Brown Carbon Formation from Nighttime Chemistry of Unsaturated

Subscriber access provided by WESTERN SYDNEY U

Environmental Processes

Brown Carbon Formation from Nighttime Chemistry of Unsaturated Heterocyclic Volatile Organic Compounds Huanhuan Jiang, Alexander L. Frie, Avi Lavi, Jin Chen, Haofei Zhang, Roya Bahreini, and Ying-Hsuan Lin Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.9b00017 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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 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 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.

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 24

Environmental Science & Technology Letters

1

Brown Carbon Formation from Nighttime Chemistry of Unsaturated Heterocyclic Volatile

2

Organic Compounds

3 4 5

Huanhuan Jiang†⊥, Alexander L. Frie†⊥, Avi Lavi‡, Jin Y. Chen§, Haofei Zhang‡§, Roya Bahreini†‡§, Ying-Hsuan Lin†§*

6 7

†

Department of Environmental Sciences, University of California, Riverside, CA 92521, USA

8

‡

Department of Chemistry, University of California, Riverside, CA 92521, USA

9

§

Environmental Toxicology Graduate Program, University of California, Riverside, CA 92521,

10

USA

11 12 13

⊥

These authors contributed equally to this work

14

*

phone: +1-951-827-3785, email: [email protected]

1 ACS Paragon Plus Environment


Page 2 of 24

15

ABSTRACT:

16

Nighttime atmospheric processing enhances the formation of brown carbon aerosol (BrC) in

17

biomass burning plumes. Heterocyclic compounds, a group of volatile organic compounds

18

(VOCs) abundant in biomass burning smoke, are possible BrC sources. Here, we investigated the

19

nitrate radical (NO3)-initiated oxidation of three unsaturated heterocyclic compounds (pyrrole,

20

furan, and thiophene) as a source of BrC. The imaginary component of the refractive index at

21

375 nm (k375), the single scattering albedo at 375 nm (SSA375) and average mass absorption

22

coefficients (290-700nm) of the resulting secondary organic aerosol (SOA) are reported.

23

Compared to furan and thiophene, NO3 oxidation of pyrrole has the highest SOA yield. Pyrrole

24

SOA (k375=0.015±0.003, SSA=0.86±0.01, 290-700nm=3400±700 cm2 g-1) is also more

25

absorbing than furan SOA (290-700nm=1,100±200 cm2 g-1) and thiophene SOA

26

(k375=0.003±0.002, SSA375=0.98±0.01, 290-700nm=3,000±500 cm2 g-1). Compared to other

27

SOA systems, MACs reported in this study are higher than those from biogenic precursors, and

28

similar to high-NOx anthropogenic SOA. Characterization of SOA molecular composition using

29

high-resolution mass spectrometric measurements revealed unsaturated heterocyclic nitro

30

products or organonitrates as possible chromophores in BrC from all three precursors. These

31

findings reveal nighttime oxidation of fire-sourced heterocyclic compounds, particularly pyrrole,

32

as a plausible source of BrC.


Page 3 of 24

33


TOC Art

34



35

Page 4 of 24

1. INTRODUCTION

36

Aerosol’s radiative effects are the most uncertain components of current climate models.1

37

One of the drivers of this uncertainty is the incomplete understanding of aerosol formation and

38

aging processes that influence aerosols’ chemical composition and optical properties. Light-

39

absorbing organic aerosols, known as brown carbon (BrC), are estimated to account for ~20% of

40

aerosol-driven atmospheric heating.2 The chemical nature of BrC chromophores has been the

41

focus of many recent studies, with field, laboratory, and modeling studies identifying aromatics,

42

conjugated systems, and highly functionalized species, such as organic nitrates, as moieties

43

responsible for absorption of tropospheric solar radiation.2-16 Specifically, nitroaromatic

44

compounds have been recognized as significant components of BrC.13

45

One previously unexplored pathway of BrC and nitroaromatic formation is the nitrate

46

radical (NO3)-initiated oxidation of unsaturated heterocyclic compounds. Unsaturated

47

heterocyclic compounds contain heteroatoms (N, O and S) within their ring structures and

48

represent a unique family of reactive compounds. These compounds are emitted during biomass

49

burning events, with estimated emission factors of 5–37% of total emitted carbon for furans.17

50

High levels of NOx emissions in biomass burning plumes can readily react with O3 to form NO3

51

radicals, the dominant atmospheric oxidant during nighttime.14,

52

compounds and NO3 radicals are likely to co-occur in the nighttime atmosphere. Major

53

unsaturated heterocyclic compounds are known to react quickly with NO3 radicals,20 forming

54

condensable nitro- or nitrate-containing reaction products that are likely to absorb light in the

55

UV-Visible range.7, 10, 21

18, 19

Thus, heterocyclic

56

Here, the NO3 oxidation of unsaturated heterocyclic compounds is investigated as a

57

probable and previously unrecognized source of BrC. Through a series of chamber experiments,


Page 5 of 24


58

we characterized chemical and optical properties of SOA from NO3-initiated oxidation of

59

pyrrole, furan, and thiophene as model compounds for 5-membered unsaturated heterocyclic

60

compounds. We used both online and offline techniques to evaluate single scattering albedo

61

(SSA, λ=375 nm), refractive index (RI, λ=375 nm) and mass absorption coefficients (MAC, 290–

62

700 nm) to provide a broad picture of the optical characteristics of SOA. Molecular composition

63

of SOA was analyzed to identify potential chromophores.

64

2. MATERIALS AND METHODS

65

2.1. Controlled Chamber Experiments

66

Experiments were performed under dry conditions (RH=2–16%) within a ~1.3 m3 FEP

67

Teflon chamber located at University of California, Riverside. Before each experiment, the

68

chamber was filled with zero air (Airgas). O3 was generated through corona discharge (Enaly,

69

1000BT–12) and injected into the chamber until a concentration of ~1500 ppbv O3 was reached.

70

Next, 2 L of NO (Praxair, 482 ppmv in air) was injected into the chamber. The chamber was

71

equilibrated for an hour to allow reservoirs of NO3 and N2O5 to develop. As shown in Fig.

72

S1, more than 9 ppbv NO3 can be generated after one hour as simulated using a kinetic box

73

model.22-24 Then, pyrrole (TCI America, >99%), furan (TCI America, >99%,) or thiophene (Alfa

74

Aesar, 99%) was added into a glass bulb and the vapors were introduced into the chamber by

75

flowing ultra-zero air (Aadco Instruments Inc., 747-30) through the bulb to achieve an effective

76

chamber concentration of ~200 ppbv. Under these conditions, the oxidation of all precursors is

77

dominated by the NO3 pathway as shown in the supplemental information (SI, Section S1 and

78

Table S1). Aerosol size distributions, NO2, NO and O3 were continuously monitored during

79

experiments with a scanning electron mobility spectrometer (SEMS, Brechtel Manufacturing



Page 6 of 24

80

Inc.), NOx analyzer (Thermo 42i-LS) and an O3 analyzer (Thermo 49i), respectively. A summary

81

of experimental conditions is provided in Table 1.

82

2.2. Online Chemical and Optical Characterization

83

Gas-phase reaction products were analyzed in real time by an iodide-adduct high-

84

resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS, Tofwerk AG,

85

Aerodyne Research Inc.). Additional details are described in Section S2.

86

Scattering and absorption coefficients at 375 nm (βscat,375 and βabs,375) were measured at 1

87

Hz using a photoacoustic extinctiometer (PAX) (Droplet Measurement Technologies, Boulder,

88

CO). PAX measurements were averaged to the SEMS scan time (140 s). PAX detection limits,

89

defined as 3 times the standard deviation of scattering and absorption measurements of filtered

90

air averaged to 140 s, were 1.29 Mm-1 and 1.08 Mm-1, respectively for βscat,375 and βabs,375. The

91

single scattering albedo at 375 nm (SSA375) was calculated using Eq. 1.

92

𝑆𝑆𝐴375 =

𝛽scat,375 𝛽scat,375 +𝛽abs,375

(1)

93

The size parameter (x) is useful in understanding SSA dynamics due to SSA’s

94

dependency on particle size and measurement wavelength. x relates the mode diameter of the

95

size distribution (dmode) to the corresponding wavelength () of optical measurements, i.e.,

96

=375 nm for PAX, as shown in Eq. 2. 𝑥𝜆 =

97 98

𝜋𝑑𝑚𝑜𝑑𝑒 𝜆

(2)

2.3. Calculation of Refractive Index

99

RI values are measures of the interactions of a material with radiation. The RI consists of

100

two components, a real component that describes scattering (n) and an imaginary component that

101

describes absorption (k). RI values were calculated using an optical closure procedure and

102

applying Mie Theory (Section S3).25-29 Variabilities reported in Section 3.2 represent standard 6 ACS Paragon Plus Environment

Page 7 of 24


103

deviations of the measurements. Uncertainties were also estimated by forcing the RI calculation

104

+0.007 +0.08 using instrumental uncertainties (Table S2);30 maximum uncertainties were 𝑘−0.005 and 𝑛−0.07

105

+0.004 +0.09 for pyrrole and 𝑘−0.002 and 𝑛−0.06 for thiophene. For furan experiments, n375 and k375 values were

106

not calculated because the majority of βabs,375 measurements were below or marginally higher

107

than PAX detection limits.

108

2.4. Offline Optical Characterization

109

Aerosol samples were collected onto polytetrafluoroethylene membrane filters (Zefluor,

110

Pall Laboratory, 47 mm, 1.0 µm pore size), after the particle volume concentration peaked.

111

Filters were extracted in methanol and analyzed for UV-Vis absorbance (Beckman DU-640).

112

This process is further detailed in Section S4.

113 114 115 116

2.5. Calculation of Mass Absorption Coefficient and Absorption Angstrom Exponent Calculation of the solution-based MAC from UV-Vis spectra has been reported previously.31, 32 The effective MAC (cm2 g-1) is calculated using Eq 3.31, 32 MAC(𝜆) =

A(𝜆)×ln10

(3)

𝑏×𝐶m

117

where A() is the absorbance at wavelength () of filter extracts, b is the path length (1 cm), and

118

Cm (g cm-3) is the mass concentration of extracted SOA compounds in solution. Cm is determined

119

by the filter sampling flow rate, average aerosol mass concentration during filter collection,

120

sampling time, and extraction efficiency. The light-absorbing properties of aerosols are estimated

121

by the average MAC () over the wavelengths range from 1=290 nm to n=700 nm).

122

〈𝑀𝐴𝐶〉 =

123 124

∑𝑛 𝑖=1 𝑀𝐴𝐶(𝜆𝑖 ) 𝑛

(4)

To understand the wavelength dependence of light absorption, the absorption Ångstrӧm exponent (AAE) was calculated in the UV (290–400 nm) and visible (400–600 nm) ranges.32



𝐴𝐴𝐸 =

125 126

𝑀𝐴𝐶(𝜆1 ) ) 𝑀𝐴𝐶(𝜆2 ) 𝜆 𝑙𝑛( 1 ) 𝜆2

Page 8 of 24

−𝑙𝑛(

(5)

2.6. Offline Chemical Characterization

127

The molecular composition of filter extracts was characterized using liquid

128

chromatography (LC) coupled with a diode array detector (DAD) and a high-resolution time-of-

129

flight mass spectrometer (HR-TOFMS, Agilent 6230 Series). The instrument was equipped with

130

an electrospray ionization source (ESI), and the detector was operated under the negative (−) ion

131

mode. Further details can be found in Section S5.

132

3. RESULTS AND DISCUSSION

133

3.1. NO3-Initiated SOA Formation from Heterocyclic Precursors

134

By assuming all of the injected VOC reacted with NO3 radicals and propagating

135

experimental uncertainties, the lower bound of SOA mass yields (Eq. S3) were estimated to be

136

109±29% for pyrrole, 7±3% for furan, and 35±8% for thiophene, assuming a density of 1.3 g cm -

137

3 33

138

comparing to more quantitative yield measures from other studies due to our small chamber size

139

and potentially high particle wall-loss rates.

140

. These values are estimates and are likely inter-comparable, but caution should be used when

The low SOA mass production from furan is consistent with the low SOA yields from

141

OH oxidation of furan reported in other studies.34,

142

furan may not be a major contributor to SOA mass under a variety of common oxidation

143

conditions. Conversely, the larger SOA mass formed from thiophene and pyrrole indicates that

144

NO3 chemistry with these compounds may be an important SOA source, particularly in biomass

145

burning plumes where these compounds are readily found.17

146

35

Together these observations reveal that

3.2. Optical Properties of SOA


Page 9 of 24


147

SSA375, n375, and k375 for pyrrole and thiophene experiments are displayed in Fig. 1A and

148

1B. For the following discussion, SSA375 values are averaged for all measurements where x375>1

149

because the size dependency of SSA is expected to be minimal within this region. 36 The n375

150

values fell within the ranges previously observed for organic aerosols, with average values of

151

1.41 ± 0.05, and 1.44 ± 0.02 for pyrrole and thiophene SOA, respectively.

152

Pyrrole SOA was the most absorbing among the three tested precursors, with an average

153

SSA375 of 0.86±0.01 and k375 of 0.015±0.003. These values are similar to those previously

154

observed for peat biomass burning (SSA375: 0.93 and 0.85, and k375: 0.009–0.015 depending on

155

fuel packing density) by Sumlin et al.37 Notably, the k375 values for pyrrole SOA decreased with

156

exposure time and seem to be inversely related to SOA mass. This decrease in k375 could be

157

driven by dilution of chromophores as more scattering components condense or by bleaching of

158

existing chromophores. Bleaching might be driven by the oxidation of conjugated species by an

159

oxidant (O3 or NO3) or particle phase processes such as hydrolysis of organonitrates.38

160

Thiophene SOA was slightly absorbing, with an average SSA375 of 0.98±0.01 and k375 of

161

0.003 ± 0.002. Thiophene k375 values are similar to those observed by Liu et al.21 for toluene

162

SOA (k405=0.0041±0.0005) and m-xylene SOA (k405=0.0030 ±0.0003) formed from OH

163

oxidation in the presence of NOx.21 These compounds are aromatic anthropogenic proxies,

164

indicating that thiophene SOA products have similar absorption properties to SOA from

165

anthropogenic precursors.

166

The majority of βabs,375 measurements for furan SOA were below the PAX detection limit,

167

which was likely caused by the low mass production of furan SOA in the chamber (Section 3.1)

168

and its low light absorption at 375 nm (Section 3.3).

169

3.3. The Absorption Spectra of BrC



Page 10 of 24

170

Similar to previously studied BrC SOA, samples collected from oxidation of unsaturated

171

heterocyclic compounds absorb strongly in UV and near UV ranges, and the absorbance

172

decreased as the wavelengths increased (Fig. 1C).15, 16 The absorbance spectra of pyrrole and

173

thiophene SOA showed a shoulder at ~330 nm. The 290-700 values of pyrrole, thiophene

174

and furan SOA were 3,400±700 cm2 g-1, 3,000±500 cm2 g-1, and 1,100±200 cm2 g-1, respectively

175

(Table 1). Notably, the offline MAC of furan SOA over the whole spectra and at 375 nm was

176

significantly lower than in the other systems, which explains its negligible βabs,375 values (Section

177

3.2). These are much higher than those of SOA (200–500 cm2 g-1) derived from

178

biogenic and anthropogenic VOCs under low or intermediate NOx conditions, but similar to the

179

SOA from anthropogenic VOCs with high NOx (VOC/NOx

Brown Carbon Formation from Nighttime Chemistry of Unsaturated

Recommend Documents