Formation and optical properties of brown carbon from small Î±

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Environmental Processes

Formation and optical properties of brown carbon from small #-dicarbonyls and amines Wilmarie Marrero-Ortiz, Min Hu, Zhuofei Du, Yuemeng Ji, Yujue Wang, Song Guo, Yun Lin, Mario Gomez-Hermandez, Jianfei Peng, Yixin Li, Jeremiah Secrest, Misti Levy Zamora, Yuan Wang, Taicheng An, and Renyi Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03995 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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Environmental Science & Technology

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Formation and optical properties of brown carbon

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from small α-dicarbonyls and amines

3

Wilmarie Marrero-Ortiz1‡, Min Hu2*, Zhuofei Du2‡, Yuemeng Ji3*, Yujue Wang2, Song

4

Guo2, Yun Lin4, Mario Gomez-Hermandez1,5, Jianfei Peng4, Yixin Li1, Jeremiah Secrest1,

5

Misti L. Zamora4,6, Yuan Wang7, Taicheng An3, and Renyi Zhang1,4* 1Department

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of Chemistry, Texas A&M University, College Station, Texas, 77840

7

2State

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of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

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Department of Atmospheric Sciences, Texas A&M University, College Station, Texas,

10

77843

Key Joint Laboratory of Environmental Simulation and Pollution Control, College

3Guangzhou

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Key Laboratory of Environmental Catalysis and Pollution Control, School

12

of Environmental Science and Engineering, Institute of Environmental Health and

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Pollution Control, Guangdong University of Technology, Guangzhou 510006, China

14

4Departments

of Atmospheric Sciences, Texas A&M University, College Station, TX 77843, USA

15 5 Department

16

of Chemistry and Biochemistry, Florida International University, Miami, FL 33199

17 6 Environmental

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Hopkins University, MD 21218

19 7 Division

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of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

21 22

Health & Engineering, Johns Hopkins School of Public Health, Johns

‡

The authors contributed equally to this work

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1

ACS Paragon Plus Environment


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Abstract: Brown Carbon (BrC) aerosols scatter and absorb solar radiation, directly

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affecting the Earth’s radiative budget. However, considerable uncertainty exists

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concerning the chemical mechanism leading to BrC formation and their optical

27

properties. In this work, BrC particles were prepared from mixtures of small α-

28

dicarbonyls (glyoxal and methylglyoxal) and amines (methylamine, dimethylamine, and

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trimethylamine). The absorption and scattering of BrC particles were measured using a

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photo-acoustic extinctometer (405 and 532 nm), and the chemical composition of the α-

31

dicarbonyl-amine mixtures was analyzed using orbitrap-mass spectrometry and thermal

32

desorption-ion drift-chemical ionization mass spectrometry. The single scattering albedo

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for methylglyoxal-amine mixtures is smaller than that of glyoxal-amine mixtures and

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increases with the methyl substitution of amines. The mass absorption cross-section for

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methylglyoxal-amine mixtures is two times higher at 405 nm wavelength than that at 532

36

nm wavelength. The derived refractive indexes at the 405-nm wavelength are 1.40-1.64

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for the real part and 0.002-0.195 for the imaginary part. Composition analysis in the α-

38

dicarbonyl-amine mixtures reveals N-heterocycles as the dominant products, which are

39

formed via multiple steps involving nucleophilic attack, steric hindrance, and dipole-

40

dipole interaction between α-dicarbonyls and amines. BrC aerosols, if formed from the

41

particle-phase reaction of methylglyoxal with methylamine, likely contribute to

42

atmospheric warming.

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1.

Introduction Atmospheric aerosols influence the Earth-atmosphere system in several distinct

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manners. For example, the effects of aerosols include modifying the Earth’s radiative

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balance, altering cloud formation and precipitation processes, reducing air quality and

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visibility, changing global biogeochemical cycles, and imposing adverse human health

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effects.1-4 In particular, the climate system is impacted by aerosols, by aerosol-radiative

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(ARI) and aerosol-cloud (ACI) interactions. Currently, the ARI and ACI effects represent

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the largest uncertainty in climate predictions using global climate models.5, 6 The large

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uncertainty of the aerosol radiative forcing is attributed in part to the chemical

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complexity of aerosols and the lack of understanding of their formation, transformation,

54

and physico-chemical properties (such as cloud-forming and optical properties).

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The optical properties of aerosols are important not only to the direct radiative

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forcing on climate but also relevant to air quality and weather. Light absorption and

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scattering by aerosols stabilize the atmosphere by retarding vertical transport, resulting in

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a negative feedback on air quality and inhibition of cloud formation.7 For example, black

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carbon (BC) particles affect radiative transfer in the atmosphere because of their strong

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ability to absorb light over a broad range of the solar spectrum, representing the second

61

most important anthropogenic climate-warming agent after carbon dioxide.8 The

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magnitude of BC direct radiative forcing depends on the mixing state (i.e., whether

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particles are externally or internally mixed with other aerosol types) and atmospheric

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aging by coating with secondary aerosol constituents (such as organics and sulfate),

65

thereby enhancing the mass absorption cross-section.9-11 There is growing evidence that

66

light-absorbing organic aerosols, known as brown carbon (BrC), also represent a

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67

significant climate-forcer.12 Typically, BC absorption is wavelength independent, while

68

BrC exhibits the strongest light absorption at shorter wavelengths.13 BrC is produced

69

from various primary and secondary sources. The main sources of primary BrC include

70

biomass burning,14 fossil fuel combustion,15 and biogenic releases.16 Additionally, light-

71

absorbing secondary organic aerosols (SOA) are generated by a variety of atmospheric

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chemical processes, such as multiphase reactions or cloud processing, yielding high

73

molecular weight light-absorbing organic compounds.17-22 Atmospheric measurements

74

have shown that BrC exists throughout the tropospheric column and its prevalence

75

relative to BC is proportional to the altitude, indicating the contribution of SOA to BrC

76

formation.23 However, light absorption by BrC has yet to be accounted for in estimation

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of the aerosol direct radiative forcing,3 since global climate models typically have

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assumed that SOA is purely scattering (non-absorbing).24, 25 The light absorption by

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organics depends on their molecular structures12, 26 and is influenced by supramolecular

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interactions.27 Currently, the understanding of formation, chemical composition, and

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optical properties of BrC is limited.

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Recent studies have shown the importance of particle-phase chemistry in

83

producing light-absorbing high molecular weight oligomeric species.28-30 Those earlier

84

studies of aqueous chemistry have focused on water-soluble volatile organic compounds

85

(VOC), such as small α-dicarbonyls, particularly glyoxal (GL) and methylglyoxal (MG).

86

As the important SOA precursors, GL and MG are produced by the photo-oxidation of

87

anthropogenic aromatics and biogenic terpenes and isoprene.31-33 Despite their high

88

volatility, GL and MG undergo polymerization to produce low-volatility oligomers.34, 35

89

In the troposphere, particularly in urban areas, the small α-dicarbonyls likely co-exist

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with amines at comparable concentrations.36, 37 Low molecular weight aliphatic amines,

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such as methylamine (MA), dimethylamine (DA), and trimethylamine (TA), are the most

92

abundant. Amines are emitted from various biogenic sources (e.g., ocean organisms,

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protein degradation, and biomass burning) and anthropogenic sources (e.g., animal

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husbandry, automobiles, industry, and treatment of sewage and waste).38, 39 The small α-

95

dicarbonyls react irreversibly with ammonium salts and primary amines, forming

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imidazoles and other nitrogen-containing and light absorbing products, but their chemical

97

identities have yet to be characterized.20, 40-42

98

Previous studies by De Haan and coworkers investigated cloud processing of GL

99

and MG via the reactions with amino acids and MA in evaporating aqueous droplets and

100

bulk solutions leading to the formation of BrC using nuclear magnetic resonance (NMR),

101

electron spray ionization - mass spectrometry (ESI-MS), and aerosol mass spectrometry

102

(AMS).19, 20, 40 The products were identified as imidazoles, and the imine formation was

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recognized as the rate-limiting step, as later confirmed by theoretical calculations.43

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Laskin and coworkers26 have studied the bulk-phase and chemical aging of biogenic SOA

105

in the presence of ammonia to form BrC. Using nano-desorption electron spray ionization

106

(DESI), the authors concluded that the light-absorbing properties of SOA are determined

107

by trace amounts of strong BrC chromophores. In another recent study, Lin et al.44 have

108

identified a relationship between the optical properties and chemical composition,

109

showing that all chromophores are nitrogen-containing compounds. It has been

110

hypothesized that the chromophores responsible for the browning are highly conjugated

111

nitrogen-containing compounds such as N-heterocycles.12, 41, 43, 45, 46 Most previous

112

studies have focused on primary amines, although tertiary amines are the most abundant

5



113

in the atmosphere.38 Most recently, it has been suggested that formation of brown carbon

114

from aqueous- and aerosol-phase reactions involving methylglyoxal occurs at rates that

115

are orders of magnitude faster than that in bulk solutions.47 In this work, particles containing BrC oligomers were synthesized from the

116 117

mixtures of small α-dicarbonyls (i.e., GL and MG) and amines (i.e., MA, DA, TA). The

118

optical properties (i.e., absorption and scattering) were measured using a commercial

119

photo-acoustic extinctometer (405 and 532 nm), and the chemical composition was

120

characterized by two complementary mass spectrometric techniques. Also, the refractive

121

indexes at the 405 nm wavelength were derived, including the real and imaginary parts.

122

Additionally, the relative radiative forcing of those BrC particles was estimated.

123

2.

124

Methodology The unbuffered model reaction systems in our study included 6 different mixtures,

125

i.e., MG-MA, MG-DA, MG-TA, GL-MA, GL-DA, and GL-TA. All reagents were used

126

as received and purchased from Sigma Aldrich except for DA, which was purchased from

127

Sinopharm Chemical Reagent. An aqueous solution of 1 M of each reagent was prepared,

128

and 1 mL of the α-dicarbonyl (MG or GL) was combined with 1 mL of the amine (MA,

129

DA, or TA) and sonicated for about 5 hours in small vials. The resulting colored products

130

were dried using nitrogen for several hours (in order to speed up the reaction) to yield 0.5

131

mL of the solution, which was then re-dissolved in up to 12 mL of Mill-Q water for

132

optical and chemical composition analysis. The procedure to synthesize the samples and

133

the concentration used in our work were similar to those by De Haan et. al.19, 20, 40, 48 who

134

suggested that drying increased the product yields.

135 136

Aliquots of 1 mL were used for determination of chemical composition using two complementary techniques, i.e., thermal desorption-ion drift-chemical ionization mass 6


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spectrometry (TD-ID-CIMS) and orbitrap-mass spectrometry (Orbitrap-MS, Thermo

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Scientific Inc.). The TD-ID-CIMS operated in the positive mode using the hydronium

139

ions (H3O+) for analysis of a 2 μL sample. The Orbitrap-MS employed electrospray

140

ionization in the positive mode (ESI+). In the Orbitrap-MS, a sample was diluted 10 times

141

with Mill-Q water before injection at a flow rate of 5 μL min-1.49

142

The experimental setup for measuring optical properties is shown in Figure S1.

143

The main components included an atomizer (DMT portable aerosol generator),

144

differential mobility analyzer (DMA, TSI 3081), condensation particle counter (CPC, TSI

145

3775), and a commercial photo-acoustic extinctometer (PAX, DMT 405). The procedures

146

for aerosol production and processing have been described elsewhere and are described

147

here only briefly.9 Aqueous solutions of the mixtures were atomized using pre-purified

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nitrogen to generate poly-disperse aerosols. The particles were diluted with a dry,

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particle-free nitrogen flow and passed through a silica gel diffusion dryer to reduce the

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relative humidity to less than 10%. The aerosols were size-selected using a DMA to

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produce a mono-disperse distribution. The mono-disperse aerosol flow was then split

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between the CPC and the PAX to measure the aerosol concentration and the optical

153

properties, respectively.

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The PAX used in-situ photoacoustic technology to measure absorption and

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reciprocal nephelometry to measure scattering.50 In the absorption cell, a laser beam

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directed through the aerosol stream was modulated at the resonant frequency of the

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acoustic chamber. The energy absorbed by particles was thermally transferred to the

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surrounding air, and subsequent air expansion produced a sound wave, which was

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recorded by a microphone. In the scattering cell, the standard nephelometer was used, but

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the locations of the light source and detectors were reversed. The instrument was

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calibrated with polystyrene latex spheres and Aquadag soot particles. The extinction was

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calculated from the sum of the absorption and scattering. Most of the measurements using

163

the PAX were made at 405 nm, and only limited data were at 532 nm.

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The effective densities (ρ) of aerosols were measured by using a DMA-APM

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(aerosol particle mass APM 3600, Kanomax) analyzer.9 The effective density is defined

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as the ratio of the measured mass to the volume, which is determined by the measured

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size by assuming spherical particles. Measurements were repeated for a range of particle

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diameters between 150 nm and 350 nm and at different concentrations. For each

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diameter, the optical properties (extinction, scattering and absorption), the density, and

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the particle concentration were determined. The data was corrected for doubly charged

171

particles by assuming that 20% of the optical properties was contributed from doubly

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charge particles.

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For a cell containing only particles in nitrogen gas, the absorption coefficient

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(babs) depends on the particle size (Dp) and concentration (N). The absorption cross

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section (abs) is calculated by normalizing babs with N. abs is a function of Dp, the

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wavelength of incident radiation (λ), and the complex refractive index (RI). Mass

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absorption cross-sections (MAC) is calculated using ρ measured,

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MAC = ρ

6abs × π × D3p

(1)

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The complex refractive index (RI) is defined by RI = n +ik, where the real part n

180

represents scattering and the imaginary part k represents absorbing. RI is independent of

181

the particle size and is calculated by using a range of different particle sizes and

182

concentrations. To determine RI, the absorption efficiency at each size is calculated for a

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given value of n and k using the Mie code for spherical particles, on the basis of the

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FORTRAN code from Bohren and Huffman.51 The calculated absorption cross-section

185

value is determined by summing the absorption from both doubly (20%) and singly

186

(80%) charged particles for a given RI. The calculated total absorption is compared to the

187

measured absorption. The best-fit refractive index is determined by minimizing the

188

reduced cumulative fractional difference (CFD),48 1

|abs(theory) ― abs(measured)|

N 𝐶𝐹𝐷r = N∑i = 1

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abs(measured)

(2)

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The CFD is calculated for a wide range of n and k values, and the lowest CFD value is

191

taken to be the refractive index for the mixture. The uncertainty in the retrieved n and k is

192

determined by varying abs with the uncertainty of the measurements.52 To estimate direct

193

radiative forcing due to the presence of our particles, the modified version of the Bond

194

and Bergstrom equation by Chylek and Wong is used.53,54 To compare the direct radiative

195

effect as a function of size, the relative forcing equation (ΔFrel) is chosen for simplicity:

∆𝐹rel = ― [(1 ― a)2β𝑄sca ― 2a𝑄abs]

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(3)

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where a is surface albedo, β is the backscatter fraction, Qsca is the scattering efficiency,

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and Qabs is the absorption efficiency. The surface albedo is set to 0.15, and the scattering

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and absorption efficiencies are calculated using the RI from the Mie Theory. In addition, we performed theoretical calculations of the structures and natural

200 201

bond orbitals (NBO) for MG, GL, MA, DA, and TA and using quantum chemical

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calculations at the M06-2X/6-311G(d,p) level,55,56 in order to elucidate the fundamental

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mechanism leading to the formation of N-Heterocycles.

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3.

Results and discussion

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3.1

Optical properties 9



206

The optical properties of particles at the 405 nm wavelength using the PAX at five

207

concentrations and five diameters are shown in Figure 1 and Figure S2. Figure 1a shows

208

the absorption coefficient (babs) as a function of the particle number concentration (N)

209

with different particle sizes for the MG-DA mixture. The measured babs increases with

210

particle number concentration and particle size, in accordance with the Beer-Lambert

211

Law. In addition, the results for all sizes and mixtures exhibit a high linearity (R2  0.99)

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and an intercept close to zero. The similar behaviors of babs are observed for all 6

213

mixtures, i.e., MG-MA, MG-DA, MG-TA, GL-MA, GL-DA, and GL-TA. The abs value

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(i.e., the slope of the lines) is plotted as a function of particle size for MG-DA in Figure

215

1b. The values for ext and abs increase with particle size, as is shown for the MG-DA

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mixture. In contrast, the single scattering albedo (SSA = sca/ext) decreases as a function

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of size, which is explained by a relatively larger increase in abs than in sca. Figure S2

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shows a comparison of the ext, abs, and SSA values for all 6 mixtures. The trend of the

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SSA variation with particle size for MG-MA is similar to that of MG-DA, except with

220

SSA lower values. On the other hand, the SSA is invariant with particle size for other

221

mixtures, because of the much smaller abs values.

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To compare the SSA values between the different mixtures, the size average SSA

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is shown in Figure 1c. At the 405 nm wavelength, the SSA values increase from MA, DA,

224

to TA (ranging from 0.75 for MG-MA to near unity for MG-TA), but is nearly invariant

225

for the GL mixtures (i.e., close to unity). For the MG mixtures, the SSA variation is

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consistent with the darkness of the solutions, i.e., the darkest for the primary amines and

227

the lightest for the tertiary amines. This trend is explained by the chemical reactivity and

228

steric effects.39 The MG mixtures exhibit lower SSA than those for the GL mixtures with

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the respective amines, consistent with the previous studies showing that the MG reactions

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form light-absorbing materials more efficiently than the analogous GL reactions.57,58

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The wavelength dependence of MAC for 250 nm particle size is shown in Figure

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1d for the MG samples. There is an enhancement of light absorption by a factor of more

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than 2 at the shorter wavelength (i.e., 405 nm), which is characteristic of BrC. The size

234

averaged MAC values for all mixtures are also listed in Table 1, in the range of 0.079 to

235

4.83 g m-2. Note that the MAC value is inversely proportional to the density, according to

236

equation (1). We measured the effective density for each particle size (Table S1). Our

237

average densities were considerably lower than those for organic aerosols under ambient

238

conditions (1.40 to 1.65 g cm-3),59, 60 explaining the higher values for MAC.

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The RI values at 405 nm are determined according to Eq. (2), and the results are

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summarized in Table 1. The contour of the CFD values for the MG-MA mixture is

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plotted against n and k in Figure S3. The reddest colors correspond to the lowest (best fit)

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CFD values. All CFD values are less than 0.01 (as shown in Figure S3 for MG-MA),

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indicating that the derived n and k values agree with the measured data within 1%. Both

244

the real part (n, scattering) and the imaginary part (k, absorption) of the RI values are

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higher for the MG mixtures than for the analogue GL mixtures (Table 1). For example,

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the retrieved RI is 1.64 (±0.10) + 0.195i (±0.023) for MG-MA and 1.53 (±0.05) + 0.004i

247

(±0.001) for GL-MA. The k values follow the same trend as the SSA values, similarly to

248

the darkness of the solutions. Figure 2 shows a comparison between the measured and

249

calculated values of abs for all six mixtures. The abs values for the MG mixtures

250

decrease with the methyl substitution (i.e., from primary to tertiary amines), and the

251

values for the GL mixtures are about an order of magnitude lower. Some of the

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differences between the measured and calculated abs values may be explained by larger

253

instrument uncertainty for PAX at smaller abs values. Also, the assumption using a fixed

254

ratio to partition the optical properties between singly and doubly charged particles

255

contributes to the differences. We estimated the uncertainty in our calculated 𝜎abs

256

associated with this assumption. For particles of 150 nm diameter at 405 nm wavelength,

257

doubly charged particles of the same electrical mobility would have a diameter of 215

258

nm. From the MIE theory, the ratio of the 𝜎abs values for the two sizes is: 𝜎abs(𝑑 = 150 nm)

259

=𝑄

260

calculated 𝜎abs increases by

261

increase in the estimated 𝜎abs by 30% of doubly charged particle contribution is by 31%

262

for the sizes from 150 nm to 350 nm. Similarly, the increases in the estimated absorption

263

cross section are by 62%, 93%, 124%, 155%, if the contribution of doubly charged

264

particle increases to 40%, 50%, 60%, and 70%, respectively. For small particles, the

265

contribution of doubly charged particles to the absorption is larger than 20%, and the

266

actual values of the calculated 𝜎abs values are higher than the calculated ones. This likely

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explains the large differences between the calculated (dashed line) and experimental

268

values (circles) for small particles (150 nm ~ 250 nm). Furthermore, the particle

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morphology likely exhibits a non-linear dependence on the particle size and chemical

270

composition, as is the case for soot particles,61,62 likely explaining some of the fluctuation

271

in measured abs. Note that the abs values for GL-DA are larger than those for GL-MA.

272

Interestingly, the measurements also show the smallest density for GL-DA (0.98 g cm-3),

273

compared to the averaged value of 1.25 g cm-3 for all mixtures. The density variation in

𝜎abs(𝑑 = 215 nm)

𝑄abs,2𝜋𝑟22 2 abs,1𝜋𝑟1

= 13.7. If doubly charged particles contribute to 30% of the 𝜎abs value, the ∆𝜎abs 𝜎abs

0.7(𝑑 = 150 nm) + 0.3𝜎abs(𝑑 = 215 nm)

= 0.8(𝑑 = 150 nm) + 0.2𝜎abs(𝑑 = 215 nm) ―1 = 36%. Hence, the

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Table S1 is within the uncertainties of the instruments.63,64 In this work, we used the

275

average values from all sizes, rather than treating the data for each size, density, and

276

mixture.

277

The measurements show a large range of values for RI, i.e., 1.41 to 1.64 for the

278

real part and 0.0 to 0.195 for the imaginary part of the various mixtures of α-dicarbonyls

279

and amines. Except for MG-MA, the uncertainty for the real part of RI ranges from 0.01

280

to 0.07. The largest uncertainty (0.10) for the real part of RI corresponds to the MG-MA

281

mixture. Zarzana et. al.48 reported a RI value of (1.55 + 0.114i) for MG-MA at 532 nm on

282

the basis of extinction measurements, which is smaller than our values at 405 nm

283

wavelength. Such a difference is explainable because absorption is enhanced at shorter

284

wavelengths (fig. 1d). For ambient humic-like substances (HULIS), Dinar et al.65 derived

285

the RI values at 532 nm and 390 nm, which are within the ranges of our k values at 405

286

nm. For comparison, we calculated the k using the equation for the bulk relationship

287

(MAC = 4πk/ρλ) and the MAC values obtained experimentally. Also, we calculated MAC

288

using the k obtained via the MIE theory calculations. The bulk values of MAC and k

289

shown in Table 1 are generally comparable to the ones obtained from the experiment and

290

MIE theory, respectively, except for the mixtures of MG-DA and MG-TA. The later

291

disparity is attributable to the combined experimental and theoretical uncertainty.

292

Because the reactions between the α-dicarbonyls and the amines lead to multiple

293

products, the RI values determined from our experiments represent those of the various

294

products within each mixture. However, errors likely occur when the RI for a mixture is

295

determined from the RI of the individual components when k is not equal to zero.66, 67

296

Most previous studies used the extinction data to retrieve RI, likely leading to large

13



297

uncertainty, particularly for k.68 Additionally, the uncertainty in our n values is large

298

because the real component for absorbing aerosols is difficult to retrieve, especially at

299

larger sizes.52 The particle sizes in our study are smaller than 400 nm, which can still

300

exhibit significant size dependence. Alternatively, our present work employed only the

301

absorption data for RI retrieval, also contributing to the uncertainty.

302

3.2.

303

Chemical Composition In the present work, two complementary mass spectrometer techniques were used

304

for the chemical composition analysis. The MG-MA and GL-MA spectra from the TD-

305

ID-CIMS and the Orbitrap-MS are shown in Figure 3. The detail peak assignments are

306

presented in Tables S2 – S7. For the same chemical composition, the spectra obtained

307

with both techniques show similar peaks in the low mass range, i.e., 111 and 125 m/z for

308

MG-MA and 143 m/z for GL-MA corresponding to the N-heterocycles. The TD-ID-

309

CIMS shows well-defined peaks typically at the lower mass range, possibly because of

310

thermal decomposition. In contrast, the spectra from the Orbitrap-MS technique exhibit

311

many peaks in the high mass range, because the ionization method does not rely on

312

thermal desorption. For the results of MG-MA using Orbitrap-MS, all the products with

313

more than 20% relative intensity contain 6 to 8 carbon atoms for the estimated chemical

314

formula; the product distribution corresponds mainly to dimers, instead of a combination

315

of dimers and trimers. In general, the GL spectra are much cleaner than the MG spectra

316

(Figure S4 – S5), indicating lower reactivity of GL with amines, less complexity of the

317

products, or a smaller number of reaction pathways available to GL.

318 319

The possible pathways leading to the identified products for MG-MA and GLMA are illustrated in Figure 4 (see also Figures S6 and S7). The reactions involve

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nucleophilic attack of amines at the reactive carbonyl site, followed by dehydration

321

before/after intermolecular cyclization of dimers or trimmers.46 The dehydration also

322

regulates the kinetics for the reactions between α-dicarbonyls and amines.69 MG reacts

323

efficiently with the amines, leading to the formation of chromophores, which are

324

identified as nitrogen-containing heterocycles (N-Heterocycles) by both the TD-ID-CIMS

325

and Orbitrap-MS techniques. However, during drying, GL-dihydrate is converted into

326

GL-monohydrate, yielding a carbonyl reactive site.42, 70 For GL, there is a competing

327

reaction between the hydration process and the nucleophilic attack of amines at the

328

carbonyl site.

329

For MG and GL, the N/C ratio is higher for the primary amines than for the

330

tertiary amines, but the O/C ratio is higher for the tertiary amines (Table S8). These ratios

331

are explained by the reactivity and steric effect of amines at the reactive carbonyl site.

332

The Double Bond Equivalent (DBE) is indicative of the molecular structure, which can

333

be related to the reactivity and optical properties. The average DBE value is the highest

334

for MG-MA (Table S8), consistent with its strongest absorption. It has been postulated

335

that supramolecular interaction governs the absorption property.27 In contrast, Aiona et al.

336

have concluded that absorption in the MG/ammonium sulfate system is dominated by

337

individual chromophores as opposed to supramolecular charge-transfer complexes.71

338

3.3.

339

Chemical Mechanism Leading to BrC The measured optical properties (e.g., SSA and MAC) for the mixtures of α-

340

dicarbonyls and amines are explainable by their distinct reactivity, i.e., the nucleophilic

341

attack of the α-dicarbonyls by amines. We calculated the NBO for the α-dicarbonyls and

342

amines. The NBO analysis reveals that the carbonyl C-atom of MG possess a strongly

15



343

positive charge (0.545 e), which is larger than that of GL (0.383 e). The N-atom of MA

344

exhibits the strongest negative charge (-0.875 e), and the negative charge decreases with

345

increasing methyl substation (i.e., -0.693 e for DA and -0.532 e for TA). The methyl

346

substituent increases the carbonyl bond polarization through the inductive electron

347

donating and hyperconjugation effects, as reflected by a more negatively charged O-atom

348

but a more positively charged C-atom for MG than GL. In addition, the dipole moments

349

for MA, DA, and TA decrease with increasing methyl substitution, with the values of

350

1.31, 1.03, and 0.61 D, respectively. The dipole moments of MG (0.44 D) is larger than

351

that of GL (e.g., 0 D because of a symmetric structure). Consequently, the electrostatic

352

attraction between the carbonyl group and the amine group facilitates the nucleophilic

353

attack, representing the key mechanism for the formation of N-Heterocycles. In addition,

354

there exists steric hindrance for DA and TA, also likely explaining their less reactivity.

355

Furthermore, the formation of N-heterocycles proceeds by nucleophilic addition

356

involving multiple α-dicarbonyls. Since there are fewer vacant sites for further

357

nucleophilic addition with increasing methyl substitution, the formation of N-

358

heterocycles is inhibited for DA and TA. Hence, nucleophilic addition occurs most

359

efficiently for the MG-MA interaction, consistent with our measurements showing the

360

smallest SSA, but the largest MAC for the MG-MA mixture.

361

4.

362

Atmospheric Implications BrC represents an important component of atmospheric fine aerosols and affects

363

the Earth’s radiative balance, directly by interfering with solar and terrestrial radiation

364

and indirectly by altering cloud formation and microphysics. In this work, BrC particles

365

containing the mixtures of small α-dicarbonyls and amines were synthesized, and the

16


Page 16 of 32

Page 17 of 32


366

optical properties of the BrC particles were measured. The SSA for MG is smaller than

367

that of GL and increases with the methyl substitution of amines. The mass absorption

368

cross-section for MG is two times higher at 405 nm wavelength than that at 532 nm

369

wavelength. The RI values at the 405 nm wavelength are in the range of 1.40-1.64 for the

370

real part and 0.002-0.195 for the imaginary part. The analysis of the chemical

371

composition of the α-dicarbonyl-amine mixtures with two complementary mass

372

spectrometry techniques reveals N-heterocycles as the dominant products, which have

373

been suggested to be responsible for browning.44 The measured optical properties of the

374

α-dicarbonyl-amine mixtures are explained by nucleophilic attack, steric hindrance, and

375

dipole-dipole interaction between α-dicarbonyls and amines via multiple steps to form N-

376

heterocycles.

377

To evaluate the impact of the optical properties and radiative forcing of BrC

378

particles, a modified version of the Chylek and Wong equation49,50 is used to estimate the

379

relative forcing (ΔFrel). The backscatter coefficient was determined from the asymmetry

380

parameter obtained from the MIE theory calculations, according to the procedure

381

described by Zarzanas et al.48 The ΔFrel values for the six α-dicarbonyl-amine mixtures as

382

a function of the particle size are shown in Figure 5, with the positive and negative values

383

representing warming and cooling, respectively. Also for comparison, four additional

384

cases are considered using the RI values previously reported at the 405 nm wavelength,

385

including ambient nucleation (1.55 + 0i),10 chamber nucleation of α-pinene-OH photo-

386

oxidation (1.40 + 0i),72 suwanee river fulvic acid (SRFA, 1.68 + 0.05i),52 and BC (1.95 +

387

0.79i).73 All results examined in this study are within the range of BC (most absorbing)

388

and nucleation of organic aerosols (less absorbing, mainly scattering). The MG mixtures

17



389

exhibit significantly less cooling than that for the GL mixtures. The MG-MA mixture

390

shows a warming effect similar to that of BC, whereas the other five cases exhibit a

391

cooling effect. For particles less than 300 nm, the magnitude of cooling is similar for

392

most of the α-dicarbonyl-amine mixtures (except for MG-MA), but for particles larger

393

than 500 nm, the ΔFrel values are distinct for the various mixtures. SRFA and MG-DA

394

show a cooling effect for smaller sizes (< 640 nm for SRFA and < 880 nm for MG-DA)

395

but a warming effect for larger sizes (> 640 nm for SRFA and > 880 nm for MG-DA).

396

Hence, our results indicate that BrC aerosols, if formed from the heterogeneous reaction

397

between MG and MA, likely contributes to atmospheric warming. Our results may be

398

more applicable to urban areas because of high concentrations of aerosol precursor gases,

399

i.e., for aged pollution plumes,74 HULIS (urban and rural),65 biomass burning,75 urban

400

plumes,76, 77 or newly formed SOA.72 While a full radiative transfer atmospheric model

401

is needed to accurately determine the direct radiative forcing for BrC particles, our results

402

provide the key optical properties (i.e., size-dependent SSA, MAC, and RI) for

403

incorporation of the α-dicarbonyl-amine mixtures into atmospheric models.

404

AUTHOR INFORMATION

405

*Corresponding Authors

406

E-mails: [email protected] (M.H.); [email protected] (Y.J.); [email protected]

407

(R.Z.).

408

Notes

409

The authors declare no competing financial interest.

410



Acknowledgements

18


Page 18 of 32

Page 19 of 32

411


This research was supported by National Natural Science Foundation of China

412

(91544214, 41675122, 41373102, 21577177, and 41425015), the Robert A. Welch

413

Foundation (A - 1417), National Basic Research Program of China (2013CB228503),

414

Science and Technology Program of Guangzhou City (201707010188), and Local

415

Innovative and Research Teams Project of Guangdong Pearl River Talents Program

416

(2017BT01Z032). W.M.-O. was supported by the National Science Foundation Graduate

417

Research Fellowship Program. Helpful discussion with Dr. Sasha Mandronich of the

418

National Center for Atmospheric Research was appreciated.

19



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Bounding the role of black carbon in the climate system: A scientific assessment. J. Geophys. Res. 2013, 118, (11), 5380-5552. Flowers, B. A.; Dubey, M. K.; Mazzoleni, C.; Stone, E. A.; Schauer, J. J.; Kim, S. W.; Yoon, S. C., Optical-chemical-microphysical relationships and closure studies for mixed carbonaceous aerosols observed at Jeju Island; 3-laser photoacoustic spectrometer, particle sizing, and filter analysis. Atmos. Chem. Phys. 2010, 10, (21), 10387-10398. 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. Guo, S.; Hu, M.; Zamora, M. L.; Peng, J.; Shang, D.; Zheng, J.; Du, Z.; Wu, Z.; Shao, M.; Zeng, L.; Molina, M. J.; Zhang, R., Elucidating severe urban haze formation in China. Proc. Natl. Acad. Sci. 2014, 111, (49), 17373-17378. Cappa, C. D.; Onasch, T. B.; Massoli, P.; Worsnop, D. R.; Bates, T. S.; Cross, E. S.; Davidovits, P.; Hakala, J.; Hayden, K. L.; Jobson, B. T.; Kolesar, K. R.; Lack, D. A.; Lerner, B. M.; Li, S.-M.; Mellon, D.; Nuaaman, I.; Olfert, J. S.; Petäjä, T.; Quinn, P. K.; Song, C.; Subramanian, R.; Williams, E. J.; Zaveri, R. A., Radiative Absorption Enhancements Due to the Mixing State of Atmospheric Black Carbon. Science 2012, 337, (6098), 1078-1081.

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Table 1: Summary of the refractive indexes derived from the measured effective density and mass absorption cross section. All values are for λ = 405 nm. For comparison, the calculated bulk properties are also included. MIE Theory

Experimental

Bulk Comparison c

Mixturesa

n

k

ρ (g cm-3) b

MAC (m2 g-1)

kb

MACb (m2 g-1)

MG-MA

1.64 ± 0.10

0.195 ± 0.023

1.28 ± 0.02

4.72 ± 0.3

0.195

4.73

MG-DA

1.60 ± 0.01

0.032 ± 0.006

1.35 ± 0.01

2.39 ± 0.2

0.104

0.74

MG-TA

1.58 ± 0.07

0.009 ± 0.001

1.37 ± 0.02

0.71 ± 0.1

0.031

0.20

GL-MA

1.53 ± 0.05

0.004 ± 0.001

1.22 ± 0.02

0.129 ± 0.04

0.005

0.10

GL-DA

1.44 ± 0.06

0.010 ± 0.002

0.98 ± 0.04

0.366 ± 0.12

0.012

0.32

GL-TA

1.41 ± 0.01

0.002 ± 0.001

1.28 ± 0.02

0.077 ± 0.01

0.003

0.02

a

The abbreviations correspond to MG-MA (methylglyoxal – methylamine), MG-DA (methylglyoxal – dimethylamine), MG-TA (methylglyoxal – trimethylamine), GL-MA (glyoxal – methylamine), GL-DA (glyoxal – dimethylamine), and GL-TA (glyoxal – trimethylamine). b Size average of the measured effective densities. Average between the systems is 1.25. c Calculated using MAC = 4πk/ρλ.

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Figure 1. Measured optical properties for MG-DA at the 405 nm wavelength (a & b). (a) Absorption coefficient (babs) as a function of the particle number concentration. (b) Single scattering albedo (SSA), absorption cross section (abs), and extinction cross section (ext) as a function of the particle size. Dependence of the optical properties on amines (c & d). (c) Size average of SSA at 405-nm wavelength. (d) MAC values of the MG-amine mixtures at 250 nm particle size.

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Figure 2. Absorption cross section experimental measurements at 405 nm (circles) and the MIE theory calculation fit (dashed lines) for MG (a) and GL (b) mixtures. The colors correspond to the different amines, MA (red), DA (green), and TA (orange). For selected particle sizes, the measurements were repeated at least twice, and the error in the figure reflects 1 of the measurements. The measured absorption cross-section was partitioned between singly (80%) and doubly (20%) charged particles and applied in eq. (2) to derive the CFD values. The lowest CFD corresponds to the combination of the real and imaginary RI and is used to determine the absorption cross-section from the MIE theory for particles of 100-400 nm (dashed lines). The experimental (exp) points correspond to singly charged particles (80%).

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Figure 3: Mass spectra from two complementary methods for the MA mixtures. TD-ID-CIMS spectra for (a) GL-MA and (b) MG-MA. Orbitrap-MS spectra for GL-MA (c) and MG-MA (d).

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Figure 4: Proposed reaction pathways leading to N-heterocycles for MA reaction with MG (a) and GL (b). The numbers correspond to the m/z values for the pronated species.

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Figure 5: Relative forcing (ΔFrel) ratios calculated from the complex refractive index at the 405nm wavelength as a function of particle size. The solid and dashed lines correspond to the αdicarbonyl-amine mixtures (MG-MA in orange, MG-DA in red, MG-TA in purple, GL-MA in blue, GL-DA in green, and GL-TA in olive) and other compositions with the RI values previously reported at the 405 nm wavelength (BC in black, SRFA in gray, ambient NPF in light blue, and chamber NPF in light pink), respectively.

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147x77mm (150 x 150 DPI)


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Formation and optical properties of brown carbon from small Î±

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