Changes in Light Absorptivity of Molecular Weight ... - ACS Publications

Subscriber access provided by University of Sydney Library

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

Environmental Science & Technology

1

Changes in Light Absorptivity of Molecular Weight

2

Separated Brown Carbon due to Photolytic Aging

3

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

4

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

5

USA

6

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

7

30331, USA

8

&

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

9 10

Patras, GR-26504, Greece ┴

Institute of Environmental Research and Sustainable Development, National Observatory of

11

Athens, Palea Penteli, GR-15236, Greece

12 13

Abstract

14

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

15

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

16

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

17

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

ACS Paragon Plus Environment

1


Page 2 of 30

18

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

19

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

20

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

21

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

22

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

23

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

24

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

25

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

26

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

28

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

29

biomass burning tracers.

30

1. INTRODUCTION

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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


2

Page 3 of 30


40

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

41

understand the persistent signatures in biomass burning aerosols.

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

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

50

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

51

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

52

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

53

chemical identity.13

54

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

55

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

56

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

57

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

58

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

59

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

60

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

61

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


3


Page 4 of 30

62

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

63

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

64

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

65

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

66

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

67

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

68

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

69

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

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

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

78

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

79

light absorptivity of different molecular weight BrC components.

Size exclusion

80

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

81

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

82

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

83

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

84

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


4

Page 5 of 30


85

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

86

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

87

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

88

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

89

highly variable and dynamic.

90

2. EXPERIMENTAL METHODS

91

2.1 Preparation of BrC Samples

92

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

93

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

94

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

95

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

96

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

97

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

98

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

99

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

100

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

101

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

102

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

103

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

104

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

105

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

106

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

107

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


5


Page 6 of 30

108

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

109

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

110

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

111

(Fisher).

112

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

113

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

114

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

115

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

116

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

117

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

118

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

119

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

120

was diluted by a factor of 7.

121

2.2 Photolysis of BrC

122

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

123

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

124

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

125

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

126

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

127

characterized by chemical actinometry using 2-nitrobenzaldehyde18 and the wavelength

128

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

129

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

130

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

131

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


6

Page 7 of 30


132

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

133

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

134

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

135

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

136

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

137

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

138

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

139

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

140

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

141

the elevated temperature conditions and not UVA radiation).

142

2.3 BrC Measurements

143

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

144

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

145

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

146

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

147

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

148

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

149

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

150

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

151

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

152

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

153

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

154

BrC 386 ± 40 µg WSOC.


7


Page 8 of 30

155

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

156

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

157

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

158

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

159

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

160

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

161

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

162

increase detection sensitivity.

163

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

164

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

165

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

166

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

167

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

168

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

169

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

170

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

171

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

172

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

173

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

174

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

175

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

176

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

177

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


8

Page 9 of 30


178

molecular weight was empirically determined using the following standards with known

179

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

180

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

181

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

182

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

183

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

184

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

185

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

186

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

187

representative of that of the BrC molecules of interest.

188

3. RESULTS AND DISCUSSIONS

189

3.1 Wood Smoke BrC

190

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

191

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

192

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

193

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

194

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

195

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

196

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

197

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

198

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

199

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

200

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

201

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


9


Page 10 of 30

202

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

203

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

204

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

205

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

206

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

207

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

208

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

209

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

210

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

211

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

212

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

213

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

214

photo-enhancement (up to 20

215

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

and nitro-aromatic

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

216

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

217

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

218

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

219

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

220

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

221

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

222

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

223

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

224

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


10

Page 11 of 30


225

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

226

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

227

the WI BrC component (shown in Figure S4).

228

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

229

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

230

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

231

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

232

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

233

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

234

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

235

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

236

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

237

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

238

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

239

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

240

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

241

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

242

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

243

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

244

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

245

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

246

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

247

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


11


Page 12 of 30

248

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

249

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

250

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

251

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

252

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

253

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

254

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

255

quantified in this study (see Section 2.3).

256

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

257

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

258

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

259

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

260

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

261

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

262

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

263

are known to depend on both factors.41

264

3.2 AS-MGL BrC

265

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

266

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

267

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

268

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

269

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

270

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

271

BrC.18,20,29,42,43


12

Page 13 of 30


272

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

273

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

274

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

275

bleaching was observed for all chromophores (Figure 5).

276

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

277

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

278

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

279

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

280

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

281

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

282

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

283

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

284

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

285

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

286

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

287

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

288

and the photon fluxes inside the respective photoreactors.20

289

4. ATMOSPHERIC IMPLICATIONS

290

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

291

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

292

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

293

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

294

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


13


Page 14 of 30

296

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

297

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

301

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

302

warranted.

303

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

304

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

305

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

306

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

307

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

308

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

309

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

310

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

311

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

312

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

313

Nonetheless, these experimental results further support earlier observations that large

314

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

315

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

316

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

317

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

318

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


14

Page 15 of 30


319

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

320

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

321

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

322

absorbing secondary organic aerosol formation from the processing of biomass burning

323

emissions in the aqueous phase.17

324

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

325

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

327

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

328

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

329

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

330

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.

334

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


Page 16 of 30

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

References

439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462

(1)

(2) (3) (4) (5) (6) (7) (8)

Boucher at al. Clouds and Aerosols. in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; Stocker et al., Ed.; Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA, 2013. Andreae, M. O.; Merlet, P. Emission of trace gases and aerosols from biomass burning. Glob. Biogeochem. Cycles 2001, 15 (4), 955–966, doi:10.1029/2000GB001382. Chung, C. E.; Kim, S.-W.; Lee, M.; Yoon, S.-C.; Lee, S. Carbonaceous aerosol AAE inferred from in-situ aerosol measurements at the Gosan ABC super site, and the implications for brown carbon aerosol. Atmos. Chem. Phys. 2012, 12 (14), 6173–6184. Feng, Y.; Ramanathan, V.; Kotamarthi, V. R. Brown carbon: a significant atmospheric absorber of solar radiation? Atmos. Chem. Phys. 2013, 13 (17), 8607–8621. He, S.; Carmichael, G. R. Sensitivity of photolysis rates and ozone production in the troposphere to aerosol properties. J. Geophys. Res. Atmospheres 1999, 104 (D21), 26307– 26324, doi:10.1029/1999JD900789. Mok, J.; Krotkov, N. A.; Arola, A.; Torres, O.; Jethva, H.; Andrade, M.; Labow, G.; Eck, T. F.; Li, Z.; Dickerson, R. R.; et al. Impacts of brown carbon from biomass burning on surface UV and ozone photochemistry in the Amazon Basin. Sci. Rep. 2016, 6, 36940. Kirchstetter, T. W.; Novakov, T.; Hobbs, P. V. Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon. J. Geophys. Res. Atmospheres 2004, 109 (D21), D21208, doi:10.1029/2004JD004999. Hecobian, A.; Zhang, X.; Zheng, M.; Frank, N.; Edgerton, E. S.; Weber, R. J. WaterSoluble Organic Aerosol material and the light-absorption characteristics of aqueous extracts measured over the Southeastern United States. Atmos. Chem. Phys. 2010, 10 (13), 5965–5977.


20

Page 21 of 30

463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508


(9) (10) (11) (12) (13) (14) (15)

(16)

(17)

(18) (19) (20) (21) (22)

(23)

Lack, D. A.; Langridge, J. M.; Bahreini, R.; Cappa, C. D.; Middlebrook, A. M.; Schwarz, J. P. Brown carbon and internal mixing in biomass burning particles. Proc. Natl. Acad. Sci. 2012, 109 (37), 14802–14807. Bond, T. C. Spectral dependence of visible light absorption by carbonaceous particles emitted from coal combustion. Geophys. Res. Lett. 2001, 28 (21), 4075–4078, doi:10.1029/2001GL013652. Zhang, X.; Lin, Y.-H.; Surratt, J. D.; Zotter, P.; Prévôt, A. S. H.; Weber, R. J. Lightabsorbing soluble organic aerosol in Los Angeles and Atlanta: A contrast in secondary organic aerosol. Geophys. Res. Lett. 2011, 38 (21), L21810, doi:10.1029/2011GL049385. Andreae, M. O.; Crutzen, P. J. Atmospheric aerosols: Biogeochemical sources and role in atmospheric chemistry. Science 1997, 276 (5315), 1052–1058. Laskin, A.; Laskin, J.; Nizkorodov, S. A. Chemistry of Atmospheric Brown Carbon. Chem. Rev. 2015, 115 (10), 4335–4382. Zhang, X.; Lin, Y.-H.; Surratt, J. D.; Weber, R. J. Sources, Composition and Absorption Ångström Exponent of Light-absorbing Organic Components in Aerosol Extracts from the Los Angeles Basin. Environ. Sci. Technol. 2013, 47 (8), 3685–3693. Mohr, C.; Lopez-Hilfiker, F. D.; Zotter, P.; Prévôt, A. S. H.; Xu, L.; Ng, N. L.; Herndon, S. C.; Williams, L. R.; Franklin, J. P.; Zahniser, M. S.; et al. Contribution of Nitrated Phenols to Wood Burning Brown Carbon Light Absorption in Detling, United Kingdom during Winter Time. Environ. Sci. Technol. 2013, 47 (12), 6316–6324. Lin, P.; Liu, J.; Shilling, J. E.; Kathmann, S. M.; Laskin, J.; Laskin, A. Molecular characterization of brown carbon (BrC) chromophores in secondary organic aerosol generated from photo-oxidation of toluene. Phys. Chem. Chem. Phys. 2015, 17 (36), 23312–23325. Gilardoni, S.; Massoli, P.; Paglione, M.; Giulianelli, L.; Carbone, C.; Rinaldi, M.; Decesari, S.; Sandrini, S.; Costabile, F.; Gobbi, G. P.; et al. Direct observation of aqueous secondary organic aerosol from biomass-burning emissions. Proc. Natl. Acad. Sci. 2016, 113 (36), 10013–10018. Sareen, N.; Moussa, S. G.; McNeill, V. F. Photochemical Aging of Light-Absorbing Secondary Organic Aerosol Material. J. Phys. Chem. A 2013, 117 (14), 2987–2996. Lee, H. J.; Aiona, P. K.; Laskin, A.; Laskin, J.; Nizkorodov, S. A. Effect of Solar Radiation on the Optical Properties and Molecular Composition of Laboratory Proxies of Atmospheric Brown Carbon. Environ. Sci. Technol. 2014, 48 (17), 10217–10226. Zhao, R.; Lee, A. K. Y.; Huang, L.; Li, X.; Yang, F.; Abbatt, J. P. D. Photochemical processing of aqueous atmospheric brown carbon. Atmos. Chem. Phys. 2015, 15 (11), 6087–6100. Liu, J.; Lin, P.; Laskin, A.; Laskin, J.; Kathmann, S. M.; Wise, M.; Caylor, R.; Imholt, F.; Selimovic, V.; Shilling, J. E. Optical properties and aging of light-absorbing secondary organic aerosol. Atmos. Chem. Phys. 2016, 16 (19), 12815–12827. Saleh, R.; Hennigan, C. J.; McMeeking, G. R.; Chuang, W. K.; Robinson, E. S.; Coe, H.; Donahue, N. M.; Robinson, A. L. Absorptivity of brown carbon in fresh and photochemically aged biomass-burning emissions. Atmos. Chem. Phys. 2013, 13 (15), 7683– 7693. Zhong, M.; Jang, M. Dynamic light absorption of biomass-burning organic carbon photochemically aged under natural sunlight. Atmos. Chem. Phys. 2014, 14 (3), 1517– 1525.


21


509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554

Page 22 of 30

(24) Forrister, H.; Liu, J.; Scheuer, E.; Dibb, J.; Ziemba, L.; Thornhill, K. L.; Anderson, B.; Diskin, G.; Perring, A. E.; Schwarz, J. P.; et al. Evolution of brown carbon in wildfire plumes: Brown Carbon in Biomass Burning Plumes. Geophys. Res. Lett. 2015, 42 (11), 4623–4630, doi:10.1002/2015GL063897. (25) Lee, A. K. Y.; Willis, M. D.; Healy, R. M.; Wang, J. M.; Jeong, C.-H.; Wenger, J. C.; Evans, G. J.; Abbatt, J. P. D. Single-particle characterization of biomass burning organic aerosol (BBOA): evidence for non-uniform mixing of high molecular weight organics and potassium. Atmos. Chem. Phys. 2016, 16 (9), 5561–5572. (26) Di Lorenzo, R. A.; Young, C. J. Size separation method for absorption characterization in brown carbon: Application to an aged biomass burning sample. Geophys. Res. Lett. 2016, 43 (1), 458-465, doi:10.1002/2015GL066954. (27) Liu, J.; Scheuer, E.; Dibb, J.; Ziemba, L. D.; Thornhill, K. L.; Anderson, B. E.; Wisthaler, A.; Mikoviny, T.; Devi, J. J.; Bergin, M.; et al. Brown carbon in the continental troposphere. Geophys. Res. Lett. 2014, 41 (6), doi:10.1002/2013GL058976. (28) Chen, Y.; Bond, T. C. Light absorption by organic carbon from wood combustion. Atmos. Chem. Phys. 2010, 10 (4), 1773–1787. (29) Tang, M.; Alexander, J. M.; Kwon, D.; Estillore, A. D.; Laskina, O.; Young, M. A.; Kleiber, P. D.; Grassian, V. H. Optical and Physicochemical Properties of Brown Carbon Aerosol: Light Scattering, FTIR Extinction Spectroscopy, and Hygroscopic Growth. J. Phys. Chem. A 2016, 120 (24), 4155–4166. (30) Galbavy, E. S.; Ram, K.; Anastasio, C. 2-Nitrobenzaldehyde as a chemical actinometer for solution and ice photochemistry. J. Photochem. Photobiol. Chem. 2010, 209 (2–3), 186– 192. (31) Strigel, Andre M.; Yau, Wallace W.; Kirkland, Joseph J.; Bly, Donald D. Modern SizeExclusion Liquid Chromatography, Second Edition.; John Wiley & Sons, Inc., 2009. (32) Gjessing, E. T. Physical and Chemical Characteristics of Aquatic Humus; Ann Arbor Science Publishers: Michigan, U.S.A., 1976. (33) Pelekani, C.; Newcombe, G.; Snoeyink, V. L.; Hepplewhite, C.; Assemi, S.; Beckett, R. Characterization of Natural Organic Matter Using High Performance Size Exclusion Chromatography. Environ. Sci. Technol. 1999, 33 (16), 2807–2813. (34) Gelencsér, A.; Hoffer, A.; Kiss, G.; Tombácz, E.; Kurdi, R.; Bencze, L. In-situ Formation of Light-Absorbing Organic Matter in Cloud Water. J. Atmospheric Chem. 2003, 45 (1), 25–33. (35) Chang, J. L.; Thompson, J. E. Characterization of colored products formed during irradiation of aqueous solutions containing H2O2 and phenolic compounds. Atmos. Environ. 2010, 44 (4), 541–551. (36) Ofner, J.; Krüger, H.-U.; Grothe, H.; Schmitt-Kopplin, P.; Whitmore, K.; Zetzsch, C. Physico-chemical characterization of SOA derived from catechol and guaiacol – a model substance for the aromatic fraction of atmospheric HULIS. Atmos. Chem. Phys. 2011, 11 (1), 1–15. (37) Smith, J. D.; Kinney, H.; Anastasio, C. Phenolic carbonyls undergo rapid aqueous photodegradation to form low-volatility, light-absorbing products. Atmos. Environ. 2016, 126, 36–44. (38) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Measurement of Emissions from Air Pollution Sources. 3. C1−C29 Organic Compounds from Fireplace Combustion of Wood. Environ. Sci. Technol. 2001, 35 (9), 1716–1728.


22

Page 23 of 30

555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600


(39) Kitanovski, Z.; Grgić, I.; Vermeylen, R.; Claeys, M.; Maenhaut, W. Liquid chromatography tandem mass spectrometry method for characterization of monoaromatic nitro-compounds in atmospheric particulate matter. J. Chromatogr. A 2012, 1268, 35–43. (40) Lin, P.; Aiona, P. K.; Li, Y.; Shiraiwa, M.; Laskin, J.; Nizkorodov, S. A.; Laskin, A. Molecular Characterization of Brown Carbon in Biomass Burning Aerosol Particles. Environ. Sci. Technol. 2016, 50 (21), 11815–11824. (41) Simoneit, B. R. T.; Rogge, W. F.; Mazurek, M. A.; Standley, L. J.; Hildemann, L. M.; Cass, G. R. Lignin pyrolysis products, lignans, and resin acids as specific tracers of plant classes in emissions from biomass combustion. Environ. Sci. Technol. 1993, 27 (12), 2533–2541. (42) Sedehi, N.; Takano, H.; Blasic, V. A.; Sullivan, K. A.; De Haan, D. O. Temperature- and pH-dependent aqueous-phase kinetics of the reactions of glyoxal and methylglyoxal with atmospheric amines and ammonium sulfate. Atmos. Environ. 2013, 77, 656–663. (43) Powelson, M. H.; Espelien, B. M.; Hawkins, L. N.; Galloway, M. M.; De Haan, D. O. Brown Carbon Formation by Aqueous-Phase Carbonyl Compound Reactions with Amines and Ammonium Sulfate. Environ. Sci. Technol. 2014, 48 (2), 985–993. (44) Fredheim, G. E.; Braaten, S. M.; Christensen, B. E. Molecular weight determination of lignosulfonates by size-exclusion chromatography and multi-angle laser light scattering. J. Chromatogr. A 2002, 942 (1–2), 191–199. (45) Gidh, A. V.; Decker, S. R.; Vinzant, T. B.; Himmel, M. E.; Williford, C. Determination of lignin by size exclusion chromatography using multi angle laser light scattering. J. Chromatogr. A 2006, 1114 (1), 102–110. (46) Liu, J.; Scheuer, E.; Dibb, J.; Diskin, G. S.; Ziemba, L. D.; Thornhill, K. L.; Anderson, B. E.; Wisthaler, A.; Mikoviny, T.; Devi, J. J.; et al. Brown carbon aerosol in the North American continental troposphere: sources, abundance, and radiative forcing. Atmos. Chem. Phys. 2015, 15 (14), 7841–7858. (47) Lee, A. K. Y.; Zhao, R.; Li, R.; Liggio, J.; Li, S.-M.; Abbatt, J. P. D. Formation of Light Absorbing Organo-Nitrogen Species from Evaporation of Droplets Containing Glyoxal and Ammonium Sulfate. Environ. Sci. Technol. 2013, 47 (22), 12819–12826. (48) Slade, J. H.; Thalman, R.; Wang, J.; Knopf, D. A. Chemical aging of single and multicomponent biomass burning aerosol surrogate particles by OH: implications for cloud condensation nucleus activity. Atmos. Chem. Phys. 2015, 15 (17), 10183–10201. (49) Jimenez, J. L.; Canagaratna, M. R.; Donahue, N. M.; Prevot, A. S. H.; Zhang, Q.; Kroll, J. H.; DeCarlo, P. F.; Allan, J. D.; Coe, H.; Ng, N. L.; et al. Evolution of Organic Aerosols in the Atmosphere. Science 2009, 326 (5959), 1525–1529. (50) Ng, N. L.; Canagaratna, M. R.; Zhang, Q.; Jimenez, J. L.; Tian, J.; Ulbrich, I. M.; Kroll, J. H.; Docherty, K. S.; Chhabra, P. S.; Bahreini, R.; et al. Organic aerosol components observed in Northern Hemispheric datasets from Aerosol Mass Spectrometry. Atmos. Chem. Phys. 2010, 10 (10), 4625–4641. (51) Hennigan, C. J.; Sullivan, A. P.; Collett, J. L.; Robinson, A. L. Levoglucosan stability in biomass burning particles exposed to hydroxyl radicals. Geophys. Res. Lett. 2010, 37 (9), L09806, doi:10.1029/2010GL043088. (52) Zhao, R.; Mungall, E. L.; Lee, A. K.; Aljawhary, D.; Abbatt, J. P. Aqueous-phase photooxidation of levoglucosan–a mechanistic study using aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS). Atmos. Chem. Phys. 2014, 14 (18), 9695–9706.


23


601 602 603 604 605 606 607 608

Page 24 of 30

(53) Bougiatioti, A.; Stavroulas, I.; Kostenidou, E.; Zarmpas, P.; Theodosi, C.; Kouvarakis, G.; Canonaco, F.; Prévôt, A. S. H.; Nenes, A.; Pandis, S. N.; et al. Processing of biomassburning aerosol in the eastern Mediterranean during summertime. Atmos. Chem. Phys. 2014, 14 (9), 4793–4807. (54) Blanchard, C. L.; Hidy, G. M.; Shaw, S.; Baumann, K.; Edgerton, E. S. Effects of emission reductions on organic aerosol in the southeastern United States. Atmos. Chem. Phys. 2016, 16 (1), 215–238.


24

Page 25 of 30


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 ... - ACS Publications

Recommend Documents