changes in light absorptivity of molecular weight...

Changes in Light Absorptivity of Molecular Weight Separated BrownCarbon Due to Photolytic AgingJenny P. S. Wong,*,† Athanasios Nenes,†,‡,§,⊥ and Rodney J. Weber†

†School of Earth and Atmospheric Sciences and ‡School of Chemical and Biomolecular Engineering, Georgia Institute of Technology,Atlanta 30331, United States§Institute of Chemical Engineering Sciences, Foundation for Research and Technology-Hellas, Patras GR-26504, Greece⊥Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Palea Penteli GR-15236,Greece

*S Supporting Information

ABSTRACT: Brown carbon (BrC) consists of those organiccompounds in atmospheric aerosols that absorb solar radiationand may play an important role in planetary radiative forcing andclimate. However, little is known about the production and lossmechanisms of BrC in the atmosphere. Here, we study how thelight absorptivity of BrC from wood smoke and secondary BrCgenerated from the reaction of ammonium sulfate withmethylglyoxal changes under photolytic aging by UVA radiationin the aqueous phase. Owing to its chemical complexity, BrC isseparated by molecular weight using size exclusion chromatog-raphy, and the response of each molecular weight fraction toaging is studied. Photolytic aging induced significant changes inthe light absorptivity of BrC for all molecular weight fractions;secondary BrC was rapidly photoblenched, whereas for woodsmoke BrC, both photoenhancement and photobleaching were observed. Initially, large biomass burning BrC molecules wererapidly photoenhanced, followed by slow photolysis. As a result, large BrC molecules dominated the total light absorption of agedbiomass burning BrC. These experimental results further support earlier observations that large molecular weight BrCcompounds from biomass burning can be relatively long-lived components in atmospheric aerosols, thus more likely to havelarger impacts on aerosol radiative forcing and could serve as biomass burning tracers.

1. INTRODUCTION

Organic aerosols (OA) are a major component of fine ambientparticles and affect the Earth’s radiative balance by directlyinteracting with solar radiation or indirectly via theirinteractions with clouds. These aerosol effects on climaterepresent the largest uncertainty in global radiative forcingassessments.1 While OA was originally thought to only scattersolar radiation, recent studies demonstrate that components inOA can absorb UV−visible radiation.2 This class of lightabsorbing OA, collectively termed brown carbon (BrC), canpotentially shift the direct radiative forcing of OA from netcooling to net warming.3,4 Additionally, modeling studies haveobserved that absorption of near UV solar radiation by BrC canresult in decreased photolysis rates for NO2 and O3, indicatingthat BrC can influence tropospheric photochemistry.5,6

Characterizing the sources and aging processes of BrC iscritical to evaluate its atmospheric impacts and to understandthe persistent signatures in biomass burning aerosols.Multiple sources of BrC have been identified, including

emissions from biomass burning,7−9 fossil fuel combustion,10,11

and release of biogenic matter, such as soils and bioaerosols.12

While many studies have established that biomass burning is

likely to be an important source of atmospheric BrC, only asmall fraction of organic chromophores has been identified,such as nitrophenols.13−16 Production of secondary BrC inaerosols and clouds has also been proposed.13,17 Althoughsecondary BrC formation from the reactions of carbonyl oraromatic compounds with nitrogen-containing compounds hasbeen studied extensively in the laboratory,13 its contribution toatmospheric BrC remains unclear. The emissions profile of BrCis poorly understood, but how aging modulates BrC levels andproperties in the atmosphere is still unclear. Part of this limitedunderstanding arises from the low mass fraction ofchromophores in the organic aerosol, as well as the uncertainand complex nature of their chemical identity.13

Most studies that have investigated BrC aging focused onsecondary BrC, which was observed to undergo rapidphotobleaching with atmospheric lifetimes on the order ofminutes to several hours.18−21 Despite the growing evidence

Received: April 19, 2017Revised: June 8, 2017Accepted: June 22, 2017Published: June 22, 2017

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that aged secondary BrC rapidly photobleaches in theatmosphere, laboratory studies investigating the effects ofaging on primary BrC have observed that biomass burning BrCcan undergo both photoenhancement and photobleach-ing.20,22,23 The results from these studies illustrated thedynamic nature of biomass burning BrC due to aging, butthe mechanisms leading to these observations remain unclear.For example, it is unknown whether all classes of compounds inbiomass burning BrC respond to photolytic aging in the samemanner (e.g., initial photoenhancement followed by photo-bleaching), or that different classes of compounds exhibitdifferent photolytic aging effects, or a combination thereof.The effects of aging on biomass burning BrC have also been

observed from ambient measurements. By following theevolution of a biomass burning plume in Western U.S.A.,airborne observations suggested that while the majority ofprimary BrC from biomass burning have short atmosphericlifetimes of 9 to 15 h, a persistent fraction may remain evenafter 50 h following emission, although the conclusion isuncertain since there are few data points for more aged BrC(>20 h).24 Other ambient measurements of aged (approx-imately 2 days of atmospheric transport) biomass burningaerosols indicated that large molecular weight organiccompounds contributed significantly to the total organicaerosol mass25 and total light absorption.26 Collectively, theseobservations suggest that atmospheric aging of biomass burningBrC decreases the light absorptivity of smaller chromophoresconsiderably more than for larger chromophores. Largerchromophores may therefore be the most persistent BrCspecies in the atmosphere, hence most influential for perturbingthe planetary radiative balance.27 The atmospheric processesleading to these observations remain unknown, and it is unclearwhether the reactivity of secondary BrC is also dependent onits molecular weight.The objective of this study is to systematically investigate the

effects of photolytic aging on the light absorptivity of differentmolecular weight BrC components. Size exclusion chromatog-raphy was coupled to UV−vis absorption spectroscopy in orderto characterize the molecular weight distributions ofchromophores in different types of BrC and to determinetheir photolytic reactivity. The photolysis of two types of BrCwere investigated: primary BrC from pyrolysis-generated woodsmoke emissions and secondary BrC generated from thereaction of ammonium sulfate with methylglyoxal (AS-MGL).Results demonstrated that both types of BrC undergosignificant changes in their optical and molecular weightsproperties due to photolytic aging. Rapid photobleaching wasobserved for AS-MGL BrC, whereas initial photoenhancement,followed by photobleaching, was observed for primary BrCfrom wood smoke emissions. These contrasting observationsillustrate that the atmospheric evolution of BrC is highlyvariable and dynamic.

2. EXPERIMENTAL METHODS2.1. Preparation of BrC Samples. Wood smoke BrC

samples, chosen to represent biomass burning BrC, weregenerated in the laboratory via controlled wood pyrolysis usingthe method of Chen and Bond that simulates the thermaldecomposition of solid organic fuel during biomass burning.28

An electronically heated combustor, with an internal volume of950 cm3, was continually flushed with 2000 sccm of N2 gas,where the lack of oxygen suppresses black carbon formationduring wood pyrolysis. For each pyrolysis event, a rectangular

piece of dry hardwood (cherry of size 3 × 2 × 2 cm,approximately 5 g) was placed in the bottom center of thecombustor, where the exterior temperature was measured. Thesmoke stream was further diluted by HEPA-filtered air (1500sccm) in a mixing volume (0.01 m3), following which particleslarger than 1.0 μm were removed using an impactor. Once thecombustor reached 210 °C, the emitted organic carbon wascollected on polytetrafluoroethylene filters (47 mm, 2 μm poresize, Pall Corporation) at 3500 sccm for 100 min. Theseconditions represent the smoldering phase of the combustionprocess. Some low volatility components of the smoke emissionmay not be measured by this method as a viscous substance wasobserved to accumulate on the tubing walls. Immediately aftercollection, the filters were stored in a freezer at −10 °C. Priorto each photolysis experiment, water-soluble BrC (WS BrC)was extracted from one particle filter by adding 15 mL ofpurified water (18.2 mΩ) in a sealed glass vial and sonicated for60 min. After the water extract was removed, 15 mL ofmethanol (HPLC grade, Merck) was added and sonicated for60 min to extract the water-insoluble BrC (WI BrC). Eachextract was filtered using a new 0.2 μm PTFE syringe filter(Fisher).Ammonium sulfate-methylglyoxal (AS-MGL) BrC was

prepared using a similar method employed by previouslaboratory studies, which simulates BrC formed by secondaryprocesses.18,20,29 The AS-MGL stock solution was prepared bycombining 98 mL of an aqueous solution of ammonium sulfate(Fisher Scientific) and 3 mL of methylglyoxal (Sigma-Aldrich,40% in water) in sealed amber bottles. The final concentrationsin the stock solution were ∼1.5 M of ammonium sulfate and∼0.17 M of methylglyoxal. The resulting solution was kept inthe dark at room temperature for 10 days. During this period oftime, the color of the solution turned dark yellow/brown froma pale yellow color. Prior to each photolysis experiment, thestock solution was diluted by a factor of 7.

2.2. Photolysis of BrC. All photolysis experiments wereconducted in a photoreactor, with a slowly rotating vial rack (4rpm, 40 vials capacity) placed in the center that was surroundedby 8 UVA lamps (F-25T8BL, Sylvania) and maintained close tonear-room temperature by continuous chamber ventilation withtwo fans. With all the UVA lamps turned on, the temperatureinside the photoreactor increased by 6° (from 24 to 30 °C).The integrated photon flux inside the photoreactor wascharacterized by chemical actinometry using 2-nitrobenzalde-hyde,30 and the wavelength dependent photon flux was directlymeasured using a spectroradiometer (StellaNet Inc.). Thechemical actinometry method is discussed in Section S1, andthe photon fluxes determined using both approaches are shownin Figure S1 (Supporting Information). Most of the radiationemitted by the UVA lamps fell in the 300−400 nm range with amaximum at 355 nm.30

Photolysis experiments using wood smoke BrC and AS-MGLBrC were conducted separately. For each experiment, multiple2 mL borosilicate glass vials (sealed with Telfon-lined caps),each containing 0.75 mL of the filter extract or dilute solution,were placed on the rotating vial rack. At different illuminationtimes, one vial was removed for offline measurements(discussed below). For the wood smoke BrC samples, filterextracts were illuminated up to 130 h in the photoreactor andup to 40 h for AS-MGL BrC. To ensure reproducibility,photolysis experiments using wood smoke BrC and AS-MGLBrC were repeated four and five times, respectively. Addition-ally, control experiments were conducted; no changes in BrC

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DOI: 10.1021/acs.est.7b01739Environ. Sci. Technol. 2017, 51, 8414−8421

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properties were observed when the vials were completelycovered by aluminum foil (i.e., exposed to only the elevatedtemperature conditions and not UVA radiation).2.3. BrC Measurements. Changes in the water-soluble

organic carbon (WSOC) concentration due to photolysis weremonitored offline using a Sievers Total Organic Carbon (TOC)Analyzer (Model 900, GR Analytical Instruments). TOCmeasurements were conducted using the bulk BrC samples(i.e., not molecular weight separated), since the use of organiccompounds in the eluent for the chromatographic molecularweight separation technique (discussed below) resulted in veryhigh background signals. Additionally, quantification of WI-BrCwas not possible due to the use of methanol as an extractionsolvent. The TOC analyzer was routinely calibrated usingsolutions of dissolved sucrose of known concentrations. BrCsamples were diluted by up to a factor of 1000 to ensure themeasured TOC concentrations were in the linear responserange of the instrument. From the TOC measurements, eachsample vial (i.e., 0.75 mL of filter extract or dilute solution) ofunreacted WS smoke BrC contained 342 ± 91 μg of WSOCand for the unphotolyzed AS-MGL BrC 386 ± 40 μg ofWSOC.Changes in molecular weight distributions of BrC due to

photolysis were measured using a high performance liquidchromatography (HPLC) system (GP40 pump with AS40autosampler, Dionex), equipped with a size exclusionchromatography (SEC) column (discussed below), coupledto an UV−vis spectrometer, consisting of a liquid waveguidecapillary (1 m optical path-length, World Precision Instru-ment), a deuterium tungsten halogen light source (DT-Mini-2,Ocean Optics), and an absorption spectrometer (USB4000,Ocean Optics) that continuously monitored all wavelengthsbetween 200 and 800 nm. The long optical path length waschosen to increase detection sensitivity.Separations were achieved by operating an aqueous size

exclusion/gel filtration chromatography column (Polysep GFCP-3000, Phenomenex). Briefly, separation by size exclusionchromatography (SEC) is controlled by differences in theextent of permeation into the pores of the column packingmaterial by analyte molecules, where larger molecules areeluted first due to weaker interactions with the packing materialcompared to smaller molecules.31 The chromatographicmethod used is similar to that developed by Di Lorenzo andYoung for the analysis of atmospheric particles;26 however, thecomposition of the mobile phase was modified to optimize theseparation of weakly interacting molecules. The chromatog-raphy system was operated in isocratic mode using a 90:10 v/vmixture of water and methanol with 25 mM ammonium acetateas the mobile phase, at a flow rate of 1 mL/min and a sampleinjection volume of 20 μL. Ammonium acetate, a pH buffer,was added to the mobile phase to minimize electrostaticinteractions between the analytes and the column, which caninterfere with the column’s ability to separate by molecular size.If electrostatic interactions are negligible, SEC separatesanalytes based solely on their hydrodynamic volume, which isa function of both molecular weight and density of thecompound.32,33 The relationship between elution volume andmolecular weight was empirically determined using thefollowing standards with known molecular weights (Sigma-Aldrich): blue dextran (2 M Da), bovine serum albuminum (66kDa), horseradish peroxidase (44 kDa), myoglobin (16.9 kDa),lysozyme (14.3 kDa), apotinin (6.5 kDa), tannic acid (1.7kDa), vitamin B12 (1.4 kDa), dichlorofluorescene (401 Da),

uridine (244 Da), and 2-nitrobenzaldehyde (151 Da). Thecalibration curve is shown in Figure S2 (SupportingInformation), where the linear region of the relationshipbetween elution volume and molecular weight represents therange of molecules that had weak interactions with the packingcolumn material. This calibration method only providesestimates of the molecular weights for BrC compounds sinceit remains unknown whether the molecular densities of thestandards are representative of that of the BrC molecules ofinterest.

3. RESULTS AND DISCUSSION3.1. Wood Smoke BrC. The change in water-soluble

organic carbon (WSOC) concentration in smoke BrC uponUV irradiation is shown in Figure 1a. Decreases in WSOC due

to photolysis were observed, resulting in a net loss in 30% ofWSOC after 125 h of UV exposure. Absorption of UV radiationby chromophores can initiate photolysis, leading to theformation of products having higher volatility (e.g., fewercarbon numbers). Evaporation of these volatile products canlead to the observed loss in WSOC. In addition, the loss ofWSOC due to photolysis exhibited an initial decay (i.e., first 8 hof UV exposure) that was rapid, followed by a slower decay,suggesting that WS smoke BrC contains multiple chromo-phores of varying degrees of photolability.In addition to changes in WSOC, changes in the absorption

per mass of water-soluble carbon (mass absorption coefficient,MAC) provide insight into the effects of photolysis on the lightabsorptivity of water-soluble chromophores. The calculationmethod for MAC at 365 and 400 nm is discussed in Section S2.Shown in Figure 1b, exposure to UV light leads to an initialincrease in MAC values at both wavelengths, indicatingphotoenhancement (i.e., increased absorptivity of near UV−vis radiation by BrC). Given that a loss in WSOC was observedduring this photoenhancement period, we speculate that thephotolysis of WS smoke BrC leads to the formation of productsof higher volatility that evaporate to the gas phase, as well asproducts that remain in the aqueous BrC solution, but are morelight absorbing. Previous studies have shown that aqueous-phase photo-oxidation of phenolic compounds34−37 and nitro-aromatic compounds,20 both of which have been identified inbiomass burning organic aerosols,38−40 lead to increasedabsorption of near UV−vis radiation. The proposed mecha-nisms leading to the increased absorption were attributed to the

Figure 1. Time series profile of the (a) changes in WSOCconcentration (normalized to initial values) and (b) WSOC massabsorption coefficients at 365 nm (black circles) and 400 nm (redsquares) for the photolysis of WS smoke BrC. The error bars representthe variability (±1σ) of multiple experiments (n = 4).



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polymerization of phenolic compounds35,37 and OH-function-alization of nitroaromatric compounds.20 Additionally, photo-enhancement has been previously observed for aged biomassburning BrC emitted from the pyrolysis of hickory, pine, andoak wood,22,23 as well as from the combustion of kaoliangstalk.20 After this initial period of photoenhancement (up to 20h), continual exposure to UV lights led to the photobleaching,behaviors previously observed from the aforementionedstudies.20,23

In addition to total light absorptivity, the molecular weightdistributions of BrC (provided by SEC) offer additional insightson the molecular nature of chromophores and the effects ofphotolysis. Typical image plots of the molecular weightseparated BrC absorption spectra from the WS and water-insoluble (WI) components of wood smoke are shown inFigure 2. Two main populations of chromophores can be

observed for unreacted BrC smoke: highly absorbing largechromophores and less absorbing smaller chromophores.Comparison of light absorption by the unreacted WS and WIcomponents indicated that the majority of light absorption canbe attributed to water-soluble chromophores, at all illuminationtimes (shown in Figure S3). On average, WI BrC contributed23 ± 9% of the total light absorption at 365 nm by wood smokeBrC (i.e., sum of absorption by both WS and WI BrC). Whilethe discussion below primarily focuses on the results from thephotolysis of WS BrC, similar trends in results were observedfor the WI BrC component (shown in Figure S4).To illustrate the evolution of chromophores with different

molecular weights, the changes in the total absorption at 365nm (Abs365) for different molecular weight fractions are shownin Figure 3. Here the total Abs365 is binned according to thestrength of interaction with the column packing material, wherea high molecular weight fraction (high-MW) is defined aschromophores that had weak interactions with the SEC column(i.e., the linear region of the calibration curve shown in FigureS2) and have approximate molecular weights between 66 kDaand 401 Da. The small molecular weight fraction (small-MW)is defined as chromophores that had strong interactions withthe SEC volume and have approximate molecular weights

smaller than approximately 400 Da. Note that the molecularweight values are only estimates, as it remains unknownwhether the molecular densities of the calibration standards arerepresentative of the molecular densities of BrC molecules. Forboth molecular weight fractions, initial photoenhancement wasobserved, followed by photobleaching with prolonged UV lightexposure. These initial increases and subsequent decays inabsorption by different molecular weight fractions exhibitedfirst-order kinetics. Shown in Table 1, the rates of photo-

enhancement (kpe) were determined by fitting first-ordergrowth curves to the first 4 h of absorption data. Forphotobleaching rates (kpb), first-order decay curves were fittedto the initial decay in absorption (e.g., between 20 and 52 h ofUV exposure for WS BrC and between 8 and 40 h of UVexposure for WI BrC), where kpb represents the rate of decayfor more photolabile species. In general, photoenhancementwas more significant for the high-MW fraction of smoke BrC,whereas the kinetics of photobleaching is similar for both high-MW and low-MW fractions. Faster photoenhancement by thehigh-MW fraction may be due to these chromophores beingmore photoreactive (e.g., larger absorption cross sections and/or quantum yields) compared to chromophores in low-MWfraction. Comparison of the photoreactivity of WS and WI BrCsuggests that WI BrC of all molecular weight fractionsundergoes more rapid photoenhancement during the first 4 h

Figure 2. Typical molecular weight separated absorption spectra ofunreacted water-soluble (WS) smoke BrC (top) and water-insoluble(WI) smoke BrC (bottom). Arrows indicate the elution volumes (Ve)of some calibration standard: bovine serum albumin (Ve = 7.6 mL, 66kDa), aprotinin (Ve = 10.3 mL, 6.5 kDa), and dichlorofluorescene (Ve= 15.1 mL, 401 Da). Note that molecular weight increases withdecreasing elution volume.

Figure 3. Time series profile of the change in absorption at awavelength of 365 nm for the high molecular weight (red circles) andlow molecular weight (black triangles) fractions in WS smoke BrC dueto photolysis. The insert is a zoomed-in view of the changes observedat longer UV illumination times.

Table 1. Observed Rate Constants for thePhotoenhancement (kpe) and Photobleaching (kpb) ofWater-Soluble (WS) Smoke BrC, Water-Insoluble (WI)Smoke BrC, and Ammonium Sulfate-Methylglyoxal (AS-MGL) BrC at 365 nma

smoke BrC fraction kpe (s−1) kpb (s

−1)

WS high-MW (9.2 ± 1.4) × 10−5 (1.5 ± 0.6) × 10−5

low-MW (5.3 ± 1.5) × 10−5 (1.8 ± 0.4) × 10−5

WI high-MW (2.0 ± 1.2) × 10−4 (1.2 ± 0.7) × 10−5

low-MW (1.7 ± 1.0) × 10−4 (2.8 ± 2.0) × 10−5

AS-MGL BrC fraction kpe (s−1) kpb (s

−1)

high-MW (8.2 ± 2.4) × 10−5

low-MW (1.6 ± 0.4) × 10−4

aRate constants for the photoenhancement of smoke BrC weredetermined from the first 4 h of UV exposure, and photobleaching rateconstants were determined between 8 and 40 h of UV exposure for WIBrC and between 20 and 52 h of UV exposure for WS BrC. Rateconstants for the photobleaching of AS-MGL BrC were determinedfrom the first 4 h of UV exposure. Uncertainties in the rate constantsrepresent the variability (±1σ) between multiple experiments.



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of UV light exposure (Figure S4). Additionally, maximumabsorption by the WI BrC was observed at this time, while WSBrC exhibited longer photoenhancement (up to 20 h of UVlight exposure). Note that the concentration of organic carbonin WI BrC was not quantified in this study (see Section 2.3).Owing to their rapid photoenhancement, chromophores in

the high-MW fraction were persistent and dominated total lightabsorption at 365 nm (with an increasing contribution with UVexposure; Figure S5). This result is consistent with previousambient measurement of molecular weight separated agedbiomass burning organic aerosols (from a boreal forest fire),where the majority of water-soluble BrC absorption wasattributed to molecules larger than 500 Da.26 It is possible thatthe allocation of a molecular weight cutoff value foratmospherically stable chromophores is dependent on thebiomass fuel type and burning conditions, as emissions areknown to depend on both factors.41

3.2. AS-MGL BrC. Although the previous set of experimentsdemonstrated that the effects of photolytic aging on the lightabsorbing properties of BrC from wood smoke are dependenton the molecular weight of its components, it remains unclearwhether other types of BrC exhibit this type of behavior. Forthe current study, we examined the changes in molecularweight distributions due to the photolysis of chromophoresgenerated from the reaction of ammonium sulfate andmethylglyoxal (AS-MGL BrC), as this reaction system iscommonly used as laboratory surrogates of secondaryBrC.18,20,29,42,43

Shown in Figure 4, two populations of chromophores wereobserved for unphotolyzed AS-MGL BrC: a population of

larger chromophores that strongly absorbs radiation at 365 nmand a less absorbing population of smaller chromophores.Upon exposure to UV lights, rapid photobleaching wasobserved for all chromophores (Figure 5).Rate constants for the photobleaching of AS-MGL BrC were

determined by fitting the observed Abs365 during the first 4 h ofUV exposure to first-order decay curves (Table 1). Similar towood smoke BrC, the change in light absorption due tophotolytic aging exhibited a molecular weight dependence,where the fastest decay was observed for the smallestmolecules. Additionally, the rate of absorption decay decreaseswith time, suggesting that this type of BrC containschromophores with different photoreactivity. To date, studiesinvestigating the photolysis of this type of BrC have notobserved photoenhancement.18,20

Observed decay rates for bulk absorption (i.e., sum of allmolecular weight fractions) at 400 nm for the current study[(1.2 ± 0.1) × 10−4 s−1] resulted in a half-life of 95 min againstphotolysis (Figure S6). This value is generally consistent withphotolysis half-life determined by Zhao et al. using bulkabsorbance measurements (∼13 min), considering differencesin the following experimental conditions between the twostudies: concentrations of BrC precursors and the photon fluxesinside the respective photoreactors.20

4. ATMOSPHERIC IMPLICATIONSUVA light exposure of BrC molecules led to significant changesin their light absorptivity and molecular weight distributions;the extent of photoenhancement and photobleaching dependedon the molecular weight fraction and source of BrC. Inparticular, the largest molecules in biomass burning BrC (i.e.,high-MW fraction) contributed to the majority of total lightabsorption, due to rapid photoenhancement of these molecules.These results indicate that molecular weight separatedtechniques, such as SEC, can be useful tools to elucidate theaging mechanisms of large molecular weight substances in theatmosphere. However, the molecular weights of BrC reportedin this work are only approximate values, as the accuracy of theSEC calibration approach depends on whether the moleculardensities of the calibration standards are representative of thatof the BrC molecules. Further work to verify the molecularweights of BrC, such as coupling SEC-UV absorptionspectroscopy with light scattering techniques, which havebeen employed to determine the absolute molecular weights oflignin and its byproducts,44,45 is warranted.From the observed decay rate for the high-MW fraction of

WS BrC, the initial atmospheric lifetime with respect tophotolysis is estimated to be approximately 14−36 h at solarnoon (calculation method discussed in Section S3). Given thatthe average lifetime of particles in the atmosphere isapproximately 1 week with respect to deposition, this veryrough estimate suggests that large water-soluble BrC moleculesfrom biomass burning could remain throughout the majority ofthe particles’ lifespan and so could be ubiquitous in theatmosphere, as observed.27 We stress that there areuncertainties in this estimate, as it assumes that the photolysisquantum yield is wavelength-independent and that photolysisof BrC in the atmosphere is restricted to the wavelength rangeconsidered (300−400 nm). The wavelengths responsible forBrC photolysis (i.e., photolysis quantum yields) are currentlyunknown.Nonetheless, these experimental results further support

earlier observations that large molecular weight BrC speciesfrom biomass burning can be long-lived components in

Figure 4. An image plot of the molecular weight separated absorptionspectra of unphotolyzed AS-MGL BrC. Arrows indicate the elutionvolumes (Ve) of some calibration standard: bovine serum albumin (Ve= 7.6 mL, 66 kDa), aprotinin (Ve = 10.3 mL, 6.5 kDa), anddichlorofluorescene (Ve = 15.1 mL, 401 Da). Note that molecularweight increases with decreasing elution volume.

Figure 5. Time series profile of the 365 nm wavelength absorptionchange compared to initial values for the high-molecular weight (redcircles) and low molecular weight (black triangles) fractions in AS-MGL BrC due to photolysis.



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atmospheric aerosols.25,26 This means they are more likely tohave larger impacts on aerosol direct radiative forcing onregional scales, whereas the contribution of smaller species toBrC, either emitted from biomass burning or formed fromsmall carbonyl compounds, are likely to be most important nearsource regions. In addition, the observed rapid photoenhance-ment of water-soluble biomass burning BrC suggests thatsecondary production of BrC in atmospheric aqueous media(e.g., wet aqueous, fog and cloud droplets) can be an importantsource of BrC in the atmosphere. In particular, Gilardoni et al.recently reported ambient observations of light absorbingsecondary organic aerosol formation from the processing ofbiomass burning emissions in the aqueous phase.17

Also, we note that the majority of total light absorption at365 nm observed in this study was contributed by the water-soluble component of wood smoke BrC (77 ± 9%), which isconsistent with the observations by Di Lorenzo and Young.26

However, dominant contributions to total light absorption at365 nm by BrC extractable in methanol or acetone wereobserved in the atmosphere,14,46 as well as from laboratorygenerated smoke BrC from the pyrolysis of pine and oak.28

These differences may be due to fuel type and burn conditions,or that only primary smoke aerosol, in isolation from otheratmospheric species, was studied here. Additionally, the samplepreparation approach employed in this study (sequential filterextraction with water, followed by methanol) does not take intoaccount the contribution of water-insoluble BrC compounds onsuspended particles that may have been removed duringfiltration of the water sample extract. Given these contrastingresults, additional work investigating the relative contributionsof water-soluble and insoluble BrC using a wide range of BrCprecursors and burn conditions is necessary.While our results continue to support the view that the

majority of AS-MGL BrC undergo rapid photobleaching, asmall fraction of these chromophores may persist in theatmosphere (e.g., in Figure 5, 5−10% of the initial absorptionby AS-MGL BrC remains after 40 h of UV exposure). As such,it is important to quantify the relative contribution of differentsources to background BrC. We note that not all secondaryBrC undergo rapid photobleaching, as the atmospheric lifetimeof secondary BrC formed from the photo-oxidation ofnaphthalene (under high NOx-conditions) has been estimatedto be approximately 20 h.19

Further investigations on the effects of other atmosphericaging processes on the light absorptivity and chemicalcomposition of different molecular weight BrC fractions areneeded. In particular, the current study examined the photolyticaging of BrC dissolved in bulk solutions. This type of approachdoes not simulate aging processes occurring on or withinsuspended particles, where parameters such as gas-particlecollision frequencies, aerosol phase state, and solute concen-trations (including pH) are different. For example, the effects ofaging processes such as cloud/fog droplets evaporation47 andheterogeneous reactions48 on the physio-chemical properties ofBrC have been demonstrated.Sources of ambient fine particle OA remains an open

question since the components all tend to evolve to a similarhighly oxygenated state49,50 and specific chemical sourcetracers, including those for biomass burning, can have aconsiderably shorter atmospheric lifetimes than aerosol.51−53

Because of this, the mass fraction of aerosol attributed tobiomass burning may be grossly underestimated for agedaerosol, leading to the view that biomass burning may be a

much more important contributor to global than currentlybelieved.53,54 The unique stability of the high-MW fraction ofBrC may provide an alternative to traditional biomass burningmarkers and enable a better estimate of the true impact ofbiomass burning emissions on the atmospheric aerosol burden.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.est.7b01739.

Experimental procedure for the determination of photonflux inside the photoreactor; details on the calculation ofmass absorption coefficients; method to convertobserved decay rates to equivalent atmospheric lifetimes;six supporting figures (photon flux inside photoreactor;molecular weight vs retention time calibration curve;evolution of light absorption by WS and WI-BrC fromwood smoke; time series of the change in absorption fordifferent molecular weight fractions of WS and WI woodsmoke BrC; time series of the change in WSOC andmass absorption coefficients of AS-MGL BrC at 365 and400 nm; table listing the estimated atmospheric lifetimesof BrC (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Phone: 404-894-1750. Fax: 404-894-5638. E-mail: [email protected].

ORCIDJenny P. S. Wong: 0000-0002-8729-8166NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFunding for this work was provided by the Electric PowerResearch Institute (EPRI) through contract #00-10003806.Additional support was also provided by NASA throughcontract NNX14A974G. A.N. acknowledges support from aGeorgia Power Faculty Scholar chair and a Johnson FacultyFellowship.

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changes in light absorptivity of molecular weight...

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