53 (1.62). Thus, the elemental ratios of the organic standard
mixtures cover the range of ambient observations.
Figures 6a and b show the error in Improved-Ambient O : C and H : C values as a
function of the number of standard molecules in the mixture of interest. It
is clear from the figure that the error becomes smaller and plateaus for
both of the elemental ratios as the number of OA species in the mixture is
increased. For O : C and H : C the errors decrease from 28 to 12 % and
13 to 4 %, respectively, as the number of species in the mixture is
increased. The plateau in the error is already reached for both elemental
ratios by the time that only 10 different standard OA species are added
together. Ambient OA is a complex mixture of hundreds of individual species.
If the standard molecule mixtures are reasonably representative of ambient
OA mixtures, these results indicate that the Improved-Ambient O : C and H : C
values calculated for ambient OA and SOA have errors close to
∼ 12 and ∼ 4 %, respectively.
Effect of Improved-Ambient method on previous measurements of AMS
elemental ratios
Given the large number of AMS data sets that have reported OA elemental
ratios calculated with the Aiken-Ambient technique, it is useful to examine
the impact that the proposed corrections will have on existing results and
their interpretation.
(a) Errors in Improved-Ambient O : C ratio of organic
standard molecule mixtures as a function of number of species in the mixture.
(b) Errors in Improved-Ambient H : C ratio of the organic standard
molecule mixtures as a function of number of species in the mixture.
Summary of elemental composition information obtained across chamber
and ambient OA measurements. The figure shows values obtained with the
Improved-Ambient method as well as the Aiken-Ambient method. Aiken-Ambient
elemental ratios are shown with errors from Aiken et al. (2007, 2008) for
reference. For the chamber data, Aiken-Explicit values measured by Chen et
al. (2011) and Hildebrandt Ruiz et al. (2014) are also shown. The elemental
composition information shown for ambient OA is averaged over the data sets
shown in Table S1 and S2 in the Supplement.
We focus on several HR-AMS data sets that have been analyzed with the
Aiken-Ambient method and for which elemental ratios have been reported in
the literature, and/or for which HR spectra are available on the HR-AMS
spectral database (http://cires.colorado.edu/jimenez-group/HRAMSsd/).
For ambient data sets, elemental ratios have been previously reported for
total OA as well as OA components (i.e., groups of organic species that
represent different OA sources and or processes). Primary OA (POA) species
are directly emitted into the atmosphere while secondary OA (SOA) are
species formed as a result of atmospheric transformation
(Ulbrich et al., 2009; Zhang et al.,
2011; Lanz et al., 2007). Several types of POA have been identified,
including hydrocarbon-like organic aerosol (HOA), which is associated with
fossil fuel combustion and other urban sources, biomass burning OA (BBOA),
cooking OA (COA), and other OA from local sources (LOA) (Zhang, et al., 2011 and references therein). SOA species, which generally dominate
ambient OA mass concentrations, consist of a continuum of oxidized organic
aerosol species (OOA), which reflects differences in extent and mechanisms of
photochemical aging as well as precursor sources
(Ng et al., 2010b; Jimenez et al., 2009). Two broad types of ambient SOA, denoted
as LV-OOA (low volatility oxidized organic aerosol) and SV-OOA
(semi-volatile oxidized organic aerosol), have been identified at many
locations. LV-OOA represent the more highly oxidized organic aerosol while
SV-OOA are the less oxidized OA.
Figure 7 shows the average O : C, H : C, OM : OC, and
OS‾C values obtained when previously
published field and chamber SOA data are analyzed using the Improved-Ambient
method. Aiken-Ambient values are shown for all data and Aiken-Explicit values
are shown for the chamber SOA. The data for Fig. 7 are available in Table 3
and the detailed values for each field data set are in Tables S1 and S2 in
the Supplement. The Improved-Ambient elemental ratios of chamber SOA are
higher than previously reported Aiken-Ambient values and the relative change
varies with the identity of the precursor. For the α-pinene + O3 and β-caryophyllene + O3 SOA (Chen et
al., 2011), the predicted increase in O : C (H : C) is smaller than that
for the isoprene + OH (Chen et al., 2011) and toluene + OH SOA
(Hildebrandt Ruiz et al., 2014). These differences are likely linked to the
specific molecular functionalities associated with SOA formed from each
precursor. Isoprene SOA, for example, is known to produce organic peroxides (Surratt et
al., 2006) and polyols (Claeys et al., 2004) while major products of toluene SOA are known to be small acids (Fisseha et al., 2004). The
largest increases are comparable to those observed for the standard molecules
with diacid and polyol functionalities while the smaller increases are
consistent with those observed for multifunctional standards. The
Improved-Ambient elemental ratios of ambient OA (individual components as
well as total OA) generally lie at the high error limit of the Aiken-Ambient
values. The Improved-Ambient O : C and H : C values of total OA, for
example are larger than the corresponding Aiken-Ambient values by
approximately 27 and 11 % on average. These relative differences are
similar to those observed for the multifunctional OA standards (Fig. 1c) and
smaller than those observed for some of the individual chamber SOA systems or
individual SOA standards.
Figure 7 shows that chamber SOA elemental ratios calculated with the
Improved-Ambient method agree well with those calculated using the
Aiken-Explicit method. This agreement is important since it confirms that the
Improved-Ambient method compares well with the Aiken-Explicit method not only
for laboratory standards but also for OA mixtures with complex compositions
and molecular functionalities that cannot be readily duplicated with
commercial standards. Recently, agreement between gas and particle phase
elemental ratio measurements of highly oxidized, extremely low-volatility
organic compounds (ELVOCs) formed in the α-pinene + O3
reaction has also been demonstrated (Ehn et al., 2014). This agreement was
found when low-volatility oxidized products (ELVOCs) were measured as
individual molecules in the gas phase using CIMS and as condensed particle
phase species using AMS and the Improved-Ambient method. The proposed
auto-oxidation mechanism in Ehn et al. (2014) indicates that the ELVOCs
probably have multiple hydroperoxide moieties. Though hydroperoxides are not
represented in the calibration set, the agreement between the individually
identified ELVOCs measured in Ehn et al. (2014) and the O : C of
low-concentration α-pinene SOA obtained from AMS data using the
Improved method describe here is encouraging.
While the Improved-Ambient only corrects the elemental ratio values obtained
with the AMS, the resulting increase in both O : C and H : C values implies an
increase in organic mass as well. On average, the OM : OC ratios obtained with
the Improved-Ambient method for total OA are 9 % higher than previously
published Aiken-Ambient values. The average Improved-Ambient OM : OC ratio of
total OA is 1.84 with variation from 1.3–1.5 for primary OA components to
1.8–2.2 for secondary OA components; chamber SOA Improved-Ambient OM : OC
ratios vary with precursor. The OM : OC ratio of the ambient OOA components is
consistent with water soluble fraction of aged ambient aerosol, which has
been measured by other techniques to be in the range of 2.1 (Turpin and
Lim, 2001) to 2.54 (Polidori et al., 2008). A fit of the
Improved-Ambient OM : OC data results in the following empirical relationship
(see Fig. S5):
OM:OCI-A=1.29×O:CI-A+1.17
In Fig. 7, it is important to note that the carbon oxidation state of total
OA calculated with the Improved-Ambient method remains relatively unchanged
from that determined by the Aiken-Ambient method (Improved-Ambient OS‾C is higher by only 0.06). The
OS‾C values from the two methods also agree
closely even for the chamber systems that display larger differences in O : C
and H : C values (see Fig. 7). This reinforces the conclusion that OS‾C is a more robust measure of OA oxidation levels than
either O : C or H : C since is not affected by hydration or dehydration
processes taking place in the atmosphere or during the measurement process
and is thus also not sensitive to other sources of H2O in aerosol
samples such as aerosol water or dehydration of inorganic acids
(Kroll et al., 2011).
The corrected elemental analysis values will have implications for the
interpretation of van Krevelen diagrams (i.e., plots of H : C vs. O : C), which
have been used to obtain insights into the chemical transformations of
ambient OA. Heald et al. (2010) first showed the
utility of this diagram for bulk total OA (including POA and SOA)
composition analysis, and demonstrated that for some data sets bulk ambient
OA evolved with a slope of -1, suggesting composition changes with aging
that are consistent with simultaneous increases in both carbonyl and alcohol
moieties. Ng et al. (2011) used the van Krevelen diagram to
follow the oxidative transformations of ambient OOA (as opposed to total OA)
from multiple field campaigns and showed that they clustered along a slope
of approximately -0.5. This slope was interpreted as being indicative of
simple oxidative mechanisms that involve net additions of both C(O)OH and
-OH/-OOH functional groups without fragmentation (i.e., C–C bond cleavage),
and/or the addition of C(O)OH groups with fragmentation. Van Krevelen plots
of ambient and chamber SOA species from Table 3 are shown in Fig. S7. The
Improved-Ambient method yields van Krevelen slopes that are approximately
20 % shallower than those determined with the Ambient-Aiken method.
Details are discussed in Chen et al., 2014. These
slopes (-0.8 for total OA and -0.4 for OOA) suggest that the ambient OA
oxidative mechanisms involve different net addition of -OH and/or -OOH
functionalities and fragmentation than previously assumed.
Effect of improved-ambient method on empirical parameterizations of
OA elemental ratios from unit mass resolution data
Empirical methods relating unit mass resolution (UMR) AMS ion tracers with
Ambient-Aiken elemental ratios obtained from high-resolution AMS data have
been previously reported by Aiken et al. (2008) and Ng et al. (2011). Here
we reassess these relationships for elemental ratios calculated with the
Improved-Ambient method.
Aiken et al. (2008) presented a parameterization to estimate O : C from
measured f44 values. High-resolution AMS measurements indicate that the
UMR signal at m/z 44 is mostly due to CO2+ , although
C2H4O+ can play a role in some cases (e.g., isoprene SOA and
BBOA). Figure 8a shows Improved-Ambient O : C values for standard, chamber,
and ambient field data vs. f44. A linear fit of the field OA components
(primary and secondary) provides the following parameterization for
Improved-Ambient O : C values:
O:CI-A=0.079+4.31×f44
Equation (11) reproduces most of the data points including all the ambient
OA components obtained by factor analysis. The O : Cs calculated with Eq. (11)
reproduce measured secondary OA component values with an error of 13 %.
The agreement for primary OA components is not as good, indicating that the
accuracy of Eq. (11) is reduced when f44 is small (< 4 % on
average). The outliers in Fig. 6a correspond to species with low acid
content and high alcohol content (i.e., polyols, and other multifunctional
species with OH groups). Thus, the inferred O : C values for some types of
marine aerosols that have been shown to contain alcohol functionalities
(Hawkins and Russell, 2010) may be somewhat underpredicted. The chamber
data outliers in Fig. 8a also indicate that Eq. (11) may underpredict O : C
values substantially in ambient environments dominated by NOx-free
isoprene chemistry and toluene chemistry.
Summary of elemental composition information obtained for chamber
and ambient OA.
Improved-Ambient
Change (with respect to Aiken-Ambient)
Literature Reference
O : C
H : C
OM : OC
OSc
O : C (%)
H : C (%)
OM : OC (%)
OSc (Absolute)
Ambient OA
Aiken et al. (2009), Chen et al. (2009),
Chen et al.,(2014), Decarlo et al. (2010),
Docherty et al. (2011), Ge et al., (2012),
Gong et al. (2012), He et al. (2011),
Wang et al. (2010), Huang et al.(2011),
Huang et al.(2012), Huang et al.(2013),
Martin et al.(2008), Mohr et al. (2012),
Ovadnevaite, et al. (2011), Poulain et al.(2011),
Robinson et al. (2011), Saarikoski et al. (2012),
Setyan et al. (2012), Sun et al. (2011)
Total OA
0.52
1.65
1.84
-0.60
27
11
9
0.06
Primary Components
HOA
0.13
1.96
1.34
-1.69
27
8
4
-0.09
BBOA
0.36
1.76
1.64
-1.04
34
11
9
0.01
COA
0.22
1.81
1.45
-1.37
32
11
6
-0.06
Secondary Components
SV-OOA
0.53
1.62
1.84
-0.57
32
12
11
0.07
LV-OOA
0.84
1.43
2.25
0.25
25
12
12
0.19
Total OOA
0.67
1.54
2.03
-0.19
28
12
11
0.13
Chamber SOA
Isoprene
0.87
1.94
2.33
-0.19
57
23
24
0.27
Chen et al. (2011)
α-pinene
0.41
1.48
1.67
-0,65
24
9
7
0.04
Chen et al. (2011)
β-caryophyllene
0.47
1.52
1.75
-0.58
29
11
10
0.06
Chen et al. (2011)
Toluene
0.85
1.67
2.28
0.10
50
25
22
0.30
Hildebrandt Ruiz et al. (2014)
Scatterplot between Improved-Ambient O : C values and f44
(fractional ion intensity at m/z 44 from unit mass resolution data).
Ambient OA component data from field campaigns are shown as black points. The
black line shows the linear fit through the ambient OA
(O : CI-A=0.079+4.31× f44). Chamber SOA and
standard OA data are also shown in the figure. (b) Scatterplot
between Improved-Ambient H : C values and f43 for ambient secondary OA
components and chamber SOA. OA standard data is shown for the few
multifunctional species which fit the criteria for this parameterization
(f44 > 0.05 and f43 > 0.04). The solid
line shows a scaled version of the Ng et al. parameterization
(H : CImprovedAmbient=1.12+6.74× f43-17.77×f432) and the dotted lines show ±10 % deviations
from the parameterization.
Ng et al. (2011) derived a method for estimating H : C values
of OOA components and SOA species from f43. This parameterization was
based on ambient OOA components and chamber SOA species with
f44> 0.05 and f43> 0.04. For these species,
m/z43 is typically dominated by C2H3O+. Only a few of the
measured multifunctional standards yield mass spectra which fall within
these prescribed valid ranges. Since these few data points do not add enough
significant information to derive a new parameterization, we use them
together with chamber and field data to evaluate a scaled version of the Ng
et al. (2011) relationship. We choose a scaling factor of 1.11 since the
Improved-Ambient method increases the H : Cs of ambient OA by 11 % on
average. The resulting scaled parameterization is as follows:
SOAH:CI-A=1.12+6.74×f43-17.77×f432
Figure 8b compares the parameterization from Eq. (12) with the measured
Improved-Ambient H : C values. The figure indicates that as in Ng et al. (2011), the measured H : C values for secondary OA components,
secondary chamber OA, and several standard molecules are reproduced to
within ±10 % by Eq. (12).
Atmospheric implications
Aerosol elemental ratios measured with the AMS have been previously used to
distinguish between different types of organic aerosol
(Jimenez
et al., 2009; Ng et al., 2010), examine the degree to which chamber SOA is
able to simulate ambient SOA
(Chhabra et al.,
2010; Ng et al., 2010), and to constrain oxidation mechanisms used in
theoretical models (Jimenez et al., 2009; Kroll
et al., 2011; Donahue et al., 2011; Daumit et al., 2013;
Chen et al., 2014). Here we show that while the
changes introduced by the Improved-Ambient method can be significant, they
do not change any fundamental conclusions made from previous AMS studies.
As shown in Fig. 7, I-A elemental ratios for ambient OA components
have the same trends with respect to each other as previously published A-A
elemental ratios. The relative levels of oxidation for the various OA
components, for example, do not change with respect to each other. The OOA
components still span a continuum of oxidation levels; LV-OOA components
remain more oxidized than SV-OOA components, and OOA components remain more
oxidized than the various POA components
(Jimenez
et al., 2009). In fact, the Improved-Ambient method enhances previous
conclusions about the high degree of oxygenation of atmospheric OOA,
indicating that ambient OA has a greater oxygen content than suggested by
previous AMS studies.
Laboratory chamber studies provide the ideal means of simulating ambient
aerosol formation and aging processes under controlled and reproducible
experimental conditions (i.e., selected reactants, photochemical conditions,
and aging times). However, previous work has shown that laboratory chamber
studies are unable to generate SOA or photochemically aged OA with the same
chemical composition as the LV-OOA species observed in the atmosphere
(Chhabra et al.,
2010; Ng et al., 2010). The elemental ratios obtained with I-A method
reconfirm this difference. Figure 7 shows, for example, that the I-A
elemental ratios observed for the SOA from terpene and sesquiterpene
precursors are significantly less oxidized than the average ambient LV-OOA
component. In fact, the terpene and sesquiterpene chamber SOA generally only
reach the O : C and OSc values observed for the less oxidized SV-OOA
components. As shown in Table 2, the I-A elemental ratios of isoprene and
toluene SOA experience large changes compared to their corresponding A-A
values. These changes are large enough to bring the O : C and OSc values of
these SOA in good agreement with LV-OOA values. However, as shown in Fig. 4a, the oxygen containing functional groups in these SOA still do not
reproduce the mass spectral signatures obtained from ambient LV-OOA. Thus,
the gap in the AMS chemical compositions measured for chamber and ambient
SOA remains even when the I-A method is used.
Many studies have used elemental ratios (O : C and H : C) or the
oxidation state values derived from them as key constraints to understand how
OA chemical composition evolves in the atmosphere. Some two-dimensional
chemical spaces that directly use these parameters as constraints are the van
Krevelen space discussed in Sect. 3.7 of this manuscript, OSc vs. carbon
number, and OSc vs. saturation vapor concentration (Jimenez et al., 2009;
Kroll et al., 2011; Donahue et al., 2011). Daumit et al. (2013) have used a
three-dimensional space (carbon number, O : C, H : C) to constrain and
define the chemically feasible back-reactions that could lead to the oxidized
LV-OOA species observed in the atmosphere. In all of these spaces the
measured bulk values of O : C, H : C, and OSc provide mechanistic insight
by limiting the reaction pathways and intermediates that are potentially
possible. Daumit et al. (2013) have compared the difference in constraints
introduced when LV-OOA elemental ratios are calculated using A-A and I-A
methods (The I-A elemental ratios in Daumit et al. (2013) were calculated by
scaling A-A O : C and H : C ratios by 1.3 and 1.11, respectively). For
the same LV-OOA volatility, elemental ratios obtained with the I-A method
constrain the LV-OOA composition to contain a higher hydroxyl / carbonyl
ratio than the elemental ratios obtained with the A-A method. Since hydroxyl
groups result in lower volatility than carbonyl groups, this implies that the
average LV-OOA carbon number calculated using the I-A constraints is
lower than that calculated using A-A constraints. From the standpoint of
chemical mechanisms, this also means that the new I-A constraints will result
in the need for new reactions that produce more hydroxyl groups relative to
carbonyl groups. This is consistent with the general trend noticed in the van
Krevelen diagrams (see Sect. 3.7) which indicate that ambient OA oxidation
increases O : C while maintaining high H : C values. This suggests that
models should explore different and or additional mechanisms for adding -OH
and/or -OOH functionalities during oxidation of ambient OA.