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Annu. Rev. Anal. Chem. 2019. 12:X–X https://doi.org/10.1146/annurev-anchem-061516-045135 Copyright © 2019 by Annual Reviews. All rights reserved Johnston • Kerecman

www.annualreviews.org • Organic Aerosol Characterization

Molecular Characterization of Atmospheric Organic Aerosol by Mass Spectrometry Murray V. Johnston and Devan Kerecman Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716,

USA; email: mvj@udel.edu

Keywords

atmospheric measurements, fine particles, high performance mass spectrometry, nanoparticles,

organic reaction mechanisms

Abstract

Atmospheric aerosol, particulate matter suspended in the air we breathe, exerts a strong impact

on our health and the environment. Controlling the amount of particulate matter in air is

difficult, since there are many ways particles can form by both natural and anthropogenic

processes. We gain insight into the sources of particulate matter through chemical composition

measurements. A substantial portion of atmospheric aerosol is organic, and this organic matter is

exceedingly complex on a molecular scale, encompassing hundreds to thousands of individual

compounds that distribute between the gas and particle phases. Because of this complexity, no

single analytical technique is sufficient. However, mass spectrometry plays a crucial role owing

to its combination of high sensitivity and molecular specificity. This review provides a survey of

the various ways mass spectrometry is used to characterize atmospheric organic aerosol at a

molecular level, tracing these methods from inception to current practice with emphasis on

current and emerging areas of research. Both offline and online approaches are covered, and

molecular measurements with them are discussed in the context of identifying sources and

elucidating the underlying chemical mechanisms of particle formation. There is an ongoing need

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to improve existing techniques and develop new ones if we are to further advance our knowledge

of how to mitigate the unwanted health and environmental impacts of particles.

1. INTRODUCTION TO ORGANIC AEROSOL AND THE ROLE OF MOLECULAR ANALYSIS FOR STUDYING AEROSOL PROCESSES

Airborne microscopic and submicroscopic particles surround us in the air we breathe. These

particles have natural and anthropogenic origins and can affect both human health and the

environment. Of greatest interest are fine particles, defined as those with aerodynamic diameters

smaller than 2.5 µm, because of their substantial health impact (1). When inhaled, fine particles

are efficiently transported deep into the respiratory tract where they induce a variety of adverse

cardiopulmonary responses. Fine particles also influence weather and climate, both directly by

scattering incoming solar radiation and indirectly by serving as seeds for cloud droplet formation

(2). Atmospheric particulate matter has been intensely studied since the mid-twentieth century

when incidents such as the London smog episode of 1952 and similar events elsewhere in

Europe and the United States brought health and environmental problems to the forefront (3).

Passage of the Clean Air Act in the United States in 1970 and related legislation around the

world since then, along with advances in emission control technology, have contributed to a

substantial reduction of atmospheric particulate matter over time. While these advances are

significant, the fine particle mass concentration (PM2.5) still exceeds the mandated level in

many locations around the United States and is a continuing problem around the world,

especially in megacities where control technology tends to be less advanced (4). Biomass

burning is a significant source of atmospheric particulate matter, and its relative contribution has

been increasing with time as control technology for fossil fuel combustion continues to improve

(4a). If events such as the 2018 California wildfires become more prevalent as our climate

changes, excursions to high PM2.5 levels may become more frequent even in locations where

control technology is mature. For these reasons, a better understanding of the source and fate of

atmospheric particulate matter is needed to develop effective strategies to mitigate environmental

and health impacts. Chemical composition measurements are essential to this effort. The Related

Resources list at the end of this review gives general references on the formation, fate, impacts,

and regulation of atmospheric aerosol including topics discussed later in the introduction.

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Atmospheric particulate matter contains both organic and inorganic components, with

organic matter constituting up to 90% of the fine particle mass depending on location and time of

year (2, 5–7). As discussed later, organic matter is extremely complex on a molecular level,

encompassing hundreds to thousands of individual compounds. These compounds span a wide

range of volatilities, and they distribute between the gas and particle phases as described by

partitioning theory (8, 9). The most important molecular property that affects partitioning is the

equilibrium vapor pressure. Volatile organic compounds (VOCs) have high vapor pressures and

reside mostly in the gas phase. Nonvolatile compounds (NVOCs) have low vapor pressures and

reside mostly in the particle phase. Semivolatile compounds (SVOCs) have intermediate vapor

pressures and appreciable concentrations in both phases.

Figure 1 summarizes the major interconnections among gas and particle phase organic

compounds in the atmosphere. Gases (VOCs) and particles are emitted into the atmosphere from

sources that may be natural or anthropogenic in origin. Organic particulate matter emitted

directly into the atmosphere is referred to as primary organic aerosol (POA). VOCs can be

oxidized to give less volatile products (SVOCs and NVOCs), which then partition into the

particle phase to form secondary organic aerosol (SOA). The major oxidants in the atmosphere

are hydroxyl radical, ozone, and nitrate radical. Newly formed POA and SOA can undergo

further oxidation in the atmosphere to produce aged organic aerosol. Figure 1 is a good reminder

that an aerosol is defined as a suspension of particles in a gas. Understanding the source, fate,

and impact of airborne particles necessarily involves characterization of the complete aerosol

i.e., both atmospheric particulate matter and the corresponding gas phase species.

<COMP: PLEASE INSERT FIGURE 1 HERE>

Figure 1 An illustration of the major sources and interconnections of gas and particle phase organic compounds in air. Acronyms are defined in the text. Major oxidants (ox.) in the atmosphere are hydroxyl radical, ozone, and nitrate radical. Gas phase species are shown in white circles, and particle phase species are shown in yellow circles. Abbreviations: NVOC, nonvolatile compound; POA, primary organic aerosol; SOA, secondary organic aerosol; SVOC, semivolatile compound; VOC, volatile organic compound.

This review focuses on molecular analysis of organic aerosol by mass spectrometry. There

are four main reasons why molecular analysis is important.

1. Molecular analysis is needed to identify molecular markers—individual compounds or

groups of compounds in particles that can be uniquely attributed to a specific source or

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process that generates organic aerosol. Markers make it possible to track the sources of

particulate matter, for example, by distinguishing primary versus secondary, natural

versus anthropogenic, and freshly formed (which implies the aerosol is produced from

local source) versus aged (which implies the aerosol has existed for a considerable time

in the atmosphere and has undergone long-range transport from one location to another).

2. Molecular analysis methods are inherently adaptable for quantitative analysis.

Quantification of molecular markers in an aerosol sample is crucial for determining the

relative contributions that various sources make to atmospheric aerosol, a process

referred to as source apportionment. Once the various sources are known and quantified,

emission control strategies can be designed for maximum impact.

3. Identification and quantification of molecular species, both intermediates and products of

chemical reactions, can elucidate the mechanisms and rates of aerosol formation and

decay that underlie Figure 1. Information of this type is needed to predict future changes

in atmospheric particulate matter in response to, e.g., long-term changes in climate or the

implementation of new control strategies.

4. Physicochemical properties of the entire aerosol can be reconstructed by combining

individual molecular properties. For example, the ability of a particle to absorb incoming

solar radiation and thereby contribute to heating of the atmosphere depends on the

distribution and concentration of light-absorbing molecules in the particle.

Mass spectrometry is ideally suited for molecular analysis because of its combination of high

sensitivity and molecular specificity. Atmospheric levels of organic particulate matter are on the

order of 1–10 µg/m3, with individual compounds in the pg/m3 to ng/m3 level. In order to keep

sampling and analysis times reasonable, low nanogram amounts of individual molecular species

must be quantifiable, which is readily achieved with gas chromatography mass spectrometry

(GC-MS), liquid chromatography mass spectrometry (LC-MS) and other methods discussed in

this review. Mass spectrometry provides molecular specificity through a combination of accurate

mass measurement at the parts per million level, which allows elemental formulas to be

determined for key ions in the mass spectrum, and tandem mass spectrometry, which provides

structural information. While this review is focused on organic molecular characterization, mass

spectrometry has had a broader impact on atmospheric aerosol research, especially through

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single particle analysis, where particle-to-particle variations in chemical composition give insight

into the source, reactivity and fate of particles in air (9a, 9b).

This review is organized by mass spectrometry technique, tracing major developments in

instrumentation and methodology over time with an emphasis on current and emerging areas of

research. In most respects, the development of instruments and methods for organic aerosol

characterization mirrors those in the broader field of mass spectrometry. That is, GC-MS and

adaptations thereof were the first to come to maturity, followed by LC-MS and a host of online

and ambient ionization techniques. These advances are described in the context of how they

have improved our understanding of the sources and sinks of atmospheric organic aerosol. The

linkage between the two is highlighted by the section titles, which connect an advance in

molecular characterization technology with one of the advances in our understanding of

atmospheric organic aerosol enabled by this technology. The final sections discuss how ambient

ionization techniques link fundamental laboratory studies with atmospheric measurements to

elucidate molecular mechanisms capable of predicting aerosol formation in the atmosphere.

2. GAS CHROMATOGRAPHY MASS SPECTROMETRY: IDENTIFYING AND APPORTIONING SOURCES OF ORGANIC PARTICUATE MATTER

Molecular characterization of atmospheric organic particulate matter has its roots in GC-MS,

which in many respects is ideally suited for organic aerosol analysis owing to high

chromatographic resolution coupled with library searchable electron ionization (EI) mass

spectra. Although applications of GC-MS date back to the late 1970s, these measurements were

constrained by a limited range of compound classes investigated and/or a limited number of

particle collection periods. In 1993, Rogge et al. (10) reported quantification of 80 compounds

by GC-MS from atmospheric particulate matter collected over the course of one year at four

sampling sites in and around Los Angeles, California. Figure 2a summarizes the results for one

particular site, showing the contribution of organic particulate matter to atmospheric aerosol and

the breakdown of the organic portion into individual compounds. Key aspects of Figure 2a are

that (a) organic matter constitutes a substantial fraction of atmospheric particulate matter, and (b)

some organic matter is well characterized and quantified at a molecular level while most is not.

Although emission control strategies and molecular measurement methods have improved

greatly since this study, the key aspects noted above are still relevant today.

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<COMP: PLEASE INSERT FIGURE 2 HERE>

Figure 2 (a) Mass balance on the chemical composition of annual mean fine particle concentrations in 1982 for West Los Angeles, California. Panel adapted with permission from Reference 10. (b) Source apportionment of atmospheric fine organic aerosol mass concentration from 1982 annual average. Panel adapted with permission from Reference 23. Because of improved emission controls and measurement methods over the intervening years, the amount and distribution of chemical species in atmospheric air and the sources identified from them have changed significantly. However, it is still true today that a significant portion of organic aerosol remains poorly characterized at a molecular level and the source(s) of this aerosol are not fully identified and quantified. Abbreviation: PAH, polycyclic aromatic hydrocarbons.

In the Rogge et al. study (10), hierarchical clustering of the seasonal concentrations of

individual compounds revealed similarities that suggested specific groups of compounds

originated from similar sources. Separate experiments in which particles from specific sources

were analyzed allowed a more rigorous identification of molecular source signatures while

simultaneously expanding the number of compounds analyzed. Some noteworthy examples

include fossil fuel combustion from motor vehicles (11), biomass burning (17), meat cooking

(18), and cigarette smoke (19). Molecular markers identifying each source could consist of an

individual compound, for example, levoglucosan from wood burning (20), or a group of related

compounds, for example, cholesterol and C16-C18 fatty acids from meat cooking (18). Many of

these markers are semivolatile, so quantitative measurements must consider the effect of

partitioning between the gas and particle phases (21, 22) and the potential of volatile and

semivolatile compounds to yield nonvolatile products when oxidized in the atmosphere (22a),

i.e. formation of SOA as shown in Figure 1. This latter point is particularly important, because

as we will see later, advances in control technology have greatly reduced the amount of POA

emitted, leaving SOA as the main source of atmospheric particulate matter.

Once molecular source signatures are in hand, they can be used for source apportionment,

defined as the process of determining the contribution of each source to atmospheric aerosol

based on the distribution of molecular species measured in the atmospheric sample. An early

example is the use of a chemical mass balance model for organic aerosol source apportionment

at four sampling sites in and around Los Angeles, as illustrated in Figure 2b (23). A key aspect

of Figure 2b is that a significant portion of the fine organic mass could not be apportioned to

specific sources. Over the intervening years, implementation of emission control strategies has

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reduced the contributions of some sources, and advances in measurement technology have

allowed unappreciated and/or unidentified sources to be quantified. These advances are

discussed in the remainder of this review.

Chemical mass balance is one of several receptor models used for identifying and/or

quantifying the contributions of different sources to atmospheric aerosol. Watson et al. (24) have

reviewed the most common receptor modeling approaches, including a discussion of results

obtained with them and best practices for proper implementation. Care must be exercised when

using receptor models, as all have inherent assumptions that must be tested. In addition,

complexities arise when applying source signatures from one location to another because the

number and types of source signatures may be different in the new location. An excellent

example of varying source signatures is given in a series of papers by Robinson et al., who

identified a coke production source (25) and assessed the variation of biomass burning (26) and

meat cooking (27) source signatures in Pittsburgh, Pennsylvania. Despite this variability, source

signatures and receptor modeling provide significant insight into the sources of atmospheric

particulate matter and how best to control them. For example, biomass burning is a significant

source of particulate matter even in the absence of extreme events such as the 2018 California

wildfires. In the work of Robinson et al., biomass burning was found to have the greatest

contribution in the fall, and was responsible for ~14% of particulate organic carbon at the time of

these measurements (26. Worldwide, a major source of biomass burning is household cooking

with solid fuels (wood, coal, charcoal and agricultural residue), which is responsible for ~12% of

PM2.5 globally. The percentage is much greater in the developing world, where it contributes

substantially to adverse health effects and warming of the atmosphere (27a).

3. ADAPTATIONS OF GAS CHROMATOGRAPHY MASS SPECTROMETRY: MAXIMIZING MOLECULAR RESOLUTION, PARTICULARLY FOR SEMIVOLATILE ORGANIC COMPOUNDS

While GC-MS is a mature, robust method for molecular identification and quantification,

significant limitations exist, which are apparent upon inspection of Figure 2a,b. First, only about

half of the organic matter in these samples could be eluted from a GC column, which is not

surprising because many organic compounds are highly polar and/or thermally labile. Second,

less than 20% of the eluted organic matter was chromatographically resolved and identified.

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Since electron ionization is normally used for quantitative GC-MS applications and causes

extensive ion fragmentation, it is difficult or impossible to disentangle the individual mass

spectra of unresolved compounds. Together, the inability to elute, resolve, and identify all

compounds in the sample meant that identified sources might be incorrectly apportioned and that

unexpected sources would not be identified at all. A third problem not directly illustrated in

Figure 2 is that the time resolution of GC-MS analysis in these early experiments was poor,

often requiring sample collection for one or more days. Sample preparation also required

substantial time and effort because it relied on manual extraction and preconcentration into a

GC-friendly solvent. Together, sample collection and preparation limitations made it difficult to

accurately identify and quantify sources when the air mass was changing rapidly. As a result,

several modifications to the GC-MS approach were developed to address these limitations,

including (a) molecular derivatization prior to GC separation, (b) multidimensional separation

strategies, and (c) automated methods of analysis.

To address the problem that many compounds are difficult to vaporize and elute by GC,

molecular derivatization methods have been developed, for example, silylation (28). Emphasis

has been placed on the development of in situ procedures, which enable faster sample

throughput, lower cost, and can be incorporated into automated sample collection and analysis

methods. Sheesley et al. (29) developed an in situ method to derivatize alcohol groups with

trimethylsilyl groups and applied it to the analysis of levoglucosan and other polar compounds

relevant to atmospheric aerosol that could not be characterized by conventional GC-MS. Other

examples include derivatization of carbonyl and carboxylic acid functionalities to detect

compounds associated with motor vehicle (30) and biogenic (31) sources. The success of this

approach is dependent on optimizing the reaction conditions so that detection limits are sufficient

for use with the low analyte concentration levels found in atmospheric aerosol (32, 33), which

may be difficult if the goal is to detect a wide range of compounds having somewhat different

optimization conditions. On the other hand, in applications where specific marker compounds are

targeted for quantification, the procedure can be fine-tuned, for example, trimethylsilylation of 2-

methyltetrols for the analysis by GC-MS to quantify the amount of SOA produced from isoprene

oxidation (34) and anhyrdo sugars produced from biomass burning to better identify and quantify

this important source (35).

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To address the problem that atmospheric aerosol is complex on a molecular level and one-

dimensional separations are unable to resolve the large number of compounds present, two-

dimensional (2D)-GC has been used as a means of increasing the peak capacity, in which the

effluent from a nonpolar primary column was collected at regular intervals and eluted through a

polar secondary column (36, 37). This approach was first applied to atmospheric aerosol samples

in the early 2000s with an emphasis on coupling 2D-GC with a time-of-flight (TOF) mass

analyzer so that complete mass spectra could be obtained on the subsecond timescale (38–42).

With this method, the number of distinct chemical species found in an atmospheric sample was

reported to be as high as 10,000 (36). However, positive identification of these species at a

molecular level turns out to be challenging and ultimately becomes limited by the lack of

relevant authentic reference standards.

Aerosol characterization by GC-MS was originally performed by collecting particles on a

filter for an extended period of time, then extracting molecular components into an appropriate

solvent and preconcentrating the solution prior to analysis. Even with high-volume aerosol

samplers, the time resolution was at best about three hours (43). The thermal desorption aerosol

GC-MS (TAG) system was developed to address this limitation through the use of an in situ

thermal desorption step to vaporize molecular species from the collection filter directly into the

GC inlet (44), allowing continuous operation with a time resolution of less than one hour (45).

The method has been coupled with 2D-GC (42, 46), and procedures have been developed to

distinguish gas and particle phase concentrations of semivolatile compounds (47). Quantitative

performance of the TAG system has been shown to be comparable to filter collection and offline

analysis (48), and the method lends itself well to source apportionment (49).

GC-MS is a mature technology, and when matched with 2D-GC and TOF-MS provides

unprecedented molecular resolution and speed, especially for semivolatile compounds that

partition between the gas and particle phases. Zhao et al. reported for atmospheric measurements

in Berkeley, California, USA that the measured abundances for semivolatile compounds detected

with a TAG system were usually about an order of magnitude higher in the gas phase than the

particle phase (47). Higher abundances in the gas phase are not surprising based on partitioning

theory and the range of molecular volatilities accessed in a GC-MS experiment. The split

between gas and particle phase concentrations is important, since atmospheric oxidation in the

gas phase is usually much faster than in the particle phase, causing gas phase species to be

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greater contributors to SOA formation. A recent study by this same group characterized

semivolatile emissions from gasoline vehicles and their potential to form SOA (22a). Among the

conclusions of this work are that recent changes in emission standards have substantially reduced

the SOA formation potential, and even greater reductions in SOA formation are possible with

tighter emission controls. Another important application of 2D-GC-TOF-MS for semivolatile

compound characterization is the investigation of personal exposure to pollutants in e.g. indoor

air (49a) and the urban environment in general (49b).

4. THERMAL DESORPTION MASS SPECTROMETRY FOR ONLINE AEROSOL ANALYSIS: FAST MOLECULAR MEASUREMENTS PROVIDE A LINK BETWEEN AEROSOL FORMATION AND CHANGING ATMOSPHERIC CONDITIONS

An alternative way to decrease the time resolution of molecular analysis is to replace the

chromatographic separation with a simple thermal desorption step. To help mitigate against the

loss of chromatographic resolution, this approach is generally coupled with a soft ionization

method. Examples include chemical ionization (50–52), proton-transfer reaction (PTR) (53–55),

vacuum ultraviolet photoionization (56), and photoelectron resonance chemical ionization (57).

High-resolution mass analysis, usually with a TOF mass spectrometer (52, 58, 59), provides

additional molecular specificity. Alternatively, successive stages of mass analysis (MSn) can be

performed in an ion trap mass analyzer (53).

Though instantaneous time response can be achieved by passing the aerosol through a heated

inlet (50), particles generally must be deposited and accumulated on a probe to achieve adequate

sensitivity for atmospheric measurements. Flash desorption can be achieved with pulsed laser

heating of the deposition probe surface, which reduces the analysis time to a few minutes (60)

and facilitates the use of fast temporal changes in molecular composition to identify organic

aerosol sources, e.g., cooking aerosol (61). Temperature-programmed desorption is more

commonly used, as it provides the opportunity to assess the volatilities, and therefore gas-particle

partitioning, of individual molecular components (59, 62, 63), though analysis times under

atmospheric conditions are typically closer to an hour. Figure 3 shows the schematic of a Filter

Inlet for Gases and AEROsols (FIGAERO) for use with chemical ionization and a high-

resolution TOF mass analyzer. Initially, gas phase species are analyzed while particles are

collected on a filter. Then, the assembly is switched so that particle phase species are thermally

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desorbed into the mass spectrometer with a temperature-controlled flow of ultrapure nitrogen.

With this setup, molecular characterization of both the gas and particle phases can be performed

repetitively on an hourly or faster time scale.

<COMP: PLEASE INSERT FIGURE 3 HERE>

Figure 3 Schematic of the Filter Inlet for Gases and AEROsols (FIGAERO) sampling assembly that alternates characterization of gas and particle phase molecular species. The main manifold (green) connects the sampling assembly to the mass spectrometer. A moveable tray (red) switches the air flow between two inlets, one designed exclusively for gases and the other for particles. While gas phase species are sampled into the mass spectrometer, particles are collected on a Teflon filter. The tray is then moved to block the atmospheric gas flow while exposing the filter to a temperature-controlled flow of ultrapure nitrogen for programmed thermal desorption. Abbreviation: IMR, ion-molecule reaction region. Adapted with permission from Reference 63. [

Timkovsky et al. (64) compared atmospheric molecular measurements by online thermal

desorption PTR-MS with offline 2D-GC-MS. A total of 153 compounds were identified with

2D-GC, and of these 123 were matched with 64 ions observed by PTR. While minor differences

were noted, results with the two methods were consistent both qualitatively and quantitatively for

molecular species at the 2 ng/m3 level or higher, illustrating the ability of online thermal

desorption to provide robust molecular information on a fast timescale.

Highly time-resolved measurements are very important in atmospheric aerosol studies since

they provide a way to link particulate matter concentrations to meteorological variables such as

wind velocity and direction, solar radiation, temperature and relative humidity (64a). For

example, wind directions that efficiently transport point source emissions to the measurement

site allow these sources to be characterized (64a). Thermal desorption methods have

substantially reduced the amount of time needed to perform molecular measurements, making it

possible to correlate molecular composition with meteorology. Although molecular

measurement timescales down to 4 min have been reported (60, 61), a time resolution of about

30 min was found to be sufficient to capture meteorological variations of greatest interest (60).

Fortunately, the 30 min timescale can be achieved with methods such as FIGAERO, TAG and

the aerosol mass spectrometer (AMS) discussed below.

The aerosol mass spectrometer (AMS) family of instruments uses thermal desorption coupled

with EI to characterize nonrefractory components, both organic and inorganic, in particles that

impact a heated probe inside the mass spectrometer source region (65). EI, which is also used in

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most GC-MS experiments, has important advantages of stability and reproducibility, which

facilitate quantitative measurements and promote uniform results across multiple instruments and

measurement locations (66). Unlike the soft ionization methods discussed above, EI also induces

substantial fragmentation, which means that the mass spectra from different molecules that

simultaneously vaporize from the probe cannot be deconvoluted. Although individual

compounds rarely can be distinguished, compounds having similar structures and functionalities

give similar fragmentation patterns allowing these chemical features to be characterized. When

coupled with high-resolution mass analysis to separate isobaric ions, quantitative information can

be gained about the elemental composition of organic matter in atmospheric particles and how it

changes with time (67). Furthermore, specific combinations of fragment ions allow different

aerosol sources to be distinguished, such as unoxidized organic matter in POA (e.g., C3H7+),

highly oxidized organic matter in SOA (e.g., CO2+), and moderately oxidized organic matter

from cooking (e.g., C3H3O+) or biomass burning (e.g., C2H4O2+) sources (68, 69). Thus, although

individual molecule analysis is not performed, the end result of identifying and quantifying

primary and secondary sources is achieved. While AMS measurements are relatively

straightforward to interpret with respect to source apportionment, methods such as GC-MS add a

molecular basis for interpreting the results (49).

An important discovery facilitated by AMS measurements in the mid-2000s is that organic

matter in atmospheric aerosol around the globe is mostly secondary rather than primary (68).

This discovery has important implications for emission control technology, since it suggests that

major reductions in PM2.5 require minimizing or eliminating the emissions of gas-phase

compounds that form SOA, rather than simply removing the POA (see Figure 1). It also

highlights the continuing need for advanced GC-MS methods to characterize semivolatile

compounds in the atmosphere, as well as the need for methods such as LC-MS to better

characterize SOA at a molecular level. Referring back to Figure 2 where molecular

measurements by GC-MS were used for source apportionment, we can surmise that significant

portions of the “nonelutable” organic matter in Figure 2a and the “other sources” in Figure 2b

were secondary. The apparent minor contribution of SOA to organic aerosol in Figure 2 may

have been a consequence of the emission control technologies in use at the time and/or the

limitations of GC-MS analysis.

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5. LIQUID CHROMATOGRAPHY MASS SPECTROMETRY: IMPROVED CHARACTERIZATION OF POLAR COMPOUNDS IN SECONDARY ORGANIC AEROSOL AND LIGHT-ABSORBING COMPOUNDS IN BROWN CARBON

GC-MS is generally preferred over LC-MS for atmospheric aerosol analysis owing to its high

separation efficiency, high sensitivity, and relative immunity to ion signal-quenching effects.

However, LC-MS is essential for characterizing polar, nonvolatile, and/or thermally labile

molecules that are not elutable by GC even with derivatization. An early report of LC-MS using

a reverse-phase column and electrospray ionization (ESI) to characterize atmospheric aerosol

showed a complex distribution of species in the 100–800 m/z range, with many nominal m/z

values exhibiting multiple chromatographic peaks, which indicated the presence of isomeric

and/or isobaric compounds (70). These results highlighted the need for high-resolution mass

measurements to distinguish isobaric elemental formulas and for the development of authentic

standards to confirm molecular assignments and perform quantitative analysis. Subsequent work

showed organosulfates to be important contributors to atmospheric SOA (71, 72), which are

formed directly in the particle phase under conditions where anthropogenic and biogenic

emissions mix. While these early studies relied mostly on reverse phase separations, recent

measurements of organosulfates have emphasized hydrophilic interaction LC owing to its higher

separation efficiency for these species (73, 74).

Other important species in atmospheric aerosol that require LC-MS analysis include

terpenoic acids and di/tri carboxylic acids (75). Terpenoic acids such as pinic and pinonic acids

are indicators of biogenic SOA produced by monoterpene oxidiation, and recent improvements

in the analysis procedure allow sub-ng/m3 amounts of these species to be routinely quantified

(76). Another important compound that requires LC-MS analysis is 3-methyl-1,2,3-

butanetricarboxylic acid (MBTCA), a molecular marker linked to aging of fresh biogenic SOA

produced by monoterpene oxidation (77). As shown in Figure 1, aging is the process by which

freshly formed organic aerosol (either primary or secondary) undergoes further oxidation during

its lifetime in the atmosphere. Distinguishing fresh and aged organic aerosol assists source

apportionment by helping to distinguish aerosol produced locally from that arriving by long-

range transport from other locations.

Brown carbon refers to organic particulate matter that absorbs ultraviolet and/or visible

radiation, which exerts a warming effect on the atmosphere (78). Because a relatively small

fraction of atmospheric organic matter is absorbing, the development of selective detection

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methods based on molecular absorption is needed. LC is well suited for this task since it can be

coupled simultaneously with an ultraviolet-visible (UV-VIS) absorption spectrometer and a mass

spectrometer. This approach was used to develop an optimized method for determining

nitrophenols and related compounds in atmospheric aerosol, which contribute to brown carbon

and are also tracers for biomass burning, and subsequently applied to atmospheric aerosol

characterization (79, 80). UV-VIS detection can also be used to isolate the absorbing fraction of

an aerosol sample that is eluting from an LC, followed by offline characterization of the fraction

by a suite of techniques including mass spectrometry (81). An example of the combined use of

UV-VIS and mass spectrometry is illustrated in Figure 4 for organic aerosol collected in

Budapest, Hungary (80). Figure 4a shows a plot of retention time versus absorptivity. The

intense absorption feature at 20.8 min is identified as 4-nitrocatechol, which is confirmed by the

extracted ion chromatogram for m/z 154 in Figure 4b. Other absorbing compounds in this

sample include isomeric phthalic acids, various methyl nitrocatechols, and azelaic acid.

<COMP: PLEASE INSERT FIGURE 4 HERE>

Figure 4 Use of LC coupled with UV-VIS and MS to identify molecular constituents of brown carbon. (a) Retention time versus UV-VIS absorbance for an atmospheric aerosol sample collected in Budapest, Hungary. (b) An extracted ion chromatogram for m/z 154, which corresponds to 4-nitrocatechol. The retention times of the absorbance and ion chromatographic peaks are offset slightly owing to different lag-times between the two detectors. Abbreviations: LC, liquid chromatography; MS, mass spectrometry; RA, relative abundance; UV-VIS, ultraviolet-visible. Adapted with permission from Reference 80.

Claeys et al. used the combined LC-UV-MS approach to compare atmospheric samples from

several locations and sampling periods (80). Aerosol samples collected from an Amazon

rainforest during a biomass burning event contained large amounts of 4-nitrocatechol, consistent

with a large impact of biomass burning on the aerosol. A significant amount of 4- nitrocatechol

was also found in aerosol collected in Budapest in March 2007, but a much smaller amount was

found in Budapest aerosol collected in June 2008, consistent with a smaller impact of biomass

burning, e.g. residential wood burning, during the summer. In contrast, June 2008 samples from

urban Budapest and a rural site in K-puszta, Hungary both showed large amounts of terpenylic

acid and MBTCA indicating substantial impact from fresh and aged biogenic SOA. This study

highlights the ability of LC-MS to characterize SOA sources.

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An emerging use of LC coupled with both UV-VIS absorption and mass spectrometry

detectors is to identify and quantify molecules that have been derivatized with a chromophore.

As the detection sensitivity of ESI is strongly compound dependent, authentic standards are

required for quantitative analysis, which can be a significant impediment given the molecular

complexity of atmospheric organic aerosol. To address this problem, Kalafut-Pettibone &

McGivern (82) used a series of derivatization reagents to replace specific functional groups with

chromophores, which allowed quantification by UV-VIS detection and molecular

characterization by mass spectrometry. Recently, this approach was refined by Ranney &

Ziemann (83), who were able to distinguish carbonyl, hydroxyl, carboxyl and ester functionalites

with detection limits for in the subnanomole range. The ability to identify and quantify

“unknown” molecular products is expected to be of great use for characterization of SOA, where

the distribution of molecular species is complex and a limited set of authentic standards exists.

6. EMERGING METHODS BASED ON AMBIENT IONIZATION: ONLINE ANALYSIS OF POLAR COMPOUNDS IN ORGANIC AEROSOL

Ambient ionization refers to methods in which ionization is initiated at atmospheric pressure

with little or no sample preparation. With respect to aerosol analysis, the methods discussed in

Section 4 (thermal desorption mass spectrometry) are not considered to be ambient ionization

methods because particles flow into at least a partial vacuum before vaporization and ionization

are initiated. Likewise, the methods in Section 5 (LC-MS with ESI) are not considered to be

ambient ionization methods, because although ionization is initiated at atmospheric pressure, the

method requires significant sample collection and preparation prior to analysis. Admittedly

though, these distinctions can be fuzzy—some methods described near the end of this section are

not precisely ambient ionization since they operate at “near-atmospheric” pressure, while others

require off-line sample handling in that particles must be collected on a substrate and transported

to the laboratory for analysis.

Several methods based on ambient ionization have been developed for characterizing

laboratory-generated aerosol, though most have not yet progressed to field measurements. A key

attribute of these methods is the ability to generate intact molecular ions without fragmentation.

When combined with accurate and precise m/z measurements (often referred to as high

resolution mass spectrometry, HR-MS), the elemental formula for each detected molecular

16

species can be determined. However, positive identification of specific compounds, especially

isomers, usually requires a separation step, e.g., GC-MS or LC-MS (84).

A method applied early on to aerosol characterization was extractive electrospray ionization

(EESI), which was originally developed in the mass spectrometry research community for

analysis of liquids that are difficult to electrospray by themselves (85, 86). When applied to

online aerosol analysis, the aerosol flow is mixed with a fine mist of charged droplets produced

by electrospray (87–90). When an aerosol particle strikes a charged droplet, soluble compounds

from the particle are extracted into the droplet. The droplet subsequently undergoes successive

stages of solvent evaporation and Coulombic fission as in conventional EI to liberate analyte ions

into the gas phase. Typically, the aerosol flow and electrospray plume are offset at an angle that

optimizes sensitivity by maximizing the probability of interaction between aerosol particles and

charged droplets (87, 88), though use of an easier to implement coaxial geometry has also been

reported (89, 90). The ion signal is linearly correlated with mass concentration of the aerosol,

indicating that the entire volume of the particle is sampled (88). Because gas phase species can

also be extracted into the charged droplets (89), EESI is normally performed with a denuder

upstream of the ion source to remove gas phase compounds so that only particle phase species

are detected (91). Normally, protonated (positive mode) or deprotonated (negative mode) ions

are monitored in an EESI experiment, though a metal salt can be added to the electrosprayed

solution to produce metal cationized species (92). Mass spectra are very similar to those obtained

with conventional ESI: both singly and multiply charged ions are produced depending on

molecular size and availability of charge sites within the molecule.

Several plasma-based methods have been developed for online characterization. Hoffmann

and coworkers reported an aerosol flowing atmospheric-pressure afterglow (AeroFAPA) method

that vaporizes and ionizes organic components with a helium glow discharge plasma (93). The

method was used to detect a variety of organic acids in air including MBTCA, the

aforementioned marker for aged aerosol. Direct analysis in real-time (DART) has been

investigated as a means for online characterization of laboratory-generated organic aerosols (94,

95). With DART, vaporization and ionization is accomplished with metastable helium atoms

created by a corona discharge. Unlike other methods described in this section, DART was found

to generate ions only from the outer few tens of nanometers of a particle, which can be either an

advantage or disadvantage depending on whether surface or bulk analysis is desired. Recently,

17

Blair et al. reported an aerosol vacuum-assisted plasma ionization (Aero-VaPI) method in which

aerosol particles are pulled through a near atmospheric pressure glow discharge connected to the

mass spectrometer inlet (96). This study also used ion mobility to partially separate molecular

species in advanced mass spectrometric analysis. While ion mobility is a fast separation method,

its ability to resolve compounds is less than either GC or LC.

Desorption electrospray ionization (DESI) is a method in which charged droplets from a

solvent spray strike a substrate containing analyte. The analyte extracts into the solvent, and

secondary droplets are expelled from the surface, leading to the production of analyte ions (86,

97). An adaptation of this method, nano-DESI, involves formation of a liquid bridge, essentially

a liquid bead, on the substrate surface. Solvent flows into the liquid bridge from a primary

capillary and is drawn away through a secondary capillary that terminates in a nanospray

ionization source (98). When applied to aerosol analysis, the aerosol is deposited into a small

spot on the substrate, a liquid bridge is established on top of the spot, analyte is extracted into the

liquid, and ions are formed by nanoelectrospray (99). This method has been used to characterize

the molecular composition of atmospheric aerosol samples from a variety of locations, including

Mexico City (100), Bakersfield, California (101, 102), as well as Shanghai, China and Los

Angeles, California (103). Up to 1,000 unique elemental formulas can be obtained from an

individual atmospheric sample using this technique. While it is impossible to positively identify

and/or quantify these compounds without the use of chromatography and authentic standards,

important information can be gained from changes in the distribution of molecular species as a

function of time. For example, organonitrates are found in the greatest numbers in the early

morning, suggesting that many are formed by NO3 oxidation of VOCs during the nighttime

(102). Organosulfate molecules differ by location: C5 to C10 species were dominant in Los

Angeles aerosol, suggesting a biogenic source, whereas C17 to C30 aliphatic chain species were

dominant in Shanghai, pointing to a different, possibly anthropogenic, source (103).

A few droplet-based techniques that do not involve charging by electrospray have been

reported for online analysis. The particle-into-liquid sampler (PILS) mixes an aerosol flow with

steam. Water condenses on the particles, which are subsequently captured into a liquid flow out

of the device (104). Clark et al. (105) reported a version of this method in which the liquid flow

from the sampler enters a commercial atmospheric pressure chemical ionization (APCI) source

that is normally used to couple a liquid chromatograph with a mass spectrometer. The liquid flow

18

into the APCI source is heated to evaporate solvent and liberate analyte molecules into the gas

phase that subsequently collide with reagent ions produced by a corona discharge. Another

method, extractive atmospheric chemical ionization (EAPCI), has been reported in which the

liquid flow into the APCI source first passes through a filter, allowing analyte molecules in the

particles collected on the filter to be extracted into the liquid flow (106). Ambient ESI and APCI

techniques for aerosol analysis are complementary in a similar manner to their broader uses as

ionization methods for LC-MS. For example, APCI tends to be well suited for detection of less

polar analytes, for example, molecular species found in automobile exhaust (106).

Zuth et al. (107) have coupled a heated (350°C) APCI source to a high resolving power Q-

Exactive Orbitrap mass spectrometer for online analysis of atmospheric aerosol. Figure 5a

shows a representative negative ion mass spectrum of atmospheric aerosol using this method.

The spectrum is very complex, encompassing 931 assigned molecular formulas. Making sense of

a spectrum like this is difficult because most molecular species are not known, and very few

authentic standards are available to confirm structures and/or enable quantitative analysis.

<COMP: PLEASE INSERT FIGURE 5 HERE>

Figure 5 Use of APCI-Orbitrap-MS for online characterization of atmospheric organic particulate matter at a molecular level. (a) Average mass spectrum in negative ion mode for atmospheric aerosol sampled on August 15, 2017. Also shown are carbon oxidation state versus number of carbon atoms of the assigned molecular formulas obtained by high-resolution mass analysis of atmospheric aerosol sampled during nighttime (b) and daytime (c). Abbreviations: APCI, atmospheric pressure chemical ionization; MS, mass spectrometry. Adapted with permission from Reference 107.

An exception is MBTCA, the aforementioned marker for aging of biogenic SOA, for which

an authentic standard exists. Using this standard, Zuth et al. were able to confirm and quantify

MBTCA down to the low ng/m3 level in air based on accurate mass MS and tandem mass

spectrometry (MS/MS) measurements, and they were able to show that its concentration was

generally highest during the daytime. Figure 5b,c show how the distribution of negative ions

from atmospheric samples differs between night (Figure 5b) and day (Figure 5c). Here, the

assigned molecular formulas are represented by a 2D plot of the average carbon oxidation state

(OSC) in the molecular formula versus the number of carbon atoms in the formula, which has

found use for summarizing the degree of oxidative processing (aging) of organic compounds in

the atmosphere (108). OSC is defined as

19

where the summation is over all noncarbon elements (hydrogen, oxygen, etc.), OSi is the

oxidation state of the element i, and ni and nC are the number of atoms of element i and carbon in

the formula, respectively. While OSC is similar in both night (Figure 5b) and day (Figure 5c) at

this particular location, the nighttime plot shows enhanced signals from ions having nitrogen in

their assigned formula as well as carbon, hydrogen and oxygen, which were attributed to organic

nitrates. The prevalence of organic nitrates in nighttime aerosol observed here is similar to that

observed with DESI discussed earlier in this section, and provides further evidence that these

species are formed by NO3 oxidation of VOCs.

With the exception of DESI, other ambient ionization techniques have rarely been used for

atmospheric aerosol measurements. DESI, like the methods discussed in Section 4, relies on a

particle collection step to obtain enough analyte for analysis. In contrast, the flow-through

methods have very low analyte mass flow rates into the mass spectrometer under atmospheric

aerosol conditions, which makes analyte detection challenging. For this reason, online ambient

methods have found their greatest use in laboratory experiments where aerosol mass loadings

tend to be large and fast analysis expands the range of experiments that can be performed, for

example following changes in molecular composition with reaction time. As advances are made

to improve the efficiency of ion formation and sampling into the mass spectrometer, online field

measurements are likely to increase.

7. METHODS FOR CHARACTERIZING NONVOLATILE ORGANIC COMPOUNDS IN THE GAS PHASE: DISCOVERY OF A NEW CLASS OF MOLECULES THAT WERE PREVIOUSLY UNDETECTED

For the most part, organic compounds in the gas phase are either volatile, i.e., exist entirely in the

gas phase, or semivolatile, i.e., partition between the gas and particle phases. Detection and

quantification of volatile organic compounds is straightforward using PTR-MS (109) for online

analysis or GC-MS (110) for off-line analysis. Quantification of semivolatile compounds is more

complicated owing to the possibility of sampling artifacts when trying to distinguish gas versus

particle phase concentrations (111). In the past decade, emphasis has shifted toward the detection

of nonvolatile compounds in the gas phase. By their nature, these compounds are hard to detect

iC i

i C

OS OS ,nn

=-å

20

in the gas phase. When they collide with any surface, whether an airborne particle or the wall of

the mass spectrometer inlet during sampling, they stick. With regard to sampling and analysis by

mass spectrometry, simply heating the inlet walls to keep these compounds in the gas phase is of

limited benefit, because they are subject to possible thermal decomposition.

Junninen et al. (112) described an atmospheric pressure interface for a time-of-flight (APi-

TOF) mass spectrometerthat minimizes the possibility of analyte contact with inlet walls during

sampling. The original design did not have an ion source but rather sampled preexisting ions in

air into the mass analyzer. Naturally occurring ions are produced by high-energy collisions of

gas molecules with, e.g., cosmic rays or products of radioactive decay. While the number

concentration of atmospheric ions is small, on the order of 1,000 cm-3 or less, ion transmission

and detection in the APi-TOF is sufficiently high for online analysis. Ambient positive ion

spectra were dominated by compounds with large proton affinities such as amines and

substituted pyridines, whereas negative ion spectra were dominated by molecular clusters with

bisulfate, nitrate, iodide, and iodate ions.

Rather than relying on atmospheric processes to produce ions, ions can be formed in a high-

pressure chemical ionization (CI) source. When implemented with an appropriately designed

inlet that minimizes wall contact while maximizing the interaction time between analyte

molecules and reagent ions, sensitive detection of nonvolatile gas-phase species can be achieved

(113–115). Reagent ions that have found the greatest use for atmospheric measurements include

adduct formation with nitrate (114, 116), acetate (113), or iodide (117) in negative ion mode, and

proton transfer (115) in positive ion mode. Zhao et al. (118) recently described an electrospray

CI source that permits alkali metal adduction in positive ion mode as well.

Ehn et al. (116) used APi-TOF in conjunction with nitrate ion CI to characterize molecules in

air associated with biogenic SOA. Negative ion mass spectra from measurements in a boreal

forest were dominated by ions up to 650 m/z, which generally corresponded to adducts of the

nitrate ion with highly oxidized organic molecules (HOMs) that were previously unknown.

These molecules are referred to as HOMs because their molecular formulas (after removing the

nitrate ion adduct) have very high oxygen-to-carbon mole ratios, typically in the 0.6 to 1.0 range,

making them extremely nonvolatile and able to nucleate and grow particles at a very fast rate.

To put this range in perspective, prior to the development of CI-APi-TOF, the detected

compound of greatest interest for new particle formation and growth was pinic acid, which only

21

has an O/C ratio of 0.44. HOMs are also detected in laboratory experiments where biogenic

SOA precursors, e.g., monoterpenes, are oxidized by ozone or hydroxyl radical (119, 120).

HOMs are thought to be formed by rapid autooxidation processes after the precursor is initially

oxidized, leading to the formation of multiple peroxy functional groups (121, 122).

The recent discovery of HOMs is an important advance in our understanding of how

nanoparticles form and subsequently grow to larger sizes in the atmosphere, and is discussed in

the next section. That a significant class of compounds went undetected for so long highlights

the need to continually develop and apply new methods of molecular characterization. As

discussed above, HOMs are difficult to sample into a mass spectrometer owing to their stickiness

when colliding with a surface. They are also highly reactive owing to the presence of peroxide

and hydroperoxide functional groups. These functionalities decompose upon heating, which

makes detection methods based on thermal desorption of limited use. Even if heating above

room temperature is avoided, these species tend to react in the particle phase at very fast rates,

making it difficult to detect them in particulate matter (122a).

8. MASS CLOSURE BETWEEN GAS- AND PARTICLE-PHASE MOLECULAR COMPOSITIONS: HOW WELL DO WE PREDICT AEROSOL FORMATION IN THE ATMOSPHERE?

An important aspect of organic aerosol research is achieving mass closure between gas- and

particle-phase measurements, in other words, demonstrating agreement between the measured

particle-phase mass concentration and the concentration that is calculated based on gas-phase

measurements. Although it is challenging to accomplish, demonstrating mass closure provides

confidence that regional and global atmospheric models accurately predict aerosol formation. It

has been known for some time that predictions of organic aerosol formation are often quite

inaccurate (5). Referring back to Figure 1, organic aerosol formation is governed by partitioning

of individual molecular species between the gas and particle phases. Nonvolatile molecules have

a negligible gas phase concentration at equilibrium, so when they collide with a particle surface,

they stick with high probability, meaning that they grow particles at their collision

(condensation) rates with the particle surface. HOMs are examples of nonvolatile molecules that

are formed directly in the gas phase. Semivolatile molecules distribute between the gas and

particle phases based on volatility. Volatility can be assessed in a variety of ways, such as

22

simultaneous GC-MS measurements of the gas and particle phases (section 3) and temperature

programmed desorption of particle samples (section 4, and ref. 59, 62, 63). For complex

mixtures of organic molecules, volatility is typically represented by a volatility basis set, which

plots the distribution of molecules in the system as a function of saturation vapor pressure

(122b, 122c). This “simple” picture of organic aerosol formation based on volatility can be

perturbed if semivolatile compounds react in the particle phase to give nonvolatile products

(122d), an example of which are organosulfates detected by LC-MS as discussed in section 5.

Mass closure is a topic that has been long studied in organic aerosol research, and could

constitute the focus of an entire review. For the purpose of this article, our discussion of mass

closure will be restricted to the recently discovered HOMs that were the focus of section 7, since

this is a topic of substantial current interest.

The first step toward mass closure is identifying and quantifying the molecular precursors to

particle formation and growth. Challenges associated with this step are that the detected

molecular species are not fully characterized (CI-APi-TOF measurements only give an elemental

formula) and authentic standards do not exist. However, insight can be gained by using gas-

phase kinetics to estimate the efficiency of creating ions from neutral molecules. If the ionization

efficiency is known, then the neutral molecule concentration in air can be determined from the

ion current measured in the mass spectrometer. This approach is based on experimental

measurements showing that the reaction rate constant in at least some CI processes is equal to the

collision rate between the molecule and reagent ion provided that the reaction is sufficiently

exothermic. An example of this principle is given by Harrison (123, figure 4) for proton transfer.

For a review of CI-MS to atmospheric chemistry, see Zhao (123a). However, this approximation

gives only an upper limit for the ionization efficiency in the aforementioned CI-APi-TOF

measurements, because the CI process may not be sufficiently exothermic for all molecules of

interest, and the anion-molecule adduct may thermally decompose after it is formed. Indeed,

significantly different products and/or yields of HOMs are obtained from nitrate (119), acetate

(124, 125), and proton transfer CI mass spectra (115).

The second step toward mass closure is modeling how gas-phase molecules migrate to the

particle phase, causing the particles to grow. As discussed above, saturation vapor pressure is

the key parameter governing partitioning between the gas and particle phases. Vapor pressures

can be determined experimentally when authentic standards are available (126). In most cases,

23

standards are not available, so vapor pressures must be estimated from molecular structures that

were deduced from mass spectrometry data, specifically elemental formulas obtained from

HRMS and structural information gained by MS/MS, in combination with known oxidation

reaction pathways of the precursor molecule. Tröstl et al. (127) used this approach to assign

candidate molecular structures to ions detected by nitrate CI-APi-TOF and thereby estimate their

vapor pressures. Figure 6a summarizes the ions detected using a plot of mass defect versus m/z.

In a mass defect plot, ions having the same number of carbon and hydrogen atoms, but an

increasing number of oxygen atoms, fall along a straight line from upper left to lower right. The

entire distribution of molecular species in the sample consists of several families of ions, with

each family consisting of its own line (and its own unique number of carbon and hydrogen

atoms) from upper left to lower right. Note that the families of ions that are prevalent at higher

m/z values generally have higher mass defects (and therefore smaller ratios of oxygen to carbon

atoms in the formulae) that the families of ions that are prevalent at lower m/z values. Figure 6b

shows the expected saturation vapor pressures of the candidate molecular structures assigned to

these ions. As might be expected, the structures span a wide range of volatilities.

<COMP: PLEASE INSERT FIGURE 6 HERE>

Figure 6 Use of nitrate ion CI-APi-TOF mass spectra to characterize molecular species formed in the gas phase by a-pinene ozonolysis. (a) Mass defect versus m/z for assigned molecular formulas of ions that were detected in the mass spectrum. (b) Volatility distribution (fraction of total ion signal intensity versus estimated vapor pressure of the corresponding molecular species) inferred from the signal intensities and assigned molecular formulas of ions that were detected. Abbreviations: CI-APi-TOF, chemical ionization with an atmospheric pressure interface for a time-of-flight; ELVOC, extremely low volatility organic compound; LVOC, low volatility organic compound; SVOC, semivolatile organic compound. Adapted with permission from Reference 127.

The third step toward mass closure is comparing measured particle mass concentrations and

growth rates to those expected from partitioning theory using gas-phase molecular

concentrations and volatilities as deduced from CI-APi-TOF mass spectra. Tröstl et al. (127)

showed that expected particle growth rates based on detected molecular species by CI-APi-TOF

are lower than the growth rates actually measured in experiments. This difference between

expected and measured particle growth could be due to the inability of CI to detect and/or

quantify all relevant species, which would underestimate the growth due to HOMs. Or, the

difference could be due to particle-phase chemical processes that quickly convert semivolatile

24

compounds into nonvolatile products and therefore increase the particle mass faster than would

be expected by partitioning alone. As of the writing of this review, a definitive answer is not yet

available.

In principle, one would expect that the molecular species detected in the gas phase would be

similar to those detected in the particle phase. Lee et al. (128) reported that gas- and particle-

phase HOMs detected in a boreal forest showed similar diurnal trends in abundance. However,

relatively few assigned molecular formulas of gas and particle HOMs are the same. For example,

Tu et al. (129) showed in a laboratory study that HOMs represent a minor fraction of the total

number of elemental formulas assigned from particle-phase mass spectra and only a few of the

particle-phase HOM formulas match those reported from gas-phase measurements. The limited

number of matches is not surprising, as gas-phase HOMs are produced by autooxidation and

contain peroxy groups, which are likely to react and/or decompose in the particle phase (122a).

A limited correspondence between gas- and particle-phase HOMs has also been reported by

Zhang et al. in laboratory experiments (131) and Mutzel et al. in atmospheric measurements

(132), who also suggested that some particle-phase species are formed by decomposition gas-

phase HOMs once they enter the particle phase. Particle-phase composition is generally

dominated by organic acids and dimer esters, which are detected in both laboratory and

atmospheric measurements (133). Dimer esters and other oligomers in atmospheric aerosol

appear to be strongly correlated with cloud condensation nuclei activity (134), which suggests a

significant impact on climate. The molecular structures of two prominent particle-phase dimer

esters known to be produced by a-pinene ozonolysis, MW 358 and MW 368, were recently

elucidated using advanced mass spectrometry techniques (135). The authors also proposed

reaction schemes to produce these species. For MW 358, they propose that formation of a dimer

in the gas phase with the formula C19H28O11 corresponds to a prominent HOM detected by CI-

APi-TOF. This dimer is produced by a radical–radical reaction (RO2• + R’O•) that subsequently

decomposes when it enters the particle phase to produce C17H26O8 (MW 358). More broadly, this

study illustrates the complex organic chemistry associated with particle formation and growth in

the atmosphere and how mass spectrometry has become an indispensible tool for elucidating

reaction pathways.

In summary, achieving mass closure with organic aerosol relies heavily on molecular

analysis. Gas-phase species must be identified, their volatilities elucidated, and their

25

concentrations quantitatively measured so that the amount of partitioning to the particle phase

can be determined. Particle-phase species must be identified and mapped back to gas-phase

precursors in order to confirm the amount of partitioning to the particle phase and/or elucidate

chemical processes in the particle phase that have perturbed the partitioning process. Relating

this back to earlier sections of the review, once mass closure has been achieved and the sources

of the gas-phase precursors have been determined, it becomes possible to accurately predict how

changes in emission control technology are likely to reduce particulate matter levels and the

impact they have on climate and human health. Mass spectrometry plays a key role in this effort.

9. FUTURE CHALLENGES FOR ATMOSPHERIC AEROSOL CHARACTERIZATION

n New and/or improved methods are needed to identify and/or quantify molecular species

for which authentic standards do not exist. For quantitative analysis, this might include

developing surrogate standards that can be applied broadly to a family of related

compounds.

n New and/or improved methods are needed to connect specific molecular species or

combinations thereof with adverse health effects, for example, to infer the oxidative

capacity of particles deposited in the respiratory tract. The recently discovered HOMs

may be important contributors to oxidative capacity owing to the presence of peroxy

groups.

n New and/or improved methods are needed to reconstruct macroscopic chemical

properties of particles from molecular-scale measurements, for example, to infer

hygroscopicity (water uptake) or the propensity for phase separation within the particle.

n New and/or improved methods are needed to perform molecular measurements of

atmospheric nanoparticles in the 3–10-nm-diameter range, where the absolute mass of

airborne particulate matter is very low. In this particle size range, the effect of particle

radius on molecular volatility is substantial, meaning that the range of compounds that

can grow these particles is much more restricted than for larger diameter particles.

DISCLOSURE STATEMENT

26

The authors are not aware of any affiliations, memberships, funding, or financial holdings that

might be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS

The authors thank Peijun Tu, Yue Wu, and Yao Zhang for assistance in surveying the published

literature. Preparation of this manuscript was supported in part by Grant Numbers CHE-1408455

and AGS-1649719 from the US National Science Foundation.

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TERMS AND DEFINITIONS

PM2.5: total mass concentration (µg/m3) of fine particles in air that are less than 2.5 µm in aerodynamic diameter Primary Organic Aerosol (POA): organic particulate matter emitted directly into the atmosphere Secondary Organic Aerosol (SOA): organic particulate matter formed in the atmosphere as a result of oxidation chemistry Aging: chemical and physical processes that change the composition and amount of organic aerosol in the atmosphere after it is initially formed Partitioning: the process by which a compound distributes between the gas and particle phases based on its volatility Volatile organic compound (VOC): a compound that exists mostly in the gas phase at equilibrium Nonvolatile organic compound (NVOC): a compound that exists mostly in the particle phase at equilibrium Semivolatile organic compound (SVOC): a compound that has appreciable concentrations in both the gas and particle phases at equilibrium Receptor modeling: the process of extracting particulate matter sources and their strengths (mass concentrations) from atmospheric chemical and physical measurements 3-Methyl-1,2,3-butanetricarboxylic acid (MBTCA): a compound found in the particle phase whose mass concentration is closely associated with the extent of atmospheric aging

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Highly oxidized molecules (HOMs): compounds that have a very high average carbon oxidation state as determined from the molecular formula; usually produced by autooxidation

ANNOTATED REFERENCES

Noziere et al. Reference (84). Review includes detailed discussion of best practices for molecular characterization of organic compounds in the atmosphere. Laskin et al. Reference (78). Detailed review of recent applications of mass spectrometry to atmospheric measurements, focusing on the years 2015 to 2017.

RELATED RESOURCES

Particulate Matter (PM) Pollution, United States Environmental Protection Agency,

https://www.epa.gov/pm-pollution (accessed 07/31/2018). Nontechnical summary of

particulate matter sources, health effects, air quality standards and control strategies.

Seinfeld JH, Pandis SN. 2016. Atmospheric chemistry and physics: From air pollution to climate

change. Hoboken, NJ: Wiley. 3rd ed. Detailed presentation of atmospheric chemical and

physical processes relevant to pollution and climate.

Harrison RM, Hester RE, Querol X. 2016. Issues in Environmental Science and Technology, Vol.

42: Airborne particulate matter sources, atmospheric processes and health. Royal Society of

Chemistry. Author contributions focus mainly on source apportionment methodologies and

applications to locations around the globe.