Changes in Light Absorptivity of Molecular Weight Separated Brown

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Changes in Light Absorptivity of Molecular Weight Separated Brown Carbon due to Photolytic Aging Jenny Pui Shan Wong, Athanasios Nenes, and Rodney J. Weber Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Environmental Science & Technology

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Changes in Light Absorptivity of Molecular Weight

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Separated Brown Carbon due to Photolytic Aging

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Jenny P. S. Wong,†* Athanasios Nenes,† ‡&┴ Rodney J. Weber†

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† School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, 30331,

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USA

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

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30331, USA

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&

Institute of Chemical Engineering Sciences, Foundation for Research and Technology-Hellas,

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Patras, GR-26504, Greece ┴

Institute of Environmental Research and Sustainable Development, National Observatory of

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Athens, Palea Penteli, GR-15236, Greece

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Abstract

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Brown carbon (BrC) are those organic compounds in atmospheric aerosols that absorb solar

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radiation and may play an important role on planetary radiative forcing and climate. However,

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little is known about the production and loss mechanisms of BrC in the atmosphere. Here, we

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study how the light absorptivity of BrC from wood smoke and secondary BrC generated from

ACS Paragon Plus Environment

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reaction of ammonium sulfate with methylglyoxal changes under photolytic aging by UVA

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radiation in the aqueous phase. Owing to its chemical complexity, BrC is separated by molecular

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weight using size exclusion chromatography and the response of each molecular weight fraction

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to aging is studied. Photolytic aging induced significant changes in the light absorptivity of BrC

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for all molecular weight fractions; secondary BrC was rapidly photo-blenched whereas for wood

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smoke BrC, both photo-enhancement and photo-bleaching were observed. Initially, large

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biomass burning BrC molecules were rapidly photo-enhanced, followed by slow photolysis. As a

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result, large BrC molecules dominated the total light absorption of aged biomass burning BrC.

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These experimental results further support earlier observations that large molecular weight BrC

27

compounds from biomass burning can be relatively long-lived components in atmospheric

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aerosols, thus more likely to have larger impacts on aerosol radiative forcing and could serve as

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biomass burning tracers.

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1. INTRODUCTION

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Organic aerosols (OA) are a major component of fine ambient particles and affect the Earth’s

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radiative balance by directly interacting with solar radiation, or indirectly via their interactions

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with clouds. These aerosol effects on climate represent the largest uncertainty in global radiative

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forcing assessments.1 While OA was originally thought to only scatter solar radiation, recent

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studies demonstrate that components in OA can absorb UV-Visible radiation.2 This class of light

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absorbing OA, collectively termed brown carbon (BrC), can potentially shift the direct radiative

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forcing of OA from net cooling to net warming.3,4 Additionally, modeling studies have observed

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that absorption of near UV solar radiation by BrC can result in decreased photolysis rates for

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NO2 and O3, indicating that BrC can influence tropospheric photochemistry.5,6 Characterizing the


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sources and aging processes of BrC are critical to evaluate its atmospheric impacts, and to

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understand the persistent signatures in biomass burning aerosols.

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Multiple sources of BrC have been identified, including emissions from biomass burning,7–9

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fossil fuel combustion,10,11 and release of biogenic matter, such as soils and bioaerosols.12 While

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many studies have established that biomass burning is likely to be an important source of

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atmospheric BrC, only a small fraction of organic chromophores have been identified, such as

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nitrophenols.13–16 Production of secondary BrC in aerosols and clouds has also been

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proposed.13,17 Although secondary BrC formation from the reactions of carbonyl or aromatic

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compounds with nitrogen-containing compounds has been studied extensively in the

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laboratory,13 its contribution to atmospheric BrC remains unclear. The emissions profile of BrC

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is poorly understood, but how aging modulates BrC levels and properties in the atmosphere is

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still unclear. Part of this limited understanding arises from the low mass fraction of

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chromophores in the organic aerosol, as well as the uncertain and complex nature of their

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chemical identity.13

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Most studies that have investigated BrC aging focused on secondary BrC, which was observed

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to undergo rapid photo-bleaching with atmospheric lifetimes on the order of minutes to several

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hours.18–21 Despite the growing evidence that aged secondary BrC rapidly photo-bleaches in the

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atmosphere, laboratory studies investigating the effects of aging on primary BrC have observed

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that biomass burning BrC can undergo both photo-enhancement and photo-bleaching.20,22,23 The

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results from these studies illustrated the dynamic nature of biomass burning BrC due to aging,

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but the mechanisms leading to these observations remain unclear. For example, it is unknown

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whether all classes of compounds in biomass burning BrC respond to photolytic aging in the


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same manner (e.g. initial photo-enhancement followed by photo-bleaching), or that different

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classes of compounds exhibit different photolytic aging effects, or a combination thereof.

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The effects of aging on biomass burning BrC have also been observed from ambient

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measurements. By following the evolution of a biomass burning plume in Western U.S.A.,

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airborne observations suggested that while the majority of primary BrC from biomass burning

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have short atmospheric lifetimes of 9 to 15 hours, a persistent fraction may remain even after 50

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hours following emission, although the conclusion is uncertain since there are few data points for

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more aged BrC (> 20 hours).24 Other ambient measurements of aged (approx. 2 days of

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atmospheric transport) biomass burning aerosols indicated that large molecular weight organic

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compounds contributed significantly to the total organic aerosol mass25 and total light

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absorption.26 Collectively, these observations suggest that atmospheric aging of biomass burning

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BrC decreases the light absorptivity of smaller chromophores considerably more than for larger

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chromophores. Larger chromophores may therefore be the most persistent BrC species in the

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atmosphere, hence most influential for perturbing the planetary radiative balance.27 The

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atmospheric processes leading to these observations remain unknown and it is unclear whether

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the reactivity of secondary BrC are also dependent on their molecular weight properties.

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The objective of this study is to systematically investigate the effects of photolytic aging on the

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light absorptivity of different molecular weight BrC components.

Size exclusion

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chromatography was coupled to UV/VIS absorption spectroscopy in order to characterize the

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molecular weight distributions of chromophores in different types of BrC and to determine their

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photolytic reactivity. The photolysis of two types of BrC were investigated: primary BrC from

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pyrolysis-generated wood smoke emissions and secondary BrC generated from the reaction of

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ammonium sulfate with methylglyoxal (AS-MGL). Results demonstrated that both types of BrC


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undergo significant changes in their optical and molecular weights properties due to photolytic

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aging. Rapid photo-bleaching was observed for AS-MGL BrC whereas initial photo-

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enhancement, followed by photo-bleaching was observed for primary BrC from wood smoke

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emissions. These contrasting observations illustrate that the atmospheric evolution of BrC is

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highly variable and dynamic.

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2. EXPERIMENTAL METHODS

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2.1 Preparation of BrC Samples

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Wood smoke BrC samples, chosen to represent biomass burning BrC, were generated in the

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laboratory via controlled wood pyrolysis using the method of Chen and Bond that simulates the

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thermal decomposition of solid organic fuel during biomass burning.28 An electronically heated

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combustor, with an internal volume of 950 cm3, was continually flushed with 2000 sccm of N2

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gas, where the lack of oxygen suppresses black carbon formation during wood pyrolysis. For

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each pyrolysis event, a rectangular piece of dry hardwood (cherry of size 3 × 2 × 2 cm, approx. 5

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g) was placed in the bottom center of the combustor, where the exterior temperature was

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measured. The smoke stream was further diluted by HEPA-filtered air (1500 sccm) in a mixing

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volume (0.01 m3), following which particles larger than 1.0 µm were removed using an

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impactor. Once the combustor reached 210˚C, the emitted organic carbon was collected on

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polytetrafluoroethylene filters (47 mm, 2 um pore size, Pall Corporation) at 3500 sccm for 100

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minutes. These conditions represent the smoldering phase of the combustion process. Some low

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volatility components of the smoke emission may not be measured by this method as a thick

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substance was observed to accumulate on the tubing walls. Immediately after collection, the

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filters were stored in a freezer at -10˚C. Prior to each photolysis experiment, water-soluble BrC

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(WS BrC) was extracted from one particle filter by adding 15 mL of purified water (18.2 mΩ) in


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a sealed glass vial and sonicated for 60 minutes. After the water extract was removed, 15 mL of

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methanol (HPLC grade, Merck) was added and sonicated for 60 minutes to extract the water-

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insoluble BrC (WI BrC). Each extract was filtered using a new 0.2 µm PTFE syringe filter

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(Fisher).

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Ammonium sulfate-methylglyoxal (AS-MGL) BrC were prepared using a similar method

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employed by previous laboratory studies, which simulates BrC formed by secondary processes.29

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The AS-MGL stock solution was prepared by combining 98 mL of an aqueous solution of

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ammonium sulfate (Fisher Scientific) and 3 mL of methylglyoxal (Sigma Aldrich, 40% in water)

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in sealed amber bottles. The final concentrations in the stock solution were ~ 1.5 M of

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ammonium sulfate and ~ 0.17 M of methylglyoxal. The resulting solution was kept in the dark at

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room temperature for 10 days. During this period of time, the color of the solution turned dark

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yellow/brown from a pale yellow color. Prior to each photolysis experiment, the stock solution

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was diluted by a factor of 7.

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2.2 Photolysis of BrC

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All photolysis experiments were conducted in a photoreactor, with a slowly rotating vial rack

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(4 rpm, 40 vials capacity) placed in the center that was surrounded by 8 UVA lamps (F-25T8BL,

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Sylvania) and maintained close to near-room temperature by continuous chamber ventilation

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with two fans. With all the UVA lamps turned on, the temperature inside the photoreactor

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increased by 6˚ (from 24 to 30˚C). The integrated photon flux inside the photoreactor was

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characterized by chemical actinometry using 2-nitrobenzaldehyde18 and the wavelength

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dependent photon flux was directly measured using a spectroradiometer (StellaNet Inc.). The

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chemical actinometry method is discussed in Section S1 and the photon fluxes determined using

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both approaches are shown in Figure S1 (Supplementary Information). Most of the radiation

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emitted by the UVA lamps fell in the 300 – 400 nm range with a maximum at 355 nm.


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Photolysis experiments using wood smoke BrC and AS-MGL BrC were conducted separately.

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For each experiment, multiple 2 mL borosilicate glass vials (sealed with Telfon-lined caps), each

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containing 0.75 mL of the filter extract or dilute solution, were placed on the rotating vial rack.

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At different illumination times, one vial was removed for offline measurements (discussed

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below). For the wood smoke BrC samples, filter extracts were illuminated up to 130 hours in the

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photoreactor and up to 40 hours for AS-MGL BrC. To ensure reproducibility, photolysis

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experiments using wood smoke BrC and AS-MGL BrC were repeated four and five times,

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respectively. Additionally, control experiments were conducted; no changes in BrC properties

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were observed when the vials were completely covered by aluminum foil (i.e. exposed to only

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the elevated temperature conditions and not UVA radiation).

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2.3 BrC Measurements

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Changes in the water-soluble organic carbon (WSOC) concentration due to photolysis were

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monitored offline using a Sievers Total Organic Carbon (TOC) Analyzer (Model 900, GR

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Analytical Instruments). TOC measurements were conducted using the bulk BrC samples (i.e.

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not molecular weight separated), since the use of organic compounds in the eluent for the

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chromatographic molecular weight separation technique (discussed below) resulted in very high

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background signals. Additionally, quantification of WI-BrC was not possible due to the use of

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methanol as an extraction solvent. The TOC analyzer was routinely calibrated using solutions of

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dissolved sucrose of known concentrations. BrC samples were diluted by up to a factor of 1000

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to ensure the measured TOC concentrations were in the linear response range of the instrument.

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From the TOC measurements, each sample vial (i.e. 0.75 mL of filter extract or dilute solution)

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of unreacted WS smoke BrC contained 342 ± 91 µg WSOC and for the unphotolyzed AS-MGL

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BrC 386 ± 40 µg WSOC.


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Changes in molecular weight distributions of BrC due to photolysis were measured using a

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high performance liquid chromatography (HPLC) system (GP40 pump with AS40 autosampler,

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Dionex), equipped with a size exclusion chromatography (SEC) column (discussed below),

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coupled to an UV-VIS spectrometer, consisting of a liquid waveguide capillary (1 m optical

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path-length, World Precision Instrument), a deuterium tungsten halogen light source (DT-Mini-

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2, Ocean Optics) and an absorption spectrometer (USB4000, Ocean Optics) that continuously

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monitored all wavelengths between 200 – 800 nm. The long optical path-length was chosen to

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increase detection sensitivity.

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Separations were achieved by operating an aqueous size exclusion/gel filtration

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chromatography column (Polysep GFC P-3000, Phenomenex). Briefly, separation by size

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exclusion chromatography (SEC) is controlled by differences in the extent of permeation into the

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pores of the column packing material by analyte molecules, where larger molecules are eluted

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first due to weaker interactions with the packing material compared to smaller molecules.31 The

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chromatographic method used is similar to that developed by Di Lorenzo and Young for the

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analysis of atmospheric particles,26 however, the composition of the mobile phase was modified

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to optimize the separation of weakly interacting molecules. The chromatography system was

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operated in isocratic mode using a 90:10 v/v mixture of water and methanol with 25 mM

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ammonium acetate as the mobile phase, at a flow rate of 1 mL/min and a sample injection

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volume of 20 µL. Ammonium acetate, a pH buffer, was added to the mobile phase to minimize

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electrostatic interactions between the analytes and the column, which can interfere with the

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column’s ability to separate by molecular size. If electrostatic interactions are negligible, SEC

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separates analytes based solely on their hydrodynamic volume, which is a function of both

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molecular weight and density of the compound.32,33 The relationship between elution volume and


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molecular weight was empirically determined using the following standards with known

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molecular weights (Sigma Aldrich): blue dextran (2M Da), bovine serum albuminum (66 kDa),

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horseradish peroxidase (44 kDa), myoglobin (16.9 kDa), lysozyme (14.3 kDa), apotinin (6.5

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kDa), tannic acid (1.7 kDa), vitamin B12 (1.4 kDa), dichlorofluorescene (401 Da), uridine (244

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Da), and 2-nitrobenzaldehyde (151 Da). The calibration curve is shown in Figure S2 (Supporting

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Information), where the linear region of the relationship between elution volume and molecular

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weight represents the range of molecules that had weak interactions with the packing column

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material. This calibration method only provides estimates of the molecular weights for BrC

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compounds since it remains unknown whether the molecular densities of the standards are

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representative of that of the BrC molecules of interest.

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3. RESULTS AND DISCUSSIONS

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3.1 Wood Smoke BrC

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The change in water-soluble organic carbon (WSOC) concentration in smoke BrC upon UV

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irradiation is shown in Figure 1a. Decreases in WSOC due to photolysis were observed, resulting

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in a net loss in 30% of WSOC after 125 hours of UV exposure. Absorption of UV radiation by

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chromophores can initiate photolysis, leading to the formation of products having higher

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volatility (e.g., fewer carbon numbers). Evaporation of these volatile products can lead to the

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observed loss in WSOC. In addition, the loss of WSOC due to photolysis exhibited an initial

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decay (i.e., first 8 hours of UV exposure) that was rapid, followed by a slower decay, suggesting

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that WS smoke BrC contains multiple chromophores of varying degrees of photolability.

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In addition to changes in WSOC, changes in the absorption per mass of water-soluble carbon

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(mass absorption coefficient, MAC) provide insight into the effects of photolysis on the light

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absorptivity of water-soluble chromophores. The calculation method for MAC at 365 nm and

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400 nm are discussed in Section S2. Shown in Figure 1b, exposure to UV light leads to initial


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increase in MAC values at both wavelengths, indicating photo-enhancement (i.e., increased

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absorptivity of near UV-VIS radiation by BrC). Given that a loss in WSOC was observed during

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this photo-enhancement period, we speculate that the photolysis of WS smoke BrC leads to the

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formation of products of higher volatility that evaporate to the gas phase, as well as products that

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remain in the aqueous BrC solution, but are more light absorbing. Previous studies have shown

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that aqueous-phase photo-oxidation of phenolic compounds34–37

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compounds,20 both of which have been identified in biomass burning organic aerosols,38–40 lead

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to increased absorption of near UV-VIS radiation. The proposed mechanisms leading to the

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increased absorption were attributed to the polymerization of phenolic compounds35,37 and OH-

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functionalization of nitro-aromatric compounds.20 Additionally, photo-enhancement has been

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previously observed for aged biomass burning BrC emitted from the pyrolysis of hickory, pine

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and oak wood,22,23 as well as from the combustion of kaoliang stalk.20 After this initial period of

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photo-enhancement (up to 20

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bleaching, behaviors previously observed from the aforementioned studies.20,23

and nitro-aromatic

hours), continual exposure to UV lights led to the photo-

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In addition to total light absorptivity, the molecular weight distributions of BrC (provided by

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SEC) offer additional insights on the molecular nature of chromophores and the effects of

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photolysis. Typical image plots of the molecular weight separated BrC absorption spectra from

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the WS and water-insoluble (WI) components of wood smoke are shown in Figure 2. Two main

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populations of chromophores can be observed for unreacted BrC smoke: highly absorbing large

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chromophores and less absorbing smaller chromophores. Comparison of light absorption by the

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unreacted WS and WI components indicated that the majority of light absorption can be

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attributed to water-soluble chromophores, at all illumination times (shown in Figure S3). On

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average, WI BrC contributed 23 ± 9 % of the total light absorption at 365 nm by wood smoke


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BrC (i.e., sum of absorption by both WS and WI BrC). While the discussion below primarily

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focuses on the results from the photolysis of WS BrC, similar trends in results were observed for

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the WI BrC component (shown in Figure S4).

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To illustrate the evolution of chromophores with different molecular weights, the changes in

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the total absorption at 365 nm (Abs365) for different molecular weight fractions are shown in

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Figure 3. Here the total Abs365 are binned according to the strength of interaction with the

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column packing material, where high molecular weight fraction (high-MW) are defined as

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chromophores that had weak interactions with the SEC column (i.e., the linear region of the

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calibration curve shown in Figure S2), which have approximate molecular weights between 66

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kDa and 401 Da. The small molecular weight fraction (small-MW) are defined as chromophores

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that had strong interactions with the SEC volume and have approximate molecular weights

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smaller than approximately 400 Da. Note that the molecular weight values are only estimates, as

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it remains unknown whether the molecular densities of the calibration standards are

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representative of the molecular densities of BrC molecules. For both molecular weight fractions,

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initial photo-enhancement were observed, followed by photo-bleaching with prolonged UV light

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exposure. These initial increases and subsequent decays in absorption by different molecular

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weight fractions exhibited first-order kinetics. Shown in Table 1, the rates of photo-enhancement

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(kpe) were determined by fitting first-order growth curves to the first 4 hours of absorption data.

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For photo-bleaching rates (kpb), first-order decay curves were fitted to the initial decay in

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absorption (e.g., between 20 to 52 hours of UV exposure for WS BrC and between 8 to 40 hours

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of UV exposure for WI BrC), where kpb represents the rate of decay for more photolabile species.

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In general, photo-enhancement was more significant for the high-MW fraction of smoke BrC

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whereas the kinetics of photo-bleaching are similar for both high-MW and low-MW fractions.


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Faster photo-enhancement by the high-MW fraction may be due to these chromophores being

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more photo-reactive (e.g., larger absorption cross sections and/or quantum yields) compared to

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chromophores in low-MW fraction. Comparison of the photoreactivity of WS and WI BrC

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suggests that WI BrC of all molecular weight fractions undergo more rapid photo-enhancement

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during the first 4 hours of UV light exposure (Figure S4). Additionally, maximum absorption by

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the WI BrC was observed at this time while WS BrC exhibited longer photo-enhancement (up to

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20 hours of UV light exposure). Note that the concentration of organic carbon in WI BrC was not

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quantified in this study (see Section 2.3).

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Owing to their rapid photo-enhancement, chromophores in the high-MW fraction were

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persistent and dominated total light absorption at 365 nm (with an increasing contribution with

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UV exposure; Figure S5). This result is consistent with previous ambient measurement of

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molecular weight separated aged biomass burning organic aerosols (from a boreal forest fire),

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where the majority of water-soluble BrC absorption was attributed to molecules larger than 500

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Da.26 It is possible that the allocation of a molecular weight cutoff value for atmospherically

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stable chromophores is dependent on the biomass fuel type and burning conditions, as emissions

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are known to depend on both factors.41

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3.2 AS-MGL BrC

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Although the previous set of experiments demonstrated that the effects of photolytic aging on

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the light absorbing properties of BrC from wood smoke is dependent on the molecular weight of

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its components, it remains unclear whether other types of BrC exhibit this type of behavior. For

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the current study, we examined the changes in molecular weight distributions due to the

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photolysis of chromophores generated from the reaction of ammonium sulfate and methylglyoxal

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(AS-MGL BrC), as this reaction system is commonly used as laboratory surrogates of secondary

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BrC.18,20,29,42,43


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Shown in Figure 4, two populations of chromophores were observed for unphotolyzed AS-

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MGL BrC: a population of larger chromophores that strongly absorbs radiation at 365 nm and a

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less absorbing population of smaller chromophores. Upon exposure to UV lights, rapid photo-

275

bleaching was observed for all chromophores (Figure 5).

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Rate constants for the photo-bleaching of AS-MGL BrC were determined by fitting the

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observed Abs365 during the first 4 hours of UV exposure to first-order decay curves (Table 1).

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Similar to wood smoke BrC, the change in light absorption due to photolytic aging exhibited a

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molecular weight dependence, where the fastest decay was observed for the smallest molecules.

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Additionally, the rate of absorption decay decreases with time, suggesting that this type of BrC

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contains chromophores with different photoreactivity. To date, studies investigating the

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photolysis of this type of BrC have not observed photo-enhancement.18,20

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Observed decay rates for bulk absorption (i.e., sum of all molecular weight fractions) at 400

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nm for the current study [(1.2 ± 0.1) × 10-4 s-1], resulted in a half-life of 95 minutes against

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photolysis (Figure S6). This value is generally consistent with photolysis half-life determined by

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Zhao et al. using bulk absorbance measurements (~13 minutes), considering differences in the

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following experimental conditions between the two studies: concentrations of BrC precursors

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and the photon fluxes inside the respective photoreactors.20

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4. ATMOSPHERIC IMPLICATIONS

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UVA light exposure of BrC molecules led to significant changes in their light absorptivity and

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molecular weight distributions; the extent of photo-enhancement and photo-bleaching depended

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on the molecular weight fraction and source of BrC. In particular, the largest molecules in

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biomass burning BrC (i.e., high-MW fraction) contributed to the majority of total light

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absorption, due to rapid photo-enhancement of these molecules. These results indicate that

295

molecular weight separated techniques, such as SEC, can be useful tools to elucidate the aging


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mechanisms of large molecular weight substances in the atmosphere. However, the molecular

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weights of BrC reported in this work are only approximate values, as the accuracy of the SEC

298

calibration approach depends on whether the molecular densities of the calibration standards are

299

representative of that of the BrC molecules. Further work to verify the molecular weights of BrC,

300

such as coupling SEC-UV absorption spectroscopy with light scattering techniques, which have

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been employed to determine the absolute molecular weights of lignin and its by-products,44,45 is

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warranted.

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From the observed decay rate for the high-MW fraction of WS BrC, the initial atmospheric

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lifetime with respect to photolysis is estimated to be approximately 14-36 hours at solar noon

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(calculation method discussed in Section S3). Given that the average lifetime of particles in the

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atmosphere is approximately one week with respect to deposition, this very rough estimate

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suggests that large water-soluble BrC molecules from biomass burning could remain throughout

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the majority of the particles’ lifespan and so could be ubiquitous in the atmosphere, as

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observed.27 We stress that there are uncertainties in this estimate, as it assumes that the

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photolysis quantum yield is wavelength-independent and that photolysis of BrC in the

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atmosphere is restricted to the wavelength range considered (300 – 400 nm). The wavelengths

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responsible for BrC photolysis (i.e., photolysis quantum yields) are currently unknown.

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Nonetheless, these experimental results further support earlier observations that large

314

molecular weight BrC species from biomass burning can be long-lived components in

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atmospheric aerosols.25,26 This means they are more likely to have larger impacts on aerosol

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direct radiative forcing on regional scales, whereas the contribution of smaller species to BrC,

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either emitted from biomass burning or formed from small carbonyl compounds, are likely to be

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most important near source regions. In addition, the observed rapid photo-enhancement of water-


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soluble biomass burning BrC suggests that secondary production of BrC in atmospheric aqueous

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media (e.g., wet aqueous, fog and cloud droplets) can be an important source of BrC in the

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atmosphere. In particular, Gilardoni et al. recently reported ambient observations of light

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absorbing secondary organic aerosol formation from the processing of biomass burning

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emissions in the aqueous phase.17

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Also, we note that the majority of total light absorption at 365 nm observed in this study was

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contributed by the water-soluble component of wood smoke BrC (77 ± 9 %), which is consistent

326

with the observations by Di Lorenzo and Young.26 However, dominant contributions to total

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light absorption at 365 nm by BrC extractable in methanol or acetone were observed in the

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atmosphere,14,46 as well as from laboratory generated smoke BrC from the pyrolysis of pine and

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oak.28 These differences may be due to fuel type and burn conditions, or that only primary smoke

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aerosol, in isolation from other atmospheric species, was studied here. Additionally, the sample

331

preparation approach employed in this study (sequential filter extraction with water, followed by

332

methanol) does not take into account the contribution of water-insoluble BrC compounds on

333

suspended particles that may have been removed during filtration of the water sample extract.

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Given these contrasting results, addition work investigating the relative contributions of water-

335

soluble and insoluble BrC using a wide range of BrC precursors and burn conditions are

336

necessary.

337

While our results continue to support the view that the majority of AS-MGL BrC undergo

338

rapid photo-bleaching, a small fraction of these chromophores may persist in the atmosphere

339

(e.g., in Figure 5, 5 - 10 % of the initial absorption by AS-MGL BrC remains after 40 hours of

340

UV exposure). As such, it is important to quantify the relative contribution of different sources to

341

background BrC. We note that not all secondary BrC undergo rapid photo-bleaching, as the


15


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342

atmospheric lifetime of secondary BrC formed from the photo-oxidation of naphthalene (under

343

high NOx-conditions) has been estimated to be approximately 20 hours.19

344

Further investigations on the effects of other atmospheric aging processes on the light

345

absorptivity and chemical composition of different molecular weight BrC fractions are needed.

346

In particular, the current study examined the photolytic aging of BrC dissolved in bulk solutions.

347

This type of approach does not simulate aging processes occurring on or within suspended

348

particles, where parameters such as gas-particle collision frequencies, aerosol phase state and

349

solute concentrations (including pH) are different. For example, the effects of aging processes

350

such as cloud/fog droplets evaporation47 and heterogeneous reactions48 on the physio-chemical

351

properties of BrC have been demonstrated.

352

Sources of ambient fine particle OA remains an open question since the components all tend to

353

evolve to a similar highly oxygenated state49,50 and specific chemical source tracers, including

354

those for biomass burning, can have a considerably shorter atmospheric lifetimes than aerosol.51–

355

53

356

underestimated for aged aerosol, leading to the view that biomass burning may be a much more

357

important contributor to global than currently believed.53,54 The unique stability of high-MW

358

fraction of BrC may provide an alternative to traditional biomass burning markers and enable a

359

better estimate of the true impact of biomass burning emissions on the atmospheric aerosol

360

burden.

361

Supporting Information.

Because of this, the mass fraction of aerosol attributed to biomass burning may be grossly

362

Experimental procedure for the determination of photon flux inside the photoreactor; details on

363

the calculation of mass absorption coefficients; method to convert observed decay rates to

364

equivalent atmospheric lifetimes; six supporting figures (photon flux inside photoreactor;


16

Page 17 of 30


365

molecular weight vs retention time calibration curve; evolution of light absorption by WS and

366

WI-BrC from wood smoke; time series of the change in absorption for different molecular

367

weight fractions of WS and WI wood smoke BrC; time series of the change in WSOC and mass

368

absorption coefficients of AS-MGL BrC at 365 and 400nm; and a table listing the estimated

369

atmospheric lifetimes of BrC.

370

Corresponding Author

371

*Email: [email protected]; phone: 404-894-1750; fax: 404-894-5638

372

Acknowledgement

373

Funding for this work was provided by the Electric Power Research Institute (EPRI) through

374

contract #00-10003806. Additional support was also provided by NASA through contract

375

NNX14A974G. AN acknowledges support from a Georgia Power Faculty Scholar chair and a

376

Johnson Faculty Fellowship.

377 378 379

Figure 1. Time series profile of the (a) changes in WSOC concentration (normalized to initial

380

values) and (b) WSOC mass absorption coefficients at 365 nm (black circles) and 400 nm (red

381

squares) for the photolysis of WS smoke BrC. The error bars represent the variability (±1σ) of

382

multiple experiments (n = 4).

383 384

Figure 2. Typical molecular weight separated absorption spectra of unreacted water-soluble

385

(WS) smoke BrC (top) and water-insoluble (WI) smoke BrC (bottom). Arrows indicate the

386

elution volumes (Ve) of some calibration standard: bovine serum albumin (Ve=7.6 mL, 66 kDa),


17


Page 18 of 30

387

aprotinin (Ve = 10.3 mL, 6.5 kDa), and dichlorofluorescene (Ve = 15.1 mL, 401 Da). Note that

388

molecular weight increases with decreasing elution volume.

389 390

Figure 3. Time series profile of the change in absorption at a wavelength of 365 nm for the high-

391

molecular weight (red circles) and low molecular weight (black triangles) fractions in WS smoke

392

BrC due to photolysis. The insert is a zoomed-in view of the changes observed at longer UV

393

illumination times.

394 395

Figure 4. An image plot of the molecular weight separated absorption spectra of unphotolyzed

396

AS-MGL BrC. Arrows indicate the elution volumes (Ve) of some calibration standard: bovine

397

serum albumin (Ve=7.6 mL, 66 kDa), aprotinin (Ve = 10.3 mL, 6.5 kDa), and

398

dichlorofluorescene (Ve = 15.1 mL, 401 Da). Note that molecular weight increases with

399

decreasing elution volume.

400

Figure 5. Time series profile of the 365 nm wavelength absorption change compared to initial

401

values for the high-molecular weight (red circles) and low molecular weight (black triangles)

402

fractions in AS-MGL BrC due to photolysis.

403 404

Figure S1. Measured photon flux in the photoreactor using chemical actinometry (red) and

405

spectroradiometer (blue) compared to the actinic flux for a clear-sky summer day (black). The

406

actinic

407

http://cprm.acom.ucar.edu/Models/TUV/Interactive_TUV/ using the following parameters: SZA

flux

was

obtained

from

“Quick

TUV

Calculator”,

available

at


18

Page 19 of 30


408

= 0, June 30, 2000, overhead ozone of 300 Dobson units, surface albedo of 0.1 and at 0 km

409

attitude.

410 411

Figure S2. Elution volume-molecular weight calibration curve for the size exclusion

412

chromatography-UV/Vis absorption spectroscopy technique used in this work. The error bars

413

represent the variabilities (± 1σ) of 5 calibration replicates. The fitted linear curve represents the

414

elution volumes between the exclusion and penetration limits of the column (i.e. molecules that

415

elute in these volumes had weak interaction with the packing material of the size exclusion

416

chromatography column). The arrows represent the range of elution volumes for “high-MW” and

417

“low-MW” BrC fractions.

418 419

Figure S3. The contributions of WS (light blue) and WI BrC (dark blue) to total absorption at

420

365 nm at different illumination times.

421 422

Figure S4. (a) Time series profile of the change in absorption for the high-molecular weight (red

423

circles) and low molecular weight (black triangles) fractions in WI smoke BrC due to photolysis.

424

The insert is a zoomed-in view of the changes observed at longer UV illumination times. The

425

relative contribution of different molecular weight fractions to total absorption by WI smoke BrC

426

is shown in (b).

427


19


Page 20 of 30

428

Figure S5. Relative contributions of the high-molecular weight (red circles) and low molecular

429

weight (black triangles) fractions in WS smoke BrC to total absorption as a function of

430

illumination time.

431 432

Figure S6. Time series profile of the (a) changes in WSOC (normalized to initial values) and (b)

433

water-soluble carbon mass absorption coefficient at 365 nm (black circles) and 400 nm (red

434

squares) for the photolysis of AS-MGL BrC. The error bars represent the variability (± 1σ) of

435

multiple experiments (n = 5).

436 437 438

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Time series profile of the (a) changes in WSOC (normalized to initial values) and (b) WSOC mass absorption coefficients at 365 nm (black circles) and 400 nm (red squares) for the photolysis of WS smoke BrC. The error bars represent the variability (±1σ) of multiple experiments (n = 4). 84x67mm (150 x 150 DPI)



Typical molecular weight separated absorption spectra of unreacted 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, 66 kDa), aprotinin (Ve = 10.3 mL, 6.5 kDa), and dichlorofluorescene (Ve = 15.1 mL, 401 Da). Note that molecular weight increases with decreasing elution volume. 84x55mm (150 x 150 DPI)


Page 26 of 30

Page 27 of 30


Time series profile of the change in absorption at a wavelength of 365 nm for the high-molecular weight (red circles) and low molecular weight (black triangles) fractions in WS smoke BrC due to photolysis. The insert is a zoomed-in view of the changes observed at longer UV illumination times. 84x53mm (150 x 150 DPI)



An image plot of the molecular weight separated absorption spectra of unphotolyzed AS-MGL BrC. Arrows indicate the elution volumes (Ve) of some calibration standard: bovine serum albumin (Ve=7.6 mL, 66 kDa), aprotinin (Ve = 10.3 mL, 6.5 kDa), and dichlorofluorescene (Ve = 15.1 mL, 401 Da). Note that molecular weight increases with decreasing elution volume. 84x37mm (150 x 150 DPI)


Page 28 of 30

Page 29 of 30


Time series profile of the 365 nm wavelength absorption change compared to initial values for the highmolecular weight (red circles) and low molecular weight (black triangles) fractions in AS-MGL BrC due to photolysis. 84x53mm (150 x 150 DPI)



84x50mm (150 x 150 DPI)


Page 30 of 30

Changes in Light Absorptivity of Molecular Weight Separated Brown

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