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Sources, Composition and Absorption Ångström Exponent of Lightabsorbing Organic Components in Aerosol Extracts from the Los Angeles Basin Xiaolu Zhang,†,§,* Ying-Hsuan Lin,‡ Jason D. Surratt,‡ and Rodney J. Weber† †

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Department of Environmental Sciences and Engineering, Gillings School of Global Public Heath, University of North Carolina, Chapel Hill, North Carolina 27599, United States

‡

S Supporting Information *

ABSTRACT: We investigate the sources, chemical composition, and spectral properties of light-absorbing organic aerosol extracts (i.e., brown carbon, or BrC) in the Los Angeles (LA) Basin during the CalNex-2010 field campaign. Light absorption of PM2.5 water-soluble components at 365 nm (Abs365), used as a proxy for water-soluble BrC, was well correlated with water-soluble organic carbon (WSOC) (r2 = 0.55−0.65), indicating secondary organic aerosol (SOA) formation from anthropogenic emissions was the major source of water-soluble BrC in this region. Normalizing Abs365 to WSOC mass yielded an average solution mass absorption efficiency (MAE365) of 0.71 m2g−1C. Detailed chemical speciation of filter extracts identified eight nitro-aromatic compounds that were correlated with Abs365. These compounds accounted for ∼4% of the overall watersoluble BrC absorption. Methanol-extracted BrC in LA was approximately 3 and 21 times higher than water-soluble BrC at 365 and 532 nm, respectively, and had a MAE365 of 1.58 m2g−1C (Abs365 normalized to organic carbon mass). The water-insoluble BrC was strongly correlated with ambient elemental carbon concentration, suggesting similar sources. Absorption Ångström exponent (Åa) (fitted between 300 and 600 nm wavelengths) was 3.2 (±1.2) for the PILS water-soluble BrC measurement, compared to 4.8 (±0.5) and 7.6 (±0.5) for methanol- and water-soluble BrC from filter extracts, respectively. These results show that fine particle BrC was prevalent in the LA basin during CalNex, yet many of its properties and potential impacts remain unknown.

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INTRODUCTION Atmospheric aerosols play a critical yet poorly understood role in determining the radiative balance of the Earth.1 Of particular importance is the role played by light-absorbing aerosols, as they influence the distribution of heat between the surface and atmosphere and overall cause a warming effect on climate.2 Light absorbing aerosol constituents include black carbon (BC), mineral dust components, and certain organic compounds (often referred to as “brown” carbon). Differing from BC that absorbs strongly over a wide wavelength range in the UV/visible spectrum, mineral dust and brown carbon (BrC) both have strong absorption-wavelength dependence and absorb more strongly toward shorter wavelengths (i.e., near UV). Such wavelength dependence can be described with an absorption Ångström exponent (Åa) that is significantly larger than one.3 Organic aerosols (OA) are ubiquitous in the atmosphere, often comprising more than half of the fine aerosol mass in both urban and rural environments.4 Over regions with high OA loadings, BrC could have a significant impact on photochemistry and regional climate.5−9 Ambient observations and laboratory studies show that BrC has multiple sources. It can be directly emitted from primary sources such as © 2013 American Chemical Society

incomplete and smoldering combustion of both fossil and biomass fuels.10−14 There is also evidence from laboratory studies for BrC as a secondary organic aerosol (SOA) component produced from a variety of chemical aging processes that often involve nitrogen.15−22 Along with these heterogeneous reaction routes, BrC can also be formed in the gas phase and then partition to condensed phase under dry conditions, especially from anthropogenic SOA precursors. Chamber studies show that brown SOA is formed directly and rapidly from photooxidation of aromatics (i.e., toluene) under high-NOx conditions, whereas oxidation of biogenic volatile organic compounds (VOCs) (e.g., α-pinene) forms little BrC under similar experimental conditions.23−25 Despite its importance, the prevalence and optical role of BrC are not well characterized, largely due to measurement limitations. Optical instruments (e.g., aethalometer) measure the light absorption by all aerosol components at a fixed number of wavelengths.26−28 Since BC is the dominant Received: Revised: Accepted: Published: 3685

December 12, 2012 March 8, 2013 March 18, 2013 March 18, 2013 dx.doi.org/10.1021/es305047b | Environ. Sci. Technol. 2013, 47, 3685−3693

Environmental Science & Technology

Article

duration of the study. The measurement uncertainty of the PILS-TOC system was 8%, estimated based on the combined uncertainties associated with various flow rates and background variability.41 The limit of detection (LOD) for WSOC was ∼0.1 μg m−3. Possible losses of semivolatile components during PILS collection could lead to an under-estimation of WSOC and water-soluble BrC. Complete light absorption spectra between wavelengths of 200 and 800 nm of the PM2.5 aqueous extracts were recorded with a UV/vis spectrometer (Ocean Optics, Dunedin, FL) at a time resolution of 15 min, and the absorption coefficients at 365 and 700 nm were recorded every 1 min. The UV/vis spectrometer reports the light absorption as the log10 of intensity of transmitted (I) to incident (I0) light at a given wavelength (Aλ), where Aλ = −log10(I/I0). The light absorption coefficient of the solution at a given wavelength (Absλ) can be calculated using eq 1.10

absorber in the atmosphere, the presence of BrC is often inferred from small deviations in the measured light absorption from what is expected for BC, and thus is subject to a high level of uncertainty. Further uncertainty arises due to other possible confounding factors such as absorption enhancement by coatings formed over BC cores.29−31 These measurement uncertainties may lead to over- or under-estimation of the presence of BrC. Alternatively, chromophores that are responsible for light absorption by BrC can be unambiguously quantified by measuring the light absorption of dissolved particles in solvents (e.g., water, methanol, or hexane)10,32,33 and complete absorption spectra obtained. This type of aqueous extract BrC measurements does not suffer from interference from light absorption by BC and mineral dust as these insoluble particles are removed from the solution. However, conversion from optical properties of solution back to aerosol is not necessarily straightforward. Analysis of water-soluble BrC data collected in urban environments with different dominant emission sources suggests that studying light-absorbing organics provides important insights on the sources and formation processes of SOA.34 Previous studies have also suggested that the contribution of BrC to the UV absorption could be significant.9,35,36 In this study, we report results on the sources, chemical composition, and spectral properties of ambient BrC chromophores in aerosol solution extracts from combined in situ and offline measurements conducted during the 2010 CalNex field campaign at two sites in the Los Angeles (LA) Basin, a region that is heavily impacted by both primary mobile source emissions and secondary aerosol formation.37−39

Absλ = (Aλ − A 700)

Vl ln(10) Va·l

(1)

where Vl is the volume of the solvent for extraction, Va is the volume of the sampled air, and l is the LWCC optical path length (0.94 ± 0.01m). Aλ at all wavelengths are referenced to the A700 to account for systematic baseline drift. The resulting aqueous extract absorption coefficient (Absλ) has a unit of Mm−1, but is different from the aerosol absorption coefficient (bap) determined from optical measurements. For ease of analysis we focus on the absorption averaged between 360 and 370 nm (Abs365) as a simple measure of BrC level. The solution absorption Ångström exponent (Åa, where Abs ∼ λ−Åa) is a measure of the spectral dependence of light absorption from chromophores in solution and was determined from the linear regression fit of Absλ between 300 and 600 nm on log−log plots. The term “solution mass absorption efficiency” (here referred to as solution MAE) is used to describe the absorption efficiency of WSOC, and has a unit of m2g−1C. We note that the solution MAE is different from the widely recognized term “mass absorption cross-section” (MAC) that is used to normalize absorption by particles suspended in the air to mass of particles.42 Sunset Laboratories OCEC. PM2.5 elemental carbon (EC) was measured semicontinuously with a Sunset Laboratories OCEC analyzer (model 3F, Forest Grove, OR). A parallel-plate carbon denuder was used to limit positive artifacts.40 EC reported in this work is optical EC, calculated by converting the absorption of laser intensity to ambient concentration using empirical parameters.43 Periodic blank measurements were made by placing a Teflon filter (47 mm, Pall Life Sciences) upstream of PM2.5 inlet. EC data are blank corrected by subtracting blank values ranging between 0 and 0.03 μg m−3. Combined uncertainty (due to the analyzer and background variability) of the EC analysis is estimated at 28%.44 Offline Filter Sampling and Analysis. During CalNex, quartz filter samples for offline chemical analyses were collected with PM2.5 high-volume filter samplers (Tisch Environmental) on the roof of the Caltech Keck Building (∼30 m above ground level) approximately 0.3 km southwest of the Pasadena site. On June 4, 10 and 13, 2010, PM2.5 samples were collected following an intensive sampling schedule, where filters were changed every 3−6 h (Supporting Information (SI) Table S1). Chemical characterization of BrC constituents was performed using ultra performance liquid chromatography

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EXPERIMENTAL SECTION Field Sites. From mid-May to mid-June 2010, as part of the CalNex (Research at the Nexus of Climate Change and Air Quality in California) field campaign, online PM2.5 watersoluble organic carbon (WSOC) mass and aqueous extract light absorption measurements were carried out simultaneously at two ground sites, that is, Pasadena and Riverside, California. Pasadena sampling was conducted on the California Institute of Technology campus (34.140582N, 118.122455W), ∼16 km NE of downtown Los Angeles (LA).38 The Riverside site was located on the University of California-Riverside campus (33.97185N, 117.32266W) approximately 80 km to the eastsoutheast of downtown LA. The sample inlet was situated 7 m above ground level at Pasadena, and approximately 4.5 m above ground at Riverside. A suite of online gas and aerosol phase measurements, as well as offline aerosol sampling, was deployed exclusively at the Pasadena site. Due to instrumentation issues, WSOC light absorption data at Pasadena from only June 1−17 were analyzed. All data are reported in local time (PDT) and at ambient temperature and pressure. PILS-LWCC-TOC. Semicontinuous measurements of watersoluble PM2.5 light absorption spectra and WSOC mass were made with a particle-into-liquid sampler (PILS) coupled in series with a long optical path length (1m) liquid waveguide capillary cell (LWCC) (World Precision Instrument, Sarasota, FL) and a total organic carbon (TOC) analyzer (GE Analytical, Boulder, CO).10 A parallel-plate carbon denuder for gas-phase WSOC removal was placed downstream of the URG PM2.5 cyclone.40 PILS liquid sample was forced through a disposable syringe filter (0.45 μm pore size, Pall Life Sciences) prior to the LWCC and TOC to remove insoluble particles. Three 20 min background measurements were performed daily for the 3686

dx.doi.org/10.1021/es305047b | Environ. Sci. Technol. 2013, 47, 3685−3693


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Table 1. Identified Nitro-Aromatic Compounds (Negative Ion Mode) and Nitrogen-Containing Compounds (Positive Ion Mode) by UPLC/ESI-Q-TOF-MS and the Correlation Coefficients between Their Ambient Concentrations and PM2.5 Aqueous Extract Light Absorption at 365 nm (Abs365) Measured from Each Filtera

a

The molecular structures of the nitrogen-containing compounds are not available. *Authentic standards available and tested via LWCC. 3687



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Figure 1. Time series plots of WSOC concentration (μg m−3) and aqueous extract light absorption at 365 nm (Abs365) (Mm−1) in Pasadena (LA) and Riverside during CalNex. The y-axis scales are identical for the two sites for better comparison.

(UPLC) coupled with diode array detection (DAD) and highresolution quadrupole time-of-flight mass spectrometry equipped with an electrospray ionization source (ESI-HR-Q-TOFMS, Agilent 6500 Series).45 Briefly, samples were chromatographically separated via a UPLC system, detected by both a DAD scanning the absorbance from 200 to 800 nm (5 nm step), and an online ESI-HR-Q-TOF-MS to acquire accurate mass data for compositional information. Commercial standards of various aromatic and nitro-aromatic compounds (Table 1) were used for compound identification and quantification. The recovery efficiency for these compounds varied between 84.6% and 95.7%. These standards were also dissolved in water and diluted in series for light absorption measurement with the same LWCC-UV/vis spectrometer used in the online PILSLWCC-TOC system. Following the field campaign, a one-inch diameter circular punch from the intensive filters was extracted in 30 mL of MilliQ water with 20 min sonication under room temperature. Another punch from the same filter was extracted in 30 mL of Methanol (HPLC grade, Fisher Chemical) following the same procedures. The liquid extracts were filtered with a disposable syringe filter (0.45 μm pore size, Pall Life Sciences) and analyzed for PM2.5 WSOC, water-soluble BrC and methanolsoluble BrC. PM2.5 OC and EC from the intensive filter samples were analyzed using a counter-top Sunset Laboratories OCEC Analyzer (Sunset Laboratory Inc., Tigard, OR) following NIOSH protocol.46 Similar filter sampling and analysis were not performed at the Riverside site.

source and age of the aerosols. It has been established that the LA SOA is mainly from anthropogenic emissions, and with a significant fraction rapidly formed within the basin, the SOA is not highly oxidized.34,38,39,49 Consistently, WSOC comprised only ∼26% of the total OC (estimated from AMS OM measurements); a much smaller fraction than reported in other urban environments, such as Atlanta (58%)51 and Tokyo (77%).52 Despite the lower fraction, WSOC was highly correlated with estimates of overall SOA concentrations (AMS-OOA) (WSOC vs OOA: slope = 0.18, intercept = −0.12, r2 = 0.74) at Pasadena.49 Figures 1 and 2 show that Abs365 closely tracked the variations of WSOC at both sites, suggesting that SOA

RESULTS AND DISCUSSION Sources of Water-Soluble Brown Carbon in the LA Basin. The WSOC fraction of OA and its aqueous extract Abs365 (i.e., water-soluble BrC) were measured online with identical PILS-LWCC-TOC systems simultaneously in Pasadena and Riverside during CalNex. A regular variability in the WSOC time series, shown in Figure 1, was often observed at both sites. Other major SOA components, including AMS oxygenated organic aerosol (OOA) and small organic acids, all had a similar diurnal pattern at the highly instrumented Pasadena site.47−50 The formation of these SOA components was apparently driven by photochemistry as they reached their daily peak concentrations in midafternoon (14:00−15:00 PDT), following earlier daily peak concentrations in primary pollutants, such as EC and VOCs (e.g., benzene, toluene), whose temporal trends were driven by transport from central LA.48 The WSOC fraction of fine aerosol OA depends on the

Figure 2. Scatter plots of Abs365 (Mm−1) versus WSOC (μg m−3), where the slope is defined as the WSOC mass absorption efficiency (MAE, aqueous extract absorption at 365 nm per WSOC mass, m2g−1C) in Pasadena (from June 1−17) and Riverside (from May 17 to June 13).

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formation was the major source of water-soluble BrC in LA. The correlation between Abs365 and WSOC was stronger at Riverside (r2 of 0.65 compared to 0.55 for Pasadena, Figure 2). Riverside is located approximately 80 km east-southeast of Pasadena, within the eastern edge of the LA Basin. During daytime, primary pollutants originating from downtown LA are advected a further distance to Riverside, on the time scale of a day, and therefore air masses are expected to be more aged and well mixed in the vicinity of Riverside relative to Pasadena,53 which may explain the tighter correlation between Abs365 and WSOC in Riverside. There are also in general higher WSOC and Abs365 in Riverside than Pasadena, probably due to a 3688



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changes in sources (e.g., greater primary sources at night and morning versus secondary sources in afternoon), or that their spectral dependence provides little information on composition. Detailed chemical analysis to identify specific chromophores was performed based on filter samples. Chemical Speciation of LA Light-Absorbing SOA Constituents. To examine the chemical composition of ambient light-absorbing OA during CalNex, fifteen intensive filter samples collected at Pasadena were analyzed using UPLC/ESI-HR-Q-TOFMS (SI Table S1). Prior to the MS detection, the samples in the UPLC system were first detected by DAD scanning absorbance at 365 nm. The observed peaks riding on a large unresolved background were later compared with the authentic standards for compound identification (SI Figure S1). A number of nitrogen-containing mono- and polyaromatic compounds that absorb light at 365 nm were quantified. SI Figure S2 shows the complete absorption spectra of these nitro-aromatic standards measured with the LWCCspectrophotomer. These compounds absorb light over a wide range of wavelengths (i.e., between 300 and 500 nm), with higher molecular weight species having a tendency of absorbing more toward longer wavelengths. Table 1 lists the identified compounds and the correlations between their ambient concentrations and the overall Abs365 measured by LWCC from the same filter samples. Some of these nitro-aromatics were highly correlated with Abs365, suggesting that they likely had similar sources and/or formation processes. Other identified aromatic SOA components that do not have a nitro group, such as aromatic acids (e.g., benzoic acid and phthalic acid, not included in the tables), absorb mostly below 300 nm and their ambient concentrations were not correlated with Abs365. The contribution of the absorption by each identified nitroaromatic compound to the total observed Abs365 was estimated from the MAE365 of the individual compounds in the aqueous extracts. Authentic standards were commercially available for eleven of the nitro-aromatic compounds identified (labeled in Table 1). The standards were dissolved in Milli-Q water and diluted in series to a range between 100 ppbC and 2 ppmC. These solutions were measured for WSOC and light absorption spectra with the LWCC-TOC (SI Figure S2). Table 2 summarizes the MAE365 and ambient concentrations of the eight nitro-aromatics that exhibited positive correlation (r > 0) with Abs365. These individual nitro-aromatics were effective light absorbers in the UV (SI Figure S2). Their MAE365 were generally more than an order of magnitude larger than that for

combination of stronger local emission in Riverside and the additional brown SOA formed in transit from LA to Riverside. For a highly polluted period during the study (June 3−7), WSOC and Abs365 were on average 55% and 37% higher in Riverside, respectively. Campaign averaged diurnal trends of PM2.5 EC and Abs365 in Pasadena (Figure 3) show some evidence for primary sources

Figure 3. Diurnal variations of PM2.5 WSOC (μg m−3), EC (μg m−3), water-soluble brown carbon (Abs365) (Mm−1) and its absorption Ångström exponent (Åa) (fitted between 300 and 600 nm) plus/ minus one standard deviation (gray shading) in Pasadena. The data are reported in local time, or PDT (Pacific Daylight Time).

contributing to the water-soluble BrC, as the increase in Abs365 before noon coincide with the daily maximum of EC, consistent with incomplete combustion being a known source for BrC.10,11,33 However, this source is clearly minor relative to photochemical formation. The correlation between Abs365 and EC was weaker (r2 = 0.38), and the amount of Abs365 per EC mass was nearly doubled on the weekends compared to weekdays, suggesting reduced primary traffic emissions on weekends have less of an impact on the observed water-soluble BrC level.34 Linear regression slopes on the scatter plots of Abs365 versus WSOC for Pasadena and Riverside (Figure 2) represents the average MAE at 365 nm (MAE365) of the solution containing water-soluble chromophores. MAE365 values are similar at the two sites, ranging from 0.70 to 0.73 m2g−1C, suggesting similar chemical composition for WSOC in the LA basin. The overall LA water-soluble BrC had an average MAE365 of 0.71 m2g−1C (a MAC365 of approximately 1.42 m2g−1C is expected for ambient particles 54), which is small compared to the MAC for pure (i.e., uncoated) BC aerosol of 11.3 m2 g−1 at 365 nm (interpolated from a MAC of 7.5 ± 1.2 m2g−1 at 550 to 365 nm assuming an Åa of 1 42). The spectral dependence of LA water-soluble BrC had an absorption Ångström exponents (Åa) that varied between 1.2 and 5.4 with a study mean of 3.2 (±1.2) for wavelengths between 300 and 600 nm. Figure 3 shows the average diurnal trend of Åa throughout the study period at Pasadena. Mean Åa was fairly constant during the day, despite the substantially varying concentrations of WSOC and Abs365, which were suggested to be mainly due to factors influencing SOA formation. The fact that Åa of the combined chromophores in the water extracts did not vary much with time suggests either the chemical composition of the water-soluble chromophores was largely similar throughout the day, despite

Table 2. Contribution of Light Absorption from Each Identified Nitro-Aromatic Compound in the Aqueous Solution to the Total Ambient Abs365. Note that the MAE365 is Reported in the Unit of m2 g−1 per Carbon mass

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formula

MAE365 (m2g−1C)

ambient concentration (ng m−3)

predicted absorption at 365 nm (Mm−1)

percentage of overall Abs365 (%)

C6H5NO4 C7H5NO5 C10H7NO3 C8H9NO3 C7H7NO3 C7H7NO4 C12H9NO4 C10H13NO3

15.4 8.1 6.4 9.5 10.1 19.6 2.22 5.52

1.67 0.92 0.14 0.92 1.24 0.36 0.047 0.50

0.012 0.0034 0.00058 0.0050 0.0069 0.0035 0.000064 0.0017

1.37 0.39 0.07 0.58 0.79 0.41 0.007 0.19



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ambient BrC measured in LA and Riverside (∼0.73 m2g−1C), or laboratory-generated SOA, such as toluene-SOA (MAE350 of 0.57 m2g−1)25 and biogenic SOA aged with NH3 (aqueous solution MAE500 0.001−1.2 m2g−1).18,22 The average mass concentrations of these compounds were, however, generally low (in the range of a few ng m−3). As a result, the light absorption of each compound, calculated as the product of their MAE365 and ambient carbon mass concentration, accounted for only a few percent of the total Abs365. Among the eight compounds, 4-nitrocatechol (C6H5NO4) was identified as the largest contributor, which also had the highest correlation coefficient (r2 = 0.84) with ambient Abs365 (Table 1), suggesting a similar formation process between the overall water-soluble BrC and 4-nitrocatechol. It is noteworthy that 4nitrocatechol was also identified as the most abundant organic species and chromophoric substances in humic-like substances (HULIS) from biomass burning (BB) emissions.55 Nitrocatechols and methyl nitrocatechols are formed by photooxidation of m-cresol emitted during wood combustion56 or by photooxidation of toluene from anthropogenic emissions57 in the presence of NOx, and have been suggested as tracer compounds for BB SOA and anthropogenic SOA.55 This is consistent with the correlations we have observed between water-soluble BrC and WSOC (SOA), and that LA fresh SOA was largely of anthropogenic origin based on radiocarbon analysis during CalNex.34 It also agrees with other studies, which have shown that PAHs emitted from anthropogenic sources are a significant class of SOA precursors in LA,38 and environmental chamber experiments that demonstrate photooxidation of aromatics (e.g., toluene) under high-NO x conditions produces brown SOA.23,25 Overall, the light absorption from these eight nitro-aromatics comprises 3.8% of the observed ambient Abs365 (Table 2). This small fraction is consistent with the presence of a large “unresolved complex mixture” that is typically seen in chromatographic separation of ambient organic aerosols.58 The overlaid chromatographs in SI Figure S1 clearly show that there are many unresolved species that contribute to the overall light absorption at 365 nm (indicated by the increased baseline between 5 and 13 min only seen in the ambient sample injection). The UPLC/ESI-Q-TOFMS operated in positive ion mode identified several more nitrogen-containing compounds (listed in Table 1) that likely absorb light at 365 nm, but their molecular structures are tentative. More work is needed to fully characterize and quantify these compounds. Other analytical techniques, such as ESI/MS, have identified additional nitrogen-containing organic compounds from BB aerosols that likely contribute to the overall near-UV light absorption.59 Water-Soluble versus Methanol-Soluble Brown Carbon in LA. As discussed earlier, since the SOA within the LA Basin is relatively fresh, the water-soluble fraction of the total OA is generally low (on the order of 25%). To more comprehensively assess the contribution of BrC, intensive filter samples collected at the Pasadena site were extracted in methanol and measured for absorption with the same LWCCspectrophotometer. Chen and Bond33 found that when using methanol approximately 98% of biomass burning OC could be extracted from filters. Figure 4a shows the average absorption spectra of water and methanol-extracts from the 15 intensive filters. The methanol-extracts have higher absorption than the water-extracts across all wavelengths between 300 and 700 nm, consistent with previous findings by Sun et al.32 and Chen and Bond.33 From our data, the light absorption by methanol-

Figure 4. Averaged absorption spectra of water and methanol (MeOH) extracted brown carbon from quartz-fiber filter samples, plotted on (a) linear scale and (b) log scale. The filter samples were collected on the roof of the Caltech Keck Building (∼30 m above ground level) approximately 0.3 km southwest of the Pasadena site during CalNex. The average absorption Ångström exponents (Åa) are given in plot b).

extracted BrC is on average 3.2 (±1.1) and 20.5 (±14.1) times higher than that by water-soluble BrC at 365 and 532 nm, respectively. Since WSOC comprised 42−82% of OC for the filter samples, the MAE365 of the methanol-extracted BrC (absorption per OC mass) was 2.2 times larger than the MAE365 for water-soluble BrC. This implies that the waterinsoluble fraction of OC was highly absorbing, contributing 16−80% of the absorption by methanol-extracts (Figure 5a) and on average was 4.2 times more absorbing than WSOC. Chen and Bond33 suggested that these strongly light-absorbing water-insoluble components are likely large molecular weight PAHs, such as quinones from biomass burning and fossil fuel combustion.32 The water-insoluble BrC, calculated as the difference between methanol- and water-extracted BrC, exhibited a tighter correlation with ambient EC concentrations (r2 = 0.81, Figure 5b) than water-soluble BrC (r2 = 0.40), suggesting that the water-insoluble BrC components and EC have similar sources (e.g., incomplete combustion from vehicle emissions and wood burning). Variations in BrC Spectral Dependence Due to Different Measurement/Extraction Techniques. It is noteworthy that the solution Åa (fitted between 300 and 600 nm) derived from the filter water- and methanol-extracted BrC spectra were significantly different (Figure 4b). Åa for watersoluble BrC from the filter samples had a mean of 7.58 (±0.49), in the same range as those reported for filter-based extract analysis of biomass burning HULIS12 and aged SOA in the southeastern U.S.,10 but significantly higher than the average Åa (3.2 ± 1.2) derived from PILS water-soluble BrC measurement (Figure 3). In contrast, the methanol-extracted BrC from filters had a mean Åa of 4.82 (±0.49), closer to the PILS Åa. The difference in Åa from various measurements may be related to the solubility of the chromophores. Molecules comprised of more aromatic rings (i.e., a higher degree of conjugation) have higher absorption that extends to longer wavelengths, resulting in lower Åa,60 and these compounds have a lower solubility in water compared to small organic compounds. Therefore, the absorption spectra of the methanol-extracts are expected to have a lower Åa than those of the water-extracts. Compared to the filters extracted in water, PILS samples may produce a more soluble environment since the solution is approximately a factor of 10 times more dilute and the sample is heated (PILS is based on steam condensation collection system), although filter extractions performed under different temperature levels suggest no noticeable difference in Åa due to heating (SI 3690



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Figure 5. (a) Extract solution absorption at 365 nm of water-extracted BrC (Mm−1) and methanol- (MeOH) minus water-extracted BrC (Mm−1) from the intensive filters (details of the filter sampling are summarized in SI Table S1), and (b) scatter plot of the water-insoluble BrC (Mm−1), calculated as the difference between MeOH- and water-extracted BrC, versus EC concentration (μg m−3) measured from the filters. (5) Jacobson, M. Z. Isolating nitrated and aromatic aerosols and nitrated aromatic gases as sources of ultraviolet light absorption. J. Geophys. Res. 1999, 104, 3527−3542. (6) Barnard, J. C.; Volkamer, R.; Kassianov, E. I. Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area. Atmos. Chem. Phys. 2008, 8, 6665−6679. (7) Park, R. J.; Kim, M. J.; Jeong, J. I.; Yooun, D.; Kim, S. A contribution of brown carbon aerosol to the aerosol light absorption and its radiative forcing in East Asia. Atmos. Environ. 2010, 44, 1414− 1421. (8) Chung, C. E.; Ramanathan, V.; Decremer, D. Observationally constrained estimates of carbonaceous aerosol radiative forcing. Proc. Natl. Acad. Sci. 2012, 109, 11624−11629. (9) Bahadur, R.; Praveen, P. S.; Xu, Y.; Ramanathan, V. Solar absorption by elemental and brown carbon determined from spectral observations. Proc. Natl. Acad. Sci. 2012, 109, 17366−17371, DOI: 10.1073/pnas.1205910109. (10) Hecobian, A.; Zhang, X.; Zheng, M.; Frank, N. H.; Edgerton, E. S.; Weber, R. J. Water-soluble organic aerosol material and the lightabsorption characteristics of aqueous extracts measured over the Southeastern United States. Atmos. Chem. Phys. 2010, 10, 5965−5977, DOI: 10.5194/acp-10-5965-2010. (11) Duarte, R. M. B. O.; Pio, C. A.; Duarte, A. C. Spectroscopic study of the water-soluble organic matter isolated from atmospheric aerosols collected under different atmospheric conditions. Anal. Chim. Acta 2005, 530, 7−14. (12) Hoffer, A.; Gelencser, A.; Guyon, P.; Kiss, G.; Schmid, O.; Frank, G. P.; Artaxo, P.; Andreae, M. O. Optical properties of humiclike substances (HULIS) in biomass-burning aerosols. Atmos. Chem. Phys. 2006, 6, 3563−3570. (13) Lukacs, H.; Gelencser, A.; Hammer, S.; Puzbaum, H.; Pio, C.; Legrand, M.; Kasper-Giebl, A.; Handler, M.; Limbeck, A.; Simpson, D.; Preunkert, S. Seasonal trends and possible sources of brown carbon based on 2-year aerosol measurements at six sites in Europe. J. Geophys. Res. 2007, 112, D23S18 DOI: 10.1029/2006JD008151. (14) Chakrabarty, R. K.; Moosmüller, H.; Chen, L.-W. A.; Lewis, K.; Arnott, W. P.; Mazzoleni, C.; Dubey, M. K.; Wold, C. E.; Hao, W. M.; Kreidenweis, S. M. Brown carbon in tar balls from smoldering biomass combustion. Atmos. Chem. Phys. 2010, 10, 6363−6370. (15) Noziere, B.; Esteve, W. Organic reactions increasing the absorption index of atmospheric sulfuric acid aerosols. J. Geophys. Res. 2005, 32, L03812 DOI: 10.1029/2004GL021942. (16) de Haan, D. O.; Corrigan, A. L.; Smith, K. W.; Stroik, D. R.; Turley, J. J.; Lee, F. E.; Tolbert, M. A.; Jimenez, J. L.; Cordova, K. E.; Ferrell, G. R. Secondary organic aerosol-forming reactions of glyoxal with amino acids. Environ. Sci. Technol. 2009, 43, 2818−2824. (17) Shapiro, E. L.; Szprengiel, J.; Sareen, N.; Jen, C. N.; Giordano, M. R.; McNeill, V. F. Light-absorbing secondary organic material

Figure S3). Thus, light absorption as a function of wavelength, and Åa, may significantly depend on the extraction method (degree of dilution). This may explain the prevalence of high Åa (6−7) observed in a variety of filter-based studies.

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ASSOCIATED CONTENT

* Supporting Information S

Table S1 and Figure S1−S3 are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 404-536-5678; e-mail: [email protected]. Present Address §

Department of Civil and Environmental Engineering, University of California, Davis, California 95616, United States

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded through National Science Foundation grants ATM-0802237 and ATM-0931492. The UNC group was funded in part by the Electric Power Research Institute (EPRI). Y.-H. L. was also supported by a Dissertation Completion Fellowship from the UNC Graduate School. We thank Jose Jimenez, Joost de Gouw, Jochen Stutz, and John Seinfeld for organizing Pasadena ground site sampling and Paul Ziemann for graciously providing space and help for Riverside sampling during CalNex.

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