Abundance and sources of phthalic acids, benzene-tricarboxylic acids

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Abundance and sources of phthalic acids, benzene-tricarboxylic acids and phenolic acids in PM at urban and suburban sites in Southern China 2.5

Xiao He, X. H. Hilda Huang, Ka Shing Chow, Qiongqiong Wang, Ting Zhang, Dui Wu, and Jian Zhen Yu ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00131 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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ACS Earth and Space Chemistry

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Abundance and sources of phthalic acids, benzene-

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tricarboxylic acids and phenolic acids in PM2.5 at

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urban and suburban sites in Southern China

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Xiao He,† X. H. Hilda Huang,† Ka Shing Chow^, Qiongqiong Wang, Ting Zhang,‡ Dui Wu, § Jian Zhen Yu†,,‡,*

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†

Division of Environment and Sustainability, The Hong Kong University of Science and Technology, Clear Water

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Bay, Kowloon, Hong Kong, China

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Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

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Hong Kong, China ^

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Environmental Science Program, The Hong Kong University of Science and Technology, Clear Water Bay,

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Kowloon, Hong Kong, China

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‡Atmospheric Research Center, HKUST Fok Ying Tung Graduate School, Guangzhou, China

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§Institute of Mass Spectrometer and Atmospheric Environment, Jinan University, Guangzhou, China

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*

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Keywords: Organic aerosol, Aerosol chemical Characterization, aromatic acids, primary sources, secondary

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formation pathways

Correspondence to: Jian Zhen Yu ([email protected]), 852‐2358‐7389 (Ph)

1 ACS Paragon Plus Environment

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Abstract

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The organic composition of airborne fine particulate matter (PM2.5, aerodynamic diameter less than 2.5

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micrometers) at a molecular level has yet to be achieved, hindering a full understanding of the climatic impacts

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and health effects of PM2.5. Compounds containing aromatic rings are closely associated with optically active

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brown carbon and toxicologically important quinones. In this work, a group of ten aromatic organic acids

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including three phthalic acids, four phenolic acids and three benzene‐tricarboxylic acids (BTCAs) in PM2.5 were

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studied for their abundance and potential sources through quantifying their ambient concentrations at four

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sites in the Pearl River Delta (PRD) region in Southern China, where biomass burning and anthropogenic

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emissions are both significant PM sources. Average concentrations of individual aromatic acids in a total of 240

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PM2.5 samples collected throughout 2012 were in the order of 0.1‐20 ng/m3, with p‐and o‐phthalic acid being

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the most abundant. Inter‐species correlation analysis with known PM source tracers reveals different source

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origins for the ten aromatic acids. The four phenolic acids, all possessing partial lignin structures, are highly

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correlated with levoglucosan, indicating their association with biomass burning emissions. Specific lignin tracer

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ratios characteristic of different types of biomass fuels (i.e., cinnamyl‐ to vanillyl‐phenol ratio) revealed

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significant influence of crop burning emissions in the PRD region. The three BTCAs have moderate correlation

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with sulfate but no correlation with levoglucosan, suggesting a strong association with secondary formation

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origins while negating a strong link with biomass burning. The three phthalic acids are moderately correlated

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with sulfate, levoglucosan, and a number of polycyclic aromatic hydrocarbons (PAHs), indicating multiple

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significant sources. This study provides a valuable data set towards establishing quantitative links between

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molecular composition of organic matter and the optical and toxicological properties of PM2.5 as well as

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assisting identification of tracers for PM2.5 sources.

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1. Introduction

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Particulate matter (PM) in the atmosphere affects the Earth’ radiative balance1 by scattering or absorbing

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solar radiation2 and modifying cloud properties.2, 3 Epidemiological studies also have established the


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association of fine PM with a range of adverse human health effects from allergic, respiratory, and

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cardiovascular diseases to increased mortality.4‐7 Unlike inorganic components that have been studied

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extensively and relatively well characterized, molecular composition of the organic matter in PM and their

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source contributions are far less understood mainly due to the large number of individual organic species

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possibly present in the atmospheric PM, a wide variety of emission sources, and multiple secondary formation

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pathways.8‐11

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In this work, we target a group of ten aromatic organic acids present in PM2.5. The list includes three phthalic

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acids (i.e., o‐, m‐, p‐phthalic acid), three benzene‐tricarboxylic acids (BTCAs) (i.e., 1,2,3‐BTCA, 1,2,4‐BTCA and

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1,3,5‐BTCA), and four phenolic acids (i.e., 3‐hydroxy benzoic acid (3‐OHBA), 4‐hydroxy benzoic acid (4‐OHBA),

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syringic acid, and vanillic acid). Their chemical structures are shown in Figure 1. Our motivations are multi‐fold.

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At a fundamental level, the benzene ring is a significant chromophore contributing to brown carbon (BrC)

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constituents (e.g., nitro‐aromatics)12 and is the basic structure leading to the atmospheric formation of the

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toxicologically important quinone compounds.13‐15 Hence, there is the need to track the PM sources for the

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aromatic constituents and one of the study goals is to evaluate the suitability of the target aromatic acids as

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source tracers. Among them, syringic and vanillic acid are known to be associated with biomass burning (BB)

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emissions.16‐19 In comparison, the sources of the other eight aromatic acids are less clear. Kleindienst et al.20

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proposed o‐phthalic acid as a tracer for secondary organic aerosol (SOA) derived from naphthalene and its

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methyl analogs on the basis of smog chamber product experiments. However, it is uncertain whether o‐phthalic

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is predominantly formed as a secondary product or its precursors are limited to naphthalene and the derivatives.



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Phthalic acids

Phenolic acids

COOH

1

COOH

0.1

3-OHBA

Syringic acid

Vanillic acid

4-OHBA

1,3,5-BTCA

1,2,3-BTCA

1,2,4-BTCA

m-phthalic acid

0.01 o-phthalic acid

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BTCAs

10

p-phthalic acid

Concentration (ng/m3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1. Measured abundance and chemical structures of phthalic acids, BTCAs, and phenolic acids. The squares and horizontal lines in the box denote the average and median, respectively. The lower and upper boundaries of the boxes represent the 25 and 75 percentile values.

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The second motivation is related to the potential of these aromatic acids serving as ligands to complex

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metals. Organic aromatic carboxylates are an important family of multiple‐dentate O‐electron‐donor ligands to

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form coordinate bonding with metal ions.21‐23 The carboxylic functional group, especially two such functional

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groups in adjacent substituents on the benzene ring, is capable of bonding with metal thereby mediating the

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bioavailability of metals.6, 24 Among the target aromatic acids, o‐phthalic acid, 1,2,3‐BTCA and 1,3,5‐BTCA serve

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as the ligands complexing with transition metals such as Copper(II), Iron(II), Manganese(II), and Zinc(II) by

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coordinating through carboxylate oxygen.25‐28 For example, Baca et al.29 enumerated the coordination modes

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of o‐phthalic acid as ligand with metals from mono‐dentate, di‐dentate to heptad‐dentate. Quantitative

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abundance measurements of carboxylate ligands provide necessary data to assist the assessment of

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toxicological effects posed by metal species.30‐32

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This suite of ten aromatic acids, along with major PM2.5 components and select known organic source tracers,

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were determined in a total of 240 filter samples collected at four locations of different pollution characteristics

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in the Pearl River Delta (PRD) in Southern China throughout 2012. Inter‐species correlations and temporal and

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spatial variation are examined to gain insights into major sources for the aromatic acids. From this real‐world


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data set, we aim to extract findings that will contribute towards establishing quantitative links between the

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measured molecular‐level composition and the end impacts of PM on climate and health.

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2. Experimental

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2.1. Sites and sampling

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Ambient PM2.5 filter samples were collected at Dongguan (DG), Guangzhou (GZ), Nanhai (NH), and Nansha

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(NS) in the PRD region. Their locations in a map are shown in Figure 2. The PRD region consists of a network of

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cities including nine prefecture‐level ones, namely Guangzhou, Shenzhen, Foshan, Dongguan, Zhuhai,

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Zhongshan, Huizhou, Jiangmen, Zhaoqing, the Hong Kong SAR, and the Macau SAR in southeast China. It is one

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of the most densely populated regions in the world. Located in a subtropical area, the PRD is subjected to

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monsoon climate, i.e. during summer time, marine wind from the Pacific Ocean to the inland dominates in the

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region bringing in clean and humid air mass, while in winter more aged and polluted air is transported into the

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region by the prevailing wind from the northern continent. The season boundaries used in this work are spring

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(16 Mar to 15 May), summer (16 May to 15 Sep), fall (16 Sep to 15 Nov), and winter (16 Nov to 15 Mar). Daily

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average relative humidity (RH) ranged from 22% to 95%, with an average value of 67%. Daily average

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temperature ranged from a very low ‐1.3 °C to an extreme high 45 °C, averaged at 17.2 °C.

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The DG site (22°57′44.85″N, 113°44′39.36″E) is the field observation site of Dongguan Meteorological Bureau,

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located inside the Dongguan Botanical Garden. The GZ site (23°7′51.08″N, 113°17′51.19″E) is on the rooftop of

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a 24‐storey building in a downtown Guangzhou residential area. The NH site (23°3′48.81″N, 113°8′42.13″) is on

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the rooftop of a 3‐storey building in Foshan Nanhai meteorological bureau in an industrial area of Foshan city.

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The NS site (22°45′08.90″N, 113°36′09.17″E) is on the rooftop of a 5‐storey building in Nansha Information

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Technology Park in Nansha District, a rural district of Guangzhou city.

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Sampling was carried out concurrently at the four sites, following a regular schedule of one 24‐h sample

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every six days throughout 2012. A total of 240 valid samples were obtained. All the sampling sites were

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equipped with one high‐volume PM2.5 sampler (Tisch Environmental, Cleves, OH) and one PM2.5 speciation

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sampler (SASS, Met One Instruments, Inc., Grants Pass, OR). Sample collection substrates in the SASS sampler



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included Teflon, Nylon, and quartz fiber filters to meet analytical needs for different constituents in PM2.5.

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Specifically, gravimetric determination of mass concentration and elemental analysis using X‐ray fluorescence

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spectrometry were carried out using the Teflon filters, nitrate and other water‐soluble ions using the Nylon

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filters, and organic carbon (OC) and elemental carbon (EC) with the quartz filters. Determination of individual

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organic compounds was carried out with the quartz fiber filters collected using the high‐volume samplers. More

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details regarding the sampling and laboratory analysis of major PM2.5 components can be found in our previous

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paper.33 Field blank filters were collected at the end of each sampling month and subject to the same suite of

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chemical analyses and the overall percentage of field blank filter was about 16.7%. No target compounds were

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detected in field blanks.

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Figure 2. Sampling site locations and the surroundings.


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2.2. Chemical analysis of organic compounds

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A portion of 20 cm2 was precisely removed from a quartz fiber filter and cut into strips before placing into an

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amber bottle for extraction. Palmitic acid‐d31, prepared in acetonitrile (ACN) was spiked onto the filter strips

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followed by ultrasonic extraction in dichloromethane (DCM) and methanol mixture (4:1 v/v) for 10 minutes.

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The extract was filtered with a syringe filter (Millipore 0.2 μm PTFE) and transferred to a 5 mL cone vial. The

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filter pieces were further extracted twice, and the extracts were combined and blown to dryness under a mild

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stream of N2 (Ultra‐high purity grade, 99.999%). The extract was then mixed with a 200 μL mixture of n‐methyl‐

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n‐(trimethylsilyl) trifluoroacetamide (MSTFA), pyridine and 8 ppm tetracosane‐d50 in hexane (2:1:1.14 v/v) for

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trimethylsilylation reactions at 70°C for 2 h. The derivatized sample was cooled to room temperature before

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analysis by gas chromatography‐mass spectrometry (GC‐MS) (Agilent 6890 GC coupled with a 5973N

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quadrupole mass spectrometer).

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The GC/MS instrumental parameters are similar to those previously published for aerosol polar

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compounds.34 Briefly, the injector, operated in splitless mode, was set at 275°C and an aliquot of 2 µL of the

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derivatized sample was injected. Compounds were separated on a DB‐5MS column (30 m×0.25 mm×0.25 µm,

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J&W Scientific) using Helium as the carrier gas at 1.2 mL/min. The GC oven temperature was initially set at 80

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°C for 5 min, increased to 200°C at 3°C/min and held for 2 min, then to 310 °C at 10°C/min and held for 25 min.

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The MS analysis was conducted in electron impact positive (EI+) mode over an m/z range of 50‐650 amu. The

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ion source, quadrupole, and interface temperatures were kept at 230, 150, and 280 °C, respectively.

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Calibration standards were prepared by spiking different volumes of a standard solution mixture onto

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ambient filter samples and analyzed following the same analytical procedure as that for ambient PM2.5 filter

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samples. This approach has the advantage of taking into account of matrix effect. All the samples were

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quantified using the calibration curves by this standard addition method. In the process of method

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development, the extent of matrix effect was examined by analyzing a parallel set of standard solutions,

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skipping the steps of spiking onto ambient filter and subsequent extraction, evaporation and reconstitution,

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while the other steps remaining the same. The result showed that the matrix effect existed to a varied degree



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from ‐5% to 145 % for the target analytes (Table S1), indicating the necessity of using standard addition method

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to generate calibration curves for quantification of this set of aromatic acids.

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3. Results and Discussion

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3.1. Abundance of aromatic acids compounds

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Figure S1 shows an example of gas chromatogram of the target aromatic acid compounds in standard and

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real sample. Their quantification ions and recoveries are shown in Table S2. Recoveries in the range of 78‐97%

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were obtained by spiking a standard solution onto a blank filter before and after solvent extraction and

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comparing the two sets of normalized peak areas.

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The annual average and concentration range of the ten aromatic acids are shown for the four‐site combined

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data set and the site‐specific data in Table 1 and Figure 1. The average concentrations of individual compounds

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in the whole data set were in the order of 0.1‐20 ng/m3, with p‐ and o‐phthalic acid being the most abundant

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(23 and 13 ng/m3, respectively). Individual phenolic acid had lower concentrations than the two abundant

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phthalic acids by one order of magnitude or more. Among the four phenolic acids, 3‐OHBA, vanillic acid and

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syringic acid were at a comparable level, with an average of 0.47, 0.55, and 0.50 ng/m3, respectively, while 4‐

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OHBA (averaged at 1.4 ng/m3) was twice as abundant. An additional phenolic acid, isovanillic acid, was

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tentatively identified based on retention time and mass spectrum in comparison with those of vanillic acid. Its

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concentration was an average of 0.48 ng/m3 (range: below detection‐4.6 ng/m3), estimated assuming the same

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GC/MS response as that of vanillic acid. Isovanillic acid was not included in the ensuing discussion, as its

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identification still awaits confirmation using an authentic standard. Of the three BTCA isomers, the

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concentration of 1,2,4‐BTCA and 1,2,3‐BTCA were of comparable abundance (average: 3.1 and 2.8 ng/m3) while

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1,3,5‐BTCA (average: 0.11 ng/m3) was 25 times lower in concentration.

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A small number of studies reported the abundance of the aromatic acids in our study region. Li and Yu35

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measured a range of 16.7‐169 ng/m3 for o‐phthalic acid at six air monitoring stations in Hong Kong from 2003

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to 2005 under different meteorological conditions (i.e. local, regional and long‐rang transport). Ho et al.36

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conducted 24‐h sampling of PM2.5 and collected 8 samples in the 2006‐2007 winter and 7 samples in 2007


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summer on the campus of Sun Yat‐Sen University in urban GZ. They reported o‐phthalic acid concentration was

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the highest among its isomers, averaging 91.8 ± 38.9 and 215 ± 86.1 ng/m3 in the winter and summer samples,

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respectively. It is difficult to speculate the cause for the lower o‐phthalic acid level observed in our current study

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in comparison with the previous studies as the latter had a much limited temporal coverage. o‐Phthalic acid

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has multiple primary and secondary sources, which most likely differ in time and in space.

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3.2. Seasonal and spatial variation of aromatic acids

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Levoglucosan and sulfate are established tracers for BB and secondary formation, respectively, thereby

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serving as useful references in the ensuing discussion of the spatiotemporal characteristics of the aromatic

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acids. Spatially, NH had the highest sulfate as well as levoglucosan while the lowest concentration for both

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occurred at NS (Figure S2). The spatial gradient was larger in levoglucosan (site average from 67 to 181 ng/m3)

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than in sulfate (9.7‐12.7 µg/m3) (Table 1). The seasonal variations of levoglucosan and sulfate are shown in

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Figure 3.Both sulfate and levoglucosan had higher abundance in fall and winter than the other two seasons,

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but their highest abundance season differed, with sulfate in the fall and levoglucosan in winter. This observation

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is consistent with the seasonal pattern of BB activities. While biofuel consumption in rural areas occurs year

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around, in winter time, BB‐related activities such as crop residue burning are quite common in northern China

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and the prevailing northerly wind in PRD at that time transports a large amount of BB emissions to the PRD

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region. The difference is larger in NS where the regional factors play a more important role than local emissions.


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Table 1. Annual average and concentration ranges of ten aromatic acids (ng/m3), K+, levoglucosan, sulfate, OC, and PM2.5 at four sites in the PRD DG

Range

Avg. Range

Avg. Range

Avg. Range

Avg. Range

Avg.

166 166 166 138 138 168 198 210 210 210

BD*‐29.3 BD‐3.4 2.0‐96.4 BD‐3.3 BD‐9.3 BD‐3.0 BD‐4.0 BD‐15.6 BD‐0.45 BD‐12.2

8.7 0.60 19.3 0.36 1.11 0.42 0.38 1.65 0.07 1.7

BD‐47.5 BD‐3.1 0.9‐70.0 0.03‐2.1 0.05‐6.6 0.05‐2.7 BD‐1.7 BD‐21.1 BD‐0.66 BD‐18.0

12.2 0.89 22.0 0.46 1.4 0.50 0.43 3.5 0.14 3.6

BD‐73.2 BD‐10.0 0.43‐178.5 BD‐3.9 BD‐10.8 BD‐4.6 BD‐5.7 BD‐47.4 BD‐0.98 BD‐43.4

22.6 1.7 35.8 0.74 2.0 0.96 0.99 5.3 0.15 6.1

BD‐28.0 BD‐2.5 BD‐86.7 BD‐1.4 BD‐5.2 BD‐1.8 BD‐2.0 BD‐3.9 BD‐0.48 BD‐6.8

7.9 0.55 16.9 0.33 1.0 0.38 0.29 0.80 0.08 1.2

BD‐73.2 BD‐10.0 BD‐178.5 BD‐3.9 BD‐10.8 BD‐4.6 BD‐5.7 BD‐47.4 BD‐0.98 BD‐43.4

12.6 0.9 23.1 0.47 1.4 0.55 0.50 2.8 0.11 3.1

2.2‐32.5 0.08‐2.1 2‐683 2.6‐27.0 11.8‐108.7

9.7 0.69 110 10.0 42.1

2.1‐31.9 0.16‐2.1 7‐308 3.3‐24.6 12.4‐108.3

11.2 0.71 90 11.8 47.2

3.0‐37.6 0.21‐3.0 2‐743 3.5‐33.7 16.5‐130.1

12.7 0.92 181 13.6 58.8

1.6‐38.0 0.04‐2.4 0.6‐613 1.2‐30.8 6.5‐124.5

10.9 0.69 67 9.2 40.1

1.6‐38.0 0.04‐3.0 0.6‐743 1.2‐33.7 6.5‐130.1

11.0 0.74 104 10.7 45.7

MW

o‐phthalic acid m‐phthalic acid p‐phthalic acid 3‐OHBA 4‐OHBA Vanillic acid Syringic acid 1.2.3‐BTCA 1,3,5‐BTCA 1,2,4‐BTCA Sulfate (µg/m3) K+ (µg/m3) Levoglucosan (ng/m3) OC (µgC/m3) PM2.5 (µg/m3)

GZ

NH

NS

All sites

* Below Detection

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300

Median

250

Avg.

20

Median Avg.

15

200

Sulfate

Levoglucosan

(a) Levoglucosan (ng/m3) and sulfate (µg/m3)

150 100

10 5

50 0

0

Spring Summer

Fall

Winter

Spring Summer

Fall

Winter

(b) Aromatic acids (ng/m3) 25

Median Avg.

o-phthalic acid

3-OHBA

1.2 0.8 0.4 0 4

20

Avg.

15 10 5 0

Spring Summer

Fall

Winter Median

2

2 1

9 6 3 0

Spring Summer

Fall

Winter Median

12

Avg.

10

1.5

1,2,4-BTCA

m-phthalic acid

3

Median

Avg.

Avg.

4-OHBA

12

Median

1,2,3-BTCA

1.6

1 0.5

Spring Summer

Fall

Winter Median Avg.

8 6 4 2

0

0

Spring Summer

Fall

Winter Median

60

1

0.5

Fall

Winter Median

40 30 20 10

0 2

0

Spring Summer

Fall

Winter Median

0.4

Spring Summer

Fall

Avg.

50

p-phthalic acid

Syringic acid

Avg.

0

Spring Summer

Winter Median Avg.

1,3,5-BTCA

1.5

0.3 0.2 0.1 0

Spring Summer

Fall

Winter

Spring Summer

Fall

Winter

Avg.

Vanillic acid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1 0.5 0

Spring Summer

182 183 184 185

Fall

Winter

Figure 3. Seasonal variations of (a) sulfate and levoglucosan; and (b) individual aromatic acids. Squares denote median value and dots denote average value. Lower and upper whiskers represent 25 and 75 percentile values.

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Concentration variations of the individual aromatic acids are compared across seasons in Figure 3 and

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spatially in Figure S2. All the aromatic acids had the highest concentration in NH and the lowest

188

concentration in NS on an annual scale. NH is well known as an industry center in PRD, which consumes a

189

higher amount of energy and emits more anthropogenic pollutants. GZ is a high‐density residential city

190

where the vehicle exhausts and other anthropogenic emissions are quite common, many of which are

191

considered to be key sources for aromatic acids. NS is at a rural‐suburban adjoining site, in which the more

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localized anthropogenic sources are fewer. DG, as a suburban site, lies in between.

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By seasonality, the ten aromatic acids separate into two groups. The four phenolic acids form one group,

194

exhibiting the highest concentration in winter followed by fall, spring and the lowest concentration in

195

summer, in line with that of levoglucosan. Phthalic acids and BTCAs form the second group, with the

196

concentrations in fall exceeding the other three seasons, similar to the seasonality of sulfate (Figure 3). The

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seasonality could be attributed to their secondary formation origins. The atmospheric oxidation processes

198

are highly effected by physical parameters such as temperature, solar radiation and atmospheric oxidants

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level. In fall, with higher temperature and stronger solar radiation than in winter while fewer rains and more

200

pollution precursors in comparison with summer, the oxidation activities in the atmosphere are stronger

201

than the other three seasons, speeding‐up the secondarily related formation processes. The largest

202

difference between fall and the other three seasons occurs in NH specifically, consistent with the site

203

characteristic of NH as a severely polluted industry city, which supplies more hydrocarbon and NOx

204

precursors to sustain a higher level of atmospheric oxidation.

205

3.3. Site by site correlation

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In order to discern the relative strength of regional and local factors that affect the pollutant

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concentrations, a site by site correlation (n=60) was conducted between every pair of the four sites for

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individual species including PAHs and select major components (i.e., sulfate, K+, OC and EC). The results are

209

exhibited in Figure 4, from which we may gain a direct knowledge of pollutants belonging to different spatial

210

scales. For those species with good site by site correlations, they are more likely to be transported from

211

other places on a regional scale originating from either primary or secondary sources while for those with

212

weak site by site correlations, they are more strongly associated with different local sources.

12


Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


213

The correlation strength varied among chemical constituents, as expected. The major components (e.g.,

214

OC, EC, and sulfate) and the two know BB emission markers (i.e., K+, levoglucosan) had strong inter‐site

215

correlation (R> 0.5). Among them, sulfate was most tightly correlated amongst the four sampling sites (R:

216

0.80‐0.94). Sulfate is known to be a regional pollutant as a result of its secondary formation origin and this

217

is confirmed by the tight inter‐site correlations. On the other hand, all PAHs from 3‐ring phenanthrene all

218

the way up to 6‐ring benzo[ghi]perylene had highly variable correlations among the sites, ranging from poor

219

correlations (R0.68). Such spatial characteristics

220

signaled significant influence of local emissions.37, 38

221

BTCAs and phenolic acids show relatively good inter‐site correlations (R: 0.58‐0.93) with a little bit of

222

fluctuation, illustrating that local and regional sources coexist to some extent. For example, the site by site

223

correlation for one of the phenolic acids, 4‐OHBA, ranges from 0.59 to 0.91, which is consistent with the

224

other phenolic acids and levoglucosan (0.58‐0.82). It is worth noting that m‐/p‐phthalic acids are

225

moderately inter‐site correlated while o‐phthalic acid shows low inter‐site correlation, further confirming

226

that the source characteristics for o‐phthalic acid is more complex than the two isomers.

13



1.00

Aromatic acids

Major components DG&GZ DG&NH DG&NS GZ&NH GZ&NS NH&NS

0.90

PAHs

0.80 0.70 0.60

R

0.50 0.40 0.30 0.20

227

K+ Sulfate OC EC Levoglucosan Oxalate

phenanthrene anthracene fluoranthene pyrene cyclopenta[cd]pyrene benzo[c]phenanthrene benz[a]anthracene+chrysene triphenylene benzo[b+k]fluoranthene benzo[a]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene benzo[ghi]perylene 1,3,5-TPB

3-OHBA 4-OHBA Syringic acid Vanillic acid

0.00

1,2,3-BTCA 1,2,4-BTCA 1,3,5-BTCA

0.10

o-phthalic acid m-phthalic acid p-phthalic acid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

228 229 230 231

Figure 4. Inter‐site correlation (R) of ten aromatic acids, PAHs and major compounds. Orange boxes cover the aromatic acids and major components of PM2.5 and dark blue box covers PAHs. (Reference R range: ൐0.9 indicates very high correlation; 0.68‐0.9 indicates strong correlation; 0.35‐0.67 indicates moderate correlation).38

232

3.4. Inter‐species correlations

233

Table S3 shows the correlation matrix of the ten aromatic acids and a few select aerosol constituents,

234

considering all the 240 samples. Figure 5 plots pairs of correlation coefficients, R (X, levoglucosan) and R (X,

235

sulfate), where X is an aromatic acid compound or a select PM2.5 species. Levoglucosan and sulfate are

236

selected as characteristic source indicators for biomass burning and secondary formation processes,

237

respectively.39‐41 It reveals that the aromatic acids fall into three sub‐groups: (1) The four phenolic acids

238

form one group that show common strong correlations with levoglucosan but low correlations with sulfate,

239

which indicates predominant BB‐related origins; (2) The three phthalic acids form a group that show

240

comparable correlations with both levoglucosan and sulfate, suggesting significant multiple sources; (3)

241

The three BTCAs form a group showing strong correlations with sulfate but weak correlations with

242

levoglucosan, favoring secondary formation sources. The three subgroups are further discussed below in

243

greater details.

244

3.4.1. Phenolic acids

245

Phenolic acids are highly correlated with each other, with correlation coefficient R ranging from 0.86 to the

14


Page 15 of 28

246

extremely high value of 0.97 between 3‐OHBA and 4‐OHBA. They are strongly correlated with levoglucosan,

247

clearly seen from the scatter correlation plots shown in Figure 5a. Levoglucosan is a specific marker for BB

248

emissions since it is abundantly and distinctively produced during pyrolysis of cellulose, which is a linear

249

polymer of glucose that widely exists in plant cell walls.39, 42 The strong correlation confirms their origins in

250

BB activities. Vanillic

1.0

Syringic

(a) R (x, levoglucosan)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) Phenolic acids vs levolucosan 4-OHBA

3-OHBA

Syringic acid

Vanillic acid

3-OHBA

0.8

4-OHBA

(b)

0.6

o-phthalic

(b) Phthalic acids vs sulfate o-phthalic acid

m-phthalic acid

p-phthalic acid

m-phthalic p-phthalic

0.4

1,2,3-BTCA (c) BTCAs vs sulfate

(c)

1,2.4-BTCA

0.2

1,2,4-BTCA

1,3,5-BTCA

1,2,3-BTCA

1,3,5-BTCA K+

0.0 0.0

0.2

0.4

0.6

R (x, sulfate)

251

0.8

1.0

NH4+ oxalate

252 253

Figure 5. Correlations of individual aromatic acids with sulfate and levoglucosan and the corresponding scatter plots of (a): Phenolic acids vs levoglucosan; (b): BTCAs vs sulfate; and (c): phthalic acids vs sulfate.

254

While levoglucosan is derived from cellulose, the phenolic acids are structurally linked with lignin, the

255

most abundant polymeric aromatic organic substance in the plant world. Lignin is the exclusive material

256

basis for the conductive tissue of vascular plants for transporting water and nutrients. Different from

257

cellulose, lignin is a highly amorphous and branched polymer without any repeating unit.42‐44 The most

258

common monomeric unit in lignin is a phenyl‐propanoid unit linked through various ether‐types.43 Figure 6

259

shows an example structure of lignin, highlighting in different colors three C9 building units of lignin, namely

260

cinnanyl‐phenols (C), vanillyl‐phenols (V), and syringyl‐phenols (S). We can see oxidation degradation acting

261

on the C=C bond on the side chain of the three types of phenols would lead to 4‐OHBA, vanillic and syringic

262

acids, respectively. Indeed, they have been intensively reported to be major components from breakdown

263

of lignin.16‐19, 39, 45, 46 4‐OHBA was also reported to be one of the major organic tracers for smoke emissions

264

from burning plastics.47 The strong correlation of 4‐OHBA with 1,3,5‐triphenylbenzene (TPB, tracer for trash

265

burning) observed in this study (R: 0.70) indicates burning plastics is likely a significant source for 4‐OHBA

266

in this region.

15



Page 16 of 28

267

We could not identify a lignin building block that has the hydroxyl and carboxyl groups in meta‐position,

268

as that of 3‐OHBA; however, the excellent correlation between 3‐OHBA and 4‐OHBA (R: 0.97) strongly

269

suggests 3‐OHBA is derived from BB origin as well. Some literature also identified 3‐OHBA as BB product.

270

For example, Pietrogrande et al.48 reported a 3‐OHBA to 4‐OHBA ratio ranging from 0.62‐0.91 in different

271

sampling periods from 2011‐2014 in urban Bologna and rural San Pietro Capofiume; Decesari et al.49

272

reported the ratio to be 0.09‐0.22 in dry season, 0.08‐0.17 in transition season, and 0 in wet season in

273

Rondonia, Brazil. In our work, the 3‐OHBA to 4‐OHBA ratio was averaged at 0.34.

C9 units Cinnamyl

Vanillyl

Syringyl

OH OMe

OH

4-OHBA Vanillic acid

274

Syringic acid

275 276

Figure 6. Example chemical structure of lignin and its three C9 units: cinnanyl‐phenol (red) vanillyl‐phenol (blue), and syringyl‐phenol (green).

277

The relative proportions of the three phenol groups (C, V, and S) convey additional source information.

278

The lignin compositions of the two most abundant vascular plants i.e., softwood and hardwood are

279

characteristically different.50 The ratios of lignin tracers (i.e., the phenols) are specific to vegetation types

280

or different taxonomical groups of plant. In particular, vanillyl‐phenols (V) are produced in all vascular plants

281

and serve as the reference for discrimination between vascular and non‐vascular plant. Syringyl‐phenols (S)

282

are produced exclusively in hardwood tissues although trace levels are found in softwood tissues in a few

283

exception cases. As a result, S/V >1 is applied to distinguish hardwood from softwood.51 Cinnamyl‐phenols

284

(C) are present in non‐woody tissues, therefore the C/V ratio is used to estimate the contributions from the

285

burning of non‐woody vegetation (e.g., crops).52 Figure 7 plots the distributions of the index phenol ratios

286

to distinguish biomass fuel types and the frequency distribution of C/V values. The S/V ratio averages at

287

0.86 and ranges from 0‐2.0, which is consistent with other similar measurements reporting S/V ratios in the

288

range of 0‐4.50 The S/V ratios more or less evenly distributed on both sides of the S/V =1 boundary line, 16


Page 17 of 28

289

indicating that BB fuels in the PRD region are a mixture of both hardwood and softwood. The C/V ratio

290

measured for hardwood was reported to approach zero and less than 0.25 for softwood and this ratio was

291

larger for non‐woody plants (0.5‐2).50, 52 In comparison, the C/V ratio in our samples (2.69±1.08) is

292

significantly larger, revealing the high load of non‐woody plants burning, which could be attributed to crop

293

residue burning. Crops such as rice and wheat are major crops in the PRD and its upwind areas. These crop

294

straws are considered non‐woody plants and their burning at the end of harvest season were common. 2.5

From source studies 2.0

S/V Distribution

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

Hardwood

Hardwood

0.443

1.0

Softwood Non-woody

Softwood

0.206

0.202

0.5

0.074

0.044

0.013 0.018

0.0 0-1

1-2

2-3

3-4

4-5

C/V Distribution

295

5-6

6-7

296 297 298 299 300 301 302

Figure 7. Distributions of characteristic lignin phenol ratios, syringyl‐ to vanillyl‐phenols (S/V) versus cinnamyl‐ to vanillyl‐phenols (C/V) and the frequency distribution of C/V values in our samples. The numbers above each column denotes the frequency of occurrence from corresponding range. S/V>1 denotes hardwood and S/V

Abundance and sources of phthalic acids, benzene-tricarboxylic acids

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