Effects of Aqueous-Phase and Photochemical Processing on

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Effects of Aqueous-phase and Photochemical Processing on Secondary Organic Aerosol Formation and Evolution in Beijing, China Weiqi Xu, Tingting Han, Wei Du, Qingqing Wang, Chen Chen, Jian Zhao, Yingjie Zhang, Jie Li, Pingqing Fu, Zifa Wang, Douglas R. Worsnop, and Yele Sun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04498 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on December 31, 2016

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Environmental Science & Technology

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Effects of Aqueous-phase and Photochemical Processing on Secondary

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Organic Aerosol Formation and Evolution in Beijing, China

3 4

Weiqi Xu1,3, Tingting Han1,3, Wei Du1,3, Qingqing Wang1, Chen Chen1, Jian Zhao1,3,

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Yingjie Zhang1,4, Jie Li1, Pingqing Fu1, Zifa Wang1, Douglas R. Worsnop5, Yele

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Sun1,2,4*

7 1

8

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric

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

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100029, China 2

11

Center for Excellence in Regional Atmospheric Environment, Institute of Urban

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Environment, Chinese Academy of Sciences, Xiamen 361021, China

13

3

4

14

University of Chinese Academy of Sciences, Beijing 100049, China

Collaborative Innovation Center on Forecast and Evaluation of Meteorological

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Disasters, Nanjing University of Information Science & Technology, Nanjing 210044,

16

China 5

17

Aerodyne Research, Inc., Billerica, Massachusetts 01821, USA

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19 20

*

To whom correspondence should be addressed: Yele Sun ([email protected]) 40 Huayanli, Chaoyang District, Beijing 100029, China. Tel.: +86-10-82021255

1

ACS Paragon Plus Environment


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Abstract

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Secondary organic aerosol (SOA) constitutes a large fraction of OA, yet remains

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a source of significant uncertainties in climate models due to incomplete

24

understanding of its formation mechanisms and evolutionary processes. Here we

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evaluated the effects of photochemical and aqueous-phase processing on SOA

26

composition and oxidation degrees in three seasons in Beijing, China, using

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high-resolution aerosol mass spectrometer measurements along with positive matrix

28

factorization. Our results show that aqueous-phase processing has a dominant impact

29

on the formation of more oxidized SOA (MO-OOA), and the contribution of

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MO-OOA to OA increases substantially as a function of relative humidity or liquid

31

water content. In contrast, photochemical processing plays a major role in the

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formation of less oxidized SOA (LO-OOA), as indicated by the strong correlations

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between LO-OOA and odd oxygen (Ox = O3 +NO2) during periods of photochemical

34

production (R2=0.59-0.80). Higher oxygen-to-carbon ratios of SOA during periods

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with higher RH were also found indicating a major role of aqueous-phase processing

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in changing the oxidation degree of SOA in Beijing. Episodes analyses further

37

highlight that LO-OOA plays a more important role during the early stage of the

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formation of autumn/winter haze episodes while MO-OOA is more significant during

39

the later evolution period.

2


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1. Introduction

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Organic aerosols (OA) comprising plenty of compounds with vastly different

42

properties such as volatilities, oxidation states and hygroscopicity, account for a

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substantial mass fraction of submicron aerosols (20-90%).1-3 OA can be either primary

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from direct emissions or secondary from oxidation of biogenic and anthropogenic

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volatile organic compounds (VOCs). While primary OA (POA) is relatively well

46

understood, our understanding of the formation mechanisms and evolutionary

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processes of secondary OA (SOA) is not complete, especially in highly polluted

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environments. As a result, traditional models often show substantial discrepancies in

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simulating SOA mass concentrations 4, 5 and oxidation states. 6

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SOA can be formed through gas-phase photochemical reactions with oxidants

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(e.g., hydroxyl radical (OH) and ozone (O3))7 followed by gas-particle partitioning

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that is affected by various factors, e.g., temperature (T), relative humidity (RH), and

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total organic aerosol mass loadings.8-10 While direct quantification of SOA in ambient

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environments is challenging, recent studies have shown that oxygenated OA (OOA)

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determined from positive matrix factorization (PMF) analysis of OA that is measured

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by aerosol mass spectrometers is a good surrogate of SOA.11,12 Therefore, OOA is

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widely used to study the formation mechanisms and evolutionary processes of SOA.

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For example, Herndon et al.

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(Ox = O3 + nitrogen dioxide (NO2)) during photochemical processing, and the

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relationship between OOA and Ox can be used as a metric to characterize SOA

13

found that OOA was well correlated with odd oxygen

3



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formation mechanisms associated with ozone production chemistry. OOA vs. Ox

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shows different slopes ranging from 0.03 to 0.16 µg m-3 ppb-1 in different megacities,

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e.g., Pasadena, Paris, Mexico City and New York City,14-17 which are all lower than

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that in Beijing, China (0.49-1.07 µg m-3 ppb-1).18,

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photochemical production of SOA and Ox in Beijing are different from other

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megacities in Europe and North America. The reasons are not clear yet, but likely due

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to different VOCs precursors and much higher NOx (= nitrogen monoxide (NO) +

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NO2) levels in Beijing. SOA can also be formed through aqueous reactions in wet

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aerosols, clouds and fogs, and has been widely observed in both ambient studies and

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lab simulations.20-23 For example, Sorooshian et al. 24 found an important dual role of

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both ambient RH and hygroscopicity in leading to an enrichment of oxygenated

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organics, and Duplissy et al.

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organics (κorg) in oxidized organics with high f44 (fraction of m/z 44 in OA) in Mexico

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City, while such a trend is not observed in CalNex.26

25

19

These results indicate that

showed an enhanced hygroscopicity parameter of

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However, Sun et al. 27 and Elser et al. 28 found that the OOA production in winter

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in Beijing and Xi’an seemed to be independent of RH, suggesting that aqueous-phase

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processing was likely not an important formation mechanism for SOA in winter. Such

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a conclusion was different from previous studies that showed an enhanced SOA

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formation at elevated RH levels (RH>70%) due to water uptake in summer in Atlanta,

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U.S.21, 29 These results likely indicate a largely different impact of aerosol liquid water

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on processing different types of SOA. In fact, Sun et al.

30

4


found that aqueous-phase

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processing has a different impact on highly oxidized OOA and freshly oxidized OOA

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in winter in Beijing. While particle liquid water exerts strong influences on the

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formation of highly oxidized OOA, it has minor impacts on freshly oxidized OOA. In

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addition, the different meteorology (RH, T) and precursors between summer (e.g.,

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U.S.) and winter (e.g., China) might also be potential factors affecting aqueous-phase

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processing SOA. Therefore, it is of vital importance to evaluate the role of

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aqueous-phase processing in the formation of SOA in different seasons in China.

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Previous studies suggested that fog and cloud processing with high RH levels

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enhanced the oxidation degree of OA,31-33 while f44, a surrogate of oxidation degree,34

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remained fairly constant at RH>50% in winter in Beijing.27 The reasons for such

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differences are not clear yet. Therefore, photochemical and aqueous-phase formation

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of SOA and its impacts on oxidation degree are far from being clearly understood,

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particularly under the heavily polluted environments found in the megacities of China.

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Owing to the high time resolution and capability in characterization of

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non-refractory submicron aerosols (NR-PM1) composition,1 Aerodyne aerosol mass

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spectrometers (AMS) are widely used to investigate the effects of photochemical and

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aqueous-phase processing on SOA formation and evolution. For instance, Zhang et al.

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33

found that aqueous-phase aging dominated the oxidation of OA in winter 2013 in

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Beijing, while photochemical aging also played a role in winter 2014. In Hong Kong,

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the degree of oxygenation and composition of OA were compared between foggy and

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hazy periods to explore the effects of aqueous-phase or photochemical processing.31 5



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further found different slopes in the Van Krevelen diagram

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Chakraborty et al.

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between foggy and non-foggy periods, suggesting the different aging mechanisms

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between fog processing and photochemical oxidation. Although these studies provide

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important insights into the effects of aqueous-phase or photochemical processing on

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OA properties, significant uncertainties of relative importance for these two pathways

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still exist,20, 37 for instance, the impacts on the transformation of less oxidized OOA to

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highly oxidized OOA, and the oxidation degree of SOA in different seasons.

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The frequent severe pollution in Beijing that is characterized by high

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contributions of secondary particles38, 39 has been widely investigated using the AMS.

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While the properties of inorganic species have been relatively well characterized,

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those of OA have mainly focused on sources, elemental ratios and mass

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concentrations, and therefore photochemical and/or aqueous-phase processing of SOA

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and its role in severe haze formation remain relatively uncertain.40-44 Moreover, few

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studies have been conducted in seasons other than winter to evaluate the different

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roles of photochemical and/or aqueous-phase processing in the formation of SOA

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during severe pollution episodes,18,

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understanding of the evolutionary processes of haze pollution in different seasons.

45, 46

preventing us from having a better

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In this study, we evaluate the impacts of photochemical and aqueous-phase

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processing on SOA composition and oxidation degree of OA in three different seasons,

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i.e., summer, autumn and winter in Beijing using High-Resolution Time-of-Flight

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AMS (HR-ToF-AMS) measurements along with PMF. A more detailed evolution of 6


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SOA composition and oxidation degree during pollution episodes is elucidated in four

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case studies in different seasons with substantially different meteorological

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parameters and precursors.

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2. Experimental methods

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Sampling and instrumentation

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All measurements took place at the Tower branch of the Institute of Atmospheric

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Physics, Chinese Academy of Sciences, a typical urban site in Beijing. The

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measurements were conducted during three seasons, i.e., winter (December 16, 2013

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– January 17, 2014), summer (June 3 – July 11, 2014) and autumn (October 14 –

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November 12, 2014) using HR-ToF-AMS. The set up and operation of the HR-ToF-AMS is similar to our previous studies.30,

134 135

44

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(model URG-2000-30EN) mounted in front of the sampling line. Ambient air

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containing the remaining particles was drawn into the sampling room and then dried

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by a diffusion silica-gel dryer, and finally was isokinetically sampled into the

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HR-ToF-AMS at a flow rate of ~0.1 L/min. The mass-sensitive V-mode alternated

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with the high mass resolution W-mode in the operation of the HR-ToF-AMS by 2 min,

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5 min and 5 min in winter, autumn and summer, respectively.

Briefly, coarse particles larger than 2.5 µm were removed by a PM2.5 cyclone

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Collocated measurements in each campaign included black carbon (BC), gaseous

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NO2, and gaseous species (carbon monoxide (CO), O3, NO/NO2, and sulfur dioxide

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(SO2)), which were measured by a two-wavelength Aethalometer (model AE22, 7



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Magee Scientific Corp.), a cavity attenuated phase shift NO2 monitor (CAPS-NO2,

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Aerodyne Research Inc.), and a series of gas analyzers from Thermo Scientific,

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respectively. The meteorological parameters (wind, T and RH) were obtained from the

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Beijing 325 m meteorological tower, which is approximately 30 m from the sampling

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site. Data collected in this study are presented in Beijing local time.

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HR-ToF-AMS data analysis

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The AMS data including mass concentrations and elemental ratios were analyzed

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using software SQUIRREL V 1.56 and PIKA V 1.15D written in Igor Pro 6.12 A

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(Wavemetrics,

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(http://cires1.colorado.edu/jimenez-group/ToFAMSResources/ToFSoftware/index.htm

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l). The ionization efficiency (IE) was calibrated using pure ammonium nitrate

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particles following the standard protocols.47-49 The relative ionization efficiency (RIE)

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of ammonium was determined from pure ammonium nitrate and the default RIEs

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were applied to other species. A collection efficiency (CE) of 0.5 was applied to the

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three datasets for the following reasons (Table S1): (1) particles were dry; and (2)

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slightly acidic as indicated by NH4+measured/NH4+predicted;50 and (3) the contribution of

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ammonium nitrate was below 40%, a threshold value affecting the CE significantly.51

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The elemental ratios of OA including organic-mass-to-organic carbon (OM/OC),

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oxygen-to-carbon (O/C) and hydrogen-to-carbon (H/C) ratios were determined with

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the approach recommended by Canagaratna et al.,52 which was referred to as

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Improved Ambient (I-A). The elemental ratios were also calculated using the

Lake

Oswego,

8


OR,

USA)

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approach proposed by Aiken et al. 34 for a comparison (Table S2), which was referred

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to as Aiken Ambient (A-A).

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OA factors representing specific sources were identified using the PMF2.exe

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algorithm (v4.2)53 applied to the high-resolution mass spectra (HRMS). The detailed

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PMF analysis and evaluation in autumn and winter have been given in Xu et al. 40 and

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Sun et al.,26 respectively, and those in summer were detailed in supplementary

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(Figures S1–S3). In this study, we use the sum of less oxidized oxygenated OA

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(LO-OOA) and more oxidized oxygenated OA (MO-OOA) as a surrogate of SOA,

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and other OA factors, e.g., cooking OA (COA), hydrocarbon-like OA (HOA), coal

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combustion OA (CCOA), and biomass burning OA (BBOA) as POA. The O/C of

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SOA was then calculated as: [LO − OOA] [MO − OOA] [LO − OOA] [MO − OOA] O/Cୗ୓୅ = ൤ × O/C୐୓ି୓୓୅ + × O/C୑୓ି୓୓୅ ൨ / ൤ + ൨ ሺOM/OCሻ୐୓ି୓୓୅ × 12 ሺOM/OCሻ୑୓ି୓୓୅ × 12 ሺOM/OCሻ୐୓ି୓୓୅ × 12 ሺOM/OCሻ୑୓ି୓୓୅ × 12

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Where O/CSOA refers to the O/C of SOA, and [LO-OOA] and [MO-OOA] represent

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the mass concentrations of LO-OOA and MO-OOA, respectively. In addition, aerosol

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liquid water content that is associated with inorganic species was predicted with the

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ISORROPIA-II model.54

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3. Results and discussion

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SOA composition and oxidation properties

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Two SOA factors, i.e., MO-OOA and LO-OOA, were identified during all three

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seasons. MO-OOA showed ubiquitously higher O/C ratios and f44 than LO-OOA,

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indicating that MO-OOA is a more oxidized SOA than LO-OOA. The mass spectra of 9



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MO-OOA between summer and autumn are similar (R2=0.98), and the oxidation

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degrees were also close (O/C = 1.15 vs. 1.23). However, MO-OOA in winter showed

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a different spectral pattern compared with those in summer and autumn. Particularly,

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the MO-OOA spectrum was characterized by high m/z 29 (mainly CHO+) and m/z 43

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(mainly C2H3O+) peaks and low O/C (0.81), suggesting that MO-OOA in winter was

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overall less aged than those in summer and autumn. Although the LO-OOA spectra

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also resembled each other during all three seasons (R2=0.92-0.98), the O/C ratios and

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mass spectra varied substantially. For instance, LO-OOA in autumn presented a much

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lower O/C (0.58) and higher hydrocarbon-like ions (CxHy+) than the other two seasons,

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indicating that LO-OOA was less aged in autumn. The differences in SOA spectral

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patterns and oxidation degrees are not only due to different meteorological conditions

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and photochemical activities, but also are related to different VOCs precursors.

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SOA dominated OA in summer, on average accounting for 57%, consistent with

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the results from previous AMS studies in Beijing.19, 40 Note that LO-OOA showed an

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overall higher contribution to SOA than MO-OOA (61% vs. 39%), indicating a more

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important role of LO-OOA in summer. The average contribution SOA to OA was

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decreased to 37% in winter. Similar to summer, SOA was also dominated by LO-OOA

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(66%) although periods with higher concentrations of MO-OOA than LO-OOA were

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also observed (Table S3). In contrast, MO-OOA showed a comparable contribution to

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LO-OOA (53% vs. 47%) in autumn. Our results showed that SOA composition and

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oxidation properties can vary substantially in different seasons and in general, the 10


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SOA contributions and O/C ratios decreased from summer, autumn to winter.

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Aqueous-phase processing of SOA

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Figure 1 shows the variations of mass fractions of OA factors, oxidation degrees,

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mass concentrations of OA, and liquid water content (LWC) as a function of RH. It is

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clear that LWC showed a linear increase as a function of RH at RH > 60% during the

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three seasons indicating the potential impacts of aqueous-phase processing at high RH

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levels. The variations of OA mass concentrations as a function of RH were different

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among the three seasons. While OA increased gradually as RH increased in autumn

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(Figure 1h), they first showed large increases at RH < 60% in summer and winter

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(Figures 1g and 1i), and then decreased instead as the RH increased. The increases of

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OA at low RH levels (< 60%) were clearly associated with the decreases of wind

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speed (Figures 1a-1c) that facilitated the accumulation of air pollutants.55 The

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decrease of OA at high RH levels in summer was mainly caused by several

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precipitation events that scavenged OA substantially, and in winter was due to two

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high pollution episodes formed under moderately high RH. In fact, after excluding the

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precipitation events in summer, MO-OOA and sulfate showed significant

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enhancements at high RH levels while most primary species remained relatively

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constant across different RH levels (Figures S4-S6). These results illustrated strong

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impacts of aqueous-phase processing in the formation of MO-OOA and sulfate.

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Figure 1 also shows that the contribution of MO-OOA to OA showed a large

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increase from 20% to 40% as RH increased from 60% to > 80%, while LO-OOA 11



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remained relatively constant (e.g., autumn and winter) and even had a decrease (e.g.,

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summer). Results here indicate that aqueous-phase processing appears to affect most

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the formation of MO-OOA while it has minor impacts on LO-OOA. After excluding

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the effects of planetary boundary layer height (PBL) using HOA as a surrogate,27

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MO-OOA and LO-OOA also showed similar variations as a function of RH (Figures

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S4-S6), further supporting our conclusion above. The conclusion that MO-OOA was

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more affected by aqueous-phase processing was further supported by the correlations

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between MO-OOA and specific fragment ions. As shown in Figure S7, MO-OOA was

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well correlated with C2H2O2+, C2O2+ and CH2O2+, which are typical fragment ions of

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methylglyoxal and glyoxal, that are precursors of SOA via cloud processing.56, 57 In

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addition, MO-OOA also showed tight correlations with CH3SO+, CH2SO2+ and

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CH3SO2+, three typical fragment ions of methanesulfonic acid (MSA), which are

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products mainly from the oxidation of dimethyl sulfide (DMS)58 and can be strongly

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enhanced by aqueous-phase processing.32,59

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As a response to the changes in OA composition, the bulk O/C of OA showed

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different variations at low and high RH levels. While O/C ratios remained small

244

changes by varying 70%.

24

These results supported that aqueous-phase processing can 12


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affect the oxidation degree of OA by changing OA composition.23,

60

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illustrating the impacts of aqueous-phase processing on SOA, we further calculated

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the O/C ratios of SOA across the three seasons. As shown in Figure 1, O/CSOA showed

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clear increases as a function of RH, yet presented different increasing rates at low and

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high RH levels. Overall, O/CSOA increased more rapidly at high RH levels than low

254

RH levels, which agrees well with the changes in MO-OOA contributions. Note that

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the increase in MO-OOA contribution in summer was associated with a decrease in

256

LO-OOA contribution, likely indicating the transformation of LO-OOA into

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MO-OOA. Together, our results suggest that aqueous-phase processing has a more

258

important impact on the formation of highly oxidized SOA during all seasons in

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Beijing.

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Photochemical production of SOA

For better

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Ox has been shown to be a more conserved tracer to indicate photochemical

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processing compared with O3 due to the reactions of NO with O3 to form NO2.13 It

263

should be noted that the air masses contained relatively more NO2 (> 90%) leading to

264

a vast decrease in fractional O3 contributions ( 70 ppb) in

265

autumn and winter (Figure S8). Hence using Ox as an indicator of photochemical

266

process13 might not be appropriate under such conditions (e.g., Ox >70 ppb).

267

The mass loadings of OA showed overall increasing trends as the increases of Ox,

268

in agreement with the variations of OA as a function of Ox in Hong Kong.61 However,

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the increasing rates of OA factors as a function of Ox were substantially different 13



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among different seasons. In summer, LO-OOA presented a linear increase with an

271

increase in Ox, while other OA factors remain relatively constant, suggesting a

272

dominant role of photochemical processing in the formation of LO-OOA (Figure S9).

273

As a comparison, LO-OOA showed synchronous increases as POA factors and

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MO-OOA in autumn and winter (Figures S10 and S11). The increases of POA factors

275

as a function of Ox were likely due to the enhanced primary emissions that were

276

supported by the much higher NO2 fractions in Ox (Figure S8). However, the faster

277

increasing rates of LO-OOA than POA factors clearly indicate additional

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photochemical production of SOA during all seasons. Note that LO-OOA presented

279

much faster increasing rates as a function of Ox than MO-OOA in summer and winter,

280

indicating that photochemical processing affects the formation of less oxidized SOA

281

in the two seasons. Comparatively, MO-OOA showed a comparable increasing rate

282

with LO-OOA in autumn, likely indicating the importance of both photochemical and

283

aqueous-phase processing in the formation of MO-OOA.

284

The relationship between O/C ratios and Ox varied among the three seasons. In

285

summer, O/C showed a clear increase as Ox increased. As shown in Figure 2a, such an

286

increase was mainly due to a significant enhancement of SOA associated with a

287

corresponding decrease in POA. Considering that the contribution of MO-OOA

288

showed a slight decrease, the increase in SOA contribution was mainly caused by

289

LO-OOA whose contribution increased from 26.4% to 63.1%. This result indicates

290

the dominant role of photochemical processing in the formation of LO-OOA and 14


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increasing oxidation degree of OA in summer. We also note that O/CSOA showed a

292

gradual decrease as a function of Ox. This can be explained by the enhanced

293

contribution of LO-OOA with lower O/C in SOA, and correspondingly a decrease in

294

the contribution of MO-OOA with higher O/C. Comparatively, SOA and O/C ratios

295

showed different relationship with Ox at low and high levels in autumn. While O/C,

296

O/CSOA, and SOA fraction all showed gradual increases at Ox < 70 ppb, they

297

decreased instead at Ox > 70 ppb. Such variations were tightly related to the dominant

298

increase in MO-OOA at low Ox levels (Figure 2b) and the increase in COA associated

299

with a decrease in MO-OOA at high Ox levels (Figure S10). These results suggest that

300

photochemical processing also played an important role in the formation of MO-OOA

301

and increasing the oxidation degree of OA at low Ox levels in autumn. Our results

302

agreed with Li et al.31 that an increase in MO-OOA on a hazy day in Hong Kong was

303

mainly due to photochemical processing. Note that the elevated LWC from 14.2 µg

304

m-3 to 28.7 µg m-3 as a function of Ox (Figure S10) might also play a role in elevating

305

MO-OOA. O/C and O/CSOA stayed relatively constant as a function of Ox in winter

306

despite the increase in LO-OOA contributions. These results supported the idea that

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photochemical processing did not affect oxidation degree of OA substantially in

308

winter. Therefore, the role of photochemical processing in the formation of SOA and

309

changing O/C was significantly different among the three seasons. While

310

photochemical processing plays a dominant role in the formation of LO-OOA and

311

enhancing oxidation degree of OA in summer, it has much smaller impacts on SOA 15



312

formation and evolution in winter. Such conclusions were further supported by the

313

correlations between LO-OOA and Ox. As shown in Figure S12, LO-OOA presented

314

the best correlation with ∆Ox (the difference of Ox and background Ox, i.e., the

315

average of the lowest 5% of the data) during 12:00-18:00 when photochemical

316

processing is the most intense in a day (R2=0.59-0.80). The regressions slopes of

317

LO-OOA vs. ∆Ox were 0.17 and 0.61 µg m-3 ppb-1 in summer and winter, respectively,

318

indicating that SOA was much more oxidized in summer than winter due to

319

photochemical processing.13 We also noticed a better correlation between LO-OOA

320

and ∆Ox in winter than summer (Figure S12). One explanation is that LO-OOA in

321

summer was much more affected by the gas-particle partitioning due to high T.

322

Combined effects of RH and Ox on SOA properties

323

The effects of photochemical and aqueous-phase processing on SOA composition

324

(LO-OOA vs. MO-OOA) and oxidation degrees during the three seasons are further

325

illustrated in Figure 3. The ratio of MO-OOA/LO-OOA in summer showed an evident

326

gradient as a function of RH and Ox, confirming the different impacts of

327

photochemical and aqueous-phase processing on SOA with different oxidation

328

degrees. The MO-OOA/LO-OOA presented the highest values on the left-top corner

329

in summer indicating the important role of aqueous-phase processing in the formation

330

of MO-OOA under low Ox conditions. Previous studies also showed that high RH in

331

summer facilitated the transformation of HNO3 into aqueous-phase and increased

332

nitrate concentrations substantially.55, 62 As a result, the ratio of NO3-/SO42- showed 16


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333

similar gradients as MO-OOA/LO-OOA in summer (Figure S13). These results might

334

indicate that the transformation of VOCs into the aqueous-phase is an important

335

source of MO-OOA in summer. The O/CSOA showed remarkably similar RH/Ox

336

dependence as MO-OOA/LO-OOA. This is rational because O/CSOA is mainly driven

337

by MO-OOA with higher O/C (1.15) than that of LO-OOA (0.78). This result suggests

338

that SOA produced via photochemical processing is generally less oxygenated than

339

that produced through aqueous-phase processing.20 Figure 3d also shows that the ratio

340

of O/CSOA experienced small changes as Ox increased when RH was less than 40%,

341

which is due to the dominance of LO-OOA in SOA (MO-OOA/LO-OOA < ~0.2).

342

This result further indicates that photochemical processing played a major role in the

343

formation of LO-OOA at low RH levels. Comparatively, the MO-OOA/LO-OOA

344

increased as Ox increased at RH=40-70%, corresponding to an increase in O/CSOA

345

from ~0.8 to ~0.9 on average. This result indicates an enhanced role of photochemical

346

processing in the formation of MO-OOA under moderately high RH levels, which is

347

also likely from the transformation of LO-OOA into MO-OOA.

348

In winter, the ratio of MO-OOA/LO-OOA and O/CSOA presented high values in

349

the left-top corner when RH was high and Ox low, which is similar to that observed in

350

summer. This result supports our conclusion that aqueous-phase reactions played a

351

more important role in processing MO-OOA than LO-OOA and affecting OA

352

oxidation degrees. As shown in Figure S13c, the RH/Ox dependence of NO3-/SO42-

353

presented the lowest values in the left-top corner, indicating much higher 17



354

concentrations of SO42- than NO3- in this area. Previous studies have shown that high

355

concentrations of SO42- in winter were mainly formed from aqueous-phase production,

356

most likely fog processing.27, 63 In contrast, low MO-OOA/LO-OOA and O/CSOA were

357

generally observed at low RH levels where high NO3-/SO42- ratios was also presented

358

(Figure S13). This result indicates a more important role of photochemical processing

359

in the formation of LO-OOA and NO3- at low RH levels. Compared with summer,

360

MO-OOA/LO-OOA and O/CSOA showed gradual decreases as Ox increased at both

361

low and high RH levels in winter, which indicates a faster formation of LO-OOA than

362

MO-OOA that is associated with photochemical processing. We note that the mass

363

loading of organics presented a large increase from ~20 to > 100 µg m-3 as Ox

364

increased in winter. High mass loadings of organic particles might be another

365

important factor in contributing to the formation of LO-OOA by absorbing VOCs.29

366

Although the mass loadings of OA also presented increases as Ox increased in summer,

367

the role of OA as an absorbing phase appears to be much smaller than winter because

368

the concentration differences between low and high Ox levels are generally less than

369

15 µg m-3.

370

The RH/Ox dependence of MO-OOA/LO-OOA and O/CSOA in autumn was

371

different from those in summer and winter. As shown in Figure 3b, both

372

MO-OOA/LO-OOA and O/CSOA showed increases as RH and Ox increased and

373

reached its highest values in the central-top corner, indicating that both photochemical

374

and aqueous-phase processing contributed to the formation of MO-OOA and 18


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Page 19 of 32


375

enhanced the oxidation degree of SOA in autumn. While the mass concentration of

376

MO-OOA was increased approximately by 6 µg m-3 as Ox increased from 30-40 ppb

377

to 60-70 ppb at RH levels of 30-40%, it was elevated by nearly 19 µg m-3 as RH

378

increased from 30-40% to > 90% at Ox levels of 30 -40 ppb. This result suggests that

379

aqueous-phase processing is still more important than photochemical processing in

380

the formation of MO-OOA in autumn.

381

Case studies of severe haze episodes

382

Figure 4 shows the evolution processes of SOA and the average bulk

383

composition of NR-PM1 and OA during four distinct episodes in the three seasons.

384

The first episode in summer (Ep1) was characterized by high humidity (~60-80%) and

385

temperature (> 25oC) that is commonly referred to as “Sauna days”, and also

386

consistently southerly winds (Figure 4a). SOA comprised the major fraction of OA,

387

on average accounting for 73% (Table S4), and the average O/CSOA was as high as

388

1.02 indicating highly oxidized SOA. Indeed, the MO-OOA concentration was more

389

than twice that of LO-OOA during this episode, highlighting the well processed SOA

390

observed under such meteorological conditions. We also note a strong diurnal cycle of

391

LO-OOA during Ep1 that is remarkably similar to that of Ox. The concentration of

392

LO-OOA showed a rapid increase after the photochemical processing started in early

393

morning, reached a maximum at noon time, and then presented a gradual decrease in

394

the late afternoon due to evaporative loss under high T. This result clearly indicates a

395

strong photochemical production of LO-OOA in “Sauna days” despite the dominance 19



396

of MO-OOA. In contrast, Ep2 in summer showed substantially different SOA

397

composition from Ep1. Particularly, the contribution of LO-OOA (58%) was

398

significantly higher than MO-OOA (8%), indicating that photochemical processing

399

was the major formation mechanisms of SOA during this episode. This is consistent

400

with the extremely high Ox level (the daily maximum is 134 ppb and 150 ppb on 29

401

and 30 June, respectively), yet RH was relatively low (< 60%). The average O/CSOA

402

(0.82) was much smaller than that during Ep1, indicating that photochemical

403

processing in summer is subject to produce freshly oxidized SOA.

404

The evolution of MO-OOA and LO-OOA in autumn and winter was largely

405

different from that in summer. During the five-day episode in autumn (Ep3),

406

MO-OOA showed a continuous increase from a few µg m-3 to approximately 30 µg

407

m-3, associated with a gradual increase in RH. Such a temporal variation likely

408

illustrated a combined effect of aqueous-phase production and accumulation

409

processes from regional transport, consistent with the fact that MO-OOA is a well

410

processed and regional SOA. The temporal variation of LO-OOA was quite different

411

from that of MO-OOA. LO-OOA showed a rapid increase during the early stage of

412

Ep3 and then remained at relatively constant levels during the evolution stage except

413

two peaks on 17 and 18 October that were due to the photochemical production

414

associated with high Ox levels. The concentration of LO-OOA was much higher than

415

MO-OOA during the early period of Ep3, and MO-OOA gradually exceeded

416

LO-OOA during the later stage as RH increased gradually. Such changes in SOA 20


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417

composition were also observed in Ep4 in winter. As shown in Figure 4d, the

418

concentration of LO-OOA increased from 15 to 50 µg m-3 in approximately half hour

419

(17:00, 15 January) while that of MO-OOA only increased from 3 to 7 µg m-3. Such a

420

rapid increase in LO-OOA in the late afternoon was most likely due to air mass

421

changes and from the transport of air masses from nearby areas where LO-OOA was

422

photo-chemically formed during the daytime.30 This is also supported by the overall

423

similar trends between LO-OOA and Ox. LO-OOA then showed a gradual decrease at

424

nighttime associated with a corresponding increase in MO-OOA, indicating that

425

aqueous-phase processing played an enhanced role during the later evolution of SOA.

426

The two episodes in autumn and winter highlight different roles of LO-OOA and

427

MO-OOA in the evolution of haze episodes. While LO-OOA appears to be always

428

important at the early formation stage of haze episodes (e.g., ~80% of OOA),

429

MO-OOA becomes more important during the later stage due to aqueous-phase

430

production under high RH levels and continuous regional transport (e.g., ~60% of

431

OOA). Nevertheless, the four episode analyses together highlight the important roles

432

of SOA in the formation of haze episodes during all seasons (65-75% of OA in

433

summer and 51-56% in other two seasons), but SOA composition (LO-OOA vs.

434

MO-OOA) can vary substantially depending on meteorological parameters that affect

435

photochemical and aqueous-phase processing differently.

436

Acknowledgements

437

This work was supported by the National Natural Science Foundation of China 21



438

(41575120), the National Key Basic Research Program of China (2014CB447900),

439

and the Strategic Priority Research Program (B) of the Chinese Academy of

440

Sciences (XDB05020501).

441 442

Supporting Information Available. The contents of Supporting Information provide

443

a detailed evaluation and selection of PMF solutions in summer, and 5 tables (Tables

444

S1-S5) and 13 figures (Figures S1–S13).

445 446

References

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(53) Paatero, P.; Tapper, U. Positive matrix factorization: A non-negative factor model with optimal utilization of error estimates of data values. Environmetrics 1994, 5, 111-126. (54) Fountoukis, C.; Nenes, A. ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+-Ca2+-Mg2+-NH4+-Na+-SO42--NO3--Cl--H2O aerosols. Atmos. Chem. Phys. 2007, 7 (17), 4639-4659. (55) Sun, Y. L.; Wang, Z. F.; Fu, P. Q.; Yang, T.; Jiang, Q.; Dong, H. B.; Li, J.; Jia, J. J. Aerosol composition, sources and processes during wintertime in Beijing, China. Atmos. Chem. Phys. 2013, 13, (9), 4577-4592. (56) Carlton, A. G.; Turpin, B. J.; Altieri, K. E.; Seitzinger, S.; Reff, A.; Lim, H.-J.; Ervens, B. Atmospheric oxalic acid and SOA production from glyoxal: Results of aqueous photooxidation experiments. Atmos. Environ. 2007, 41 (35), 7588-7602. (57) Altieri, K. E.; Seitzinger, S. P.; Carlton, A. G.; Turpin, B. J.; Klein, G. C.; Marshall, A. G. Oligomers formed through in-cloud methylglyoxal reactions: Chemical composition, properties, and mechanisms investigated by ultra-high resolution FT-ICR mass spectrometry. Atmos. Environ. 2008, 42 (7), 1476-1490. (58) Zorn, S. R.; Drewnick, F.; Schott, M.; Hoffmann, T.; Borrmann, S. Characterization of the South Atlantic marine boundary layer aerosol using an aerodyne aerosol mass spectrometer. Atmos. Chem. Phys. 2008, 8 (16), 4711-4728. (59) Barnes, I.; Hjorth, J.; Mihalopoulos, N. Dimethyl sulfide and dimethyl sulfoxide and their oxidation in the atmosphere. Chem Rev 2006, 106 (3), 940-975. (60) Chen, Q.; Farmer, D. K.; Rizzo, L. V.; Pauliquevis, T.; Kuwata, M.; Karl, T. G.; Guenther, A.; Allan, J. D.; Coe, H.; Andreae, M. O.; et al. Submicron particle mass concentrations and sources in the Amazonian wet season (AMAZE-08). Atmos. Chem. Phys. 2015, 15 (7), 3687-3701. (61) Sun, C.; Lee, B. P.; Huang, D.; Jie Li, Y.; Schurman, M. I.; Louie, P. K. K.; Luk, C.; Chan, C. K. Continuous measurements at the urban roadside in an Asian megacity by Aerosol Chemical Speciation Monitor (ACSM): particulate matter characteristics during fall and winter seasons in Hong Kong. Atmos. Chem. Phys. 2016, 16 (3), 1713-1728. (62) Sun, Y. L.; Wang, Z. F.; Du, W.; Zhang, Q.; Wang, Q. Q.; Fu, P. Q.; Pan, X. L.; Li, J.; Jayne, J.; Worsnop, D. R. Long-term real-time measurements of aerosol particle composition in Beijing, China: seasonal variations, meteorological effects, and source analysis. Atmos. Chem. Phys. 2015, 15 (17), 10149-10165. (63) Quan, J.; Liu, Q.; Li, X.; Gao, Y.; Jia, X.; Sheng, J.; Liu, Y. Effect of heterogeneous aqueous reactions on the secondary formation of inorganic aerosols during haze events. Atmos. Environ. 2015, 122, 306-312.

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677 678 679

Figure 1. Variations of the mass fractions of MO-OOA, LO-OOA and POA in OA,

680

and the O/C, O/CSOA, WS, LWC and OA as a function of RH in (a,d,g) summer, (b,e,h)

681

autumn and (c,f,i) winter. The data were binned according to the RH (10% increment),

682

and mean (circle), median (horizontal line), 25th and 75th percentiles (lower and

683

upper box), and 10th and 90th percentiles (lower and upper whiskers) are shown for

684

each bin. Note that the LWC and mass concentrations of MO-OOA and SO42- during

685

the three seasons are scaled on the same right axis on the top and middle panel,

686

respectively.

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687

688 689 690

Figure 2. Variations of mass fractions of MO-OOA, LO-OOA, and POA in OA, and

691

the O/C, O/CSOA, LO-OOA, MO-OOA and OA as a function of Ox in (a,d) summer,

692

(b,e) autumn and (c,f) winter. The data were binned according to the Ox concentration

693

(10 ppb increment), and mean (circle), median (horizontal line), 25th and 75th

694

percentiles (lower and upper box), and 10th and 90th percentiles (lower and upper

695

whiskers) are shown for each bin.

29



696

697 698 699

Figure 3. RH vs. Ox dependence of the ratio of MO-OOA/LO-OOA and O/CSOA in

700

(a,d) summer, (b,e) autumn and (c,f) winter. The lines are colored by the mass

701

concentrations of organics. Grids with the number of points less than five were

702

excluded.

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703

704 705 706 707

Figure 4. Time series of meteorological parameters (RH, T, WS, WD), mass

708

concentrations of MO-OOA, LO-OOA and Ox during four episodes, i.e., (a) Ep1 and

709

(b) Ep2 in summer, and (c) Ep3 and (d) Ep4 in autumn and winter, respectively.

710

Below panels show average mass concentrations (e,f) and mass fractions (g,h) of

711

NR-PM1 species and OA factors during the four episodes. Also shown in (f) are the

712

average O/C and O/CSOA for each episode.

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203x112mm (150 x 150 DPI)


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Effects of Aqueous-Phase and Photochemical Processing on

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