First Chemical Characterization of Refractory Black Carbon Aerosols

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First chemical characterization of refractory black carbon aerosols and associated coatings over the Tibetan Plateau (4730 m a.s.l) Junfeng Wang, Qi Zhang, Min-Dong Chen, Sonya Collier, Shan Zhou, Xinlei Ge, Jianzhong Xu, Jinsen Shi, Conghui Xie, Jianlin Hu, Shun Ge, Yele Sun, and Hugh Coe Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03973 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Environmental Science & Technology

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First chemical characterization of refractory black carbon aerosols

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and associated coatings over the Tibetan Plateau (4730 m a.s.l)

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Junfeng Wang,† Qi Zhang,‡,† Mindong Chen,† Sonya Collier,‡ Shan Zhou,‡

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Xinlei Ge,†,* Jianzhong Xu,§,* Jinsen Shi, Conghui Xie,§, Jianlin Hu,† Shun Ge,†

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Yele Sun, and Hugh Coe#







6



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Control, Collaborative Innovation Center of Atmospheric Environment and

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Equipment Technology, School of Environmental Science and Engineering, Nanjing

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University of Information Science and Technology, Nanjing 210044, China

Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution

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11

CA 95616, USA

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§

13

Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000,

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China

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Department of Environmental Toxicology, University of California at Davis, Davis,

State

Key

Laboratory

of

Cryospheric

Sciences,

Northwest

Institute

of



College of Atmospheric Science, Lanzhou University, Lanzhou 730000, China



State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric

17

Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing

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100029, China

19

#

20

Manchester, UK

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*Corresponding authors, Email: [email protected]; [email protected]

School of Earth and Environmental Sciences, University of Manchester, M13 9PL,

22 23

Phone: +86-25-58731394

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For Environ. Sci. Technol.

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Abstract: Refractory black carbon (rBC) aerosol is an important climate forcer, and

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its impacts are greatly influenced by the species associated with rBC cores. However,

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relevant knowledge is particularly lacking at the Tibetan Plateau (TP). Here we report,

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for the first time, highly time-resolved measurement results of rBC and its coating

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species in central TP (4730 m a.s.l), by using an Aerodyne soot particle aerosol mass

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spectrometer (SP-AMS), which selectively measured rBC-containing particles. We

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found that the rBC was overall thickly coated with an average mass ratio of coating to

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rBC (RBC) of ~7.7, and the coating species were predominantly secondarily produced

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by photochemical reactions. Interestingly, the thickly coated rBC was less oxygenated

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than the thinly coated rBC, mainly due to influence of the transported biomass

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burning organic aerosol (BBOA). This BBOA was relatively fresh but formed very

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thick coating on rBC. We further estimated the “lensing effect” of coating

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semi-quantitatively by comparing the measurement data from a multi-angle

40

absorption photometer and SP-AMS, and found it could lead to up to 40% light

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absorption enhancement at RBC>10. Our findings highlight that BBOA can

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significantly affect the “lensing effect”, in addition to its relatively well known role as

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light-absorbing “brown carbon.”

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1. Introduction

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Atmospheric refractory black carbon particles (rBC), or soot, are a product of

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incomplete combustion of biomass or fossil fuel.1 It is harmful to human health and

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impacts air quality significantly.1 rBC can affect the radiative forcing by direct

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absorption of sunlight, or indirectly through alteration of cloud properties as it can be

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coated with hydrophilic materials and activated into cloud condensation nuclei

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(CCN).2, 3 Some studies estimate that the positive climate forcing of rBC is second

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only to CO2.1, 4 In addition, rBC can also deposit on snow and ice, reducing surface

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albedo and accelerating melting.1,

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Tibetan Plateau (TP), the “Third Pole” of earth,7 as it is the world's third-largest store

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of glacial ice;8 and since TP is in the subtropical latitude, the radiation energy per unit

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area reflected into space by its glaciers was estimated to be four times more than that

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intercepted between 60 and 70 oN or S.9 However, glaciers in TP are retreating at a

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higher speed than any other part of the world, which may lead to a shortage of

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seasonal fresh water supply for Asian Rivers, and may severely influence the

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Indian/Asian summer monsoon and hence global climate too.10, 11 A recent study

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revealed that rBC and fly ash particles could occupy a large fraction of the cryoconite

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deposited on the TP glaciers.12 rBC has been shown to be an important contributor to

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the TP glacier retreat and Asia monsoon,13 and adds to the much faster temperature

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increase (0.3 oC per decade in the past three decades) in areas above 4000 m than the

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observed global warming.7

5, 6

This effect is particularly important for the

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Nevertheless, the climate effects of rBC are highly uncertain, largely due to its

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interactions/mixing with other aerosol components.1, 14-16 rBC can be co-emitted with

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a variety of species, and become internally mixed with various organic and inorganic

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species through coagulation or condensation processes during atmospheric transport.1,

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17

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change dynamically during aging and greatly affect the radiative behaviors of

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rBC-containing particles.18, 19 For instance, the radiative absorption of rBC can be

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enhanced due to the non-rBC components coated on rBC cores;4, 20-22 although the

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degree of enhancement is highly variable (from 6%17 to ~200%23) - dependent on the

The chemical composition, optical properties, and thickness of rBC-coating also

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composition, mixing state, coating thickness and volatility of the coating materials.24

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The coating can also alter rBC’s hygroscopicity, for example, increases in water

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uptake and CCN activity have been observed when it is coated with secondary

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species.25-28 Clearly, rBC properties are diverse in different environments, thus

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measurements of the concentrations, compositions, and sources of rBC and its coating

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over TP, are critical to assess its environmental and climate impacts in this region.

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A number of studies have been conducted previously to investigate rBC in TP.

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For example, mean rBC concentrations of 0.75 µg m-3 and 0.50 µg m-3 (both in

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southeast TP, ~3300 m a.s.l.),29 0.36 µg m-3 and 0.16 µg m-3 (both in Qinghai-TP,

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~3200 m a.s.l.),30, 31 0.077 µg m-3 (western trans-Himalayas, ~4520 m a.s.l.)32 and

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0.133 µg m-3 (a western TP site, ~4500 m a.s.l.)33 were reported. A modeling study

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simulated surface rBC concentration of 0.24-0.40 µg m-3 over TP for 1980-2010.34

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Sources of rBC in TP mainly include biofuel/biomass burning from South Asia and

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fossil fuel emissions from South and East Asia.

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dominant contribution from fossil fuel combustion to rBC in northern TP but a more

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significant contribution from local sources (such as yak dung combustion) for rBC in

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inner TP.36 A few studies also investigated the light-absorption characteristics of

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carbonaceous aerosols (including rBC) in TP,30,

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characterize the rBC-coating directly and its influence on the optical properties.

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A recent report further revealed a

37-39

yet none of these studies

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To probe the properties of rBC-coating, single particle soot photometer (SP2)40, 41

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is a powerful instrument as it can exclusively detect the rBC-containing particles and

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quantify rBC mass and coating thickness by assuming a core-shell configuration. The

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SP2 can respond to metals and probably some non-rBC refractory material (e.g.,

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hematite), but overall it is very limited in determining the chemical compositions of

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coating. On the other hand, the Aerodyne aerosol mass spectrometers (AMS)42-44 have

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been widely used to determine the fine aerosol composition and some studies45-48 used

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the AMS-measured non-refractory aerosol components to infer sources/aging

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processes of rBC. However, such treatment is largely uncertain as the AMS measured

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the total aerosol species without differentiating the portion that actually coats on rBC.

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An advanced instrument, which physically integrates the laser from SP2 into the

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Aerodyne high-resolution time-of-flight AMS (HR-AMS)49, termed as soot-particle

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AMS (SP-AMS),50-55 is the only instrument that can selectively detect rBC and

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simultaneously determine the sizes, concentrations and compositions of its associated

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coating. Of course, single-particle mass spectrometry technique44, 56 can also detect

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rBC and relevant mixed material, but in a much less quantitative manner. So far, only

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a limited number of studies applied the SP-AMS technique to explore the

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rBC-containing particles and/or the light absorption properties, such as in

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California,17, 57, 58 Toronto,59-61 and London.20 This work, for the first time, conducted

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the real-time and highly time-resolved measurement on rBC-containing particles in

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the remote and high-altitude region (central TP). The temporal variations, chemical

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compositions, sources of rBC and its coating are elucidated, and influences of the

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coating on the light absorption of rBC are also discussed.

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2 Experiments

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2.1 Sampling site

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The field measurement was carried out at the Comprehensive Observation Station

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of Chinese Academy of Science in Nam Co (4730 m a.s.l, 30o46’N, 90o59’E, Figure

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S1 of the supporting information (SI)) from May 30 to June 30, 2015. This is the

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highest elevation site in the TP enabling deployment of a suite of real-time aerosol

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monitoring instruments. It is located south of the Lake Nam Co and north of the

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Nyenchen Tanglha Mountains. The surroundings are mainly pastureland with a few

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residents herding yaks and sheep, a tourist resort is situated ~9 km west.

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2.2 Instrumentation

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Besides the SP-AMS, an HR-AMS, and a multi-angle absorption photometer

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(MAAP) (Model 5012, Thermo Scientific Corp., USA) were also operated in parallel.

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This work focused on the SP-AMS data. The SP-AMS and HR-AMS shared the same

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sampling line with a PM2.5 cyclone in front to remove coarse particles and

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subsequently a silica gel diffusion dryer to eliminate moisture. Both AMSs used 130

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µm orifices to maintain ~0.1 L min-1 air flow as the pressure at the site (~56 kpa) is

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much lower than that at sea level. Due to the transmission efficiency of the lens

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system, both AMSs measured submicron particles (PM1).

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The standard SP-AMS is equipped with an intracavity infrared laser vaporizer

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and a thermal tungsten heater.50 However, to exclusively measure rBC-containing

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particles, the tungsten vaporizer was physically removed, since it could still reach

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~200 oC62

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instrument was operated with the laser only configuration throughout the campaign.

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We only collected V-mode data with high sensitivity due to typically low mass

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loadings at this site. Each 5-minute cycle consisted of two menu settings (2.5 minutes

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each) for quantification of concentration/mass spectra and chemically-resolved size

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distributions, respectively.

even when it was turned off due to heating by the filament. The

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Mass quantification (i.e., ionization efficiency) of the SP-AMS was calibrated

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following the protocol reported in Jayne et al.42 using ammonium nitrate. Relative

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ionization efficiencies (RIEs) of sulfate and rBC to nitrate were determined separately

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by using ammonium sulfate and Regal Black (REGAL 400R pigment black, Cabot

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Corp.),50 respectively. Note under laser-only configuration, direct calibrations using

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ammonium nitrate/sulfate are impossible, thus the calibrations aforementioned were

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performed before removal of the tungsten vaporizer. RIEs of ammonium, nitrate,

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sulfate and BC were determined to be 3.2, 1.05, 1.18, and 0.33, respectively, and RIE

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of organics used the default value of 1.4.43 rBC calibration was repeated under the

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laser-only configuration to determine its IE (IErBC). Based on IErBC, and by assuming

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RIEs of the aerosol species are constants, we can quantify their concentrations.60

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Without the tungsten vaporizer, the SP-AMS rBC quantification is not influenced by

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particle bouncing which heavily affects the collection efficiency in other AMS

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measurements,43 but is mainly governed by the overlap of particle beam and laser

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beam. Size calibration was done by using polystyrene latex (PSL) spheres (100-700

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nm) (Duke Scientific Corp., Palo Alto, CA) before removal of tungsten heater

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according to the method of Canagaratna et al.43

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The MAAP was connected to a separate sampling inlet with a 16.7 L min-1 and 1

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µm size cut cyclone (URG2000-30EHB, URG Corp., Chapel Hill, NC). It determined

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the rBC mass at a single wavelength of 670 nm.

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Concentrations of gases including carbon monoxide (CO), ozone (O3), nitrogen

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dioxide (NO2), and sulfur dioxide (SO2) measured by Thermo Scientific gas analyzers

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were acquired from the Nam Co Observation Station of Tibet Environmental

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Protection Agency (~50 m away from our instruments). Meteorological parameters,

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including temperature (T), relative humidity (RH), visibility (km), wind speed (WS)

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and wind direction (WD) were measured at our site.

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2.3 Data Analysis

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The SP-AMS data were processed by the standard Igor-based AMS analysis

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software.63 All mass concentrations were calculated from the high-resolution fitting of

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V-mode data. Positive matrix factorization (PMF)64 was performed using the PMF

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Evaluation Tool from Ulbrich et al.65 to resolve sources of organics coated on rBC.

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We conducted the analysis on the combined organic and inorganic high-resolution

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mass spectra (HRMS) using the method described in Sun et al.66 and Zhou et al.67

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Different solutions were evaluated carefully according to the procedures outlined in

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Zhang et al.68 Finally, the 4-factor solution was deemed as optimal, and after

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exclusion of the inorganic ion fragments, we were able to identify four distinct

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organic aerosol (OA) factors including a transported biomass burning OA (TBBOA),

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a regional background oxygenated OA (RBOOA), a semi-volatile oxygenated OA

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(SVOOA), and a low-volatility oxygenated OA (LVOOA). Key diagnostic plots of

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this solution are presented in Figure S2. Results of the 3-factor and 5-factor solutions

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are shown in Figures S3 and S4: the 3-factor solution was unable to well separate

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RBOOA and SVOOA, while the 5-factor solution presented clearly a splitting of

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LVOOA.

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Backward trajectory analysis was conducted by using the HYSPLIT4 model (http://www.arl.noaa.gov/ready/hysplit4.html).69

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3. .Results and Discussion

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3.1 Mass concentrations, size distributions of rBC and rBC-coating species

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In Figure 1a, we present the highly time-resolved mass concentrations of both

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rBC and its coating (sum of organics, sulfate, nitrate, chloride and ammonium)

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determined by the SP-AMS. A detailed combo plot depicting the temporal changes of

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meteorological

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contributions of individual components throughout the sampling period is provided in

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Figure S5. The campaign-averaged concentration of rBC-containing particles (= rBC

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+ coating) was 1.06 µg m-3, and the most abundant component was organics, followed

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by sulfate (Figure 1b). The average (±1ơ) concentration of rBC was 0.12(±0.085) µg

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m-3, close to the average concentration of elemental carbon (0.09 µgm-3) measured in

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PM1 during 2012 at the same location.70 An elevated PM pollution event occurred

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from June 10 to 13, during which the average concentrations of rBC-containing

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particles and rBC increased to 2.70 µg m-3 and 0.26 µg m-3, respectively, with

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maximums reaching 7.24 µg m-3 and 0.54 µg m-3, respectively. This episode was

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associated with transport of biomass burning emissions (details in Section 3.2). In

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addition, the coated BC particles were overall neutralized since the mean equivalent

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molar ratio of the major cations (SO42-, NO3- and Cl-) to NH4+ was very close to 1

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(Figure S6; slope of 0.95 and r2 of 0.97).

parameters,

gaseous

species,

concentrations

and

fractional

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Our measurement period covered a transition from pre-monsoon to monsoon on

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June 14 based on the changes of meteorological conditions (Figure S5). Before

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monsoon, the rBC-containing particles had a higher mass loading (1.70 µg m-3) and

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the particles were thickly coated with an average RBC (mass ratio of coating to rBC)

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of 8.9; while during monsoon, the concentration was notably low (0.31 µg m-3) and

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the coating appeared to be relatively thin (average RBC of 4.5). The air mass back

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trajectories showed that the air parcels arriving at the site in general had longer

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transportation pathways before monsoon than those during monsoon (Figure S7).

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Overall, this result indicates that the properties of rBC-containing particles before

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monsoon were heavily influenced by long-range transported air pollutants whereas the

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low mass loading and thin coating during monsoon was likely caused by strong

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scavenging of thickly coated rBC particles,71 and influences from local/regional

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sources.

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We further compared the non-refractory PM1 species (sulfate, nitrate, chloride,

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ammonium and organics) measured by the SP-AMS and HR-AMS in Figure 1c. As

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our laser-only SP-AMS only detected the portion coated on rBC cores, the slope

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represent the average mass ratio of the rBC-associated species to their total mass

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determined by the HR-AMS. Temporal variations of PM1 mass concentrations from

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both instruments generally correlated well (r2 of 0.84), and ~51% of the

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non-refractory PM1 species was mixed with rBC, which is larger than the value of 35%

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observed in California coastal air. 57 Note this ratio varied dramatically, which was on

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average much higher before monsoon (~71%) than it during monsoon (~27%). The

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comparisons for the major species (organics, sulfate and nitrate) are presented in

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Figure S8, which shows that ~55% organics, ~64% nitrate and 46% sulfate were

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internally mixed with rBC, also much higher than 7-20% of those determined for

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urban aerosols in Fontana, California.58 In addition, sometimes the SP-AMS measured

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species had higher concentrations than those from the HR-AMS (above 1:1 line,

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Figures S8 and 1c). This is probably in large part due to measurement uncertainties of

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the two instruments. However, we want to point out that these extra species might be

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real. One reason is that the SP-AMS can measure refractory species (metal

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sulfate/nitrate and refractory organics) while the HR-AMS cannot. Moreover, recent

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studies21,

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restructuring into compacted spheres. It is possible that not all species inside voids of

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rBC cores (either ones from aging or co-emitted with rBC) are detected by the

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HR-AMS. We propose both hypotheses should be investigated in future.

22, 72

found that BC aging was initiated by the filling of voids and

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The campaign-averaged size distributions of the major species of rBC-containing

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particles are shown in Figure 2a. Nitrate, sulfate and organics all had very similar

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patterns, and their dependences on RBC and the aerodynamic dimeter (Dva) of rBC

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(Figures 2b-d) were also similar. The data strongly indicates that nitrate, sulfate and

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organics were well internally mixed, and thickly coated on rBC, implying a likely

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core-shell configuration of the rBC-containing particles. The data (Figures 2a and S9)

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also shows that large particles were dominated by thickly coated rBC, and only small

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particles (10. In particular, the contribution

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of transported BBOA to Eabs appeared to be very significant, since the particles with

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larger mass fractions of the transported BBOA tended to have thicker coating (Figure

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5).

364

In future, a more accurate quantification of Eabs can be performed by directly

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comparing the light absorptions of coated rBC to that of uncoated rBC (obtained by

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using a thermodenuder to remove coating at high temperatures20). Such measurement

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can eliminate the measurement uncertainties of SP-AMS. In addition, implementation

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of other photoacoustic absorption measurements can also help to improve the Eabs

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quantification.

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4. Atmospheric implications

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Our real-time measurement conducted in central TP elucidated for the first time

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the detailed chemical characteristics of rBC and its associated coating. Our results

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revealed that in TP the thickly coated rBC was not necessarily highly oxygenated,

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indicating that the rBC aging in remote and clean environment may be diverse, and

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highly dependent upon the pollutants co-emitted with rBC or being intercepted along

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its way to the receptor site. In our case, BBOA was found to play a vital role: it was

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relatively fresh but formed very thick coating on rBC, and well mixed with other

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species (See Figure S11 for the chemically-resolved size distributions of

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rBC-containing particles during the period heavily influenced by the transported

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BBOA).

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Moreover, it is relatively well understood that BBOA is an important source of 81, 83-85

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light-absorbing BrC,

and our analyses of simultaneously collected filter

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samples indeed revealed that BB was important to the water-soluble BrC in this site.86

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The findings in this work, on the other hand, underscore that BBOA can also

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contribute remarkably to the light absorption via “lensing effect” as it can coat thickly

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on rBC. A previous study87 observed high organic carbon to elemental carbon ratios in

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biomass burning aerosols, here we pinpoint that the BB organics can heavily coat on

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rBC cores. Nevertheless, the BBOA here was likely attributed to regional biomass

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mass/yak dung/incense burning. It is essential to conduct further investigations on

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different biomass burning sources to clarify their specific properties and interplays

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with the light absorption enhancement of rBC.

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At last, Mie theory88 assuming a spherical rBC core and a well-defined core-shell

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structure is currently used in many climate models to evaluate rBC’s climate effects,

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while our size distribution data of different species likely suggest a well

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internally-mixed structure of the coated rBC in TP. Nevertheless, further analyses

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including single particle imaging techniques89, 90 and SP2 measurement41 are still very

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necessary to investigate the morphology and mixing states of rBC-containing particles

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under different meteorological conditions and locations in TP. Such measurements are

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the subject of our future work, to further understand the suitability and limitations of

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Mie theory, and improve the assessment of rBC’s effects in this region.

402 403

List of acronyms

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rBC: refractory black carbon

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BrC: Brown carbon

406

TP: Tibetan Plateau

407

HR-AMS: High-resolution aerosol mass spectrometer (Aerodyne research Inc.)

408

SP-AMS: Soot-particle aerosol mass spectrometer (Aerodyne research Inc.)

409

MAAP: Multi-angle absorption photometer (Thermo Scientific Model 5012)

410

SP2: Single particle soot photometer

411

RBC: mass ratio of rBC coating to rBC core

412

CCN: Cloud condensation nuclei

413

IE: Ionization efficiency

414

RIE: Relative ionization efficiency

415

HRMS: High resolution mass spectra

416

PMF: Positive matrix factorization

417

OA: Organic aerosol

418

SOA: Secondary organic aerosol

419

BB: Biomass burning

420

TBBOA: Transported biomass burning organic aerosol

421

RBOOA: Regional background oxygenated organic aerosol

422

SVOOA: Semi-volatile oxygenated organic aerosol

423

LVOOA: low-volatility oxygenated organic aerosol

424

Dva: Vacuum aerodynamic diameter

425

O/C: Oxygen-to-carbon ratio

426

H/C: Hydrogen-to-carbon ratio

427

OSc: Average oxidation state, equal to 2*O/C-H/C

428

Eabs: Light absorption enhancement

429

MAC: Mass absorption cross section

430 431

Supporting Information

432

Sampling site (Figure S1), PMF results (Figures S2-S4), overview of measurement

433

(Figure S5), ion balance (Figure S6), air mass back trajectories (Figure

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S7),comparisons of SP-AMS and HR-AMS data (Figure S8), rBC size distribution

435

(Figure S9), average HRMS of total OA (Figure S10) and TBBOA (Figure S11), and

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size distributions during the BB-influenced period (Figure S12).

437 438

Acknowledgements. This work was supported by the National Key R&D program of

439

China (2016YFC0203501), Natural Science Foundation of China (21777073 and

440

91544220), International ST Cooperation Program of China (2014DFA90780),

441

Jiangsu Natural Science Foundation (BK20150042), and Jiangsu Provincial

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Specially-Appointed Professors Foundation. J. Wang also would like to thank the

443

support from China Scholarship Council and the Innovative Project for Graduate

444

Student of Jiangsu Province (KYZZ16_0347). We thank the Nam Co Station for

445

Multispheres Observation and Research, Chinese Academy of Science and the Tibet

446

Environment Protection Agency for the supporting data and logistical assistance.

447 448

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Apel, E.; Diskin, G. S.; Fisher, J. A.; Fuelberg, H. E.; Hecobian, A.; Knapp, D. J.; Mikoviny, T.; Riemer, D.; Sachse, G. W.; Sessions, W.; Weber, R. J.; Weinheimer, A. J.; Wisthaler, A.; Jimenez, J. L., Effects of aging on organic aerosol from open biomass burning smoke in aircraft and laboratory studies. Atmos. Chem. Phys. 2011, 11, (23), 12049-12064. 76. He, L.; Lin, Y.; Huang, X.; Guo, S.; Xue, L.; Su, Q.; Hu, M.; Luan, S.; Zhang, Y., Characterization of high-resolution aerosol mass spectra of primary organic aerosol emissions from Chinese cooking and biomass burning. Atmos. Chem. Phys. 2010, 10, (23), 11535-11543. 77. Lee, T.; Sullivan, A. P.; Mack, L.; Jimenez, J. L.; Kreidenweis, S. M.; Onasch, T. B.; Worsnop, D. R.; Malm, W.; Wold, C. E.; Hao, W. M.; Collett, J. L., Chemical smoke marker emissions during flaming and smoldering phases of laboratory open burning of wildland fuels. Aerosol Sci. Tech. 2010, 44, (9), i-v. 78. Du, W.; Sun, Y.; Xu, Y.; Jiang, Q.; Wang, Q.; Yang, W.; Wang, F.; Bai, Z.; Zhao, X.; Yang, Y., Chemical characterization of submicron aerosol and particle growth events at a national background site (3295 m a.s.l.) on the Tibetan Plateau. Atmos. Chem. Phys. 2015, 15, (18), 10811-10824. 79. Ge, X.; He, Y.; Sun, Y.; Xu, J.; Wang, J.; Shen, Y.; Chen, M., Characteristics and formation mechanisms of fine particulate nitrate in typical urban areas in China. Atmosphere 2017, 8, (3), 62. 80. Kroll, J. H.; Donahue, N. M.; Jimenez, J. L.; Kessler, S. H.; Canagaratna, M. R.; Wilson, K. R.; Altieri, K. E.; Mazzoleni, L. R.; Wozniak, A. S.; Bluhm, H.; Mysak, E. R.; Smith, J. D.; Kolb, C. E.; Worsnop, D. R., Carbon oxidation state as a metric for describing the chemistry of atmospheric organic aerosol. Nat. Chem. 2011, 3, (2), 133-139. 81. Laskin, A.; Laskin, J.; Nizkorodov, S. A., Chemistry of atmospheric brown carbon. Chem. Rev. 2015, 115, (10), 4335-4382. 82. Ahern, A. T.; Subramanian, R.; Saliba, G.; Lipsky, E. M.; Donahue, N. M.; Sullivan, R. C., Effect of secondary organic aerosol coating thickness on the real-time detection and characterization of biomass-burning soot by two particle mass spectrometers. Atmos. Meas. Tech. 2016, 9, (12), 6117-6137. 83. Chakrabarty, R. K.; Moosmuller, H.; Chen, L. W. A.; Lewis, K.; Arnott, W. P.; Mazzoleni, C.; Dubey, M. K.; Wold, C. E.; Hao, W. M.; Kreidenweis, S. M., Brown carbon in tar balls from smoldering biomass combustion. Atmos. Chem. Phys. 2010, 10, (13), 6363-6370. 84. Budisulistiorini, S. H.; Riva, M.; Williams, M.; Chen, J.; Itoh, M.; Surratt, J. D.; Kuwata, M., Light-absorbing brown carbon aerosol constituents from combustion of indonesian peat and biomass. Environ. Sci. Technol. 2017, 51, (8), 4415-4423. 85. Chen, Y.; Bond, T. C., Light absorption by organic carbon from wood combustion. Atmos. Chem. Phys. 2010, 10, (4), 1773-1787. 86. Zhang, Y.; Xu, J.; Shi, J.; Xie, C.; Ge, X.; Wang, J.; Kang, S.; Zhang, Q., Light absorption by water-soluble organic carbon in atmospheric fine particles in the central Tibetan Plateau. Environ. Sci. Pollut. Res. 2017, 24, (26), 21386-21397. 87. Zhang, Y.; Shao, M.; Zhang, Y.; Zeng, L.; He, L.; Zhu, B.; Wei, Y.; Zhu, X., Source profiles of particulate organic matters emitted from cereal straw burnings. J. Environ. Sci. 2007, 19, (2), 167-175. 88. Bohren, C. F.; Huffman, D. R., Absorption and scattering of light by small particles. John Wiley & Sons, Inc.: New York, 1983. 89. Li, W.; Sun, J.; Xu, L.; Shi, Z.; Riemer, N.; Sun, Y.; Fu, P.; Zhang, J.; Lin, Y.; Wang, X.; Shao, L.; Chen, J.; Zhang, X.; Wang, Z.; Wang, W., A conceptual framework for mixing structures in individual aerosol particles. J. Geophys. Res. - Atmos. 2016, 121, (22), 13,784-13,798.

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Environmental Science & Technology

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90. Li, W.; Chen, S.; Xu, Y.; Guo, X.; Sun, Y.; Yang, X.; Wang, Z.; Zhao, X.; Chen, J.; Wang, W., Mixing state and sources of submicron regional background aerosols in the northern Qinghai–Tibet Plateau and the influence of biomass burning. Atmos. Chem. Phys. 2015, 15, (23), 13365-13376.

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Environmental Science & Technology

-3

4

10.1%

6.6% 2.8%

3 2

Monsoon

Pre-monsoon

18.2% 4.3% 2.4% 9.4% 65.7% -3 0.31µg m

13.5% 67.0% 1.70µg m

8

-3

6 4

1

2

0

0 6/3

rBC coating

Mass Conc.(µg m ) rBC

(a)

Page 24 of 30

6/7 6/11 6/15 6/19 6/23 6/27 Date and time (UTC+8)

(c) (b) 11.5% 6.2%

PM1,SP vs.PM1,HR 1:1 2 r = 0.84

2.8%

12.7%

4 2015/6/30 2015/6/20

2

2015/6/10

Date and time

66.8%

Org. 2SO4 NO3 rBC + NH4

SP-AMS

6

2015/5/31

-3

Camp. Avg.:1.06 µg m

740

0 0

2

4

6

HR-AMS

741

Figure 1. (a) Time series of rBC and its coating (sum of organics, sulfate, nitrate,

742

chloride and ammonium) (inset pie charts show the average compositions of the

743

rBC-containing particles before and during monsoon); (b) Campaign-averaged

744

compostion of the rBC-containing particles; (c) Scatter plot of the total PM1

745

measured by the SP-AMS and the HR-AMS (colored by time).

746

ACS Paragon Plus Environment

Environmental Science & Technology

-3

6 5

SO4

4

NO3 rBC

0.8

2-

0.3 0.8

5

0.6 0.2

0.6

0.4 0.4

2

0.1

1 2

4

0.0

6 8

100

8

4

6

3

4

2

0.2 0.0

0.0

2 0

100

1000

4

6

Dva (nm)

8

10

12

14

RBC

(c)

1000

9 8 7 6

1000

-

0.1

1.0

4

0.8

3

0.6 0.4

2

100

0.0 100

4

747

)

0.2

0.0

-3

2

5

2-

0.2

-3

3

1.2

SO4 (µg m

0.3

4

Dva (nm)

0.4

5

(d)

9 8 7 6

NO3 (µg m )

Dva (nm)

-3

0.2

6 8

9 8 7 6

1.0

-

3

0

(b)

1000

Org.

Org. (µg m )

dM/dlogDva (µg m )

(a)

Dva (nm)

Page 25 of 30

6

8

10

12

14

4

6

RBC

8

10

12

14

RBC

748

Figure 2. (a) Campaign-averaged size distributions of major species of coated rBC

749

(rBC size distribution was scaled from that of m/z 24), and their dependences on Dva

750

of rBC and RBC (mass ratio of coating to rBC) (b-d).

751

ACS Paragon Plus Environment

Environmental Science & Technology

Page 26 of 30

752 O/C = 0.51, H/C = 1.48, N/C = 0.011, OM/OC = 1.82

(a) 10

(e)

TBBOA

Pre-monsoon 3.0

RBOOA

(f)

0.00

-3

5 0 O/C = 0.69, H/C = 1.22, N/C = 0.007, OM/OC = 2.03

LVOOA

4

RBOOA

0.6 0.4 0.2 0.0

(g)

2

1.6

r =0.95

1.2

2-

(LVOOA vs. SO4 )

0.8

(d)

(h)

SVOOA +

+

+

+

+

CxHy CxHyO1 CxHyO2

2 0

2

1.2

r =0.97

0.8

(SVOOA vs. NO3 )

-

40

60

80

m/z (amu)

100

120

0.2 0.1

0.4 0.0

20

0.3

-

HyO1 CxHyNp CxHyOzNp

+

0.0

1.6

NO3

4

2-

0.0

O/C = 0.61, H/C = 1.35, N/C = 0.005, OM/OC = 1.93

0.4 0.2

0.4

0

0.6

SO4

Mass Conc.(µg m

% of total

)

10

2

753

C2H4O2+

0.0

0.02

O/C = 0.48, H/C = 1.31, N/C = 0.004, OM/OC = 1.75

6

(c)

+

(TBBOA vs. C2H4O2 )

1.0

0

(b)

0.04

r =0.96

2.0 5

Monsoon 2

0.0

6/3

6/7 6/11 6/15 6/19 6/23 6/27 Data and Time (UTC+8)

754

Figure 3. (a-d) High resolution mass spectra of the PMF-resolved OA factors, and (e-h)

755

time series of corresponding factors with C2H4O2+, nitrate and sulfate.

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ACS Paragon Plus Environment

Environmental Science & Technology

Pre-monsoon TBBOA 29.6%

40 10 5 20 0

(c)

0 0.24

1.2

0.20

1.0

0.16

0.8

0.12

0.6

(d)

Monsoon

1.2

0.4 0.2

0.3 RBOOA/rBC

0.0

6.5% O3(ppb)

(e)

SVOOA/rBC

0.0

90

0.6

80

0.5

70

0.4

60

0.3

50

0.2 1

757

0.6

0.6

0.4

36.5%

TBBOA/rBC

0.9

0.8

43.4%

13.7%

LVOOA/rBC

1.2

4

8

12

16

20

Hour of Day

2-

-

41.3% LVOOA

8 6 4 2 0

15

60

SO4 /rBC

14.5% SVOOA

o

T( C)

80

RBC

RBOOA 14.6%

(b) RH(%)

(a)

NO3 /rBC

Page 27 of 30

2.5 2.0 1.5 1.0 0.5 0.0 3.6 3.2 2.8 2.4

24 NO2 SO2

758

Figure 4. (a) Average mass contributions of the four factors to the total OA before and

759

during monsoon, diurnal cycles of RH, T and RBC (b), mass ratios of NO3-, SO42- (c),

760

four factors (d) to rBC, and concentrations of gaseous species (O3, SO2 and NO2) (e).

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Environmental Science & Technology

(b) Monsoon

(a) Pre-monsoon NO3

-

Cl

-

1.0 0.8 0.6 0.4 0.2 0.0

NH4 -0.2 -0.3 -0.4 -0.5 -0.6

LVOOA TBBOA

+

1.0 0.8 0.6 0.4 0.2 0.0

0.0 -0.3 -0.6 -0.9 2 4 6 8 10 12

2 4 6 8 10 12

RBC

RBC

(c)

0.4

OSc

RBC

12 8 4

SVOOA RBOOA 0.3

OSc

Mass Fraction

SO4

2-

Page 28 of 30

(d)

0.0 -0.4 -0.8

0 Neg. Less Sig. BB BB BB

Neg. Less Sig. BB BB BB

763 764

Figure 5. Mass fractions of the non-rBC components coated on rBC cores (left y-axis).

765

and the average oxidation states of organic coating (OSc, right y-axis) as a function of

766

RBC during (a) pre-monsoon and (b) monsoon periods, and box plots of (c) RBC and (d)

767

OSc (d) for periods with negligible BB influences (Neg. BB, June 15-30), less BB

768

influences (Less BB, May 30-June 10), and significant BB influences (Sig. BB, June

769

10-13) (the bounds of boxes represent quartiles, the whiskers indicate the 90th and 10th

770

percentiles, and the lines and dots inside the boxes are median and mean values).

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Environmental Science & Technology

rBCMAAP/rBCAMS (Eabs)

Page 29 of 30

2.5 0.0

2.0

0.2

0.4

TBBOA/OA

2.5 Sized by conc. of rBC-containing particles 2.0

1.5

1.5

1.0

1.0

0.5

0.5 4

6

8

10

RBC

12

14

773 774

Figure 6. Light absorption enhancement (Eabs) estimated by the mass ratios of rBC

775

measured by MAAP and SP-AMS versus RBC (Only data from pre-monsoon period

776

are included here; data points are colored by the mass fractions of transported BBOA

777

to total organics, and the marker sizes are proportional to the mass concentrations of

778

rBC-containing particles; Symbols of the boxes are the same as those described in

779

Figure 5).

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Environmental Science & Technology

781

TOC Chart

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