Molecular Characterization of Nitrogen-Containing Organic

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Molecular Characterization of Nitrogen-Containing Organic Compounds in HULIS Emitted from Straw Residue Burning Yujue Wang, Min Hu, Peng Lin, Qingfeng Guo, Zhijun Wu, Mengren Li, Limin Zeng, Yu Song, Liwu Zeng, Yusheng Wu, Song Guo, Xiaofeng Huang, and Lingyan He Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Environmental Science & Technology

ACS Paragon Plus Environment


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Molecular Characterization of Nitrogen-Containing Organic

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Compounds in HULIS Emitted from Straw Residue Burning

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Yujue Wang,1 Min Hu,*,1,2 Peng Lin, *, §,4,5 Qingfeng Guo,1 Zhijun Wu,1 Mengren Li,1 Limin

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Zeng,1 Yu Song1, Liwu Zeng,3 Yusheng Wu,1 Song Guo,1 Xiaofeng Huang,3 Lingyan He 3

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

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University, Beijing 100871, China

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State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of

Beijing Innovation Center for Engineering Sciences and Advanced Technology, Peking

Key Laboratory for Urban Habitat Environmental Science and Technology, School of

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Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055,

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China

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4

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& Technology, Clear Water Bay, Kowloon, Hong Kong, China

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Corresponding author:

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* Min Hu

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Email: [email protected]

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Telephone: 86-10-62759880

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Fax: 86-10-62751920

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* Peng Lin

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Email: [email protected]

Division of Environment and 5Department of Chemistry, Hong Kong University of Science

ACS Paragon Plus Environment 1

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Telephone: 1-509-554-6358

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ABSTRACT: The molecular composition of humic-like substances (HULIS) in different

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aerosol samples were analyzed using an ultrahigh resolution mass spectrometer, to investigate

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the influence of biomass burning on ambient aerosol composition. HULIS in background

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aerosols were characterized with numerous molecular formulas similar to biogenic secondary

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organic aerosols. Differently, the abundance of nitrogen-containing organic compounds

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(NOC), including nitrogen-containing bases (N-bases) and nitroaromatics, increased

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dramatically in ambient aerosols affected by crop residue burning in the farm field. The

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molecular distribution of N-bases in these samples exhibited similar patterns with those

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observed in smoke particles freshly emitted from lab-controlled burning of straw residues,

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but significantly different with those from wood burning. Signal intensity of the major

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N-bases correlated well with the atmospheric concentrations of potassium and levoglucosan.

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These N-bases can serve as molecular markers distinguishing HULIS from crop residue

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burning with from wood burning. More nitroaromatics were detected in ambient aerosols

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affected by straw burning than in fresh smoke aerosols, indicating that many of them are

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formed in secondary oxidation processes as smoke plumes evolving in the atmosphere. This

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study highlights the significant contribution of crop residue burning to atmospheric NOC.

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Further study is warranted to evaluate their roles on climate and human health.

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TOC Art

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INTRODUCTION

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Atmospheric HUmic-Like Substances (HULIS) are important fractions in organic

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aerosols (OA).1 They are so termed due to their resemblance to terrestrial and aquatic humic

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and fulvic acids.1 The concentration of HULIS in the atmosphere was observed to vary from

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0.04 to 15 µgC/m3, comprising 9-72% of water soluble organic carbon.1-4 Due to their surface

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active and UV-light absorbing properties, HULIS could influence the formation of cloud

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condensation nuclei, solar radiation balance and photochemical processes in the

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atmosphere.5-10 Recently, HULIS were demonstrated to be a potential source of aerosol

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toxicity, resulting in health risks to humans.11,12 It was proposed that HULIS contain

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reversible redox sites, thereby serving as electron carriers to catalyze the formation of

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reactive oxygen species (ROS).11

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Large amounts of nitrogen-containing organic compounds (NOC) have been observed in

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HULIS.13 It has been suggested that biomass burning is an important source of water soluble

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NOC.14,15 Both nitrogen-containing bases (N-bases) and nitroaromatics are among the major


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constituents of NOC in biomass burning aerosols (BBA) and ambient aerosols influenced by

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biomass combustion.13,16-21 N-bases in BBA are usually formed from the pyrolysis of plant

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materials with chemical compounds mostly containing basic nitrogen atoms. Postulated

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reaction pathways include: pyridine and alkyl-pyridines generated from alanine, polypeptides

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or chitin; pyrrole, imidazole and their derivatives generated from histidine or the

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proteinaceous materials of plant residues.17 Nitroaromatic compounds can be resulted from

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both primary emission and secondary oxidation of emitted volatile or semi-volatile organic

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compounds in smoke plumes.22-25 For instance, nitrophenols and nitrocatechols are usually

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formed by nitration of phenols in the presence of NOx in smoke plumes.25,26

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Methyl-nitrocatechols, produced from the oxidation of cresol in biomass burning smokes,

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have been identified as molecular markers for biomass burning secondary organic aerosols

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(SOA).24

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Atmospheric particulate NOC are proposed to affect the environmental system in a

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variety of ways. As a significant fraction of total nitrogen in aerosols,27 NOC have

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considerable effects on the nitrogen concentration in terrestrial and aquatic ecosystems

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through dry and wet deposition.28 Depending on their composition, NOC can be either

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beneficial or harmful to ecosystems. For example, amino acids are essential nutrients for

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many organisms. Nitrophenols are known for their phytotoxic properties and their ability to

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penetrate into plant tissues, so that they are responsible for forest deterioration in polluted

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area.29 Nitroaromatic compounds are important contributors to light absorption in the

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near-UV regions,25,30,31 affecting both radiative balance and photochemical reactions in the

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atmosphere. The unprotonated nitrogen atom in N-bases is suggested to act as a H-bonding

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acceptor to facilitate hydrogen-atom transfer in the redox reaction cycle, which in turn

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enhance the ROS generation by atmospheric HULIS.32 Therefore, a fundamental



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understanding on the molecular characteristics of NOC is essential to accurately assess their

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environmental impacts.

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Crop residues burning is a very common and widespread type of open burning in

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Southeast Asia, especially in China.33 Emission profiles and characterizations of NOC in

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BBA in this area may differ from other areas with wood burning as the major type of biomass

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burning. In order to obtain a comprehensive understanding of the influence of straw burning,

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we investigated the molecular characterization of HULIS isolated from different aerosol

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samples, including ambient aerosols and smoke particles from lab-controlled burning of straw

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residues and wood. We employed an ultrahigh resolution mass spectrometer (UHRMS),

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coupled with soft ionization techniques, to examine the overall molecular composition of

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HULIS,34 with special focus on the NOC. We demonstrated some typical distributions on the

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molecular characteristics of NOC, particularly N-bases, in straw burning aerosols (SBA).

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METHODOLOGY

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Sampling

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Ambient aerosols were collected from May 30 to June 27, 2013 during local harvest

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season at Suixi (33.91° N, 116.72° E), located in a typical agriculture region of China.35

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PM2.5 (particulate matter with aerodynamic diameter less than 2.5 µm) were collected on 47

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mm prebaked quartz fiber filters (Whatman Inc.) and Teflon filters (Whatman Inc.) using a

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4-channel sampler (TH-16A, Tianhong, China). The sampling flow rate was 16.7 L/min for

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each channel. A high-volume sampler (TH-1000C, Tianhong, China) was also employed to

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collect PM2.5 on prebaked quartz fiber filters from June 8 to 17, during local straw burning

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period. The sampling flow rate was 1050 L/min. Generally, daytime samples were collected


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from 7:40 to 18:00 and nighttime ones from 18:40 to 7:00 the next morning. Field blank

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samples were collected by placing filters in samplers for 30 min with the pump off.

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The lab-controlled burning experiments were also designed to simulate the emissions of

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“real-world” field burning activities in China (Fig. S1). The experiments were conducted in

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the Laboratory of Biomass Burning Simulation at Peking University Shenzhen Graduate

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School. The simulation system was designed and constructed according to the one described

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by He et al. (2010).36 Corn and wheat, two primary grain crops in China, were chosen to

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represent straw materials burned in fields. Eucalypt is one kind of trees widely distributed in

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southern China, whose branches are usually used for domestic heating and cooking in this

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area. The burning of eucalypt branches was simulated to compare with straw (corn and wheat)

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burning. During each burning experiment, 1.5-2.5 kg of biomass was ignited on the

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combustion pan. The smoke was collected by the hood over the pan, diluted with zero air (21

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mol% O2 and 79 mol% N2) in the dilution tunnel and went into the residence chamber. The

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PM2.5 smoke aerosols were then collected on Teflon and quartz fiber filters. The sampling

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flow rate of each port was 16.7 L/min. Each burning experiment lasted for 20-35 min.

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Lab-controlled corn burning aerosols (lab CBA), wheat burning aerosols (lab WBA) and

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eucalypt burning aerosols (lab EBA) were collected. During each burning experiment, CO

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and CO2 were measured continuously by CO and CO2 analyzers (Thermo Scientific Inc.,

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Bremen, Germany) to estimate the burning conditions by determining its modified

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combustion efficiency (MCE).37 Blank samples were collected following the same

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procedures, but without igniting any biomass fuels on the combustion pan. The burning

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experiments were conducted six times for corn, six times for wheat and four times for

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eucalypts. Two lab CBA and two lab WBA under different burning conditions (Appendix S1,

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Fig. S2) and one lab EBA were selected for further analysis using UHRMS.



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Isolation of the HULIS Fraction

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Isolation of HULIS was based on an established method with minor modifications,

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which has been proved to be suitable for the isolation and characterization of this fraction in

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aerosols.2,38-40 A portion of filter was extracted with ultrapure water in an ultrasonic bath for

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40 min. The water extracts were then filtered through a 0.45 µm PTFE syringe filter (Gelman

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Sciences). The extracts were then acidified to pH=2 using HCl and loaded on a solid phase

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extraction (SPE) cartridge (DSC-18, Sigma-Aldrich, USA). Through the SPE cartridge,

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majority of inorganic ions, low molecular weight organic acids and sugars appeared in the

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effluent.2 After rinsing the cartridge with ultrapure water, the fractions retained in the

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cartridge were eluted with CH3OH. Then the elution was dried under a gentle stream of N2

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and re-dissolved in ultrapure water to quantify the concentration of HULIS-C (carbon

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component of HULIS) or re-dissolved in acetonitrile/water (1:1) solvent for UHRMS

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analysis.

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UHRMS Analysis

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The molecular composition of HULIS was analyzed by an Exactive Plus-Orbitrap mass

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spectrometer (Thermo Scientific Inc., Bremen, Germany) equipped with a heated

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electrospray ionization (ESI) source. Samples were injected into the mass spectrometer

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through the ESI interface at a flow rate of 5 µL/min. The system was operated in positive

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(ESI+) and negative (ESI-) ion modes with a spray voltage of 3.0-4.0 kV. The sheath gas

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flow was 7 (arb units) and capillary temperature was 320 °C. The spectrometer was calibrated

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using a standard mixture of N-butylamine, caffeine, MAFA (L-methionyl-arginyl-

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phenylalanyl-alanine acetate·H2O), sodium dodecyl sulfate, sodium taurocholate and

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Ultramark 1621. The scan range was m/z 90-900 with a resolution of 140,000 at m/z = 200.

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Each sample was analyzed for three times with at least 100 full-scan spectra acquired at each


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analysis. The one with most stable ESI signals was selected to obtain an averaged mass

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spectrum for each sample. The background spectra were obtained by analyzing the

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corresponding field blank sample following the same procedures.

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The mass spectra were processed using Xcalibur software (V2.2, Thermo Scientific).

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Peaks with signal-to-noise ratio (S/N) ≥ 10 were exported. All the mathematically possible

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formulas for each ion were calculated with a mass tolerance of ± 2ppm. Details of the data

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analysis have been described in Lin et al. (2012).13 In brief, each molecular formula was

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allowed containing certain elements and limited by several conservative rules. The H/C, O/C,

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N/C and S/C ratios were limited to 0.3−3.0, 0−1.2, 0−1.0 and 0−0.8 in ESI+ and 0.3−3.0,

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0−3.0, 0−0.5 and 0−2.0 in ESI-. The double bond equivalents (DBE) and aromaticity index

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(AI) value were calculated as follows:41

1 DBE = 1 + (2ܿ − ℎ + ݊) 2 AI =

1 + ܿ − 0.5‫ ݋‬− 0.5݊ − 0.5ℎ ܿ − 0.5‫ ݋‬− ݊

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where c, h, o, and n correspond to the number of C, H, O and N atoms respectively in the

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assigned formulas. All the formulas with DBE < 0 or that disobeyed the nitrogen rule for

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even electron ions were excluded from the list.42 Peaks were also eliminated from the list if

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their intensities were lower than ten times of those in field blank sample. All the molecules

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detected in ESI+ and ESI- modes were reported as neutral molecules, including molecular

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weights and molecular formulas, in the following discussion.

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Other Chemical Analysis

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PM2.5 mass concentrations were measured using the gravimetric method. Water soluble

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inorganic ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3- and SO42-) and low molecular weight



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organic acids (formic acid, acetic acid, propionic acid, methane sulfonic acid, succinic acid

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and oxalic acid) were analyzed by ion chromatograph (DIONEX, ICS2500/ICS2000)

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following procedures described in Guo et al. (2010).43 K+ could be used to indicate the

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influence of biomass burning on ambient aerosols.44 The portion that isn’t attributed to soil

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dust or sea salt is considered to be contributed by biomass burning (Kbb), which could better

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represent the intensity of biomass burning than the total K+. The sampling site is located

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inland with negligible impact of sea salt. The amount of Kbb was estimated by subtracting the

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contribution of soil from the total K+. The contribution of soil was calculated by multiplying

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[Ca2+] by K/Ca ratio (0.71).44 Organic carbon (OC) and elemental carbon (EC) were analyzed

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using thermal/optical carbon analyzer (Sunset Laboratory). Particulate organic species,

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including levoglucason, were quantified with authentic standards and internal standards using

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Agilent gas chromatograph-mass spectrometer system (6890 plus GC-5973N MSD)

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following the same procedures outlined by Guo et al. (2012).45 The concentration of

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HULIS-C was detected using a total organic carbon (TOC) analyzer (AnalytikJena multi N/C

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3100).

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RESULTS AND DISCUSSION

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Chemical Composition of Different Samples

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The average PM2.5 mass concentration was 110.73 ± 130.61 µg/m3 during the sampling

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period at Suixi site. Temporal variations of mass concentrations of PM2.5, K+ and

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levoglucosan are shown in Fig. 1. The rain scavenged ambient particles during the nighttime

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of June 10. The daytime of June 11 was a very clean day, with the lowest concentrations of

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K+ and levoglucosan during the sampling period, so this sample was considered as the

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background ambient aerosols (background AA). Straw burning contributed 51% of PM2.5 and

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76% of OC from June 8 to 18 due to local open burning according to our previous study.35


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Very intense straw burning activities occurred near the site during the nighttime of June 12-

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15. Because straw residues burning in open fields were forbidden by the local government,

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burning activities usually occurred at night to evade supervision of the government. This,

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besides weaker mixing state of boundary layer, was the major reason for much higher

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nighttime concentrations than the corresponding daytime ones (Fig. 1). The most intense

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local straw burning activities occurred during the nighttime of June 15 (Fig. S1), with the

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nearest fire one to several kilometers away from the sampling site, when PM2.5, OC, K+ and

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levoglucosan increased dramatically to the highest level during the sampling period (Fig. 1).

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Therefore, the nighttime sample of June 15 was chosen as the straw burning aerosol in the

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field (field SBA).

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The following analysis will be focused on the five representative types of samples:

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background AA, field SBA, lab-controlled straw burning aerosols (lab SBA), including lab

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CBA and lab WBA, and lab EBA. The concentrations of PM2.5, OC, EC, K+, Kbb, HULIS-C

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and levoglucosan in different samples are shown in Table S1. The Kbb concentration of

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background AA was 0.00 µg/m3 after subtracting the K+ contributed by soil, reflecting

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negligible impact of straw burning. All the K+ in BBA from lab-controlled burning was

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primarily emitted from biomass burning, thus the Kbb was considered equal to K+.

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Overall Molecular Characterization of HULIS

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The mass spectra of HULIS emitted from lab-controlled burning of corn and wheat

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straws under different burning conditions are compared in Fig S3 and S4. More species were

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detected in ESI+ than in ESI- (Table S2, S3), in consistence with previous studies showing

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that spectra observed in ESI+ were usually more informative than those of ESI-.46,47 It is also

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showed that mass spectra of samples from low MCE conditions contained more peaks than

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those from high MCE ones (Table S2, S3), and most of the peaks detected in high MCE



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samples were also detected in the low MCE samples (Fig. S3, S4). Therefore, further

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discussion on lab-controlled burning samples is mainly focused on one lab CBA sample

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(MCE = 0.78) and one lab WBA sample (MCE = 0.71). We further found that most of the

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peaks detected in HULIS from field burning aerosols were also detected in the samples from

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lab-controlled burning experiments (Fig. S5). The mass spectra of HULIS extracted from the

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five representative samples observed in ESI+ and ESI- are compared in Fig. 2 and Fig. S6,

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respectively. Each spectrum in ESI+ contained 380-1720 peaks, with the most intense ones

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falling between m/z 90 and 350. The intensity weighted average molecular weight of HULIS

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was 177-210 Da, much smaller than that of aquatic fulvic acids.48,49 This may be attributed to

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their different origins and formation mechanisms.1,50-53 The background AA contained the

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most numerous compounds among different types of samples (Table S2, S3). This is because

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there are also numerous secondary organic compounds in ambient aerosols, in addition to

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primary emitted compounds.19,53-55 Directly emitted organic aerosols or volatile organic

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compounds (VOCs) can react in smoke plumes and form secondary organic compounds

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within hours,23,24,26 so more compounds are identified in field SBA than in lab SBA or lab

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EBA (Table S2, S3).

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The identified formulas were classified into several major compound categories based

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on their elemental compositions, including CHO+, CHN+, CHON+ detected in ESI+ and

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CHO-, CHON-, CHOS-, CHONS- detected in ESI-. For example, CHON+ refers to

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compounds that contain C, H, O and N elements detected in ESI+. Other compound

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categories are defined analogously. Compounds excluded from the above major categories

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only accounted for 0.01-9.92% of the total ions in terms of peak intensity (Table S2, S3) and

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will not be discussed further. CHOS- and CHONS- are usually related to organosulfates or

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nitrooxy-organosulfates, most of which are formed through oxidation of biogenic VOCs in

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ambient atmosphere.56-60 These two categories account for more than half of all the


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compounds detected in ESI- in background AA, while very small fractions in lab SBA or lab

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EBA (Table S3). Many CHON compounds emitted from biomass burning (e.g., nitrocatechol,

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nitrophenol)24-26,61 are preferentially detected in ESI- due to the presence of some readily

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deprotonated groups.13,62 These compounds were either solely detected in ESI- or had higher

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intensity in ESI- spectra than in ESI+, so that they provided complementary information in

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addition to the spectra of ESI+.

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In background AA, CHO+ compounds were the most abundant in terms of peak

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intensity (Fig. 2a and Table S2) and 75% of them were not detected in lab SBA or EBA.

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Among all the 557 CHO+ compounds detected in background AA, 118 compounds shared

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the same formulas with species found in biogenic SOA (Table S4). For example, the highest

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peak in background AA is C10H16O3. This molecule is likely pinonic acid, a major

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photooxidation product of α-pinene,63,64 which was not observed in SBA or EBA. Table S4

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lists other intense CHO+ compounds (relative intensity > 10%), whose formulas are identical

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to species in biogenic SOA reported in chamber studies. These compounds can be formed by

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oxidation of monoterpenes or other biogenic VOCs in ambient atmosphere. The NOC are

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classified by whether the compounds contained O element or not: CHON (CHON+ and

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CHON-) and N-bases (CHN+ compounds contain only C, H and N elements without O

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element). In field or lab SBA, the percentage and intensities of NOC increased significantly

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compared with those in background AA and were much higher than those of CHO in each

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sample (Fig. 2, Table S2). The enhancement of CHON- in field SBA was more significant

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than those in lab SBA (Fig S6, Table S3). This is because many CHON compounds (e.g.,

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nitrocatechols, nitrophenols), preferentially detected in ESI-, are formed by secondary

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oxidation of primary emitted compounds (e.g., phenols, cresol) in smoke plumes.23,24,26 There

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is more time for the oxidation of burning smoke in the open field, while the lab-controlled

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burning experiment and its sampling only lasted for 20-35 min. In addition, the dark



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condition in biomass burning simulation system is less favorable for oxidation processes than

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that in the field.

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The oxidation of primarily emitted BBA can form nitroaromatics in burning smoke

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plumes.23-25,65 Molecules with O/N ≥ 2 or 3 allow the assignment of nitro (-NO2) or nitrooxy

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(-ONO2) groups. The number of CHON compounds with O/N ≥ 2 or 3 in field SBA is larger

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than that in lab SBA (Table S5). This is because the condition in the field was more favorable

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for the oxidation of primary BBA and formation of nitroaromatics than that in the

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lab-controlled burning system. For example, the molecule C7H7NO4 was only observed in

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field SBA, but not in background AA, lab SBA or EBA. This molecule is probably

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methyl-nitrocatechol, formed via oxidation of phenols or cresols in smoke plumes.24,61

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C6H5NO4 was detected as the highest peak in field SBA in ESI-, but was not detected in lab

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SBA or EBA. This molecule may be nitrocatechol, formed via nitration of phenols in the

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presence of NOx in smoke plumes.25,61 Other CHON compounds such as C7H9NO, C10H9NO,

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C12H11NO and C9H7NO were observed with high intensity in ESI+ in SBA. They contain

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only one N atom and one O atom in each molecule, which indicates that they are composed

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of reduced nitrogen functional groups rather than nitro or nitrooxy groups. Imines, produced

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by reversible reactions between ammonia and carbonyls, are unstable under our experimental

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conditions and usually quickly hydrolyze in aqueous solutions.13,66 Thus, the reduced

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nitrogen was unlikely to be imines but N-heterocyclic compounds formed by additional

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cyclization and disproportionation reactions after the formation of imines.66,67 There were

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also numbers of CHON compounds, as well as those with O/N ≥ 2 or 3 in background AA.

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Different from SBA, the formation mechanisms of CHON compounds in the ambient

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atmosphere include daytime photooxidation, nighttime chemistry and hydrolysis of some

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nitrooxy-organosulfate,23,60,68,69 which is out of the scope of this study.


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Another important NOC category is the N-bases composed of C, H, N elements. Their

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intensity and percentages in SBA showed the highest enhancement ratios relative to those in

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background AA or lab EBA among all compound categories (Fig. 2, Table S2). Increased

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emissions of N-bases were observed under more smoldering conditions (Table S2), which are

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usually the cases of straw burning in the farm field (Fig. S1). The N-bases are probably

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emitted into smoke by direct volatilization/stream stripping and thermal alteration of fiber

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materials or recombination of primary combustion products during straw burning.70,71

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Molecular Distribution of N-bases

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The peak distributions among mass spectra of field SBA, lab CBA and lab WBA exhibit

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similar patterns (Fig. 2 b, c, d), especially the N-bases. 74 same N-bases (Table S6) were

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identified in field SBA, lab CBA and WBA, which were generally among the most intense

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ions in Fig. 2. 42 of the 74 N-bases were identified in the three SBA samples, but not in

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background AA or lab EBA. The other 32 N-bases also detected in background AA or lab

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EBA, but their intensities were much lower than those in SBA. 59% of the 74 N-bases had AI

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larger than 0.5, which is the threshold value used as the minimum criterion for the existence

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of aromatic structures in a molecule.41 The average AI of N-bases in lab SBA (0.51-0.56) was

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larger than that in background AA (0.49), which suggests more aromatic structures existed in

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lab SBA. Most of the N-bases contained two N atoms in their formula. Identification of their

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exact molecular structures based on the elemental composition alone is impractical due to the

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existence of stable isomers. Alternatively, the N-bases containing two N atoms could be

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assigned to a variety of N-heterocyclic alkaloids with two nitrogen atoms embedded into

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five-membered (pyrazole, imidazole and their derivatives) or six-membered rings (pyrazine,

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pyrimidine, pyridazine and their derivatives) and molecules with 1N-containing heterocyclic

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rings fused together.



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To further understand the molecular distribution of N-bases in SBA, we obtained their

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van Krevelen diagrams by plotting the H/C ratio versus N/C ratio (Fig. 3), in which DBE was

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employed to reflect the degree of molecular unsaturation. This plot shows the wide presence

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of CH2-based homologous series with two nitrogen atoms in SBA, which can be chemically

324

related to each other by methylation/demethylation or alkyl chain elongation. The most

325

abundant N-bases were three 2N-containing homologous series, indicated by dark blue, light

326

blue and green dotted lines in Fig. 3. The three major N-base homologous series accounted

327

for 47-77% of all the N-bases in terms of peak intensity. Distribution of the three major

328

homologous series are plotted in Fig. 4, of which the S/N are normalized against sampled air

329

volumes and ion injection time to make them comparable.72 The C5H8N2(CH2)n homologous

330

series dominated the spectra of field SBA, lab CBA and WBA (Fig. 4 f, i, j). According to

331

previous MS/MS studies,13,19 they are likely homologous series of imidazole compounds,

332

which is five-membered heterocyclic ring with two nitrogen atoms as the core structure and

333

alkyl as side chains. The core structure of the next abundant C5H6N2(CH2)n series has an AI

334

of 0.67. Pyrazine, pyrimidine or amine pyridine homologous series could be the candidate,

335

composed of six-membered heterocyclic ring with nitrogen atoms and alkyl side chains.13,19

336

In the C7H6N2(CH2)n series, the core molecule (C7H6N2) has an AI of 0.80, which is

337

characteristic of compounds with fused five-membered and six-membered rings (e.g.,

338

azaindole). Other homologous series such as C8H6N2(CH2)n or C11H8N2(CH2)n series have

339

DBE ≥ 7, indicated by grey dotted lines in Fig. 3, can only be explained by the presence of

340

N-heterocyclic compounds with fused aromatic rings. Overall, Fig. 3 demonstrates that

341

HULIS emitted from straw residue burning contain a variety of N-base homologous series.

342

N-bases as Potential Tracers of Straw Burning


Page 17 of 36


343

In background AA, the intensities of the three major N-base homologous series are

344

much lower than those in SBA (Fig. 4). During nighttime of background day (Fig. 4b), the

345

intensities of these N-bases were 1.2-5.3 times of those during the daytime (Fig. 4a), mainly

346

due to the weaker boundary layer mixing state at night. For the field SBA collected during

347

the nighttime of June 15 (Fig. 4f), the intensities of these N-bases increased several orders of

348

magnitude higher than those during the corresponding daytime (Fig. 4e) or those during

349

background day (Fig. 4a). This was principally due to the emission of local intense straw

350

burning. The same trend was also observed during the nighttime of June 12 (Fig. 4d) under

351

the influence of straw burning. After the intense straw burning period from June 12 to 15, the

352

N-bases were gradually diluted on June 16 (Fig. 4g, h). High levels of N-bases were also

353

observed in lab SBA (Fig. 4i, j), which further suggest they were primarily emitted from

354

straw burning. They are likely N-heterocyclic alkaloids, which have been observed in

355

biomass burning organic aerosols (BBOA) extracts in previous studies.13,17,19 The

356

N-heterocyclic core structures remain stable under the thermal conditions of smoldering

357

burning, so that they can be emitted into the smoke with minor pyrolytic and oxidative

358

processing.73

359

To further confirm the influence of straw burning on N-bases, we examined the

360

interspecies relationships between major N-bases and BBA tracers (Kbb and levoglucosan),

361

Cl- or secondary components (SO42- and oxalate) in ambient aerosols. The correlation

362

coefficients are listed in Table S7, with the representative ones shown in Fig. 5. The good

363

correlations between N-bases and BBA tracers and their similar temporal variations further

364

confirm the contribution of straw burning to heterocyclic N-base homologous series. Chloride

365

aerosols emitted from biomass burning normally mix in BBA.74 The intensities of N-bases

366

also show good correlations with chloride ion (Table S7). In contrast, they don’t have

367

obvious relationship with sulfate or oxalate (Table S7). The K+, levoglucosan concentrations



Page 18 of 36

368

and intensities of major heterocyclic N-bases in each ambient aerosol sample were compared

369

with those in background AA. The enhancement ratios were calculated by taking the ratio of

370

enhancement relative to background AA (Fig. 6, Table S8). During intense straw burning

371

period (the nighttime of June 12-15), the enhancement ratios of N-bases were much higher

372

than those of K+ or levoglucosan (Fig. 6, Table S8). While during other periods, the

373

enhancement ratios of N-bases were comparable to those of two BBA tracers. The

374

heterocyclic N-bases seem to be more sensitive molecular markers than K+ or levoglucosan,

375

when used to identify the periods of intense straw burning.

376

Atmospheric Implications

377

As a traditional molecular tracer of BBOA for aerosol source apportionment,71,75

378

levoglucosan was previously used to quantitatively differentiate the contribution of biomass

379

burning emission to HULIS with those from other major sources.76 However, based on the

380

operational definition of HULIS, levoglucosan doesn’t belong to HULIS fraction. It is mainly

381

produced from the pyrolysis of cellulose, which is a universal and also the most abundant

382

component of green plants. Thus, levoglucosan is widely present in various BBOA,

383

hampering its capability to distinguish HULIS emitted from the burning of various biomass

384

fuels. The results in this study demonstrate that the heterocyclic N-bases, most likely

385

alkaloids, constitute significant fractions of HULIS in ambient aerosols affected by crop

386

residue burning. The molecular distribution of N-bases in these samples (Fig. 4d, f) are

387

similar with those observed in smoke particles freshly emitted from lab-controlled burning of

388

crop residues (Fig. 4i, j), demonstrating the identical sources. The major homologous series

389

of N-bases exhibited very different UHRMS spectra patterns between crop residue burning

390

and wood burning,19 reflecting differences in the chemical natures of these biomass materials.

391

Therefore, it may be reasonable using them as molecular markers to distinguish HULIS


Page 19 of 36


392

emitted from burning of agricultural waste (especially wheat and corn straws) with those

393

from other biomass burning sources (e.g., wood burning).

394

The results of this study demonstrated that the chemical composition of HULIS emitted

395

from crop residue burning is dominated by NOC. Though the size of each individual crop

396

residue burn is small, its widespread nature across the farm fields worldwide makes it

397

actually contribute significantly to the overall global biomass burning emissions,77 so crop

398

residue burning could be an important source of atmospheric NOC. The NOC have

399

significant effects on environment and human health. Both N-bases and nitroaromatics have

400

been recognized as major components of brown carbon aerosols.78 They can affect

401

atmospheric radiative transfer and photochemical processes due to their light absorption in

402

UV or near-UV spectral range.25,31 Moreover, the light absorption property of nitroaromatics,

403

particularly nitrophenols, was demonstrated to be enhanced as mixing with mineral or

404

chloride aerosols or at high pH values.30 This is caused by the red-shift in absorption spectra

405

as a result of deprotonation.30 In this study, both acidic nitroaromatics and alkaline N-bases

406

are abundant in HULIS from crop residue burning. The acid-base reactions between them

407

may significantly enhance the light absorbing properties of HULIS. In addition, the N-bases

408

can modulate aerosol acidity due to their buffering capacity, thus influencing many

409

acid-catalyzed heterogeneous reactions in atmospheric condensed phase. Their alkaline

410

property also favorites the partition of acid gases (e.g., HCl) into the particle phase. These

411

reactions in turn change particle composition, and then influence the hygroscopic properties,

412

phase transitions and optical properties of aerosols.74,79-81 Furthermore, the unprotonated

413

nitrogen atoms in N-bases can facilitate electron transfer in redox reactions via accepting

414

protons from the reductants.32,82 It is further demonstrated to catalyze ROS generation,

415

resulting in adverse health effects of HULIS.19,32,83 Once intense straw burning happened, a



Page 20 of 36

416

vast number of NOC would be emitted into the atmosphere, of which the impacts on

417

environment and human health deserve comprehensive exploration.

418 419

ASSOCIATED CONTENT

420

Supporting Information

421 422

This supporting information contains one appendix, 6 figures, 8 tables and 23 pages in total. The Supporting Information is available free of charge on the ACS Publications website.

423

Additional descriptions of results: The burning conditions in lab-controlled burning

424

experiments (Appendix S1, Fig. S2).

The photo of intense straw burning in the field (Fig.

425

S1). Chemical composition of aerosol samples (Table S1) and their spectra detected in ESI-

426

(Fig. S6). Comparison of lab-controlled SBA under different burning conditions (Fig. S3, S4)

427

and comparison of SBA in the field and from lab-controlled burning experiments (Fig. S5).

428

The number and intensity percentages of different compound categories (Table S2, S3). Some

429

CHO compounds in background AA with formulas identical to species in SOA (Table S4).

430

Characterization of CHON (Table S5) and intensity of the same N-bases in SBA (Table S6).

431

Correlation coefficients between major N-bases and other chemical components (Table S7)

432

and their enhancement ratios relative to those in background AA (Table S8).

433

AUTHOR INFORMATION

434

Corresponding Author

435

*

E-mail: [email protected] (Min Hu); phone: 86-10-62759880; fax: 86-10-62751920

436

*

E-mail: [email protected] (Peng Lin); phone: 1-509-554-6358

437

Present Address


Page 21 of 36


438

§

439

Richland, WA 99532.

440

Notes

441

The authors declare no competing financial interest.

442

ACKNOWLEDGMENT

443

This work was supported by National Natural Science Foundation of China (91544214,

444

41421064); the National Basic Research Program of China (973 Program) (2013CB228503);

445

the China Ministry of Environmental Protection’s Special Funds for Scientific Research on

446

Public Welfare (20130916). We also thank Prof. Jianzhen Yu for her helpful suggestions to

447

this work and Mr. Jianfeng Li for the photo of intense straw burning in SI and his helping

448

with sampling.

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

449 450

REFERENCES

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53. Lin, P.; Engling, G.; Yu, J. Z. Humic-like substances in fresh emissions of rice straw burning and in ambient aerosols in the Pearl River Delta Region, China. Atmos. Chem. Phys. 2010, 10, 6487-6500. 54. Gomez-Gonzalez, Y.; Surratt, J. D.; Cuyckens, F.; Szmigielski, R.; Vermeylen, R.; Jaoui, M.; Lewandowski, M.; Offenberg, J. H.; Kleindienst, T. E.; Edney, E. O., et al. Characterization of organosulfates from the photooxidation of isoprene and unsaturated fatty acids in ambient aerosol using liquid chromatography/(-) electrospray ionization mass spectrometry. J. Mass Spectrom. 2008, 43, 371-382. 55. Altieri, K. E.; Turpin, B. J.; Seitzinger, S. P. Composition of Dissolved Organic Nitrogen in Continental Precipitation Investigated by Ultra-High Resolution FT-ICR Mass Spectrometry. Environ. Sci. Technol. 2009, 43, 6950-6955. 56. Surratt, J. D.; Gomez-Gonzalez, Y.; Chan, A. W.; Vermeylen, R.; Shahgholi, M.; Kleindienst, T. E.; Edney, E. O.; Offenberg, J. H.; Lewandowski, M.; Jaoui, M., et al. Organosulfate formation in biogenic secondary organic aerosol. J. Phys. Chem. 2008, 112, 8345-8378. 57. Lin, P.; Yu, J. Z.; Engling, G.; Kalberer, M. Organosulfates in humic-like substance fraction isolated from aerosols at seven locations in East Asia: a study by ultra-high-resolution mass spectrometry. Environ. Sci. Technol. 2012, 46, 13118-13127. 58. Romero, F.; Oehme, M. Organosulfates - A New Component of Humic-Like Substances in Atmospheric Aerosols? J. Atmos. Chem. 2005, 52, 283-294. 59. He, Q. F.; Ding, X.; Wang, X. M.; Yu, J. Z.; Fu, X. X.; Liu, T. Y.; Zhang, Z.; Xue, J.; Chen, D. H.; Zhong, L. J., et al. Organosulfates from pinene and isoprene over the Pearl River Delta, South China: seasonal variation and implication in formation mechanisms. Environ. Sci. Technol. 2014, 48, 9236-9245. 60. Darer, A. I.; Cole-Filipiak, N. C.; O'Connor, A. E.; Elrod, M. J. Formation and stability of atmospherically relevant isoprene-derived organosulfates and organonitrates. Environ. Sci. Technol. 2011, 45, 1895-1902. 61. Chow, K. S.; Huang, X. H. H.; Yu, J. Z. Quantification of nitroaromatic compounds in atmospheric fine particulate matter in Hong Kong over 3 years: field measurement evidence for secondary formation derived from biomass burning emissions. Environ. Chem. 2016, 13, 665-673. 62. Gaskell, S. J. Electrospray: principles and practice. J. Mass Spectrom. 1997, 32, 677-688. 63. Yu, J.; Griffin, R. J.; Cocker, D. R.; Flagan, R. C.; Seinfeld, J. H.; Blanchard, P. Observation of gaseous and particulate products of monoterpene oxidation in forest atmospheres. Geophys. Res. Lett. 1999, 26, 1145-1148. 64. Kavouras, I. G.; Mihalopoulos, N.; Stephanou, E. G. Secondary Organic Aerosol Formation vs Primary Organic Aerosol Emission: In Situ Evidence for the Chemical Coupling between Monoterpene Acidic Photooxidation Products and New Particle Formation over Forests. Environ. Sci. Technol. 1999, 33, 1028-1037. 65. Kroflic, A.; Grilc, M.; Grgic, I. Unraveling Pathways of Guaiacol Nitration in Atmospheric Waters: Nitrite, A Source of Reactive Nitronium Ion in the Atmosphere. Environ. Sci. Technol. 2015, 49, 9150-9158. 66. Laskin, J.; Laskin, A.; Roach, P. J.; Slysz, G. W.; Anderson, G. A.; Nizkorodov, S. A.; Bones, D. L.; Nguyen, L. Q. High-resolution desorption electrospray ionization mass spectrometry for chemical characterization of organic aerosols. Anal. Chem. 2010, 82, 2048-2058. 67. Bones, D. L.; Henricksen, D. K.; Mang, S. A.; Gonsior, M.; Bateman, A. P.; Nguyen, T. B.; Cooper, W. J.; Nizkorodov, S. A. Appearance of strong absorbers and fluorophores in limonene-O3 secondary organic aerosol due to NH4+-mediated chemical aging over long time scales. J. Geophys. Res. 2010, 115. 68. Ng, N. L.; Kwan, A. J.; Surratt, J. D.; Chan, A. W. H.; Chhabra, P. S.; Sorooshian, A.; Pye, H. O. T.; Crounse, J. D.; Wennberg, P. O.; Flagan, R. C., et al. Secondary organic aerosol(SOA) formation from reaction of isoprene with nitrate radicals(NO3). Atmos. Chem. Phys. 2008, 8, 4117-4140. 69. Ng, N. L.; Brown, S. S.; Archibald, A. T.; Atlas, E.; Cohen, R. C.; Crowley, J. N.; Day, D. A.; Donahue, N. M.; Fry, J. L.; Fuchs, H., et al. Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol. Atmos. Chem. Phys. 2017, 17, 2103-2162. 70. Mayol-Bracero, O. L.; Guyon, P.; Graham, B.; Roberts, G.; Andreae, M. O.; Decesari, S.; Facchini, M. C.; Fuzzi, S.; Artaxo, P. Water-soluble organic compounds in biomass burning aerosols over Amazonia - 2. Apportionment of the chemical composition and importance of the polyacidic fraction. J. Geophys. Res., [Atmos.] 2002, 107. 71. Applied GeochemistrySimoneit, B. R. T. Biomass burning - A review of organic tracers for smoke from incomplete combustion. Appl. Geochem. 2002, 17, 129-162.


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Figures

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672 673

Figure 1 Temporal variation of PM2.5, K+ and levoglucosan during the sampling period at

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Suixi site.



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675

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Figure 2 Intensity distribution, number and intensity percentages of CHO+, CHN+ and

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CHON+ compound categories in HULIS isolated from the five representative aerosol

679

samples detected in ESI+: (a) background ambient aerosol (background AA), (b) straw

680

burning aerosol in the field (field SBA), (c) aerosols from lab-controlled corn burning (lab

681

CBA, MCE=0.78), (d) aerosols from lab-controlled wheat burning (lab WBA, MCE=0.71),

682

and (e) aerosols from lab-controlled eucalypt burning (lab EBA, MCE=0.87).

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Figure 3 Van Krevelen plots of N-bases in (a) field SBA, (b) lab CBA and (c) lab WBA. The

686

color bar denotes the double bond equivalent and the text markers denote the number of

687

nitrogen atoms in each assigned formula. The points, which are expressed as the same

688

number of nitrogen atoms and aligned along the same line, are members of the same

689

CH2-based homologous series, formulated as a core molecule plus (CH2)n (n≥0).

690



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Figure 4 Intensity distributions of the three major N-base homologous series. The names of

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13/06/11D (SX) and 13/06/11N (SX) refer to the daytime and nighttime sample of June 11,

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2016 collected at Suixi (SX) site and others are named analogously.

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Figure 5 The correlations of major N-bases intensities with (a) Kbb, (b) levoglucosan and (c)

699

their temporal variations in ambient aerosols collected at Suixi site.

700



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Figure 6 The enhancement ratios of major N-bases, K+ and levoglucosan in each ambient

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aerosol sample compared with those in background AA at Suixi site.

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TOC Art 170x85mm (300 x 300 DPI)



Figure 1 Temporal variation of PM2.5, K+ and levoglucosan during the sampling period at Suixi site. 355x165mm (300 x 300 DPI)


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Figure 2 Intensity distribution, number and intensity percentages of CHO+, CHN+ and CHON+ compound categories in HULIS isolated from the five representative aerosol samples detected in ESI+: (a) background ambient aerosol (background AA), (b) straw burning aerosol in the field (field SBA), (c) aerosols from labcontrolled corn burning (lab CBA, MCE=0.78), (d) aerosols from lab-controlled wheat burning (lab WBA, MCE=0.71), and (e) aerosols from lab-controlled eucalypt burning (lab EBA, MCE=0.87). 381x304mm (300 x 300 DPI)



Figure 3 Van Krevelen plots of N-bases in (a) field SBA, (b) lab CBA and (c) lab WBA. The color bar denotes the double bond equivalent and the text markers denote the number of nitrogen atoms in each assigned formula. The points, which are expressed as the same number of nitrogen atoms and aligned along the same line, are members of the same CH2-based homologous series, formulated as a core molecule plus (CH2)n (n≥0). 304x508mm (300 x 300 DPI)


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Figure 4 Intensity distributions of the three major N-base homologous series. The names of 13/06/11D (SX) and 13/06/11N (SX) refer to the daytime and nighttime sample of June 11, 2016 collected at Suixi (SX) site and others are named analogously. 355x279mm (300 x 300 DPI)



Figure 5 The correlations of major N-bases intensities with (a) Kbb, (b) levoglucosan and (c) their temporal variations in ambient aerosols collected at Suixi site. 508x457mm (300 x 300 DPI)


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Figure 6 The enhancement ratios of major N-bases, K+ and levoglucosan in each ambient aerosol sample compared with those in background AA at Suixi site. 288x330mm (300 x 300 DPI)


Molecular Characterization of Nitrogen-Containing Organic

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