Seasonal variability of carbon in humic-like matter of ambient size-segregated water soluble organic aerosols from urban background environment

Accepted Manuscript Seasonal variability of carbon in humic-like matter of ambient size-segregated water soluble organic aerosols from urban background environment Sanja Frka, Irena Grgić, Janja Turšič, Maria I. Gini, Konstantinos Eleftheriadis PII:

S1352-2310(17)30758-6

DOI:

10.1016/j.atmosenv.2017.11.013

Reference:

AEA 15670

To appear in:

Atmospheric Environment

Received Date: 10 July 2017 Revised Date:

7 November 2017

Accepted Date: 10 November 2017

Please cite this article as: Frka, S., Grgić, I., Turšič, J., Gini, M.I., Eleftheriadis, K., Seasonal variability of carbon in humic-like matter of ambient size-segregated water soluble organic aerosols from urban background environment, Atmospheric Environment (2017), doi: 10.1016/j.atmosenv.2017.11.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Seasonal variability of carbon in humic-like matter of ambient

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size-segregated water soluble organic aerosols from urban

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background environment

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Sanja Frkaa,b, Irena Grgićb,*, Janja Turšičc, Maria I. Ginid, Konstantinos

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Eleftheriadisd

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Croatia

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Division for Marine and Environmental Research, Ruđer Bošković Institute, 10000 Zagreb,

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Slovenia

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“Demokritos”, 15341 Athens, Greece

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*Corresponding author. E-mail address: [email protected] (I. Grgić)

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Abstract

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Long-term measurements of carbon in HUmic-LIke Substances (HULIS-C) of ambient size-

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segregated water soluble organic aerosols were performed using a ten-stage low-pressure

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Berner impactor from December 2014 to November 2015 at an urban background

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environment in Ljubljana, Slovenia. The mass size distribution patterns of measured species

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(PM - particulate matter, WSOC - water-soluble organic carbon and HULIS-C) for all seasons

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were generally tri-modal (primarily accumulation mode) but with significant seasonal

Department of Analytical Chemistry, National Institute of Chemistry, 1000 Ljubljana,

Environmental Agency of the Republic of Slovenia, 1000 Ljubljana, Slovenia

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Institute of Nuclear & Radiological Sciences & Technology, Energy & Safety, N.C.S.R.

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ACCEPTED MANUSCRIPT variability. HULIS-C was found to have similar distributions as WSOC, with nearly the same

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mass median aerodynamic diameters (MMADs), except for winter when the HULIS-C size

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distribution was bimodal. In autumn and winter, the dominant accumulation mode with MMAD

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at ca. 0.65 µm contributed 83 and 97% to the total HULIS-C concentration, respectively.

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HULIS-C accounted for a large fraction of WSOC, averaging more than 50% in autumn and

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40% in winter. Alternatively, during warmer periods the contributions of ultrafine (27% in

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summer) and coarse mode (27% in spring) were also substantial. Based on mass size

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distribution characteristics, HULIS-C was found to be of various sources. In colder seasons,

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wood burning was confirmed as the most important HULIS source; secondary formation in

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atmospheric liquid water also contributed significantly, as revealed by the MMADs of the

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accumulation mode shifting to larger sizes. The distinct difference between the spring and

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summer ratios of HULIS-C/WSOC in fine particles (ca. 50% in spring, but only 10% in

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summer) indicated different sources and chemical composition of WSOC in summer (e.g.,

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SOA formation from biogenic volatile organic compounds (BVOCs) via photochemistry).

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The enlarged amount of HULIS-C in the ultrafine mode in summer suggests that the

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important contribution was most likely from new particle formation during higher emissions

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of BVOC due to the vicinity of a mixed deciduous forest; the higher contribution of HULIS-C

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in the coarse mode demonstrated that beside soil erosion other sources, such as pollen and

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plant fragments, could also be responsible.

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Keywords: Size-segregated aerosols, Organic carbon, WSOC, HULIS, Levoglucosan,

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Biomass burning.

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1.

Introduction Atmospheric aerosols are recognized to play an essential role in diverse environmental

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problems, such as climate change and eutrophication of remote areas; they are involved in many

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different physico-chemical processes in the atmosphere and also represent an important risk

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factor for various adverse health effects in humans (Pöschl, 2005; Hallquist et al., 2009; Liu et

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al., 2013; Pöschl and Shiraiwa, 2015; Ravishankara et al., 2015; Seinfeld and Pandis, 2016).

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Atmospheric aerosols exhibit a wide range in diameter, from a few nanometers to several tens

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of micrometers, making size the most important parameter used to describe their properties.

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Thus, besides chemical composition, information on particle size offers vital information not

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only for deducing origin and (trans)formation pathways, including secondary formation and

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potential impacts on human health, but also on visibility, radiative forcing, and cloud

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formation (e.g., Kanakidou et al., 2005; Seinfeld and Pandis, 2016). Although various

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categories for particle size exist in the literature, in general, nucleation (Aitken) (particle size

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< 0.1 µm, mostly called ultrafine) and accumulation modes (from ∼ 0.1 to ∼ 2 µm) are

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together defined as “fine” particles (Seinfeld and Pandis, 2016). In number of cases the

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accumulation mode comprises two modes: the condensation mode (∼ 0.1–0.5 µm), produced

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by primary particle emissions and growth of smaller particles by coagulation and

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condensation of gases, and the droplet mode (∼ 0.5–2.0 µm), likely formed from aqueous-

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phase processing of condensation mode particles. Material in the coarse mode (particle size >

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2.5 µm) is of primary origin.

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In rural background conditions (i.e. for 12 European rural background sites following a one-

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year sampling campaign within the European Monitoring and Evaluation Programme,

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EMEP), up to 90% of the carbonaceous fraction, which accounts for 30 ± 9% of atmospheric

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PM10, can be attributed to organic carbon (OC) with the remaining fraction comprising

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ACCEPTED MANUSCRIPT elemental carbon (EC) (Yttri et al., 2007). Elemental carbon results exclusively from primary

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combustion processes and is typically found in fine mode aerosols; OC can be of primary or

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secondary origin; hence, extends over a wider aerosol size range (Pio et al., 2007).

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Accumulation mode particles often exhibit a bimodal distribution consisting of condensation

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(gas-to-particle conversion; 0.14 - 0.42 µm) and droplet mode particles (chemical processing;

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0.42 - 1.2 µm) (e.g. John et al., 1990; Meng and Seinfeld, 1994; Sandrini et al., 2016).

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The water-soluble organic fraction of atmospheric aerosols, usually quantified as

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water-soluble organic carbon (WSOC), represents a highly variable fraction (10–80%) of

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organic carbon and depends on season, location, time-of-day, and particle size (Agarwal et al.,

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2010; Duarte and Duarte, 2011). Secondary organic aerosols (SOA), in particular, are

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associated with a large water-soluble organic fraction, comprising more oxidized and soluble

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organic compounds, induced by photooxidation of anthropogenic or biogenic precursors

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(Pöschl, 2005; Hallquist et al., 2009; Ervens et al., 2011). Besides altering the hygroscopic

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properties (Fors et al., 2010), surface tension (Salma et al., 2006), and effective density of the

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aerosol (Dinar et al., 2006)—thus indirectly influencing the ability of particles to act as cloud

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condensation nuclei (CCN) (Padró et al., 2010)—a significant fraction of WSOC also exhibits

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light-absorbing properties, contributing to atmospheric brown carbon (BrC) (Zhang et al.,

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2013; Nguyen et al., 2012; Laskin et al., 2015).

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One of the most important classes of water-soluble organics in atmospheric aerosols

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and fog and cloud waters are HUmic-LIke Substances (HULIS), a term that underscores their

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physico-chemical similarities to natural humic matter (Graber and Rudich, 2006; Duarte et al.,

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2007; Zheng et al., 2013). In contrast to humic substances from terrestrial and/or aquatic

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sources, atmospheric HULIS have smaller molecular weights and are composed of fewer

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acidic and aromatic compounds (Graber and Rudich, 2006). HULIS are of special interest due 4

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mechanisms, physico-chemical properties, and chemical composition on water soluble

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organics, including HULIS have already been the subject of numerous studies (e.g., Decesari

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et al., 2000, 2001, 2006; Salma et al., 2007, 2008, 2010; Ziese et al., 2008; Claeys et al., 2012;

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Pavlovic and Hopke, 2012; Zheng et al., 2013). Field experiments have observed HULIS in a

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variety of aerosols of different origins, and carbon fractions (HULIS-C), though variable,

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generally constitute 24–72% of WSOC in bulk PM (Zheng et al., 2013 and references

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therein). The observed seasonal variabilities of HULIS mass concentrations over Europe

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result from different processes responsible for emissions and formation of HULIS, such as

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biomass burning (BB) in winter and photooxidation in summer (Feczko et al., 2007; Baduel et

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al., 2010; Poulain et al., 2011, Amato et al., 2016).

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Although the size distributions of WSOC have been studied widely in different

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environments (e.g., Timonen et al., 2008; Agarwal et al., 2010; Lin et al., 2010; Pavlovic and

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Hopke, 2012), information on the size distributions of HULIS is quite limited (Lin et al.,

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2010; Salma et al., 2013). In addition, very little is known about seasonal variations of HULIS

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in size-segregated ambient aerosols, in part due to the highly demanding procedure of

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sampling and analyzing low-level material concentrations. In the present study, emphasis was

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given to the seasonal variability of size-resolved WSOC and HULIS-C mass concentrations in

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ambient aerosols collected by a ten-stage, low-pressure Berner impactor at an urban

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background environment in Ljubljana, Slovenia. According to current literature, these are the

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first comprehensive long-term measurements of HULIS water-soluble carbon content in size-

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segregated atmospheric aerosols. The obtained data were tested for correlation with

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levoglucosan, a primary tracer for biomass burning emissions (Simoneit et al., 1999), as well

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as with aerosol PM mass and total carbon (TC), to identify possible sources. Mass size

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distribution patterns of PM, WSOC, and HULIS-C for all seasons, and also levoglucosan for

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winter and autumn, were obtained and approximated by a sum of fitted characteristic log-

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normal Gaussian modes. Based on the characteristics of HULIS-C in the water-soluble

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organic aerosols, possible sources and formation mechanisms are discussed.

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2.

Experimental section

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2.1. Aerosol sampling

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Size-segregated aerosol samples were collected with a Berner low-pressure cascade

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impactor (HAUKE, LPI 25/0,015/2) at an urban background site of Ljubljana, Slovenia

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(approx. 279,000 inhabitants, 298 m a.s.l.) within the AERONAR project campaign. The

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sampling site was located ca. 1.5 km away from the motorway ring-road, in the near vicinity

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of hill Rožnik covered mostly with deciduous trees (oak and beech) and ca. 400 m away from

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a residential area, where substantial wood combustion for domestic heating during winter,

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was expected to be used.

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The impactor was operated at a nominal flow rate of 25.8 L min-1 (at 20 °C); it has 10

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collection stages for the nominal size ranges expressed in aerodynamic equivalent diameter,

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dae (stage 1, 0.038–0.067 µm; stage 2, 0.067–0.104 µm; stage 3, 0.104–0.16 µm; stage 4,

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0.16–0.305 µm; stage 5, 0.305–0.56 µm; stage 6, 0.56–1.01 µm; stage 7, 1.01–2.1 µm; stage

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8, 2.1–3.99 µm; stage 9, 3.99–8.06 µm; stage 10, 8.06–15.6 µm). The samples were collected

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on aluminum foils for 48–72 h from 3 December 2014 until 11 November 2015, resulting in

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the overall collection of 52 samples, each comprising 10 stages. Thus, during winter, spring,

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summer and autumn 240, 90, 90 and 100 foil samples were collected, respectively. To remove

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organic contaminants foils were pre-baked at 500 °C for 24 h prior to sampling. Collected

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aerosol samples and filter field blanks (no air was drawn through the filter) were stored at -18

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°C prior to analysis. Standard meteorological parameters (e.g., RH, T, precipitation, sun

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duration, wind) were obtained from the nearest meteorological station (Table S1).

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2.2.

Sample treatment and chemical analysis Aerosol mass was determined by weighing (Sartorius M3P microbalance, sensitivity of

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1 µg; reproducibility of ±3 µg) each impaction foil before and after sampling, after

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conditioning for at least 24 h (20 ± 1 °C; RH = 50 ± 5). Two steps were taken to ensure a

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sufficient amount of carbonaceous material for chemical analysis: (1) stages 1 and 2 were

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processed together for each sample (called stage 2a herein); and (2) 2–5 consecutive samples

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were combined, providing seven sample sets for winter, three for spring, three for summer,

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and three for autumn. Thus, each sample set, comprising nine stages (from 2a to 10)

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represents the average of 6 to 12 sampling days. The number of samples combined into

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sample set was dependent on the PM mass deposition on aluminum foils, i.e. to ensure at least

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90 µg of PM deposit for each size fraction (Tables S2-S5). Specific PM mass size fractions

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were obtained by combining the corresponding deposits as follows and used hereafter: PM<0.1

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(size below 0.16 µm, stages 2a and 3), PM0.1-1 (size 0.16–1.01 µm, stages 4, 5 and 6), PM1-2

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(size 1.0–2.01 µm, stage 7), PM2 (size 0.038–2.1 µm, stages 2a, 3, 4, 5, 6 and 7), PM>2 (size

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2.1–15.6 µm, stages 8, 9 and 10), and PM15.6 (all stages together, size: 0.038–15.6 µm; stages

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from 2a to 10). As schematically presented in Fig. 1, combined foils with deposits were

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extracted jointly for 2 min in 20 mL of high-purity water (18.2 MΩ cm, Milli-Q purification

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water system; Millipore, Bedford, MA, USA) in an ultrasonic bath, left for 24 h (at 4 °C), and

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filtrated through 0.22 µm pore size filter (Supelco, USA). An aliquot of 0.5 mL was used for

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WSOC and levoglucosan determination. Another aliquot representing 14.8 mL of overall

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water extract was further adjusted to pH 2 with HCl and used for HULIS separation. Due to

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ACCEPTED MANUSCRIPT its simplicity and selective isolation, solid-phase extraction (SPE) is the most frequently-used

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approach for simultaneous concentration and fractionation of HULIS from other dissolved

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constituents (Kiss et al., 2002; Salma et al., 2007, 2008, 2013, Samburova et al., 2007) and

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was also applied in this work. For SPE, C-18 cartridges were chosen; organics that represent

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HULIS were retained on C-18 cartridges (SEP-PAK VAC, 3 mL, 500 mg, Waters),

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afterwards they were eluted with methanol, dried gently with N2, and re-dissolved in 14.8 mL

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of Milli-Q. Aliquots of HULIS were then used for organic carbon analysis. Procedure blanks

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were obtained with Milli-Q water subjected to the same processing steps as the samples

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analyzed for WSOC. Blank WSOC values were below limit of detection (LOD, 0.1 mgC L-1).

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Fig. 1: Scheme of experimental set up.

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Organic carbon in water soluble extracts (WSOC) and in HULIS (HULIS-C) were

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determined by Combustion TOC Analyzer (Teledyne, Apollo 9000 HS) with NDIR

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(nondispersive infrared gas analyzer) detection (e.g. Sandrini et. al., 2016). Levoglucosan in water-soluble extracts were analyzed by ion-exchange chromatography

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(Dionex ICS-3000, column Dionex CarboPac MA1) with electrochemical detection (Pulse

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amperometric detection with standard quadruple wave form) at a flow rate of 0.4 mL min-1

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(e.g., Engling et al., 2006). Limit of detection (LOD) of the used method was 0.005 mg L-1 and

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limit of quantification 0.015 mg L-1.

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A portion of each Al foil filter (depending on the number and surface area of spots at

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each impactor stage) was used for TC analyses performed by OC-EC Analyzer (Sunset

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Laboratory) using He/O2 mixture following two-step procedure: (1) 450 °C for 120 s and (2)

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650 °C until signal reaches the baseline. The accuracy of the used procedure was confirmed

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with the EUSAAR 2 protocol using quartz fiber filters (Cavalli et al., 2010) and the agreement

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was within 5%.

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2.3. Multi-modal analysis of mass size distributions Further data analysis was conducted on the dataset of each 10 size fractions of PM mass

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and of WSOC, HULIS-C, and levoguclosan mass concentrations obtained by the analysis.

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Aerosol fractionation by impactors is non ideal and each stage always collects a fraction of

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particles with smaller sizes than the aerodynamic 50% cut off size, while it fails to collect

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some of the particles with sizes larger than its prescribed cut off size. The redistribution of

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particle mass to the correct one can be achieved by an inversion algorithm considering the

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individual stage collection efficiency curves of the Berner impactor. In order to achieve a

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meaningful solution for this inversion problem some assumptions are made, mainly

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ACCEPTED MANUSCRIPT considering a mass closure for all stages and that aerosol mass size distributions can be

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described by a sum of lognormal functions. The inverted continuous size distribution curves

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were reconstructed by means of the MICRON inversion algorithm (Wolfenbarger and

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Seinfeld, 1990). The result is a continuous size distribution instead of the step-type

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distribution of raw data available for the individual stages. In order to derive quantitative

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statistics and details of the dynamics and evolution of the size distribution for PM mass and

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the chemical components the metrics of lognormal size distribution modes resolved is again

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used by simulating an optimum sum of such log-normal modes to fit the inverted size

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distributions. Each mode is described by a mass median aerodynamic diameter (MMAD) (i.e.,

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characteristic parameter to define the mean size of aerosol particles for each size mode) and a

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geometric standard deviation (GSD) applying the methodology as described elsewhere

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(Hussain et al., 2005; Zwozdziak et. al., 2017). The position of the size distribution peaks and

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their broadness are particularly useful in discussing the similarity of the aerosol modes

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containing the individual chemical components. Therefore, their existence in the atmospheric

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environment as internal or external mixtures resulting from the same or different sources of

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origin and aging processes can be discussed.

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3.

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3.1 Seasonal varability of atmospheric concentrations of PM, WSOC and HULIS-C in

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Results and discussion

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different size fractions

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Seasonal PM masses and atmospheric concentrations of WSOC and HULIS-C in ultrafine

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(denoted as PM<0.1 for particles < 0.16 µm), accumulation (PM0.1-1, size: 0.16–1.01 µm; and

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PM1-2, size: 1.0–2.01 µm) and coarse (PM>2, size: 2.01–15.6 µm) fractions are shown in Fig. 2.

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Since the determination of HULIS-C requires demanding handling and sufficient amount of

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carbonaceous material present after SPE procedure, HULIS-C was determined for 6 sample 10

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one sample set represents the average of 6 to 12 sampling days in each season. The average

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PM15.6 mass concentrations of 32.8 ± 14.0 µg m-3 (average conc. ± standard deviation; N = 7),

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11.6 ± 1.2 µg m-3 (N = 3), 16.8 ± 2.2 µg m-3 (N = 3) and 27.4 ± 11.9 µg m-3 (N = 3) were

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determined for winter, spring, summer and autumn samples, respectively. In winter, the

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average PM15.6 WSOC concentration was 5.1 ± 3.0 µgC m-3 (N = 7) with the highest

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concentration of 11.9 µgC m-3 measured in mid-February (Fig. 2). In autumn and summer, and

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especially in spring, the average PM15.6 WSOC concentrations were lower (autumn, 1.7 ± 0.4

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µgC m-3, N = 3; summer, 1.8 ± 0.2 µgC m-3, N = 3; spring: 0.6 ± 0.2 µg m-3, N = 3). The

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average PM2 WSOC concentrations of 4.6 ± 2.6 µg m-3, 0.5 ± 0.1 µg m-3, 1.6 ± 0.3 µg m-3

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and 1.6 ± 0.3 µg m-3 were determined for winter, spring, summer and autumn samples,

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respectively. In particular in winter and autumn, the highest concentrations were measured in

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PM0.1-1 fraction. In addition, spring and summer were characterized also with some enhanced

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concentrations in PM>2 coarse fraction.

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Further discussion is related to particular HULIS-C contribution to WSOC of

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corresponding sample set (Fig. 2). For winter, the average PM15.6 HULIS-C concentration was

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1.7 ± 1.3 µgC m-3 (N = 6) with the highest concentration of 3.2 µgC m-3 observed also in mid-

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February (Fig. 2). The average PM2 fraction of HULIS-C was 1.6 ± 1.2 µgC m-3 with 1.4 µgC

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m-3 in PM0.1-1 fine particles. In autumn (5–11 November 2015), more than 50% of the PM15.6

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WSOC represents HULIS-C (1.3 µgC m-3), 70% of which was in the PM0.1-1 fraction, and the

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rest largely in the coarse mode PM>2. The PM15.6 HULIS-C concentrations for selected spring

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(15–25 May 2015) and summer (10–21 August 2015) periods were similar (0.36 and 0.40 µg m-

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26% in PM>2 for summer (Fig. 2).

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, respectively) with about 60% in PM0.1-1 and 21% in PM>2 for spring and 44% in PM0.1-1 and

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ACCEPTED MANUSCRIPT Our measurements showed that the contributions of HULIS-C to WSOC in PM15.6

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were about 20% in summer, 30% in winter, almost 50% in spring and nearly 70% in autumn.

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Similarly, contributions of HULIS-C to WSOC in PM10 aerosol samples from French cities

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(Grenoble, Strasbourg, Lille, Tolouse, Marseille, and Paris) during all seasons were found to

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be between 28 and 43% (Baduel et al., 2010). Pavlovic and Hopke (2012) investigated

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seasonal patterns (fall vs. summer) of HULIS in three different PM size fractions (PM<0.1,

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PM0.1-1 and PM1-2.5) for a rural environment in Potsdam, NY (close to the park with mixed

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coniferous/deciduous forest); about 40% of HULIS was found in PM0.1-1 during summer

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increasing to 56% in fall, which is similar to our measurements, i.e. 44% of HULIS-C in

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PM0.1-1 in summer and more than 50% in autumn. The ultrafine fraction (PM<0.1) in the fall

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season contributed 45% to the total HULIS and was much higher than in summer.

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Fig. 2. Long-term measurements of particulate mass (PM), water-soluble organic carbon

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(WSOC) and carbon in water-soluble humic-like substances (HULIS-C) in different size

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fractions of aerosol particles (ultrafine denoted as PM<0.1 for particles < 0.16 µm; accumulation

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as PM0.1-1 and PM1-2 for sizes 0.16–1.01 and 1.0–2.01 µm, respectively; and coarse as PM>2,

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size: 2.01–15.6 µm) collected in the period from December 2014 to November 2015 at an

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urban background site in Ljubljana, Slovenia. Concentrations for PM mass are in µg m-3 (right

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y-axis), and for WSOC and HULIS-C in µgC m-3 (left y-axis). Note that HULIS-C was

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determined for six sample sets during winter and for one sample set in spring, summer and

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autumn period.

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3.2 Mass size distributions of PM, WSOC and HULIS-C The seasonal characteristics of the PM mass, WSOC, and HULIS-C size-distribution

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patterns represented by one selected data set from each season are shown in Fig. 3 and their

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seasonal contributions to different modes are presented in Fig 4. Namely, HULIS-C

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determination was limited to particular seasonal sample sets, thus, further discussion is related

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to the selected data being the average of 6 to 12 sampling days in each season. Three modes

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(ultrafine, particle size < 0.16 µm; accumulation, from 0.16 to ∼ 2 µm and coarse, above 2

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µm) are evident for PM mass (Figs. 3a and b) and WSOC (Figs. 3c and d) for all seasons.

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However, significant seasonal variability among modes can be seen between the winter-autumn

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and spring-summer periods. In general, accumulation was the major mode for both PM mass

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and WSOC for all seasons with a MMAD for both of 0.66, 0.60, 0.50, and 0.51–0.56 µm for

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autumn, winter, spring, and summer, respectively (Table 1). For winter and autumn, this

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mode represents 91% and 84% of the total WSOC concentration and 83% and 70% of the

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total PM mass concentration, respectively (Fig 4). Alternatively, accumulation mode

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accounted for slightly more than 50% of the total PM mass and WSOC in spring, whereas in

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summer this mode represented 60% of the total PM mass and 70% of the total WSOC

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concentration. Ultrafine mode was quite persistent, and its contribution to total PM mass

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increased from 4% in winter (MMAD = 0.13 µm) to 10% in summer (MMAD = 0.17 µm),

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while ultrafine contributions to WSOC were around 7% for all seasons (av MMAD =

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0.13 µm) with an increase in spring to 12%. The coarse modes of PM mass and WSOC in

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winter (13% and 2%, respectively) and autumn (23% and 10%, respectively) increased

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significantly during spring (37% and 34%, respectively) and summer (30% and 22%,

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respectively). The prominent increase from winter to spring was observed especially in coarse

299

WSOC contribution; the MMAD values did not change significantly. As shown in Figs. 3e and f, as well as in Table 1, the mass size distribution of HULIS-C

301

follows that of WSOC with nearly the same MMADs; three modes were characteristic for all

302

seasons, except for winter where HULIS-C size distribution had a bimodal structure. Thus, for

303

autumn and winter, the dominant accumulation mode contributed 83 and 97%, respectively; the

304

minor ultrafine mode represented 7 and 3% of the total HULIS-C concentration, respectively. In

305

autumn, the coarse mode contributed 10%. In contrast, for spring and summer, the ultrafine

306

mode reached 16 and 27% and the coarse mode 27 and 16%, respectively, while significantly

307

lower contributions of the dominant accumulation mode were observed (ca. 57%) during spring

308

(Table 1, Fig. 4). The structure of the distribution of HULIS-C for summer was different than

309

for WSOC. Although three modes (Fig. 3f) were still resolved, they appeared much broader

310

and therefore merged, with larger mean diameters and GSDs, especially for the coarse and

311

accumulation modes and different than those of WSOC for the same season (Table 1). This

312

could also be an indication of aging and the result of small contributions by several processes

313

of HULIS-C in the summer. In addition, the WSOC appear to be internally well mixed within

314

the ultrafine and accumulation modes (similar GSDs). In the coarse mode, modal diameters

315

for the WSOC and HULIS-C, and PM differed to some extent indicating that they still appear

316

as a rather external mixture with mineral dust.

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Mass size distributions for levoglucosan were characterized by one mode in winter

318

aerosols and two modes in autumn aerosols with similar MMADs as for WSOC and HULIS-C

319

with major contribution to the accumulation mode (Table 1). Our results are in agreement with

320

those found for aerosols affected by biomass burning, where most of the levoglucosan (up to

15

ACCEPTED MANUSCRIPT 321

89%) was present in small particles (<1.5 µm) (Cerasi Urban et al., 2012). The concentrations

322

of levoglucosan during spring and summer were below the detection limit.

323

60.0

a

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PM-Winter (Inv) PM-Winter

PM-Autumn (Inv)

40.0

PM-Autumn

30.0

SC

dM/dlogda (µg/m3)

50.0

20.0

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18.0

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PM-Spring PM-Summer (Inv) PM-Summer

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ACCEPTED MANUSCRIPT 10.0 WSOC-Winter (Inv) WSOC-Winter

8.0

WSOC-Autumn (Inv)

4.0 2.0 0.0 0.01

0.1

1 da, (µm)

WSOC-Spring (Inv)

d

WSOC-Spring WSOC-Summer (Inv)

2.0

WSOC-Summer

1.5 1.0 0.5

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0.1

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da, (µm)

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dM/dlogda (µg/m3)

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dM/dlogda (µg/m3)

c

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ACCEPTED MANUSCRIPT 6.0

e

HULIS-C-Winter (Inv) HULIS-C-Winter

4.0

HULIS-C-Autumn (Inv) HULIS-C-Autumn

3.0

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dM/dlogda (µg/m3)

5.0

2.0

0.0 0.01

0.1

1 da, (µm)

0.5

HULIS-C-Spring (Inv) HULIS-C-Spring

0.4

HULIS-C-Summer (Inv) HULIS-C-Summer

0.3

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dM/dlogda (µg/m3)

f

0.2

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0.1

1 da, (µm)

10

100

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0.0 0.01 330

100

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Fig. 3. Mass size distributions of particulatte mass (PM) (a, b), water-soluble organic carbon

332

(WSOC) (c, d), carbon in water-soluble humic-like substances (HULIS-C) (e, f) in aerosols

333

from an urban background site of Ljubljana, Slovenia for a typical data set from winter and

334

autumn (a, c and e), and spring and summer (b, d and f), 2015.

335

----- Berner Impactor data with mass concentrations measured at every stage;  inverted size

336

distributions by the Micron algorithm.

337 18

ACCEPTED MANUSCRIPT 338

Table 1: Modal characteristics of particulatte mass (PM), water-soluble organic carbon

339

(WSOC), carbon in water-soluble humic-like substances (HULIS-C) and levoglucosan mass

340

size distributions for winter, spring, summer and autumn 2015. GSD – geometric standard

341

deviation, MMAD – mass median aerodynamic diameter.

GSD

MMAD (µm)

Mass conc. (µg m-3)

GSD

MMAD (µm)

Mass conc. (µg m-3)

GSD

MMAD (µm)

Mass conc. (µg m-3)

1.29 1.40 1.20

0.13 0.15 0.13

1.53 0.40 0.08

1.59 1.70 1.48 1.72

0.60 0.60 0.64 0.57

28.9 5.47 2.36 0.79

1.57 1.33

4.32 4.14

4.50 0.14

1.33 1.34 1.46

0.13 0.12 0.13

0.80 0.09 0.05

1.56 1.49 1.62

0.50 0.51 0.58

1.53 2.84 2.25

5.20 4.37 4.92

3.77 0.25 0.10

1.71 1.30 2.61

0.17 0.11 0.17

1.53 0.14 0.11

1.29 1.31 1.32 1.25

0.13 0.12 0.13 0.12

2.81 0.14 0.09 0.03

343

5.57 0.39 0.20

1.66 1.53 3.36

0.56 0.51 0.78

8.90 1.32 0.23

1.82 1.77 2.46

5.05 4.03 6.68

4.51 0.42 0.07

1.63 1.65 1.66 1.65

0.66 0.65 0.66 0.71

29.9 1.76 1.00 0.53

1.88 1.66 1.56

4.18 3.15 3.39

9.96 0.21 0.12

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Winter PM WSOC HULIS-C Levoglucosan Spring PM WSOC HULIS-C Summer PM WSOC HULIS-C Autumn PM WSOC HULIS-C Levoglucosan

Coarse mode

M AN U

Species

Accumulation mode

SC

Ultrafine mode

3.3 Possible sources and formation mechanisms of HULIS In the Ljubljana region, the highest PM mass concentrations are typically in winter and

345

late autumn due to more intensive emissions from high-temperature processes, especially

346

biomass burning, enhanced by accelerated condensation of semivolatile organics due to lower

347

temperatures and frequent temperature inversions (Morawska et al., 2008; Hitzenberger et al.,

348

2006)). Elevated concentrations of fine particles during cold periods have been reported

349

previously for many urban areas (e.g., Huang et al., 2015; Zwozdiak et al., 2017). As shown

350

in Fig. 4b and explained above, the accumulation mode is the major mode of PM mass.

351

Moreover, the similar size distributions of the measured species (PM mass, WSOC, HULIS-

352

C, levoglucosan) indicate that they were internally mixed within this mode (Table 1).

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ACCEPTED MANUSCRIPT Together, these suggest a common evolution and aging mechanism. Higher RH conditions in

354

winter and autumn (av RH > 80%) and low temperatures especially during winter (av T = 4

355

°C) lead to increased uptake of water-soluble organic gases and also to aqueous-phase

356

chemistry. Importantly, these contribute to the secondary formation of WSOC, thus of

357

HULIS-C as well, possibly through heterogeneous reactions or oligomerization through in-

358

cloud processing (Lin et al., 2013). Consequently, MMADs were shifted to larger sizes

359

(Sandrini et al., 2016). The correlations of levoglucosan as a specific tracer for primary BB

360

aerosols (Urban et al., 2012) with HULIS-C were highly correlated (R2 = 0.76 and 0.99 for

361

winter and autumn samples, respectively), indicating that wood-burning emissions from

362

domestic heating near the sampling site was the major source of HULIS-C during the autumn

363

and winter months.

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In spring and summer, the accumulation mode was also dominant (Fig. 4b) but with

365

lower contributions and at smaller MMADs (ca. 0.50 µm). This can be attributed to higher

366

temperatures (av 16 °C in spring and 22 °C in summer) and more intensive solar radiation. On

367

the other hand, the distribution pattern of HULIS-C for summer is quite different than that for

368

the WSOC, indicating that different formation pathways attributed to the observed WSOC

369

and HULIS-C concentrations (Figs. 3d, f).

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For the coarse mode, which accounted for 10% in the autumn, but reached nearly 30%

371

of the total HULIS-C in spring and over 20% in summer (Fig. 4a), the major source could be

372

assigned to primary emissions of aged material deposited on the ground enhanced by

373

mechanical processes (Salma et al., 2013), but also could be the result of wind driven

374

transport of plant fragments, such as pollen and other particles of biological origin (Jaenicke,

375

2005), due to their increased production during spring time.

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ACCEPTED MANUSCRIPT The ultrafine mode was low during cold periods; however, its contribution to the total

377

HULIS-C in warmer seasons, especially in summer, was relatively high (27%) (Fig. 4a). In

378

that period, in addition to primary emissions caused by high-temperature processes, the

379

significant contribution of HULIS-C was most likely the result of new particle formation

380

processes. Such processes include heightened emissions of biogenic volatile organic

381

compounds (BVOCs) (e.g., isoprene), due to the vicinity of a mixed deciduous forest (Lin et

382

al., 2013; Yu et al., 2014), followed by growth to CCN-active sizes (ca. 100 nm) and further

383

processing by more intensive atmospheric photochemical reactions in summer (Song et al.,

384

2012). Summer maximums of HULIS-C are typically observed in sites where anthropogenic

385

sources are not significant, especially when biogenic emissions (e.g., from forests) dominate.

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Before this work, explicit mass size distributions of HULIS-C have been reported only

387

for a rural location in South China at a time of harvest season (7 days in November) with a

388

visible presence of crop residue burning (Lin et al., 2010) and for a kerbside within a street

389

canyon in central Budapest, Hungary for 12 days in spring (Salma et al., 2013). In both

390

studies, two submodes of the accumulation mode (condensation and droplet) were reported,

391

although at different MMADs (condensation modes were 0.23-0.28 and 0.31 µm, and droplet

392

modes were 0.63-0.87 and 1.22 µm for Lin et al., 2010 and Salma et al., 2013, respectively).

393

In our case, we did not determine two submodes of the accumulation mode, but the dominant

394

accumulation mode for HULIS-C (Table 1) is close to the droplet mode determined during

395

intensive emissions of biomass burning aerosols in South China (Lin et al., 2010). In addition,

396

previous reports showed that accumulation mode was the main contributor to the total

397

HULIS-C (81%), in agreement with our findings (97% in winter and 83% in autumn).

398

Similarly, as found by Salma et al. (2013) for spring urban aerosols, the coarse mode HULIS-

399

C represented an important contribution to atmospheric HULIS-C during dry seasons,

400

especially in spring in Ljubljana, Slovenia (Table 1).

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ACCEPTED MANUSCRIPT The concentration ratio of WSOC/TC demonstrates that the contribution of WSOC to the

402

TC was much higher in winter aerosols (av ca. 50% in PM1) than autumn aerosols (ca. 20%)

403

(Figs. S1 a and b); the higher WSOC fraction in winter aerosols likely results from greater SOA

404

and levoglucosan output (Irei et al., 2014; Sandrini et al., 2016). HULIS-C accounted for a large

405

fraction of WSOC, i.e. about 60% for autumn particles and interestingly contributed similarly to

406

all size fractions. In winter, HULIS-C contributed on average 40% to WSOC with the highest

407

contribution in fine particles (ca. 50%). Similarly, as we measured for autumn aerosols, the

408

ratios of about 60% (Lin et al., 2010) and 63–76% (Salma et al., 2010) were found for typical

409

biomass burning aerosols; however, in fresh BB aerosols also lower values (e.g. 30%) were

410

observed (Lin et al., 2010), leading us to conclude that other sources were also important

411

contributors to WSOC. Interestingly, an average HULIS/WSOC ratio (56%) similar to what we

412

determined for PM0.1-1 autumn aerosols was also reported for fall fine PM0.1-1 aerosols collected

413

in a rural environment in Potsdam, NY (Pavlovic and Hopke, 2012). In addition, the WSOC and

414

HULIS-C concentrations for PM2 fraction (2 and 1.14 µg m-3, respectively) (Fig. 2) determined

415

in our study are in good agreement to those measured for PM2.5 (2.44 ± 0.41 and 1.33 ± 0.12 µg

416

m-3, respectively) (Pavlovic and Hopke, 2012).

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In summer, the contribution of WSOC to TC (Figs. S1c and d) was much higher than in

418

spring, especially in fine particles (ca. 50 and 30%, respectively), clearly the result of

419

increased secondary processing in summer (as photochemistry). Moreover, the distinct

420

difference between the spring and summer ratios of HULIS-C/WSOC (ca. 50% for spring, but

421

as low as 10% for summer fine particles (Figs. S1c and 1d)) also supports different origins of

422

WSOC in summer. Namely, in addition to the expected higher amounts of low molecular

423

weight carboxylic acids due to the vicinity of mostly oak and beech forests (e.g., formic,

424

acetic, and oxalic acids (Tsai and Kuo, 2013)), the meteorological conditions in August

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22

ACCEPTED MANUSCRIPT (Meeningen et al., 2016) also likely caused more intensive emissions of volatile plant

426

metabolites (isoprene, monoterpenes, etc.) followed by the formation of SOA via

427

photochemistry contributing to higher amounts of highly oxygenated species within WSOC

428

(Carlton et al., 2009). On the other hand, it is known that SPE-isolated HULIS fraction has a

429

pronounced hydrophobic and neutral character, being mostly composed of aliphatic and

430

branched structures and hydroxyl groups (primarily C9–C18), while highly oxygenated

431

species, such as levoglucosan, high MW tetracarboxylic acid, glucose, and galacturonic acid

432

will not be retained by extraction methods (Zheng et al., 2013). In the Po Valley, Italy, HULIS

433

contributed the least to WSOC in summer as well, but was the most abundant class for other

434

seasons (Decesari et al., 2001).

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23

ACCEPTED MANUSCRIPT

contribution (%)

50

a

40

coarse mode

PM mass WSOC HULIS-C

30 20

RI PT

10 0 100

accumulation mode

60 40

SC

contribution (%)

80

b

M AN U

20 0 30

c

ultrafine mode

20 15 10 5 0 winter

435

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contribution (%)

25

spring

summer

autumn

Fig. 4. Seasonal variability of contributions of PM, WSOC and HULIS-C mass concentrations

437

to coarse (a), accumulation (b), and ultrafine (c) mode (% of mass per mode; reference value

438

100% is the sum of ultrafine, accumulation, and coarse mode) for one selected sample set from

439

each season (winter: 7– 14 January 2015; spring: 15–25 May 2015; summer: 10–21 August and

440

autumn: 5–11 November 2015).

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441

24

ACCEPTED MANUSCRIPT 442

4.

Summary and conclusions The present study focused on the seasonal variabilities of carbon in humic-like

444

substances (HULIS-C) in ambient size-segregated water soluble organic aerosols collected by

445

a ten-stage low-pressure Berner impactor at an urban background environment in Ljubljana,

446

Slovenia. Our long-term measurements showed strong seasonal variations of size-resolved

447

WSOC and HULIS-C. The average total PM15.6 HULIS-C concentration during winter was 1.7

448

± 1.3 µg m-3 accounting for more than 30% of WSOC and contributing the most to fine particles

449

PM0.1-1. In contrast, in autumn, more than 50% of the total WSOC represented HULIS-C (1.3 µg

450

m-3) with almost 70% in PM0.1-1. The total HULIS-C concentrations for both spring and summer

451

were similar: 0.40 µg m-3 with about 60% in PM0.1-1 and 21% in PM>2 for spring and about 44%

452

in PM0.1-1 and 26% in PM>2 in summer (with some also in the ultrafine particles).

M AN U

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443

The mass size distribution patterns of PM and WSOC for all seasons were tri-modal, with

454

a predominant peak in the accumulation mode. The mass size distributions of HULIS-C and

455

WSOC were similar with nearly the same MMADs. Three modes were characteristic for all

456

seasons, except for winter where the HULIS-C size distribution was bimodal. Thus, for autumn

457

and winter, the dominant accumulation mode contributed 83 and 97%, respectively. In contrast,

458

during spring and summer the contributions of ultrafine (16 and 27%, respectively) and coarse

459

mode (27 and 16%, respectively) were also substantial, with a much lower contribution of the

460

dominant accumulation mode (ca. 57%).

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461

Primary emissions from wood burning were confirmed to be the most important source

462

of HULIS-C in the aerosol accumulation mode during the autumn and winter months. In

463

addition, due to higher RH conditions, secondary formation by heterogeneous reactions and/or

464

oligomerization through in-cloud processing also likely contributed to HULIS-C, as evidenced

465

by the shifting of MMADs to some larger sizes. The higher contribution of WSOC to TC in 25

ACCEPTED MANUSCRIPT fine particles in summer (ca. 50%) further supports enhanced secondary processing via

467

photochemistry. Moreover, the distinct difference between the spring and summer ratios of

468

HULIS-C/WSOC in fine particles (ca. 50% in spring, but only 10% in summer) implicate

469

additional sources in the summer (e.g., the more intensive emissions of volatile plant

470

metabolites followed by SOA formation via photochemistry), which did not contribute to

471

HULIS-C.

RI PT

466

The enhanced coarse mode contribution of WSOC, as well as HULIS-C, during spring

473

time emphasized that, besides soil erosion, additional primary sources may be responsible

474

(e.g., pollen and plant fragments). Alternatively, the reason for higher contributions of the

475

ultrafine mode to total HULIS-C levels, especially in summer, was most likely due to new

476

particle formation followed by growth to CCN-active sizes, a result of higher emissions of

477

BVOCs and more intensive photochemical reactions.

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Acknowledgements

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479

This work was supported by the European Commission and the Croatian Ministry of

481

Science, Education and Sports through Marie Curie FP7-PEOPLE-2011-COFUND, project

482

NEWFELPRO and by the Slovenian Research Agency (Contract no. P1-0034-0140). We

483

thank the Faculty of Chemistry and Chemical Technology, University of Ljubljana for the

484

permission to install the Berner impactor in the area of the faculty.

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Long-term measurements of water soluble HULIS carbon (HULIS-C) in ambient aerosols. Strong seasonal variability of size-resolved WSOC and HULIS-C. Except for winter, mass size distributions of HULIS-C were tri-modal. Main contribution to accumulation mode from wood burning in winter-autumn. Substantial contribution of ultrafine and coarse modes to HULIS-C in spring-summer.

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