Aerosol composition and the contribution of SOA formation over Mediterranean forests

Abstract. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx), a series of aerosol and gas-phase measurements were deployed aboard the SAFIRE ATR42 research aircraft in summer 2014. The present study focuses on the four flights performed in late June early July over two forested regions in the south of France. We combine in situ observations and model simulations to aid in the understanding of secondary organic aerosol (SOA) formation over these forested areas in the Mediterranean and to highlight the role of different gas-phase precursors. The non-refractory particulate species measured by a compact aerosol time-of-flight mass spectrometer (cToF-AMS) were dominated by organics (60 to 72 %) followed by a combined contribution of 25 % by ammonia and sulfate aerosols. The contribution from nitrate and black carbon (BC) particles was less than 5 % of the total PM1 mass concentration. Measurements of non-refractory species from off-line transmission electron microscopy (TEM) showed that particles have different mixing states and that large fractions (35 %) of the measured particles were organic aerosol containing C, O, and S but without inclusions of crystalline sulfate particles. The organic aerosol measured using the cToF-AMS contained only evidence of oxidized organic aerosol (OOA), without a contribution of fresh primary organic aerosol. Positive matrix factorization (PMF) on the combined organic–inorganic matrices separated the oxidized organic aerosol into a more-oxidized organic aerosol (MOOA), and a less-oxidized organic aerosol (LOOA). The MOOA component is associated with inorganic species and had higher contributions of m∕z 44 than the LOOA factor. The LOOA factor is not associated with inorganic species and correlates well with biogenic volatile organic species measured with a proton-transfer-reaction mass spectrometer, such as isoprene and its oxidation products (methyl vinyl ketone, MVK; methacroleine, MACR; and isoprene hydroxyhydroperoxides, ISOPOOH). Despite a significantly high mixing ratio of isoprene (0.4 to 1.2 ppbV) and its oxidation products (0.2 and 0.8 ppbV), the contribution of specific signatures for isoprene epoxydiols SOA (IEPOX-SOA) within the aerosol organic mass spectrum (m∕z 53 and m∕z 82) were very weak, suggesting that the presence of isoprene-derived SOA was either too low to be detected by the cToF-AMS, or that SOA was not formed through IEPOX. This was corroborated through simulations performed with the Polyphemus model showing that although 60 to 80 % of SOA originated from biogenic precursors, only about 15 to 32 % was related to isoprene (non-IEPOX) SOA; the remainder was 10 % sesquiterpene SOA and 35 to 40 % monoterpene SOA. The model results show that despite the zone of sampling being far from industrial or urban sources, a total contribution of 20 to 34 % of the SOA was attributed to purely anthropogenic precursors (aromatics and intermediate or semi-volatile compounds). The measurements obtained during this study allow us to evaluate how biogenic emissions contribute to increasing SOA concentrations over Mediterranean forested areas. Directly comparing these measurements with the Polyphemus model provides insight into the SOA formation pathways that are prevailing in these forested areas as well as processes that need to be implemented in future simulations.

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Aerosol composition and the contribution of SOA formation over Mediterranean forests

10.5194/acp-2017-482 ◽

2017 ◽

Cited By ~ 3

Author(s):

Evelyn Freney ◽

Karine Sellegri ◽

Mounir Chrit ◽

Kouji Adachi ◽

Joel Brito ◽

...

Keyword(s):

Gas Phase ◽

Organic Aerosol ◽

Oxidation Products ◽

Dust Particles ◽

Large Contribution ◽

Mediterranean Forests ◽

Aerosol Composition ◽

Tem Analysis ◽

Organic Species ◽

Inorganic Species

Abstract. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx), a series of aerosol and gas phase measurements were deployed aboard the SAFIRE ATR-42 research aircraft in summer 2014. The present study focuses on the 4 flights performed in late June early July over two forested regions in the south of France. We combine in situ observations and model simulations to aid in the understanding of secondary organic aerosol (SOA) formation over these forested areas in the Mediterranean and to highlight the role of different gas-phase precursors. The non-refractory particulate species measured by C-ToF-AMS instrument were dominated by organic species (60 to 72 %) followed by a combined contribution of 25 % by ammonia and sulphate aerosols. The contribution from the anthropogenic nitrate and black carbon (BC) concentrations, measured by an SP2, never contributed to more than 5 % each to the total PM1 mass concentration. Measurements of non-refractory species from off-line transmission electron microscopy (TEM) were coherent with the C-ToF-AMS instrument, showing a large contribution of externally mixed organic aerosol and externally mixed sulphate particles. Externally mixed organic aerosols, were equally identified with S signals, which may suggest the presence of organo-sulphates. Measurements of refractory species from TEM analysis showed a significant contribution of both sea salt and dust particles depending on the air mass trajectory. The organic aerosol measured by the C-ToF-AMS contained only evidence of oxidised organic aerosol (OOA), without a contribution of fresh primary organic aerosol. Positive matrix factorization (PMF) on the combined organic/inorganic matrices separated the oxidised organic aerosol into a more oxidised organic aerosol (MOOA), and a less oxidised organic aerosol (LOOA). The MOOA component is associated with inorganics species and had higher O : C ratios than the LOOA factor. The LOOA factor is not associated with inorganic species and correlates well with biogenic volatile organic species measured with a PTR-MS, such as isoprene and its oxidation products (methylvinylketone (MVK), methacroleine (MACR), and isoprene hydroxyhydroperoxides (ISOPOOH)). Despite a significantly high mixing ratio of isoprene (2– ppbV) and oxidation products (0.6 and 1.2 ppbV), the contribution of specific signatures for isoprene epoxydiols SOA (IEPOX) within the aerosol organic mass spectrum (m / z 53 and m / z 82) were very weak, suggesting that isoprene SOA may be formed through a non-IEPOX route here, or with different precursors without clear mass spectral signatures in the C-ToF-AMS. This was corroborated through simulations performed with the Polyphemus model showing that 60 to 80 % of SOA originated from biogenic precursors: about 15 to 32 % isoprene (non-IEPOX) SOA, 10 % sesquiterpenes SOA and 35 to 40 % monoterpenes SOA). A total of 20 to 34 % was attributed to purely anthropogenic precursors (aromatics and intermediate/semi volatile compounds).

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Characterization of fresh and aged organic aerosol emissions from meat charbroiling

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-7143-2017 ◽

2017 ◽

Vol 17 (11) ◽

pp. 7143-7155 ◽

Cited By ~ 24

Author(s):

Christos Kaltsonoudis ◽

Evangelia Kostenidou ◽

Evangelos Louvaris ◽

Magda Psichoudaki ◽

Epameinondas Tsiligiannis ◽

...

Keyword(s):

Mass Spectrometer ◽

Urban Areas ◽

Fine Particulate Matter ◽

Organic Aerosol ◽

Proton Transfer Reaction ◽

Gas Phases ◽

Uv Illumination ◽

Aerosol Mass ◽

Inorganic Species ◽

Aerosol Emissions

Abstract. Cooking emissions can be a significant source of fine particulate matter in urban areas. In this study the aerosol- and gas-phase emissions from meat charbroiling were characterized. Greek souvlakia with pork were cooked using a commercial charbroiler and a fraction of the emissions were introduced into a smog chamber where after a characterization phase they were exposed to UV illumination and oxidants. The particulate and gas phases were characterized by a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and a proton-transfer-reaction mass spectrometer (PTR-MS) correspondingly. More than 99 % of the aerosol emitted was composed of organic compounds, while black carbon (BC) contributed 0.3 % and the inorganic species less than 0.5 % of the total aerosol mass. The initial O  :  C ratio was approximately 0.09 and increased up to 0.30 after a few hours of chemical aging (exposures of 1010 molecules cm−3 s for OH and 100 ppb h for ozone). The initial and aged AMS spectra differed considerably (θ =  27°). Ambient measurements were also conducted during Fat Thursday in Patras, Greece, when traditionally meat is charbroiled everywhere in the city. Positive matrix factorization (PMF) revealed that cooking organic aerosol (COA) reached up to 85 % of the total OA from 10:00 to 12:00 LST that day. The ambient COA factor in two major Greek cities had a mass spectrum during spring and summer similar to the aged meat charbroiling emissions. In contrast, the ambient COA factor during winter resembled strongly the fresh laboratory meat charbroiling emissions.

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Secondary organic aerosol formation and primary organic aerosol oxidation from biomass-burning smoke in a flow reactor during FLAME-3

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-11551-2013 ◽

2013 ◽

Vol 13 (22) ◽

pp. 11551-11571 ◽

Cited By ~ 162

Author(s):

A. M. Ortega ◽

D. A. Day ◽

M. J. Cubison ◽

W. H. Brune ◽

D. Bon ◽

...

Keyword(s):

Mass Spectrometer ◽

Biomass Burning ◽

Flow Reactor ◽

Organic Aerosol ◽

Proton Transfer Reaction ◽

Oxidation Products ◽

Chemical Transformations ◽

Aerosol Formation ◽

Aerosol Mass ◽

Smog Chambers

Abstract. We report the physical and chemical effects of photochemically aging dilute biomass-burning smoke. A "potential aerosol mass" (PAM) flow reactor was used with analysis by a high-resolution aerosol mass spectrometer and a proton-transfer-reaction ion-trap mass spectrometer during the FLAME-3 campaign. Hydroxyl (OH) radical concentrations in the reactor reached up to ~1000 times average tropospheric levels, producing effective OH exposures equivalent to up to 5 days of aging in the atmosphere, and allowing for us to extend the investigation of smoke aging beyond the oxidation levels achieved in traditional smog chambers. Volatile organic compound (VOC) observations show aromatics and terpenes decrease with aging, while formic acid and other unidentified oxidation products increase. Unidentified gas-phase oxidation products, previously observed in atmospheric and laboratory measurements, were observed here, including evidence of multiple generations of photochemistry. Substantial new organic aerosol (OA) mass ("net SOA"; secondary OA) was observed from aging biomass-burning smoke, resulting in total OA average of 1.42 ± 0.36 times the initial primary OA (POA) after oxidation. This study confirms that the net-SOA-to-POA ratio of biomass-burning smoke is far lower on average than that observed for urban emissions. Although most fuels were very reproducible, significant differences were observed among the biomasses, with some fuels resulting in a doubling of the OA mass, while for others a very small increase or even a decrease was observed. Net SOA formation in the photochemical reactor increased with OH exposure (OHexp), typically peaking around three days of equivalent atmospheric photochemical age (OHexp~3.9 × 1011 molecules cm−3 s), then leveling off at higher exposures. The amount of additional OA mass added from aging is positively correlated with initial POA concentration, but not with the total VOC concentration or the concentration of known SOA precursors. The mass of SOA formed often exceeded the mass of the known VOC precursors, indicating the likely importance of primary semivolatile/intermediate volatility species, and possibly of unidentified VOCs as SOA precursors in biomass burning smoke. Chemical transformations continued even after mass concentration stabilized. Changes in the biomass-burning tracer f60 ranged from substantially decreasing to remaining constant with increased aging. With increased OHexp, oxidation was always detected (as indicated by f44 and O/C). POA O/C ranged from 0.15 to 0.5, while aged OA O/C reached up to 0.87. The rate of oxidation and maximum O/C achieved differs for each biomass, and appears to increase with the initial O/C of the POA.

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Oxidation product characterization from ozonolysis of the diterpene ent-kaurene

10.5194/acp-2021-881 ◽

2021 ◽

Author(s):

Yuanyuan Luo ◽

Olga Garmash ◽

Haiyan Li ◽

Frans Graeffe ◽

Arnaud P. Praplan ◽

...

Keyword(s):

Gas Phase ◽

Mass Spectrometer ◽

Atmospheric Chemistry ◽

Ambient Air ◽

Proton Transfer Reaction ◽

Oxidation Products ◽

Ionization Mass ◽

Product Characterization ◽

First Time ◽

Clustering Patterns

Abstract. Diterpenes (C20H32) are biogenically emitted volatile compounds that only recently have been observed in ambient air. They are expected to be highly reactive, and their oxidation is likely to form condensable vapors. However, until now, no studies have investigated gas-phase diterpene oxidation. In this paper, we explored the ozonolysis of a diterpene, ent-kaurene, in a simulation chamber. Using state-of-the-art mass spectrometry, we characterized diterpene oxidation products for the first time, and we identified several products with varying oxidation levels, including highly oxygenated organic molecules (HOM) monomers and dimers. The most abundant monomers measured using a nitrate chemical ionization mass spectrometer were C19H28O8 and C20H30O5, and dimers were C38H60O6 and C39H62O6. The exact molar yield of HOM from kaurene ozonolysis was hard to quantify due to uncertainties in both the kaurene and HOM concentrations, but our best estimate was a few percent, which is similar to values reported earlier for many monoterpenes. We also monitored the decrease of the gas-phase oxidation products in response to an increased condensation sink in the chamber to deduce their affinity to condense. The oxygen content was a critical parameter affecting the volatility of products, with 4–5 O-atoms needed for the main monomeric species to condense. Finally, we report on the observed fragmentation and clustering patterns of kaurene in a Vocus proton transfer reaction time-of-flight mass spectrometer. Our findings highlight similarities and differences between diterpenes and smaller terpenes during their atmospheric oxidation, but more studies on different diterpenes are needed for a broader view of their role in atmospheric chemistry.

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Secondary organic aerosol formation and primary organic aerosol oxidation from biomass burning smoke in a flow reactor during FLAME-3

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-13799-2013 ◽

2013 ◽

Vol 13 (5) ◽

pp. 13799-13851 ◽

Cited By ~ 4

Author(s):

A. M. Ortega ◽

D. A. Day ◽

M. J. Cubison ◽

W. H. Brune ◽

D. Bon ◽

...

Keyword(s):

Mass Spectrometer ◽

Biomass Burning ◽

Ion Trap ◽

Flow Reactor ◽

Organic Aerosol ◽

Proton Transfer Reaction ◽

Oxidation Products ◽

Chemical Transformations ◽

Aerosol Formation ◽

Aerosol Mass

Abstract. We report the physical and chemical effects of photochemically aging dilute biomass-burning smoke. A potential aerosol mass "PAM" flow reactor was used with analysis by a high-resolution aerosol mass spectrometer and a proton-transfer reaction ion-trap mass spectrometer during the FLAME-3 campaign. Hydroxyl (OH) radical concentrations in the reactor reached up to ~ 1000 times average tropospheric levels, producing effective OH exposures equivalent to up to 5 days aging in the atmosphere. VOC observations show aromatics and terpenes decrease with aging, while formic acid and other unidentified oxidation products increase. Unidentified gas-phase oxidation products, previously observed in atmospheric and laboratory measurements, were observed here, including evidence of multiple generations of photochemistry. Substantial new organic aerosol (OA) mass ("net SOA"; secondary OA) was observed from aging biomass-burning smoke, resulting in an total OA average of 1.42 &pm; 0.36 times the initial primary OA (POA) after oxidation. This study confirms that the net SOA to POA ratio of biomass burning smoke is far lower on average than that observed for urban emissions. Although most fuels were very reproducible, significant differences were observed among the biomasses, with some fuels resulting in a doubling of the OA mass, while for others a very small increase or even a decrease was observed. Net SOA formation in the photochemical reactor increased with OH exposure (OHexp), typically peaking around three days of equivalent atmospheric photochemical age (OHexp ~ 3.9 × 1011 molecules cm−3 s−1), then leveling off at higher exposures. The amount of additional OA mass added from aging is positively correlated with initial POA concentration, but not with the total VOC concentration or the concentration of known SOA precursors. The mass of SOA formed often exceeds the mass of the known VOC precursors, indicating the likely importance of primary semivolatile/intermediate volatility species, and possibly of unidentified VOCs as SOA precursors in biomass burning smoke. Chemical transformations continue even after mass concentration stabilizes. Changes in the biomass-burning tracer f60 ranged from substantially decreasing to remaining constant with increased aging. With increased OHexp, oxidation was always detected (as indicated by f44 and O/C). POA O/C ranged 0.15–0.5, while aged OA O/C reached up to 0.87. The rate of oxidation and maximum O/C achieved differs for each biomass and appears to increase with the initial O/C of the POA.

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Enhanced SOA formation from mixed anthropogenic and biogenic emissions during the CARES campaign

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-2091-2013 ◽

2013 ◽

Vol 13 (4) ◽

pp. 2091-2113 ◽

Cited By ~ 108

Author(s):

J. E. Shilling ◽

R. A. Zaveri ◽

J. D. Fast ◽

L. Kleinman ◽

M. L. Alexander ◽

...

Keyword(s):

Mass Spectrometer ◽

Organic Aerosol ◽

Proton Transfer Reaction ◽

Chemical Mechanism ◽

Biogenic Emissions ◽

Anthropogenic Emissions ◽

Radiation Balance ◽

Particle Phase ◽

Aerosol Formation ◽

Research Aircraft

Abstract. The CARES campaign was conducted during June, 2010 in the vicinity of Sacramento, California to study aerosol formation and aging in a region where anthropogenic and biogenic emissions regularly mix. Here, we describe measurements from an Aerodyne High Resolution Aerosol Mass Spectrometer (AMS), an Ionicon Proton Transfer Reaction Mass Spectrometer (PTR-MS), and trace gas detectors (CO, NO, NOx) deployed on the G-1 research aircraft to investigate ambient gas- and particle-phase chemical composition. AMS measurements showed that the particle phase is dominated by organic aerosol (OA) (85% on average) with smaller concentrations of sulfate (5%), nitrate (6%) and ammonium (3%) observed. PTR-MS data showed that isoprene dominated the biogenic volatile organic compound concentrations (BVOCs), with monoterpene concentrations generally below the detection limit. Using two different metrics, median OA concentrations and the slope of plots of OA vs. CO concentrations (i.e., ΔOA/ΔCO), we contrast organic aerosol evolution on flight days with different prevailing meteorological conditions to elucidate the role of anthropogenic and biogenic emissions on OA formation. Airmasses influenced predominantly by biogenic emissions had median OA concentrations of 2.2 μg m−3 and near zero ΔOA/ΔCO. Those influenced predominantly by anthropogenic emissions had median OA concentrations of 4.7 μg m−3 and ΔOA/ΔCO ratios of 35–44 μg m−3 ppmv. But, when biogenic and anthropogenic emissions mixed, OA levels were enhanced, with median OA concentrations of 11.4 μg m−3 and ΔOA/ΔCO ratios of 77–157 μg m−3 ppmv. Taken together, our observations show that production of OA was enhanced when anthropogenic emissions from Sacramento mixed with isoprene-rich air from the foothills. After considering several anthropogenic/biogenic interaction mechanisms, we conclude that NOx concentrations play a strong role in enhancing SOA formation from isoprene, though the chemical mechanism for the enhancement remains unclear. If these observations are found to be robust in other seasons and in areas outside of Sacramento, regional and global aerosol modules will need to incorporate more complex representations of NOx-dependent SOA mechanisms and yields into their algorithms. Ultimately, accurately predicting OA mass concentrations and their effect on radiation balance will require a mechanistically-based treatment of the interactions of biogenic and anthropogenic emissions.

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Atmospheric β-Caryophyllene-Derived Ozonolysis Products at Interfaces

10.26434/chemrxiv.7228694 ◽

2018 ◽

Author(s):

Ariana Gray Bé ◽

Hilary M. Chase ◽

Liu, Yangdongliu ◽

Mary Alice Upshur ◽

Zhang, Yue ◽

...

Keyword(s):

Gas Phase ◽

Structural Information ◽

High Surface ◽

Organic Aerosol ◽

Oxidation Products ◽

Laboratory Studies ◽

Sum Frequency ◽

Sum Frequency Generation Spectroscopy ◽

Surface Active ◽

Cloud Activation

<p>By integrating organic synthesis, secondary organic aerosol synthesis and collection, DFT calculations, and vibrational sum frequency generation spectroscopy, we identify close spectral matches between the surface vibrational spectra of β-caryophyllene-derived secondary organic material and those of β-caryophyllene aldehyde and β-caryophyllonic acid at various interfaces. Combined with the record high surface tension depression described previously for these same oxidation products, we discuss possibilities for an intrinsically chemical origin for cloud activation by terpene-derived surfactants. Although the present study does not unequivocally identify the synthesized and analyzed oxidation products on the β-caryophyllenederived SOM surfaces, these two compounds appear to be the most surface active out of the series, and have also been foci of previous β-caryophyllene field and laboratory studies.</p><p>An orientation analysis by phase-resolved SFG spectroscopy reveals a “pincer-like” configuration of the β-caryophyllene oxidation products, albeit on a model quartz surface, that somewhat resembles the orientation of inverse double-tailed surfactants at the surfaces biological systems. The structural information suggests that the less polar moiety of a surface-localized oxidation product, such as those studied here, may be the first site-of-contact for a gas-phase molecule approaching an SOA particle containing surface-active β-caryophyllene oxidation products.</p>

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Characterization of secondary organic aerosol from heated-cooking-oil emissions: evolution in composition and volatility

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-5137-2021 ◽

2021 ◽

Vol 21 (6) ◽

pp. 5137-5149 ◽

Cited By ~ 1

Author(s):

Manpreet Takhar ◽

Yunchun Li ◽

Arthur W. H. Chan

Keyword(s):

Gas Phase ◽

Organic Compounds ◽

Secondary Organic Aerosol ◽

Carbon Number ◽

Complex Mixture ◽

Organic Aerosol ◽

Oxidation Products ◽

Atmospheric Evolution ◽

Oxidative Aging ◽

Cooking Oil

Abstract. Cooking emissions account for a major fraction of urban organic aerosol. It is therefore important to understand the atmospheric evolution in the physical and chemical properties of organic compounds emitted from cooking activities. In this work, we investigate the formation of secondary organic aerosol (SOA) from oxidation of gas-phase organic compounds from heated cooking oil. The chemical composition of cooking SOA is analyzed using thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS). While the particle-phase composition of SOA is a highly complex mixture, we adopt a new method to achieve molecular speciation of the SOA. All the GC-elutable material is classified by the constituent functional groups, allowing us to provide a molecular description of its chemical evolution upon oxidative aging. Our results demonstrate an increase in average oxidation state (from −0.6 to −0.24) and decrease in average carbon number (from 5.2 to 4.9) with increasing photochemical aging of cooking oil, suggesting that fragmentation reactions are key processes in the oxidative aging of cooking emissions within 2 d equivalent of ambient oxidant exposure. Moreover, we estimate that aldehyde precursors from cooking emissions account for a majority of the SOA formation and oxidation products. Overall, our results provide insights into the atmospheric evolution of cooking SOA, a majority of which is derived from gas-phase oxidation of aldehydes.

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Evaluation of the chemical composition of gas- and particle-phase products of aromatic oxidation

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-9783-2020 ◽

2020 ◽

Vol 20 (16) ◽

pp. 9783-9803

Author(s):

Archit Mehra ◽

Yuwei Wang ◽

Jordan E. Krechmer ◽

Andrew Lambe ◽

Francesca Majluf ◽

...

Keyword(s):

Gas Phase ◽

Mass Spectrometer ◽

Urban Areas ◽

Minor Component ◽

Proton Transfer Reaction ◽

Chemical Mechanism ◽

Particle Phase ◽

Experimental Conditions ◽

Radical Oxidation ◽

Product Ions

Abstract. Aromatic volatile organic compounds (VOCs) are key anthropogenic pollutants emitted to the atmosphere and are important for both ozone and secondary organic aerosol (SOA) formation in urban areas. Recent studies have indicated that aromatic hydrocarbons may follow previously unknown oxidation chemistry pathways, including autoxidation that can lead to the formation of highly oxidised products. In this study we evaluate the gas- and particle-phase ions measured by online mass spectrometry during the hydroxyl radical oxidation of substituted C9-aromatic isomers (1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, propylbenzene and isopropylbenzene) and a substituted polyaromatic hydrocarbon (1-methylnaphthalene) under low- and medium-NOx conditions. A time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) with iodide–anion ionisation was used with a filter inlet for gases and aerosols (FIGAERO) for the detection of products in the particle phase, while a Vocus proton-transfer-reaction mass spectrometer (Vocus-PTR-MS) was used for the detection of products in the gas phase. The signal of product ions observed in the mass spectra were compared for the different precursors and experimental conditions. The majority of mass spectral product signal in both the gas and particle phases comes from ions which are common to all precursors, though signal distributions are distinct for different VOCs. Gas- and particle-phase composition are distinct from one another. Ions corresponding to products contained in the near-explicit gas phase Master Chemical Mechanism (MCM version 3.3.1) are utilised as a benchmark of current scientific understanding, and a comparison of these with observations shows that the MCM is missing a range of highly oxidised products from its mechanism. In the particle phase, the bulk of the product signal from all precursors comes from ring scission ions, a large proportion of which are more oxidised than previously reported and have undergone further oxidation to form highly oxygenated organic molecules (HOMs). Under the perturbation of OH oxidation with increased NOx, the contribution of HOM-ion signals to the particle-phase signal remains elevated for more substituted aromatic precursors. Up to 43 % of product signal comes from ring-retaining ions including HOMs; this is most important for the more substituted aromatics. Unique products are a minor component in these systems, and many of the dominant ions have ion formulae concurrent with other systems, highlighting the challenges in utilising marker ions for SOA.

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Characterization of a real-time tracer for isoprene epoxydiols-derived secondary organic aerosol (IEPOX-SOA) from aerosol mass spectrometer measurements

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-11807-2015 ◽

2015 ◽

Vol 15 (20) ◽

pp. 11807-11833 ◽

Cited By ~ 121

Author(s):

W. W. Hu ◽

P. Campuzano-Jost ◽

B. B. Palm ◽

D. A. Day ◽

A. M. Ortega ◽

...

Keyword(s):

Mass Spectrometer ◽

Secondary Organic Aerosol ◽

Transport Model ◽

Organic Aerosol ◽

Oxidation Products ◽

Chemical Transport Model ◽

Aerosol Mass Spectrometer ◽

Aerosol Mass ◽

Multiple Field

Abstract. Substantial amounts of secondary organic aerosol (SOA) can be formed from isoprene epoxydiols (IEPOX), which are oxidation products of isoprene mainly under low-NO conditions. Total IEPOX-SOA, which may include SOA formed from other parallel isoprene oxidation pathways, was quantified by applying positive matrix factorization (PMF) to aerosol mass spectrometer (AMS) measurements. The IEPOX-SOA fractions of organic aerosol (OA) in multiple field studies across several continents are summarized here and show consistent patterns with the concentration of gas-phase IEPOX simulated by the GEOS-Chem chemical transport model. During the Southern Oxidant and Aerosol Study (SOAS), 78 % of PMF-resolved IEPOX-SOA is accounted by the measured IEPOX-SOA molecular tracers (2-methyltetrols, C5-Triols, and IEPOX-derived organosulfate and its dimers), making it the highest level of molecular identification of an ambient SOA component to our knowledge. An enhanced signal at C5H6O+ (m/z 82) is found in PMF-resolved IEPOX-SOA spectra. To investigate the suitability of this ion as a tracer for IEPOX-SOA, we examine fC5H6O (fC5H6O= C5H6O+/OA) across multiple field, chamber, and source data sets. A background of ~ 1.7 ± 0.1 ‰ (‰ = parts per thousand) is observed in studies strongly influenced by urban, biomass-burning, and other anthropogenic primary organic aerosol (POA). Higher background values of 3.1 ± 0.6 ‰ are found in studies strongly influenced by monoterpene emissions. The average laboratory monoterpene SOA value (5.5 ± 2.0 ‰) is 4 times lower than the average for IEPOX-SOA (22 ± 7 ‰), which leaves some room to separate both contributions to OA. Locations strongly influenced by isoprene emissions under low-NO levels had higher fC5H6O (~ 6.5 ± 2.2 ‰ on average) than other sites, consistent with the expected IEPOX-SOA formation in those studies. fC5H6O in IEPOX-SOA is always elevated (12–40 ‰) but varies substantially between locations, which is shown to reflect large variations in its detailed molecular composition. The low fC5H6O (< 3 ‰) reported in non-IEPOX-derived isoprene-SOA from chamber studies indicates that this tracer ion is specifically enhanced from IEPOX-SOA, and is not a tracer for all SOA from isoprene. We introduce a graphical diagnostic to study the presence and aging of IEPOX-SOA as a triangle plot of fCO2 vs. fC5H6O. Finally, we develop a simplified method to estimate ambient IEPOX-SOA mass concentrations, which is shown to perform well compared to the full PMF method. The uncertainty of the tracer method is up to a factor of ~ 2, if the fC5H6O of the local IEPOX-SOA is not available. When only unit mass-resolution data are available, as with the aerosol chemical speciation monitor (ACSM), all methods may perform less well because of increased interferences from other ions at m/z 82. This study clarifies the strengths and limitations of the different AMS methods for detection of IEPOX-SOA and will enable improved characterization of this OA component.

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