Molecular-Size-Separated Brown Carbon Absorption for Biomass

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Molecular size separated brown carbon absorption for biomass burning aerosol at multiple field sites Robert A Di Lorenzo, Rebecca A. Washenfelder, Alexis Rae Attwood, Hongyu Guo, Lu Xu, Nga Lee Ng, Rodney J. Weber, Karsten Baumann, Eric S. Edgerton, and Cora J Young Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06160 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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Environmental Science & Technology

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Molecular size separated brown carbon absorption

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for biomass burning aerosol at multiple field sites

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Robert A. Di Lorenzo1, Rebecca A. Washenfelder2.3, Alexis R. Attwood2,3,†, Hongyu Guo4, Lu Xu5,

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Nga L. Ng4,5, Rodney J. Weber4, Karsten Baumann6, Eric Edgerton6, Cora J. Young1*

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Department of Chemistry, Memorial University, St. John’s, Newfoundland, Canada.

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Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder,

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Boulder, Colorado, USA.

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Atmospheric Administration, Boulder, Colorado, USA.

Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and

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USA.

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Georgia, USA

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School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia,

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta,

Atmospheric Research & Analysis Inc., Cary, North Carolina, USA

†

Now at Droplet Measurement Technologies, Boulder, Colorado, USA

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ACS Paragon Plus Environment

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Abstract Biomass burning is a known source of brown carbon aerosol in the atmosphere. We

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collected filter samples of biomass burning emissions at three locations in Canada and the United

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States, with transport times of 10 h to >3 days. We analyzed the samples with size exclusion

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chromatography coupled to molecular absorbance spectroscopy to determine absorbance as a

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function of molecular size. The majority of absorption was due to molecules >500 Da, and these

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contributed an increasing fraction of absorption as the biomass burning aerosol aged. This

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suggests that the smallest molecular weight fraction is more susceptible to processes that lead to

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reduced light absorption, while larger molecular weight species may represent recalcitrant BrC.

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We calculate that these large molecular weight species are composed of more than 20 carbons

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with as few as two oxygens, and would be classified as extremely low volatility organic

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compounds (ELVOCs).

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1. Introduction

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The role of atmospheric aerosol in the global radiative budget is important, but poorly

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quantified1. Direct aerosol absorption is responsible for positive radiative forcing, in contrast to

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negative radiative forcing (RF) impacts of light scattering and indirect cloud effects. Although

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black carbon (BC) dominates carbonaceous particle absorption, organic aerosol (OA) may also

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include absorbing species that absorb light and are described as brown carbon (BrC). Compared

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to BC, BrC is known to have greater chemical functionality, water solubility, and absorption that

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increases rapidly in the near ultraviolet spectral region, with a λ-2 to λ-6 dependency, but its

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molecular composition is poorly characterized2,3. After accounting for the absorption and lensing

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properties of BrC, model-based RF estimates are positive and more closely aligned with

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experimentally-constrained estimates4. Only recently have BrC properties determined from

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experimental data been included in global radiative forcing models4–7. Many primary and


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secondary sources for BrC have been proposed, and the majority of mechanistic laboratory

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studies have focused on secondary organic aerosol (SOA) formation and processing8. Potential

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BrC sources include various multi-phase polymerization mechanisms from gas-phase sources9–11,

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SOA generated from processing of toluene12 and other precursors13, and production from

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combustion sources14. Improved knowledge of BrC sources, composition, and optical properties

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is necessary for accurate global models.

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A few spectroscopic detection methods exist for field measurements of BrC.

First,

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photoacoustic spectroscopy or aethalometry can directly measure total absorption by an aerosol

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sample. BrC absorption can be inferred from measurements at multiple wavelengths, with the

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assumption that absorption at visible wavelengths can be attributed to BC, while absorption in

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the near-UV is due to both BrC and BC15–17. Second, because BrC is more volatile than BC, BrC

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can be selectively removed by thermal denuding.

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ringdown spectrometer, or broadband cavity enhanced spectrometer can be used to measure the

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difference between a denuded and undenuded sample18,19. Third, a Particle-into-Liquid Sampler

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(PILS) can be coupled to a liquid waveguide capillary cell (LWCC) and an absorbance

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spectrometer20. A similar approach uses offline filter extracts where solubility differences

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between BC and BrC are exploited, such that only soluble aerosol components are dissolved and

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measured using absorbance spectrophotometry. The BrC content is inferred from the average

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absorbance in the UV, such as between 360 and 370 nm (Abs365)20. None of these optical

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methods provide any direct information about the chemical composition of BrC.

A photoacoustic spectrometer, cavity

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Our group recently developed a method for offline determination of BrC absorption for

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molecules from 250-75,000 Da using size exclusion chromatography and molecular absorbance

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spectroscopy21. This method allows quantification of BrC absorbance as a function of molecular


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weight, and provides new information about BrC composition. Our previous results showed BrC

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in a biomass burning (BB) plume was primarily composed of large molecular weight species21.

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This is consistent with observations that BrC from BB consists of extremely low volatility

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organic compounds (ELVOCs)22. A recent field study also demonstrated that large molecular

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weight organics that cannot be removed by thermal denuding at 250 °C are major contributors of

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BrC in an aged biomass-burning plume23,24. The composition of BrC from BB is important

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because field measurements have shown that water-soluble organic aerosol absorption is

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predominantly correlated with BB in the southeastern U.S., despite the larger mass contribution

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of biogenic SOA252518,25–29.

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In this work, we apply our method to analyze filters from multiple locations with different

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aerosol sources, thereby determining whether the molecular weight distribution of absorbing

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species is consistent across BB plumes derived from wildfires, and whether these distributions

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are consistent in regions not directly influenced by discrete BB events. Less aged BB aerosol was

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collected during a wildfire event in Vancouver, British Columbia in July 2015, aged BB aerosol

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was collected in St. John’s, Newfoundland and Labrador in July 2013, and background aerosol

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was acquired during the Southern Oxidant and Aerosol Study (SOAS) in Alabama during June

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and July of 2013 (Figure 1). We use these results together with the extensive instrumentation

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available at the SOAS field site to gain information about the sources, the molecular species that

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compose, and the mechanisms involved in the formation of BrC.

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2. Methods

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2.1 Sampling Sites

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2.1.1 Less Aged Biomass Burning Aerosol in Vancouver, British Columbia


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Less aged BB emissions were collected during summer 2015 as part of the Lower Fraser

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Valley Air Quality Monitoring Network in British Columbia, Canada. These samples capture

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early July wildfire events in nearby temperate coniferous forests, where PM2.5 concentrations

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exceeded 200 µg/m3 (Figure S2). Samples were obtained from two sites: Burnaby/Kensington

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Park (BKP) (49.2792°N, 122.9707°W, 110 m above sea level) and North Vancouver/Second

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Narrows (NVSN) (49.3015°N, 123.0204°W). Vancouver BKP is in an urban neighbourhood near

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residential, industrial, commercial and parkland areas. Vancouver NVSN is in a commercial and

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industrial area, within 50 m of a chemical plant known to emit NOx. Both sites were

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approximately 100 km east of the main burn location.

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Aerosol samples were collected using real-time beta attenuation particle monitors (5030

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SHARP Monitor at Vancouver BKP, 5030i SHARP Monitor at Vancouver NVSN, Thermo

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Fisher Scientific, Waltham, MA, USA). Particles were collected at 16.67 L/min on glass

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microfiber filter tape at 8 h intervals. Filter tape was stored in the dark at -20 °C until analysis.

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2.1.2 Aged Biomass Burning Aerosol in Newfoundland, Canada

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Aged BB aerosol samples were collected in St. John’s, Newfoundland (47.572°N, 52.722°W,

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42 m above sea level) two days after a large-scale forest fire erupted in northern Quebec and

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Labrador, Canada on 4 July 2013. Sample collection is described in detail in Di Lorenzo and

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Young21. Briefly, PM2.5 samples (n=6) were collected in parallel on 47 mm quartz fibre filters

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(Pall Life Sciences, Washington, NY, USA) using a multi-channel, medium volume air sampler

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(URG-3000ABC, URG Corp., Chapel Hill, NC, USA) for 25.5 h during the BB plume intrusion.

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Filters were stored in the dark at -20 °C until analysis.

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2.1.3 Regional Background of Aged Biomass Burning Aerosol in the southeastern U.S.


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Background air samples were collected over six weeks during summer 2013. PM2.5 samples

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were collected as part of SOAS in the Talladega National Forest in Central Alabama

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(32.90328°N, 87.24994°W, 76.5 m above sea level). Samples represent typical rural air masses

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characterized by biogenic emissions of surrounding forests and occasional intrusion from

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neighboring urban areas of Birmingham and Tuscaloosa26. Forests are primarily composed of

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pine and hardwoods with isoprene emissions representative of the southeastern US30.

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PM2.5 samples were collected on the back filter assembly of a custom-built broadband cavity

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enhanced spectrometer (BBCES)31 on 47 mm PTFE filters (Pall Life Sciences, Washington, NY,

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USA) housed in a molded PFA 47 mm filter assembly (Savillex Corp., Eden Prairie, MN, USA)

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across two channels. Each channel was sampled at 2035 standard cm3 min-1 for 3, 12, or 24 h

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through a 6m copper inlet and conductive silicone tubing within the instrument. The earliest

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samples (10 - 13 June 2013) may have been affected by contamination from the off-gassing of

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siloxanes from the black silicone tubing used in the instruments32,33, based on the absorbance of

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these filters at 300 nm34, and these three filter samples were excluded from analysis.

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These measurements were co-located with the Electric Power Research Institute Southeastern

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Aerosol Research and Characterization (SEARCH) Network, where a suite of gas- and particle-

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phase measurements were performed: CO by non-dispersive infrared (NDIR) detection, K+ by

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the Monitor for Aerosols and Gases in Ambient Air (MARGA)35, and OA fractions by high

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resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS)36. Particle absorption was

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also measured in real time using PILS-LWCC for water soluble OA absorption20 and multi-angle

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absorption photometer (MAAP) for BC absorption37.

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2.2 Fire identification and transport time


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Fire locations and burn areas (Figure 1) were obtained from the British Columbia Wildfire

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Service38 and the Canadian Wildland Fire Information System39 for Vancouver and St. John’s

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samples, respectively. Fires occurring within seven days prior to sampling time were included.

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Airmass back trajectories computed using the Hybrid Single Particle Lagrangian Integrated

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Trajectory Model (HYSPLIT) are shown as grey traces of Figure 140. Back trajectories were

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calculated from the ground-level sampling location, using archived Global Data Assimilation

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System (GDAS1) meteorology. Vertical motion was computed using the vertical velocity field

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from the meteorological data. New trajectories were computed every 1 hour for 48 and 72 hours

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for Vancouver and St. John’s locations, respectively.

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2.3 Sample Extraction and Analysis by Size Exclusion Chromatography Absorption

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Spectrophotometry (SEC-UV)

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Each filter was sub-sampled using a 10 mm arch punch and placed in a glass vial sealed with

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an acid washed cap containing a Teflon-lined septum. 750 µL of ultrapure deionized water

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(Barnstead Infinity Ultrapure Water System, Thermo Scientific, Waltham, MA, USA) was added

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and samples sonicated for 45 minutes. For SOAS samples, seven punches from each filter were

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combined per extraction. For Vancouver samples, one punch from each filter spot was used per

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extraction. Due to the hydrophobicity of the SOAS PTFE filters, a 38 mm, 20-gauge needle (BD

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Precision Glide, Fisher Scientific, Waltham, MA, USA) was inserted through the septa to keep

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filter media below water surface during sonication, and the exposed needle opening was wrapped

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in parafilm to prevent contamination. After sonication, all extracts were filtered with 0.2 µm

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PTFE syringe filters (Iso-Disc, Supelco, Bellefonte, PA, USA), transferred to pre-muffled glass

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sample vials and stored at 4 °C until analysis.


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Samples were analyzed according to the procedure outlined in Di Lorenzo and Young21.

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Briefly, samples were separated using high performance liquid chromatography (HPLC) (1260

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Infinity, Agilent Technologies, Santa Clara, CA, USA) on an aqueous gel filtration column

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(Polysep GFC P-3000, Phenomenex, Torrance, Ca, USA) at 1 mL min-1 with 25 mM aqueous

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ammonium acetate eluent. Compounds were detected using a diode array detector (1260, Agilent

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Technologies, Santa Clara, CA, USA). Integrations of total absorbance were performed at 300,

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365 and 405 nm. Conversion of absorption peak areas (units of AU·min) to absorption

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coefficients (units of Mm-1), as well as blanks and quality control, can be found in the supporting

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information (Text S1).

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3. Results and Discussion

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3.1 Calculated Ages for Biomass Burning Aerosol

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Although the back trajectories and transport can be uncertain, they give a useful indication of

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transport age for the BB samples (Figure 1). Plume ages were estimated by determining the

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HYSPLIT-calculated age at which a plume traversed over a likely fire source. Back trajectories

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for Vancouver NVSN/BKP sites show aerosol transport times between 8 and 30 h. Airmasses

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sampled at the St. John’s site were more aged, with transport times between 48 and 72 h.

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Finally, the aerosol at the SOAS field site was significantly aged, and was generally not

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associated

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Spectroradiometer (MODIS) fire locations and back trajectories showed the SOAS site was not

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influenced by large, discrete fires, and more likely was affected by a regional background of

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small agricultural fires41. In this case, aerosol age is difficult to quantify, but we estimate a value

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of up to 5 days from determining the aerosol lifetime over terrestrial southeastern US with

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HYSPLIT back trajectories.

with

identifiable

fires.

Examination

of

Moderate

Resolution

Imaging


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3.2 BrC Absorbance as a Function of Molecular Weight

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Absorbance density plots as a function of molecular weight of BrC for less aged

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subalpine/montane wildfire, more aged boreal plume, and typical background rural aerosol

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sample are shown in Figure 2. Absorption density plots show marked similarity in molecular

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weight profile distribution across sample locations. Molecular weights of absorbing species were

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estimated using the calibration method outlined previously21 (Text S2). Samples from each site

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contain three apparent populations with consistent absorbance maxima, all occurring at 2400 Da,

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1400 Da and 250 Da. Each sample shows a maximum observable molecular weight of 10,000

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Da. Further, this molecular weight absorbance profile distribution remained constant throughout

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all days of the SOAS campaign with measurable absorbance (Figure S3). The only day in which

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the molecular weight profile was different was 11 June 2013, which may be a result of siloxane

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contamination, as described above.

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Previous studies reported the amount of BrC emitted from BB depends on burn conditions,

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with a smaller influence from fuel type22. Although the two wildfire (Vancouver and St John’s)

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samples are of different origin, age and fuel source (boreal vs. temperate coniferous forests), they

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have similar absorption characteristics. This suggests that although the quantity of BrC produced

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is variable depending on the burning event, molecular characteristics responsible for absorption

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of BB-derived BrC may be constant.

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To assess the relative absorbance and aging of BB aerosol from each fire, the ratio of BrC to a

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conserved CO tracer (∆BrC/∆CO) was plotted as a function of aerosol transport time in Figure

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3A42. Background CO values were calculated from average ground-level CO measurements

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made at the same site 8 h before plume intrusion. These background levels were consistent

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within 0.2 ppb across days with little anthropogenic input. Background CO concentrations were


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calculated as 0.15, 0.11 and 0.14 parts per million by volume (ppmv) for the Vancouver BKP,

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Vancouver NVSN and St. John’s sites, respectively. Background BrC values were assumed to be

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zero, determined from the negligible BrC absorbance (10 h old. Furthermore, total

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BrC absorption at the Vancouver and St. John’s sites is the same after correcting for plume

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dilution. Differences between values observed in this study and those of Forrister et al.27 may be

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because of uncertainty in determining plume ages from back trajectories, difficulty in

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determining background CO levels in comparison to aircraft flights in and out of BB plumes, or

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because Forrister et al. reported the sequential sum of methanol and water extracts.

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Although total water-soluble absorption of measured BB-derived samples appears constant

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with age, the relative contributions of higher molecular weight components (>500 Da) increase

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in comparison to smaller molecular weight components (

Molecular-Size-Separated Brown Carbon Absorption for Biomass

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