Role of glyoxal in SOA formation from aromatic hydrocarbons : gas-phase reaction trumps reactive uptake

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Introduction
Aerosol contributes to climate change and adversely affects air quality (Seinfeld and Pandis, 2006;Finlayson-Pitts and Pitts, 1999).Secondary organic aerosol (SOA) is formed from oxidative processing of volatile organic compounds in the atmosphere.Previous researchers have estimated approximately 70 % of organic aerosols are secondary in nature (Hallquist et al., 2009  formation is described solely by gas-to-particle partitioning of semi-volatile oxidation products of volatile organic compounds (VOCs) (Odum et al., 1996;Pankow, 1994).However, recent works have observed enhanced SOA formation from the oligomerization of volatile species (Kalberer et al., 2004;Tolocka et al., 2004).Glyoxal was previously ignored as a SOA precursor due to its high vapor pressure (6 orders of magnitude too high; Volkamer et al., 2007); however, the current view is that glyoxal can contribute to SOA formation by uptake into water (cloud, fog, and wet aerosols) followed by radical and non-radical reactions to produce low volatility products (Lim et al., 2010, and references therein).The global emission of glyoxal is estimated to be 45 Tg yr −1 (Fu et al., 2008); globally, the major precursor of glyoxal is isoprene (21 Tg yr −1 ) (Fu et al., 2008), while aromatic hydrocarbons are the main precursors in urban areas (e.g., 70-79 % in Mexico City, Volkamer et al., 2007).
The liquid water content of typical cloud droplets are orders of magnitude higher than that of aerosols (Seinfeld and Pandis, 2006); therefore, early work on glyoxal SOA formation focused on aqueous reactions in cloud and fog water (e.g., Ervens et al., 2004).However, SOA formation from glyoxal uptake onto wet aerosols has acquired increasing attention during the last few years.Volkamer et al. (2007) observed significantly lower glyoxal concentration than model predictions for Mexico City, indicating a large missing sink of glyoxal.The discrepancy was resolved by introducing glyoxal uptake onto aerosols; ∼ 15 % of the SOA formation in Mexico City was attributed to glyoxal uptake onto aerosols (Volkamer et al., 2007).Additionally, recent laboratory studies suggest formation of SOA via oligomerization of glyoxal in aerosol aqueous phase (Volkamer et al., 2009;Corrigan et al., 2008;Kroll et al., 2005;Galloway et al., 2009Galloway et al., , 2011;;Liggio et al., 2005).
Glyoxal uptake onto particles is observed to be strongly dependent on seed composition.Acidity is suggested to enhance glyoxal partitioning to the aqueous phase (Jang and Kamens, 2001).However, Kroll et al. (2005) did not observe the acidity effect; instead they suggested that ionic strength of the seed aerosols ("salting in") could explain the enhanced glyoxal uptake onto aqueous ammonium sulfate seeds (by Introduction

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Full a factor of ∼ 70 compared to uptake by water).A more recent work by Ip et al. (2009) suggested that sulfate was a more important factor than the ionic strength in affecting glyoxal's Henry's law constant.Organic seeds are also reported to enhance glyoxal uptake (e.g., Volkamer et al., 2009): fulvic acid, humic acid sodium salt, and amino acids; Corrigan et al. (2008): amino acids and carboxylic acids).However, understanding of the composition of SOA formed from aromatic hydrocarbon oxidation is currently limited; typically only less than ∼ 10 % of aromatic SOA composition is identified (Sato et al., 2007;Cocker et al., 2001b;Hamilton et al., 2005).Therefore, the impact of organic aerosol on glyoxal uptake is highly uncertain.
Glyoxal is a major product of aromatic hydrocarbon photooxidation (e.g., 8-24 % from toluene-NO x photooxidation Calvert et al., 2002).Aromatic hydrocarbons comprise ∼ 20 % of nonmethane hydrocarbons in the urban atmosphere and are considered to be one of the major precursors to urban SOA (Calvert et al., 2002).A large number of studies have investigated gas-phase photooxidation of aromatic hydrocarbons (e.g., Olariu et al., 2002;Volkamer et al., 2002;Takekawa et al., 2003;Johnson et al., 2004Johnson et al., , 2005;;Coeur-Tourneur et al., 2006;Arey et al., 2009;Calvert et al., 2002, and references therein;Birdsall et al., 2010).Although multigenerational reactions have been suggested to contribute to aromatic SOA formation (Hurley et al., 2001;Ng et al., 2007;Sato et al., 2007;Nakao et al., 2011a), the extent of the contribution from second or later generation products to SOA are poorly understood.Based on previous studies on SOA formation by glyoxal uptake, glyoxal oligomerization has been inferred to be a substantial intermediate reaction in SOA formation from aromatic hydrocarbon under humid conditions (Zhou et al., 2011;Kalberer et al., 2004;Kamens et al., 2011).
According to previous studies on glyoxal uptake, glyoxal is expected to partition to aqueous phase of SOA and subsequently undergo radical or non-radical reactions to produce low-volatility products.However, the applicability of these previous studies of relatively pure systems (wet inorganic/organic seed) to complex aromatic SOA system remains uncertain.The aim of this work is to shed light on the role of glyoxal in SOA formation from aromatic hydrocarbon oxidation -specifically as an OH radical source Introduction

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Full or an oligomer precursor.

Environmental chamber
The experiments were conducted in the UC Riverside/CE-CERT environmental chamber described in detail in Carter et al. (2005)

Gas analysis
Glyoxal was measured by a custom-built incoherent broadband Cavity Enhanced Absorption Spectrometer (CEAS) (Washenfelder et al., 2008;Langridge et al., 2006).In CEAS, a continuous wave incoherent light is injected into a cavity, where the intensity Introduction

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Full reaches its limiting value and absorption spectra are obtained (Engeln et al., 1998).
The absorption coefficient is obtained from (Fiedler et al., 2005).
where α is the absorption coefficient, d is the length of the cavity, I 0 is the intensity of light exiting the cavity without any absorber present, I is the light intensity of the cavity with absorber, and R is the reflectivity of the mirrors.In CEAS, the transmitted light intensity through the optical cavity of two high reflectivity mirrors provides sensitive measurements of trace species with a long effective optical path (Engeln et al., 1998;Paul, 2001;Fiedler, 2003;Langridge et al., 2006;Washenfelder, 2008).CEAS allows for simultaneous analysis of multiple absorbers in the same spectral region (e.g., both NO 2 and glyoxal in the 440-460 nm region).In this work, the CEAS system for the glyoxal measurements was based on the previous work by Langridge et al. (2006) and Washenfelder et al. (2008).The major components of the CEAS system include a glass cell housing the optical cavity (65 cm long, 2.54 cm diameter with 1/16 inch wall thickness), two high reflectivity (R = 0.9998) mirrors (Los Gatos), a light emitting diode (LED) (Luxeon) light source, a monochromator and a charge-coupled device (CCD) light detector (Andor).The light from the LED was focused and coupled into the optical cavity; the output light from the cavity was dispersed by the monochromator and collected by the CCD detector.Gas flow rate through the CEAS was 1 l min −1 , while the pressure inside the optical cavity ranged from 714-720 Torr (0.939-0.947 atm).The CCD collected the transmission spectra from the cavity using an exposure time of 0.5 s with 112 samples accumulated during an overall sampling time of 1 min.The 0.5 s exposure time was chosen to prevent saturation of the signal at the peak LED emission spectrum at the maximum operating power of 200 mW for this CEAS system.The background signal under the same acquisition conditions was collected with the LED off and the background spectra was subtracted from the transmission spectra from the cavity.Introduction

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Full The glyoxal number density was calculated from where λ is the wavelength, σ gly is glyoxal absorption cross-section, n gly is the glyoxal number density, α(λ) is the measured absorption coefficient for a given λ, the polynomial terms of the equation account for light extinction by background molecules (e.g., N 2 and O 2 ) and α NO 2 and n NO 2 account for light absorption by NO 2 .Wavelengthdependent absorption cross-sections (σ(λ)) were obtained from literature and data evaluation web sites (e.g., IUPAC, 2006;NASA, 2006;Volkamer et al., 2005).The glyoxal number densities were extracted using the spectra between 446.5 and 450.0 nm.The Agilent 6890 Gas Chromatograph -Flame Ionization Detector was used to measure concentrations of parent hydrocarbons (toluene and 2-tert-butylphenol) and an inert tracer (perfluorohexane).A GS-Alumina column (30 m × 0.53 mm) and a DB-5 column (30 m × 0.53 mm) were used for perfluorohexane and toluene analysis, respectively.2-tert-butylphenol was collected on a sorbent tube packed with Tenax-TA/Carbopack/Carbosive (CDS Analytical, Inc, MX062171) and was thermally desorbed at 290 • C (CDS Analytical, Inc, ACEM9305) onto a Restek ® Rtx-35 Amine (30 m × 0.53 mm ID, 1.00 micron) column.Toluene measurements were calibrated using a dilute gas cylinder (SCOTT-MARIN, Inc); perfluorohexane was calibrated by introducing a known amount of the liquid into the reactor; and 2-tert-butylphenol was calibrated by impregnation of the glass tube and subsequent thermal desorption.

Particle analysis
Particle size distribution between 27 and 686 nm was monitored by a custom built Scanning Mobility Particle Sizer (SMPS) similar to that described by Cocker et al. (2001a).
The chemical evolution of organic particulates was observed by a high-resolution timeof-flight aerosol mass spectrometer (HR-ToF-AMS) (DeCarlo et al., 2006;Jayne et al., 2000).The HR-ToF-AMS operation was alternated between the high resolution Wmode and high sensitivity V-mode.The high resolution capability allowed determination 30605 Introduction

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Full ).SQUIRREL v1.49 and PIKA v1.08 were used for data analysis.The default fragmentation table was used without modification.Particle volatility was monitored with a volatility tandem differential mobility analyzer (VTDMA) (Nakao et al., 2011b;Qi et al., 2010;Rader and McMurry, 1986), in which monodisperse particles of mobility diameter D mi were selected by the 1st differential mobility analyzer (DMA) followed by transport through a Dekati thermodenuder (TD, residence time: ∼ 17 s, typically at 100 • C).The particle size after the TD (D mf ) was then measured by fitting a log-normal size distribution curve acquired by the 2nd DMA.Volume fraction remaining (VFR) was calculated by taking a (cubed) ratio of particle mobility diameter after the TD (D mf ) to initial particle size (D mi ), i.e., VFR = (D mf /D mi ) 3 .D mi was adjusted during the experiment according to mode diameters of particle size distribution within the environmental chamber to maximize the signal-to-noise ratio (typically D mi = 50 ∼ 150 nm).

Thermodenuder characterization
As a basis to evaluate the VFR in terms of vapor pressure, thermodenuder characterization was performed by measuring the VFR of select compounds with known vapor pressure (D mi = 150 nm).The vaporization profile was previously evaluated based on the temperature at which 50 % of the mass evaporates (T 50 ) (Faulhaber et al., 2009).The vaporization profiles for hexanedioic acid and decanedioic acid acquired in this study agreed reasonably with Fualhaber et al. ( 2009) (T 50 agreed within 2 • C), consistent with the similar residence times of the thermodenuders (This study: ∼ 17 s, Fualhaber et al. ( 2009): ∼ 15 s).Therefore we applied their vapor pressure calibration for approximate evaluation of SOA vapor pressure in this study (Fig. 1).The volatility of glyoxal oligomer was evaluated by generating glyoxal oligomer from evaporating droplets (De Haan et al., 2009b).Glyoxal solution in water (40 wt%) was aerosolized by an atomizer into a 0.6 m 3 Teflon chamber.As water evaporated from the droplet, dihydrated glyoxal lost water to form more reactive monohydrated glyoxal, Introduction

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Full which then self-oligomerized to form low-volatility compounds (De Haan et al., 2009a).
The vaporization profile suggested that the vapor pressure of glyoxal oligomer was much lower than 10 −8 Pa, where reliable vapor pressure measurement is not available.

Chamber experiments
The experimental test matrix is summarized in Table 1.A known volume of high purity liquid hydrocarbon was injected through a heated glass injection manifold system and flushed into the chamber with pure N 2 .Injection of 2-tert-butylphenol and H 2 O 2 was performed in the same way as described in Nakao et al. (2011a).Since phenolic compounds are less volatile than hydrocarbons typically used for chamber experiments, injections into the chambers were carefully performed using a heated oven (50-80 • C) through a heated transfer line maintained at a temperature higher than the oven.The glass manifold inside the oven was packed with glass wool to increase the mass transfer surface area.H 2 O 2 was used as an additional OH radical source to test the role of glyoxal.H 2 O 2 50 wt% solution was injected through the same oven system.Particlefree water vapor was injected using a two-unit system (Warren et al., 2009).Unit one contained Milli-Q water (Millipore, 18.2 MΩ) with submerged heaters to maintain a desired water temperature, which determined the water vapor concentration in the air stream, while unit two contained a 1 µm filter.Purified air was bubbled through the water and then passed through the filter before entering the reactors.Humidity in the reactor was monitored by a humidity and temperature transmitter (VAISALA HMT334).Deliquesced (NH 4 ) 2 SO 4 seed particles were generated by aerosolizing dilute (NH 4 ) 2 SO 4 solution in Milli-Q water by a custom-build atomizer followed by Kr-85 neutralizer (TSI, model 3077) without drying.Seed particles were confirmed to be deliquesced by using VTDMA; evaporation of water from particles was observed by loss of volume after passing particles through a thermodenuder.Particle wall-loss correction was performed by using exponential decay rates of particle number (Carter et al., 2005).Introduction

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Full Significant SOA formation by glyoxal uptake onto deliquesced (NH 4 ) 2 SO 4 was observed (Fig. 2).Wet ammonium sulfate seed particles were injected into the chamber (RH = 74 %) and allowed to equilibrate followed by glyoxal injection.Immediately following the glyoxal injection, the organic/sulfate ratio measured by the AMS increased due to glyoxal uptake, reaching a maximum value of ∼ 0.4 around 6 h after glyoxal injection.
After reaching maximum organic/sulfate ratio, the environmental chamber was diluted; organic/sulfate ratio decreased due to evaporation of glyoxal oligomers, suggesting glyoxal oligomerization is reversible as also observed by Galloway et al. (2009).The slower decrease of glyoxal concentration than tracer concentration is consistent with the large glyoxal-reservoir effect of chamber surface (Loza et al., 2010).In a separate experiment (not shown), no increase in organic/sulfate ratio is observed for a similar experiment conducted under dry conditions (RH < 0.1 %), confirming the critical role of aqueous phase of (NH 4 ) 2 SO 4 seed particles in glyoxal oligomerization (Galloway et al., 2009;Liggio et al., 2005;Kroll et al., 2005).

Evaluation of glyoxal uptake onto toluene SOA
Glyoxal uptake during SOA formation from toluene photooxidation was investigated under humid conditions (RH 40-80 %).A representative toluene photooxidation experiment including toluene decay, SOA formation, and glyoxal formation is shown in Fig. 3. Typically, glyoxal concentration remained below 10 ppb.The impact of glyoxal on SOA formation for the toluene photooxidation system was evaluated by injecting 80 ppb additional glyoxal into the system (Fig. 4).The addition of 80 ppb glyoxal in the toluene + NO x oxidation system resulted in enhanced OH radical concentrations from glyoxal photolysis (confirmed by faster toluene decay) and higher SOA formation (green trace in Fig. 4).To elucidate the role of increased OH radical versus glyoxal Introduction

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Full uptake, another toluene-NO x photooxidation experiment with elevated H 2 O 2 concentration (added to match OH levels in previous experiment) was performed (red trace in Fig. 4).Addition of H 2 O 2 resulted in practically identical toluene decays between the H 2 O 2 and glyoxal experiments indicating successful matching of OH levels in the H 2 O 2 and glyoxal added experiments.For these two experiments, SOA formation was nearly identical, suggesting that the major impact of glyoxal occurred in the gas-phase chemistry with insignificant contributions of glyoxal to SOA formation by direct uptake.The absence of glyoxal uptake onto the toluene SOA was further investigated by addition of glyoxal after SOA formation in the dark as a "SOA seed" experiment (Fig. 5).Blacklights were turned off after ∼ 9 h of irradiation and nearly 120 ppb glyoxal was injected; no significant increase in particle volume concentration was observed.
In addition to particle volume, particle volatility was monitored to evaluate glyoxal uptake onto SOA.The evolution of VFR at 100 • C of toluene SOA was monitored by VTDMA (Fig. 6).The VFR profiles of toluene SOA rapidly increased and plateaued after ∼ 6 h.Addition of glyoxal to the photooxidation system resulted in increased VFR.
One might interpret this as contribution of glyoxal oligomer.However, the addition of H 2 O 2 as a radical source resulted in nearly identical profile, again suggesting that the role of glyoxal in this system was as a radical source, not an oligomer precursor.et al., 2008); lack of these enhancements by sulfate ion and ammonium ion in water associated with SOA can contribute to the lack of reactive uptake of glyoxal onto SOA.

Evaluation of glyoxal uptake onto 2-tert-butylphenol SOA
The absence of significant SOA formation from glyoxal uptake onto toluene SOA is further probed by using 2-tert-butylphenol as a parent aromatic compound.When SOA formed from 2-tert-butylphenol was introduced into the HR-ToF-AMS, significant signals of the C 4 H + 9 fragment from the tert-butyl substituent were observed.Since glyoxal oligomerization can not produce C 4 H + 9 , C 4 H + 9 can be used as a tracer for the SOA from 2-tert-butylphenol oxidation.The phenolic functionality (-OH) was used to enhance the reactivity of aromatic ring (e.g., o-cresol is 7 times more reactive than toluene, Calvert et al., 2002) and to minimize the reaction of the tert-butyl substituent.Although the steric hindrance by the tert-butyl group remains uncertain, adequately similar aromatic oxidation reaction is expected for the purpose of evaluating glyoxal uptake.The result of 2-tert-butylphenol oxidation is shown in Fig. 8. Glyoxal addition to this system during photooxidation (at 10 h after lights on) resulted in enhanced SOA formation; however the fraction of C 4 H + 9 in the total organics (fC 4 H 9 ) did not change significantly indicating that aerosol formation from products not containing C 4 H + 9 fragments (glyoxal and its products) was not enhanced after glyoxal injection and oxidation.This further confirms that glyoxal's influence on SOA formation in the aromatic photooxidation systems under humid conditions (RH 51 % for this experiment) is limited to increasing SOA formation by increasing gas-phase OH radical concentrations and not by reactive uptake of glyoxal into the SOA.Introduction

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Conclusions
The significance of glyoxal uptake in SOA formation from aromatic hydrocarbon photooxidation was evaluated for the first time.Glyoxal uptake onto deliquesced (NH 4 ) 2 SO 4 seed resulted in rapid SOA formation as shown in previous studies; however, no significant glyoxal uptake onto SOA formed from aromatic hydrocarbon oxidation was observed.Instead of contributing to SOA formation by reactive uptake, glyoxal acted as an OH radical source following photolysis.This study suggests that uptake and/or subsequent reaction of glyoxal in aqueous phase to form low-volatility compounds is not favored in the water associated with aromatic SOA up to RH ∼ 80 %.This study highlights the need for evaluating glyoxal uptake onto SOA seed.This study does not preclude glyoxal uptake onto SOA at RH above 80 % or glyoxal cloud processing.Introduction

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Full and references therein).Traditionally, SOA Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | of molecular formula of ion fragments of SOA (e.g., C 4 H + 9 Discussion Paper | Discussion Paper | Discussion Paper | uptake onto deliquesced (NH 4 ) 2 SO 4 Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4 ) 2 SO 4 seed on toluene SOA formation Deliquesced (NH 4 ) 2 SO 4 was confirmed to rapidly form SOA in the presence of glyoxal (Fig. 2).If glyoxal is a major reaction intermediate in toluene SOA formation, the presence of deliquesced (NH 4 ) 2 SO 4 seed particles is expected to enhance the toluene SOA formation significantly.SOA growth curves (SOA formation vs. hydrocarbon consumption) for non-seeded (nucleation) experiments and deliquesced (NH 4 ) 2 SO 4 seed experiments are shown in Fig. 7.No significant difference in those two systems was observed, which is attributed to the formation of a condensed secondary organic coating inhibiting the rapid glyoxal uptake onto deliquesced (NH 4 ) 2 SO 4 .Previous studies reported enhanced partitioning of glyoxal into water with the presence of sulfate ion (Discussion Paper | Discussion Paper | Discussion Paper | et al., 2009) and catalytic effect of ammonium ion on glyoxal oligomerization (Nozi ère Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 1 .
Fig. 1.Thermograms of hexanedioic acid, decanedioic acid, and glyoxal oligomer (produced from evaporating droplets of glyoxal/water solution) where T TD is the set temperature of the thermodenuder and P 25 is the vapor pressure of a compound at 25 • C.