evidence for ambient dark aqueous soa formation …...8096 a. p. sullivan et al.: evidence for...

Atmos. Chem. Phys., 16, 8095–8108, 2016www.atmos-chem-phys.net/16/8095/2016/doi:10.5194/acp-16-8095-2016© Author(s) 2016. CC Attribution 3.0 License.

Evidence for ambient dark aqueous SOA formation inthe Po Valley, ItalyAmy P. Sullivan1, Natasha Hodas2, Barbara J. Turpin3, Kate Skog4, Frank N. Keutsch4,5, Stefania Gilardoni6,Marco Paglione6, Matteo Rinaldi6, Stefano Decesari6, Maria Cristina Facchini6, Laurent Poulain7,Hartmut Herrmann7, Alfred Wiedensohler7, Eiko Nemitz8, Marsailidh M. Twigg8, and Jeffrey L. Collett Jr.11Colorado State University, Department of Atmospheric Science, Fort Collins, Colorado 80523, USA2California Institute of Technology, Division of Chemical Engineering, Pasadena, California 91125, USA3Rutgers University, Department of Environmental Sciences, New Brunswick, New Jersey 08901, USA4University of Wisconsin – Madison, Department of Chemistry, Madison, Wisconsin 53706, USA5Harvard University, Department of Chemistry and Chemical Biology, Cambridge, Massachusetts 02138, USA6Istituto di Scienze dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, 40129 Bologna, Italy7Leibniz Institute for Tropospheric Research, 04318 Leipzig, Germany8Centre for Ecology and Hydrology, Bush Estate, Penicuik, EH26QB, UK

Correspondence to: Amy P. Sullivan ([email protected])

Received: 31 October 2015 – Published in Atmos. Chem. Phys. Discuss.: 16 December 2015Revised: 25 March 2016 – Accepted: 8 June 2016 – Published: 4 July 2016

Abstract. Laboratory experiments suggest that water-solubleproducts from the gas-phase oxidation of volatile organiccompounds can partition into atmospheric waters where theyare further oxidized to form low volatility products, pro-viding an alternative route for oxidation in addition to fur-ther oxidation in the gas phase. These products can remainin the particle phase after water evaporation, forming whatis termed as aqueous secondary organic aerosol (aqSOA).However, few studies have attempted to observe ambient aq-SOA. Therefore, a suite of measurements, including near-real-time WSOC (water-soluble organic carbon), inorganicanions/cations, organic acids, and gas-phase glyoxal, weremade during the PEGASOS (Pan-European Gas-AeroSOls-climate interaction Study) 2012 campaign in the Po Valley,Italy, to search for evidence of aqSOA. Our analysis fo-cused on four periods: Period A on 19–21 June, Period Bon 30 June and 1–2 July, Period C on 3–5 July, and Period Don 6–7 July to represent the first (Period A) and second (Pe-riods B, C, and D) halves of the study. These periods werepicked to cover varying levels of WSOC and aerosol liquidwater. In addition, back trajectory analysis suggested all sitessampled similar air masses on a given day. The data collectedduring both periods were divided into times of increasing rel-ative humidity (RH) and decreasing RH, with the aim of di-

minishing the influence of dilution and mixing on SOA con-centrations and other measured variables. Evidence for localaqSOA formation was only observed during Period A. Whenthis occurred, there was a correlation of WSOC with organicaerosol (R2

= 0.84), aerosol liquid water (R2= 0.65), RH

(R2= 0.39), and aerosol nitrate (R2

= 0.66). Additionally,this was only observed during times of increasing RH, whichcoincided with dark conditions. Comparisons of WSOC withoxygenated organic aerosol (OOA) factors, determined fromapplication of positive matrix factorization analysis on theaerosol mass spectrometer observations of the submicronnon-refractory organic particle composition, suggested thatthe WSOC differed in the two halves of the study (PeriodA WSOC vs. OOA-2 R2

= 0.83 and OOA-4 R2= 0.04,

whereas Period C WSOC vs. OOA-2 R2= 0.03 and OOA-4

R2= 0.64). OOA-2 had a high O / C (oxygen / carbon) ra-

tio of 0.77, providing evidence that aqueous processing wasoccurring during Period A. Key factors of local aqSOA pro-duction during Period A appear to include air mass stagna-tion, which allows aqSOA precursors to accumulate in theregion; the formation of substantial local particulate nitrateduring the overnight hours, which enhances water uptake bythe aerosol; and the presence of significant amounts of am-monia, which may contribute to ammonium nitrate formation

Published by Copernicus Publications on behalf of the European Geosciences Union.

8096 A. P. Sullivan et al.: Evidence for ambient dark aqueous SOA formation

and subsequent water uptake and/or play a more direct rolein the aqSOA chemistry.

1 Introduction

The formation of secondary organic aerosol (SOA) remainsa major source of uncertainty in predicting organic aerosolconcentrations and properties that affect visibility, health,and climate (Kanakidou et al., 2005). SOA can form throughgas-to-particle partitioning of semi-volatile organic com-pounds formed from gas-phase oxidation of VOCs (volatileorganic compounds) (Seinfeld and Pankow, 2003). However,laboratory experiments and predictions suggest that water-soluble products from the gas-phase oxidation of VOCs canalso partition into atmospheric waters (i.e., clouds, fogs, andaerosol water) and react to form low volatility products.These products can remain in the particle phase after wa-ter evaporation, forming what is termed aqueous secondaryorganic aerosol (aqSOA) (e.g., Blando and Turpin, 2000; Al-tieri et al., 2006; Carlton et al., 2007; de Haan et al., 2009;Galloway et al., 2009; Ervens and Volkamer, 2010; Sun etal., 2010; Lee et al., 2012; Monge et al., 2012; Nguyen et al.,2012; Tan et al., 2012; Gaston et al., 2014).

Evidence that aqSOA may be a contributor to ambientSOA includes a gap between observed SOA and SOA pre-dicted by models that only include SOA formed via gas-phase oxidation and gas-particle partitioning (de Gouw et al.,2005; Heald et al., 2005). In addition, there is a tendency forsmog chamber experiments (generally conducted under dryconditions) to form SOA that is less oxygenated and hygro-scopic than ambient SOA, suggesting a missing source ofSOA (Aiken et al., 2008). In some locations, SOA surrogateshave been shown to be more strongly correlated with liquidwater than organic aerosol (Hennigan et al., 2008; Zhang etal., 2012), contrary to partitioning theory. Lastly, the abun-dance of ambient oxalate, an important product of aqSOAmechanisms (Carlton et al., 2007; Ervens et al., 2011), can-not be explained solely by gas-phase chemistry.

While it is important to study aqSOA, there have beenfew studies designed to observe aqSOA formation in the am-bient atmosphere. Therefore, a suite of near-real-time mea-surements was assembled, with the goal of identifying evi-dence of aqSOA formation in the Po Valley of Italy duringthe summer of 2012. A key measurement for this analysiswas water-soluble organic carbon (WSOC), which previousresearch has suggested is a good proxy for SOA (e.g., Sulli-van et al., 2004; Miyazaki et al., 2006; Kondo et al., 2007).Fog measurements in the Po Valley have been well docu-mented (e.g., Facchini et al., 1999; Fuzzi et al., 2002). Fogis unlikely to occur in the summer; but even in summer, theregion does have high relative humidity (60 to 80 %) and ispolluted, providing favorable conditions for aqSOA forma-tion in wet aerosols.

Herein, we present an approach for the investigation ofaqSOA formation in the ambient atmosphere and provide re-sults from such analyses. We examine WSOC as a function ofknown parameters likely to be associated with aqSOA, suchas relative humidity (RH), aerosol liquid water (ALW), andorganic aerosol (OA) concentration. We also look at the re-lationship of oxalate with sulfate and gas-phase glyoxal; ox-alate and sulfate are both produced by cloud processing andglyoxal is a known precursor to aqSOA formation (Yu et al.,2005; Tan et al., 2009; Ervens and Volkamer, 2010; Lim etal., 2010; Sorooshian et al., 2010). This study aims to iden-tify conditions conducive to aqSOA formation in this region.

2 Methods

Measurements were conducted within the Italian field cam-paign of the European Project PEGASOS (Pan-EuropeanGas-AeroSOls-climate interaction Study) in June andJuly 2012, focusing on the Po Valley. PEGASOS was aEuropean-wide study to address regional to global feedbacksbetween atmospheric chemistry and climate in different loca-tions as well as in the laboratory. The observations includedairborne measurements using a Zeppelin and multiple groundsites to study surface–atmosphere exchange, assess the ver-tical structure of the atmosphere, and study boundary layerphotochemistry. An auxiliary site was located in Bologna.Our measurements were made at the main ground site inSan Pietro Capofiume (SPC, Figs. 1 and 2). The SPC fieldstation is located approximately 40 km northeast of Bolognaand 30 km south of the Po River in flat terrain of agriculturalfields (Fig. 1).

Our measurements included running a Particle-into-LiquidSampler – Ion Chromatography (PILS-IC) (Orsini et al.,2003) system for inorganic cations, inorganic anions, andlight organic acids, and a Particle-into-Liquid Sampler –Total Organic Carbon (PILS-TOC) system (Sullivan et al.,2004) for particle-phase WSOC. A PILS collects the ambi-ent particles into purified water, providing the liquid sam-ple for analysis. Both systems operated at 15 L min−1 with a2.5 µm size-cut cyclone. Two annular denuders coated withsodium carbonate and phosphorous acid to remove inorganicgases were placed upstream of the PILS-IC, and for thePILS-TOC an upstream activated carbon parallel plate de-nuder (Eatough et al., 1993) was used to remove organicgases. In addition, for the PILS-TOC, a normally open actu-ated valve controlled by an external timer was periodicallyclosed every 2 h for 30 min, forcing the airflow through aTeflon filter before entering the PILS. This was to allow fora real background measurement to be determined. AmbientPM2.5 WSOC concentrations were calculated as the differ-ence between the filtered and non-filtered measurements. Thebackground was assumed to be constant between consecutivebackground measurements. Based on comparison with inte-grated quartz filter WSOC measurements, it appears the dif-

Atmos. Chem. Phys., 16, 8095–8108, 2016 www.atmos-chem-phys.net/16/8095/2016/

A. P. Sullivan et al.: Evidence for ambient dark aqueous SOA formation 8097

(a)

(b)

Figure 1. Maps created using Google Earth (version 7.1.5.1557) toshow the areas surrounding the (a) Bologna and (b) SPC samplingsites.

ference between filtered and non-filtered measurements wasbeing overestimated by ∼ 20 % before the carbon denuderwas replaced on 25 June. Therefore, the WSOC concentra-tions before this date have been corrected for this.

For the PILS-IC, the liquid sample from the PILS was splitbetween two Dionex ICS-1500 ion chromatographs. Thesesystems include an isocratic pump, a self-regenerating an-ion or cation SRS-ULTRA suppressor, and a conductivitydetector. The cations were separated using a Dionex Ion-Pac CS12A analytical (4× 250 mm) column with eluent of

46

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38

Latit

ude

(deg

rees

)

16128Longitude (degrees)

(a)San PietroCapofiume

(SPC)

Bologna

46

44

42

40

38

Latit

ude

(deg

rees

)


(b)

46

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40

38

Latit

ude

(deg

rees

)


(c) 46

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40

38

Latit

ude

(deg

rees

)


(d)

Figure 2. Characteristic 72 h air mass back trajectories for (a) Pe-riod A, (b) Period B, (c) Period C, and (d) Period D at the PE-GASOS ground sites of Bologna and SPC. All back trajectories arebased on the NOAA ARL HYSPLIT trajectory model.

18 mM methanesulfonic acid at a flow rate of 1.0 mL min−1.A Dionex IonPac AS15 analytical (4× 250 mm) column,employing an eluent of 38 mM sodium hydroxide at a flowrate of 1.5 mL min−1, was used for the anion analysis. Anew chromatogram was obtained every 30 min with a sampleloop fill time of 8 min. The limit of detection (LOD) for thevarious anions and cations was approximately 0.02 µg m−3.These inorganic PILS data were also used to determineALW from the Extended Aerosol Inorganics Model (E-AIM,Wexler and Clegg, 2002) run in a metastable state. More in-formation on the ALW calculations can be found in Hodas etal. (2014).

In the PILS-TOC, the liquid sample obtained from thePILS was pushed through a 0.2 µm PTFE liquid filter by aset of syringe pumps to ensure any insoluble particles wereremoved. The flow was then directed into a Sievers Model800 Turbo TOC (Total Organic Carbon) Analyzer. This an-

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alyzer works by converting the organic carbon in the liquidsample to carbon dioxide through chemical oxidation involv-ing ammonium persulfate and ultraviolet light. The conduc-tivity of the dissolved carbon dioxide formed is determined.The amount of organic carbon in the liquid sample is propor-tional to the measured increase in conductivity. The analyzerwas run in online mode, providing a 6 min integrated mea-surement of WSOC with a LOD of 0.1 µg C m−3.

Other measurements presented here include 2.5 min inte-grated organic aerosol (OA) concentrations determined bya High-Resolution Time-of-Flight Aerosol Mass Spectrom-eter (HR-ToF-AMS) (Drewnick et al., 2005; DeCarlo et al.,2006; Canagaratna et al., 2007). Positive matrix factorization(PMF) analysis of the AMS OA data was performed using theMultilinear Engine algorithm (ME-2) (Paatero, 1999) imple-mented within the toolkit Solution Finder (SoFi) developedby Canonaco et al. (2013). More details on the AMS ME-2analysis can be found in the Supplement and Figs. S1–S7.Gas-phase glyoxal was determined by the Madison Laser-Induced Phosphorescence (Mad-LIP) instrument (Huismanet al., 2008) at a time resolution of 51 s, hourly integrated am-monia was determined by a Monitor for AeRosols and Gases(MARGA) (ten Brink et al., 2007) in SPC and 30 min ammo-nia was determined by AiRRmonia (Erisman et al., 2001) inBologna, and relative humidity was collected at a 1 min timeresolution from a Vaisala weather transmitter WXT510. Alldata presented throughout are hourly averaged starting at thebeginning of the hour.

3 Results and discussion

3.1 Overview

As previously mentioned, WSOC is key to our analysis, sincein the absence of biomass burning (see Supplement for moredetails on the source apportionment of the AMS OA), themain contributor to WSOC has been found to be SOA (Sul-livan et al., 2006). Figure 3 shows the time series for WSOCduring the entire study at SPC. Overall, the WSOC con-centration was higher in the first half of the study (before25 June) compared with the second half. The WSOC con-centration peaked on 19 June then steadily decreased through22 June. During this time the concentration ranged fromabout 1 to 7 µg C m−3. During July, the WSOC was relativelyconstant at around 2 µg C m−3.

Therefore, our analysis will focus on comparing these twodifferent halves of the study. Given our interest in examiningfor evidence of aqSOA we picked four periods with vary-ing levels of WSOC and ALW. We also picked cases withboth sites sampling similar air masses on a given day. Pe-riod A represents the first half of the study and covers 19–21 June. Period A has elevated WSOC and moderate ALW.As indicated by the difference in the length of the back tra-jectories (Draxler and Rolph, 2013; Rolph, 2013) shown in

8

6

4

2

0WS

OC

(g

C m

)-3

6/21/2012 6/25/2012 6/29/2012 7/3/2012 7/7/2012Date and time (LT)

60

40

20

0ALW

(

10080604020

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RH

(%

)

403530252015T

empe

ratu

re (

o C)

Period A Period C (a)

(c)

(b)

(d)

Period B Period D

g m

)

-3

Figure 3. Time series of hourly averaged measured (a) WSOC,(b) calculated ALW, (c) RH, and (d) temperature at SPC. Any gapsin ALW are due to missing PILS-IC data. The dashed vertical linesindicate midnight local time (UTC+ 2). Periods A, B, C, and D arealso indicated.

Fig. 2, Period A occurred during the end of a stagnation. Pe-riod B (30 June, 1–2 July), Period C (3–5 July), and Period D(6–7 July) represent three different cases in the second halfof the study. Period B has moderate ALW, Period C has lowALW, and Period D has the highest ALW observed during thestudy. As indicated by Fig. 2, all three of these periods rep-resent typical background conditions influenced by regionaltransport, but with slightly different flow patterns. The flowsof Periods A and C are most similar. Due to changes in theWSOC concentrations and a non-consistent flow pattern ona daily basis, no periods between 23 and 29 June were exam-ined.

Cloud cover, as indicated from satellite measurements,showed that the days preceding Period A were generallycloud-free, whereas clouds developed west of the groundsites preceding Periods B, C, and D (not shown). The pres-ence of clouds was determined by examining satellite pic-tures set to the view of Europe at 11:00 LT provided bySat24 (http://en.sat24.com/en/eu). Additionally, wet scav-enging was not likely important as there was very little pre-cipitation at SPC or west of the site during the entire studyperiod. Only two cases of light rain lasting ∼ 30 min, whichoccurred on the afternoons of 23 June and 6 July, wererecorded at SPC. Table 1 provides a comparison of the var-ious concentrations and parameters observed during all fourperiods. With the exception of WSOC mentioned above, onlythe OA and NOx (nitrogen oxides) concentrations across allof Period A are noticeably elevated compared to Periods B,C, and D.


http://en.sat24.com/en/eu


Tabl

e1.

Ave

rage

conc

entr

atio

nsof

aero

sol

and

gas-

phas

esp

ecie

sal

ong

with

vari

ous

met

eoro

logi

cal

para

met

ers

obse

rved

duri

ngth

etim

esof

RH

incr

easi

ngan

dde

crea

sing

duri

ngPe

riod

sA

,B,C

,and

Dat

SPC

.

OA

WSO

CG

lyco

late

Ace

tate

Form

ate

Chl

orid

eSu

lfate

Oxa

late

Nitr

ate

Sodi

umA

mm

oniu

mPo

tass

ium

Mag

nesi

umC

alci

umA

LW(µ

gm−

3 )(µ

gC

m−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )(µ

gm−

3 )

Peri

odA

RH

incr

easi

ng8.

934.

730.

280.

400.

430.

133.

490.

242.

91N

AN

AN

AN

AN

A6.

81

Peri

odA

RH

decr

easi

ng9.

635.

090.

300.

330.

470.

173.

230.

235.

61N

AN

AN

AN

AN

A7.

29

Peri

odB

RH

incr

easi

ng4.

062.

870.

220.

240.

240.

093.

220.

121.

670.

011.

040.

430.

100.

374.

21

Peri

odB

RH

decr

easi

ng3.

782.

890.

220.

240.

230.

092.

690.

111.

560.

011.

040.

480.

090.

134.

34

Peri

odC

RH

incr

easi

ng2.

051.

550.

240.

280.

230.

112.

800.

131.

180.

040.

920.

510.

110.

262.

89

Peri

odC

RH

decr

easi

ng2.

011.

540.

220.

320.

230.

102.

750.

121.

280.

040.

940.

540.

090.

062.

64

Peri

odD

RH

incr

easi

ng2.

891.

920.

170.

180.

210.

113.

380.

121.

310.

021.

070.

480.

100.

324.

10

Peri

odD

RH

decr

easi

ng3.

021.

990.

190.

190.

240.

144.

890.

133.

560.

032.

000.

550.

100.

207.

90

Ozo

neN

Ox

SO2

Ben

zene

Tolu

ene

Xyl

ene

Gly

oxal

TR

H(µ

gm−

3 )(µ

gm−

3 )(p

pb)

(µg

m−

3 )(µ

gm−

3 )(µ

gm−

3 )(p

pb)

(◦C

)(%

)

Peri

odA

RH

incr

easi

ng47

.42

28.9

00.

650.

211.

210.

260.

0524

.47

64.4

9

Peri

odA

RH

decr

easi

ng63

.70

17.7

51.

140.

271.

780.

340.

0926

.09

57.6

6

Peri

odB

RH

incr

easi

ng76

.610

.94

0.68

0.19

0.83

0.53

0.06

26.7

460

.87

Peri

odB

RH

decr

easi

ng51

.69.

300.

690.

291.

430.

660.

0726

.261

.20

Peri

odC

RH

incr

easi

ng61

.29

9.72

0.40

0.17

1.18

0.40

0.05

23.3

160

.60

Peri

odC

RH

decr

easi

ng75

.40

8.08

0.51

0.17

1.11

0.44

0.07

25.0

253

.88

Peri

odD

RH

incr

easi

ng87

.21

8.93

0.30

0.12

0.52

0.23

0.05

25.6

363

.45

Peri

odD

RH

decr

easi

ng93

.73

5.12

0.38

0.15

0.85

0.28

0.07

27.3

254

.92



Each period will be examined in terms of the times whenRH increased from 40 to 70 % (times of RH increasing) andthen when the RH decreased from 70 back to 40 % (times ofRH decreasing). This was done to try to diminish the influ-ence of dilution and mixing on SOA concentrations and mea-surements of other key variables, since measurements of aconserved tracer were not available. The idea behind this ap-proach was that the times of RH increasing would representa time with a stable nocturnal boundary layer. The switch inregimes on average occurs at 05:00 LT, but varied from 03:00to 08:00 LT. Therefore, the times of RH increasing primarilyoccurred in the dark. Table S1 in the Supplement providesa list of the exact times used for the times of RH increasingand decreasing in each period.

We first will compare all four periods to examine for ev-idence of aqSOA. Then we will provide a further examina-tion of aqueous aerosol tracers and WSOC for the two pe-riods with similar air flow (Periods A and C). Our analysiswill largely be based on least squares regression correlationanalysis to examine the relationship between various speciesand provide a general approach to examine for evidence ofaqSOA. We have chosen to examine R2 values as opposedto p values since R2 values can provide a useful tool forexplaining the amount of observed variance in a dependentvariable that is explained by variation in an independent vari-able. p values merely indicate whether a relationship is sta-tistically significant without information about the amountof variance explained. To help categorize the fraction of vari-ance explained, we consider a high correlation as R2 valuesgreater than 0.7, a moderate correlation as R2 values between0.3 to 0.7, and a low correlation as R2 values less than 0.3.

3.2 Evidence for aqSOA

WSOC is shown as a function of RH for the times of RHincreasing (Fig. 4a and b) and decreasing (Fig. 4c and d)during Periods A, B, C, and D. For Periods B, C, and D,WSOC had no relationship with RH. Only during the timesof increasing RH did Period A have a relationship of increas-ing WSOC with RH, consistent with local aqSOA forma-tion. This can further be illustrated by examining the cor-relation of WSOC vs. organic aerosol (OA), aerosol liquidwater (ALW), and RH for Periods A, B, C, and D during thetimes of RH increasing (Figs. 5 and S8). In general, WSOChad a strong relationship with OA, but only Period A addi-tionally had a moderate correlation of the WSOC with bothALW (Period A R2

= 0.65 vs. Period B R2= 0.15, Period C

R2= 0.29, and Period D R2

= 0.01) and RH (Period AR2= 0.39 vs. Period B R2

= 0.01, Period C R2= 0.12, and

Period D R2= 0.07). The good correlation between WSOC

and ALW is in agreement with a previous smog chamberstudy that found that ALW is a key determinant of SOA yield(Zhou et al., 2011). This also supports a recent study thatobserved ambient aqSOA formation during the nighttime asevident by the increased partitioning of gas-phase WSOC

RH increasing

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OC

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100806040200

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A C

3 7 2 14

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0

WS

OC

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OC

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A C 7 5 4 15

7 6 11 3

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0

WS

OC

(

100806040200

RH (%)

B D

(d)8 2 3

4 3 7 2

g C

m

-3)

g C

m

-3)

g C

m

-3)

Figure 4. Hourly averaged WSOC as a function of RH for (a) Pe-riods A and C and (b) Periods B and D during the times of RHincreasing and (c) Periods A and C and (d) Periods B and D dur-ing the times of RH decreasing at SPC. The WSOC was binnedinto 10 % RH bands starting at 40 % RH. The error bars representthe standard deviation at each bin. Numbers above or below pointsrepresent the number of data points in each bin.

to the particle phase with increasing RH (El-Sayed et al.,2015). The study by El-Sayed et al. (2015) found the increasein the fraction of total WSOC in the particle phase (Fp) atthe two highest RH levels (70–80 %, > 80 %) to be statis-tically significant compared to the Fp values at RH < 60 %.The main focus of their work was to investigate whether theuptake of gas-phase WSOC to aerosol water occurs throughreversible or irreversible pathways. The data suggested theaqSOA was formed irreversibly. We investigate this with ourdata in Sect. 3.3.2.

Figures 6 and S9 show the correlation of WSOC vs. ni-trate, oxalate, and sulfate for the times of RH increasing. Ni-trate and WSOC are strongly correlated only during the timesof RH increasing for Period A. Early morning nitrate peakswere observed at SPC during the first part of the study, butwere absent at the upwind Bologna site (Fig. 7). The occur-rence of these peaks overlaps with Period A. (Note, the ni-trate event observed on 6 and 7 July during Period D will bediscussed in Sect. 3.4.) This additionally suggested that thenitrate formation or the ammonium–nitrate–ammonia–nitricacid equilibrium at SPC was locally controlled since theback trajectory analysis indicated both the SPC and Bolognasites were sampling similar upwind air masses to each otherin each period (Fig. 2). Therefore, the correlation with lo-cally formed particulate nitrate suggests local formation ofWSOC. (Note, increased nitrate also results in higher ALW



Period A Period C 8

6

4

2

0

WS

OC

(g

C m

-3)

12840

OA (g m -3)

R2 = 0.84

(a)

4

3

2

1

0

WS

OC

(

43210

OA (

R2 = 0.66

(b)

8

6

4

2

0

WS

OC

(

2520151050

ALW (

R2 = 0.65

(c)

4

3

2

1

0

WS

OC

(

6543210

ALW (

R2 = 0.15

(d)

8

6

4

2

0

WS

OC

(

100806040200RH (%)

R2 = 0.39

(e)

4

3

2

1

0

WS

OC

(

100806040200RH (%)

R2 = 0.01

(f)

g C

m -3

)g

C m

-3)

g C

m -3

)

g m -3)

g m -3) g m -3)

g C

m -3

)

g C

m -3

)

Figure 5. Correlation of hourly averaged WSOC vs. OA for (a) Pe-riod A and (b) Period C, ALW for (c) Period A and (d) Period C,and RH for (e) Period A and (f) Period C at SPC. All plots are forduring the times of RH increasing.

at the same RH.) This argues that aqSOA formation was pre-dominately local during Period A.

3.3 Further examination of oxalate, sulfate, andWSOC during Periods A and C

3.3.1 Oxalate and sulfate

To help better understand the potential for aqSOA forma-tion, correlations with oxalate and sulfate can be examined.Oxalate and sulfate are known tracers for aerosol formationthrough cloud processing (Yu et al., 2005; Sorooshian et al.,2010), although sulfate does also have a substantial, albeitslower, gas-phase formation pathway (Seinfeld and Pandis,2006). As shown in Fig. 8a and b for Periods A and C, dur-ing both the times of RH increasing and decreasing, thereis a positive linear relationship between oxalate and sulfate(R2 ranged from 0.39 to 0.68). The association between ox-alate and sulfate but not oxalate and WSOC in Period A sug-gests that the local aqSOA formed in wet aerosols during Pe-riod A has little effect on oxalate. This result supports the

Period A Period C 8

6

4

2

0

WS

OC

(g

C m

-3)

86420

Nitrate (g m -3)

R2 = 0.66

(a)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

WS

OC

(

2.01.51.00.50.0

Nitrate (

R2 = 0.01

(b)

8

6

4

2

0

WS

OC

(

0.40.30.20.10.0

Oxalate (

R2 = 0.16

(c)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

WS

OC

(

0.200.100.00

Oxalate (

R2 = 0.77

(d)

8

6

4

2

0

WS

OC

(

6543210

Sulfate (

R2 = 0.07

(e)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

WS

OC

(

543210

Sulfate (

R2 = 0.04

(f)

g C

m -3)

g C

m -3)

g C

m -3)

g C

m -3)

g C

m -3)

g m -3)

g m -3)g m -3

)

g m -3) g m -3

)

Figure 6. Correlation of hourly averaged WSOC vs. nitrate for(a) Period A and (b) Period C, oxalate for (c) Period A and (d) Pe-riod C, and sulfate for (e) Period A and (f) Period C at SPC. Allplots are for during the times of RH increasing.

12

10

8

6

4

2

0

Nitr

ate

(g

m -3

)

6/21/2012 6/26/2012 7/1/2012 7/6/2012

Date and time (LT)

12

10

8

6

4

2

0

(a)

(b)

Period A Period CPeriod B Period D

Figure 7. Time series of hourly averaged AMS nitrate observed at(a) SPC and (b) Bologna. The dashed vertical lines indicate mid-night local time (UTC+ 2). Periods A, B, C, and D are also indi-cated.



RH increasing RH decreasing 0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Oxa

late

(g

m -3)

543210

Sulfate (g m -3 )

Period A

R2 = 0.56

Period C

R2 = 0.39(a)

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Oxa

late

(

43210

Sulfate (

Period A

R2 = 0.52

Period C

R2 = 0.68(b)

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Oxa

late

(

80x10-36040200

Glyoxal (ppb)

Period A

R2 = 0.10

Period C

R2 = 0.05(c)

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Oxa

late

(

0.120.080.040.00Glyoxal (ppb)

Period A

R2 = 0.01

Period C

R2 = 0.44(d)

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Oxa

late

(

151050

ALW (

Period A

R2 = 0.01

Period C

R2 = 0.07(e)

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Oxa

late

(

12840

ALW (

Period A

R2 = 0.17

Period C

R2 = 0.04(f)

g m

-3)

g m

-3)

g m

-3)

g m

-3)

g m

-3)

g m -3 )

g m -3 )g m -3 )

Figure 8. Correlation of hourly averaged oxalate vs. sulfate for Pe-riods A and C during the times of RH (a) increasing and (b) de-creasing, gas-phase glyoxal for Periods A and C during the times ofRH (c) increasing and (d) decreasing, and ALW for Periods A andC during the times of RH (e) increasing and (f) decreasing at SPC.

supposition that oxalate is not a universal marker for aqSOA.This is further illustrated in our data by examining the cor-relation of oxalate vs. gas-phase glyoxal, a known precursorfor aqSOA (Tan et al., 2009; Ervens and Volkamer, 2010;Lim et al., 2010), and ALW (Fig. 8c–f). Laboratory experi-ments suggest a relationship between oxalate and gas-phaseglyoxal when there is in-cloud processing as oligomers havebeen proposed to be the dominant products from processingin aerosol water when hydroxyl radical concentrations are onthe order of 10−12 M (Lim et al., 2010; Tan et al., 2010). Ox-alate could be produced in aerosol water at lower hydroxylradical concentrations, such as 10−13 M, due to insufficientorganic radical concentrations for oligomer formation (Leeet al., 2011). Although the hydroxyl radical concentrationsare unknown, there is only a relationship between oxalateand gas-phase glyoxal for Period C during times of RH de-creasing (R2

= 0.44), which is when clouds were observedwest of the site. In addition, there is no important relation-ship observed between oxalate and ALW for either period(all R2 < 0.17).

86420

OO

A o

r W

SO

C (g

C m

-3)

6/21/2012 6/25/2012 6/29/2012 7/3/2012 7/7/2012

Date and time (LT)

86420864208

6

4

2

0

(a)

(b)

Period A Period C

(c)

(d)

WSOC OOA-1 OOA-2 OOA-3 OOA-4

Period B Period D

Figure 9. Time series of hourly averaged WSOC with AMS ME-2 factors (a) OOA-1, (b) OOA-2, (c) OOA-3, and (d) OOA-4 atSPC. The units for each factor have been converted from µg m−3

to µg C m−3 using their calculated OM / OC ratio (OOA-1= 1.81,OOA-2= 2.15, OOA-3= 2.13, and OOA-4= 1.62). The dashedvertical lines indicate midnight local time (UTC+ 2). Periods A,B, C, and D are also indicated.

3.3.2 WSOC

The above analysis suggests that the majority of the WSOCobserved during the first half of the study, as illustrated byPeriod A, is formed locally via chemistry in aerosol liquidwater. Clearly, WSOC in the second half of the measure-ments appears to be different and to derive from differentsources. As illustrated by Period C, the WSOC during thistime is likely more regional, with contributions from gas-to-particle partitioning and possibly in-cloud aqSOA.

To further explore this idea of different types of WSOC,the WSOC observations were compared to positive matrixfactorization (PMF) analysis of the AMS OA data collectedat SPC. Five factors, one HOA (hydrocarbon-like OA) andfour OOA (oxygenated OA), were found. The four OOA fac-tors include one semi-volatile type (OOA-1) and three lowvolatility types (OOA-2, OOA-3, and OOA-4). More detailson the AMS ME-2 analysis can be found in the supportinginformation.

As shown in Fig. 9, the measured WSOC from the firsthalf of the study is dominated by OOA-2 and the secondhalf by OOA-4. This can be further illustrated by looking atthe correlation of WSOC vs. OOA-2 and OOA-4 during thetimes of RH increasing for Periods A and C (Fig. 10). TheWSOC in Period A is most strongly correlated with OOA-2(R2= 0.83) and in Period C with OOA-4 (R2

= 0.64).To estimate how each AMS ME-2 factor contributed to

WSOC and what fraction of each factor was water-soluble,a multilinear regression analysis was tentatively performedusing the method proposed by Timonen et al. (2013). The re-sults are shown in Table S2 and Fig. 11. This approach seeks



Period A Period C 8

6

4

2

0

WS

OC

(g

C m

-3)

0.60.40.20.0

OOA-1 (g C m -3)

R2 = 0.41

(a)

4

3

2

1

0

WS

OC

(

0.80.60.40.20.0

OOA-1 (

R2 = 0.02

(b)

8

6

4

2

0

WS

OC

(

43210

OOA-2 (

R2 = 0.83

(c)

4

3

2

1

0

WS

OC

(

40x10-33020100

OOA-2 (

R2 = 0.03

(d)

8

6

4

2

0

WS

OC

(

3.02.01.00.0

OOA-3 (

R2 = 0.06

(e)

4

3

2

1

0

WS

OC

(

1.00.80.60.40.20.0

OOA-3 (

R2 = 0.01

(f)

8

6

4

2

0

WS

OC

(

2.01.51.00.50.0

OOA-4 (

R2 = 0.04

(g)

4

3

2

1

0

WS

OC

(

1.20.80.40.0

OOA-4 (

R2 = 0.64

(h)

g C

m -3)

g C

m -3)

g C

m -3)

g C

m -3)

g C

m -3)

g C

m -3)

g C

m -3)

g C m -3)

g C m -3)g C m -3)

g C m -3) g C m -3)

g C m -3)g C m -3)

Figure 10. Correlation of hourly averaged WSOC vs. AMS ME-2factors OOA-1 for (a) Period A and (b) Period C, OOA-2 for (c) Pe-riod A and (d) Period C, OOA-3 for (e) Period A and (f) Period C,and OOA-4 for (g) Period A and (h) Period C at SPC. All plots arefor during the times of RH increasing.

to reproduce the total WSOC as a linear combination of thedifferent factors, whilst minimizing the residuals and, unlikein Timonen et al. (2013), capping the individual factor con-tributions at 1 to allow conservation of the carbon mass. Theregression analysis was carried out with a zero intercept likein Timonen et al. (2013), as well as with a non-zero inter-cept to account for possible instrumental biases between theAMS and PILS methods. Only the four OOA factors wereconsidered, while HOA was assumed to be completely insol-uble. All concentrations are in carbon mass units, which forthe AMS factors were derived from organic mass concentra-tions through factor-specific OM / OC ratios. The results ofthe regression are reported for the whole PILS measurementperiod and also for Periods A and C separately.

Figure 11. Time series of hourly averaged AMS ME-2 OOA fac-tors, WSOC measured, and WSOC reconstructed for the wholemeasurement period (top), Period A (bottom left), and Period C(bottom right) at SPC. The units for each OOA factor havebeen converted from µg m−3 to µg C m−3 using their calculatedOM / OC ratio.

The results for the whole measurement period indicate thatthe largest contributions to the WSOC must be attributed tothe OOA types which were simply the most abundant (OOA-3 and OOA-4), but the water-soluble fractions as reflectedin the regression coefficients were greatest for OOA-2 andOOA-4, in agreement with their high correlation coefficientswith WSOC. Interestingly, OOA-2 and OOA-4 are also thefactors possessing the highest O / C ratios (0.77 and 0.76, re-spectively), with respect to the other two (O / C= 0.48 forOOA-1 and 0.36 for OOA-3). Therefore, in this study thefactor-specific WSOC fractions seem to be related to the oxy-gen contents measured by the AMS.

The multilinear regression analysis performed on the Pe-riod A measurements suggests that the largest water-solublefractions are exhibited by OOA-1 and OOA-2, whose con-centrations were observed to increase along with RH andWSOC for all the days in this period of the campaign.Due to the very different absolute average concentrations,the second factor (OOA-2) provided the largest contribu-tion to WSOC, accounting for more than one-third of the to-tal water-soluble organic carbon concentration. Interestingly,the diurnal trend of OOA-1 indicated that its partitioning tothe aerosol phase was largely reversible, and its concentra-tions declined steeply in the late morning hours when RHand ALW decreased (Fig. 12a). In the same hours of theday, the OOA-2 concentrations were largely unaffected byRH, indicating (a) that OOA-2 mainly accounted for oxidizedcompounds stable in the aerosol phase and (b) that boundarylayer growth is not the reason for the decrease in OOA-1 asthis should have affected all factors. OOA-1 and OOA-2 can



Period A

12

10

8

6

4

2

0

Nitrate (

20151050Hour of the day

4

3

2

1

0

WS

OC

(

90

80

70

60

50

40

30

20

RH

(%)

10

8

6

4

2

0

ALW

(g m

)

-3

0.6

0.5

0.4

0.3

0.2

0.1

0.0

OO

A-1

(

2.0

1.5

1.0

0.5

0.0

OO

A-2

(g

C m

)

-3

40

35

30

25

20

15

Tem

perature ( oC)

ALW Nitrate OOA-1 OOA-2 RH T WSOC

(a)

Period C 2.0

1.5

1.0

0.5

0.0

Nitrate (

20151050Hour of the day

2.5

2.0

1.5

1.0

0.5

0.0

WS

OC

(

90

80

70

60

50

40

30

20

RH

(%)

4

3

2

1

0

ALW

(

0.5

0.4

0.3

0.2

0.1

0.0

OO

A-1

(

50x10-3

40

30

20

10

0

OO

A-2

(

40

35

30

25

20

15

Tem

perature ( oC)

ALW Nitrate OOA-1 OOA-2 RH T WSOC

(b)

g C

m

)-3

g C

m

)-3

g C

m

)-3

g C

m

)-3

g C

m

)-3

g m

)

-3

g m

)-3

g m

)

-3

Figure 12. Diurnal profile of WSOC, OOA-1, OOA-2, RH, temper-ature, ALW, and nitrate for (a) Period A and (b) Period C at SPC.

therefore be hypothesized as two aging stages of aqSOA for-mation during Period A.

Interestingly, some OOA-2 is also produced in Periods Band D. Although the concentration levels of OOA-2 observedare similar between Periods A and D, OOA-2 concentrationsare much more sustained across the day in Period A. In ad-dition, as illustrated in the diurnal profiles for these periods(Fig. S10) the OOA-2 follows along more closely with OOA-1, RH, and ALW in Periods B and D, likely due to the dif-ferences in meteorology and/or chemistry of these periodscompared to Period A. Regardless of these differences, theobservations all still point to the strong relationship betweenOOA-1, OOA-2, and ALW.

The results obtained for Period C show again that thegreatest coefficients (hence the largest water-soluble frac-tions) were found for OOA-2 and OOA-4. However, due toits very small concentrations in this period, OOA-2 provideda negligible contribution to WSOC (1 %), while OOA-4 wasestimated to account for more than half of the WSOC carboncontent. The examination of time trends indicates that OOA-4 is mainly a background component of the aerosol, showingno appreciable increase at the time when RH increased fora few hours on the mornings of 5 and 6 July. Similar to Pe-riod A, here again the times when RH and ALW were highshowed relatively high concentrations of OOA-1 (Fig. 12b),which represented an additional (though small compared toOOA-4) contribution to WSOC. Period C provides a casewhere significant OOA-1 is formed, but not OOA-2.

Overall, whilst not without uncertainty, the above findingssupport the idea that two different types of WSOC occurred

during these two different periods. They also support the ideathat aqueous processing is dominating during the times ofRH increasing during Period A, and OOA-2 represented themost important component. The high O / C ratio of OOA-2 is expected for SOA formed through aqueous-phase reac-tions, because precursors are water-soluble and thus have lowcarbon numbers and high O / C ratios. Average O / C ratiosof ∼ 0.7 to 1.1 have been observed in the oligomeric prod-ucts formed from laboratory experiments involving hydroxylradical oxidation and/or aqueous photolysis of methylgly-oxal, glycolaldehyde, and phenolic compounds (Altieri et al.,2008; Tan et al., 2009; Perri et al., 2010; Sun et al., 2010).The high O / C ratios observed for the other main WSOCcomponent, OOA-4, which dominates Period C, could be ex-plained by extensive aging of non-aqueous SOA (Lambe etal., 2011). However, in-cloud aqueous-phase reactions couldhave occurred upwind of the Po Valley, as indicated by theoccurrence of oxalate and clouds previously discussed. Ourmeasurements are fully consistent, in indicating that OOA-4was mainly transported to the site and was not a product ofthe local aqueous-phase heterogeneous chemistry in the PoValley atmospheric surface layer.

3.4 Conditions for local aqSOA

What leads to strong local aqSOA formation during PeriodA at SPC? High ALW was present throughout the study(Fig. 3b). It was observed that the days with the highest ALWalso had the highest aerosol loading in the lowest layers ofthe atmosphere. However, no other day outside of Period A,except for 23 June, had a relationship of WSOC with RH dur-ing the times of RH increasing. This suggests that high ALWor aerosol loading alone are not sufficient for local aqSOAformation.

As previously mentioned, during Period A early morningnitrate peaks were observed only at the SPC ground site andnot at the urban site. However, just the presence of high ni-trate (above 2 µg m−3) does not seem to lead to aqSOA asno relationship of WSOC as a function of RH was observedon 6 and 7 July (Period D) when nitrate in concentrationssimilar to those of Period A were observed at SPC. Interest-ingly, the nitrate observed on these days was also observedin Bologna (Fig. 7). The timing of the peak nitrate concen-tration also differed from Period A; it occurred later in themorning, around 07:00 LT, whereas during Period A nitratepeaked around midnight or 01:00 LT and then again around07:00 LT. This suggests that the presence and timing of ele-vated nitrate, which is a strong determinant of ALW, may beimportant for local aqSOA production and resulting WSOCaerosol concentrations in this region.

As previously mentioned, an examination of possible gas-phase precursors (e.g., aromatic VOCs and glyoxal, Table 1)shows no noticeable decline in concentration from the first tosecond half of the measurement period. Therefore, a possi-ble explanation for the difference between Period A and the



80

60

40

20

0

Am

mo

nia

(g

m -3

)

6/21/2012 6/26/2012 7/1/2012 7/6/2012

Date and time (LT)

80

60

40

20

0

(a)

(b)

Period A Period CPeriod B Period D

Figure 13. Time series of hourly averaged ammonia observed at(a) SPC and (b) Bologna. The dashed lines indicate midnight localtime (UTC+ 2). Periods A, B, C, and D are also indicated.

other periods is meteorology. Period A featured an anticy-clonic condition that led to air stagnation; the other periodsfeatured stronger transport and ventilation. Therefore, duringPeriods B, C, and D intermediate products that are needed toform appreciable concentrations of aqSOA are less likely toquickly accumulate in the local boundary layer.

It is possible that another key ingredient in the chemistryis ammonia. Recent studies have suggested possible aqSOAformation processes mediated by ammonia and other atmo-spheric bases (Galloway et al., 2009; Nozière et al., 2009;Ortiz-Montalvo et al., 2014; Yu et al., 2011). Ammonia isprevalent in the Po Valley due to agricultural activities. Dur-ing Period A, high ammonia concentrations (greater than∼ 30 µg m−3) were observed only at SPC (Fig. 13a).

Overall, the data suggest that local aqSOA production dur-ing the stagnation of Period A is not due to cloud processing.Our results also suggest that this aqueous chemistry occursin the dark, which likely provides the favorable low tem-peratures and high RH for nitrate aerosol and ALW (Ho-das et al., 2014). Based on other measurements at SPC, thestagnation conditions and elevated nitrate around midnightoccurred each day from 14 June through 23 June, suggest-ing that the local aqSOA formation actually commenced 5days earlier. When all these conditions were met, each day∼ 1 µg C m−3 of new WSOC (determined as the change inWSOC concentration during the times of RH increasing) canbe attributed to this process.

4 Summary

Measurements were conducted during the PEGASOS studyin the Po Valley of Italy during June and July 2012 in SanPietro Capofiume (SPC). The goal was to look for evidenceof aqSOA in the ambient atmosphere. Measurements in-

cluded near-real-time WSOC (a good proxy for SOA), in-organic anions/cations, and organic acids. The data were an-alyzed in terms of the times when RH increased from 40 to70 % (times of RH increasing) and then when the RH de-creased from 70 back to 40 % (times of RH decreasing) inorder to diminish influences from dilution and mixing on am-bient measurements. The analysis focused on four periods:Period A on 19–21 June, Period B on 30 June, 1–2 July, Pe-riod C on 3–5 July, and Period D on 6–7 July.

Evidence for local aqSOA formation in wet aerosols wasobserved during Period A. When this occurred there was acorrelation of WSOC with OA, ALW, RH, and nitrate. Addi-tionally, this was only observed during times of RH increas-ing, suggesting the aqSOA was formed in the dark. The aq-SOA formation is thought to be local because elevated ni-trate, the driver for aerosol water, was only observed at themain ground site in SPC even though the auxiliary site inBologna was sampling similar upwind air masses at the time.

A comparison of Periods A and C suggested Period Cdiffered from Period A. The WSOC during Period C waslikely formed regionally. Interestingly, during Period C aswell as Period A, a correlation was found between oxalateand sulfate. This suggests that oxalate concentrations werenot strongly affected by local aqSOA formation. More im-portantly, it indicates that oxalate is not a good universalmarker for aqSOA.

A comparison of WSOC with the AMS PMF OOA factorsshowed that Period A featured high O / C ratios, consistentwith aqSOA formation. However, they also reinforce the con-clusion that the composition of the WSOC differed betweenthe two halves of the study. Periods A and C were dominatedby two different OOA factors, OOA-2 (locally produced) andOOA-4 (long-range transported), respectively.

Overall, by examining the conditions observed in PeriodA, the data suggest that the local aqSOA formation observedis not due to cloud processing and occurs in the dark. Thetiming of elevated nitrate concentrations is critical (aroundmidnight local time) to provide the liquid water reservoirneeded for aqueous chemistry. Approximately 1 µg C m−3 ofnew WSOC was formed through this process each day theseconditions were met, indicating the importance of aqSOA asa source of ambient OA in this region.

5 Data availability

The data are available by request from the individual re-search groups contributing to this work. PILS data are fromA. P. Sullivan, AMS data are from L. Poulain, glyoxaldata are from K. Skog/F. Keutsch, MARGA data are fromE. Nemitz/M. M. Twigg, and meteorological data are fromS. Decesari.

The Supplement related to this article is available onlineat doi:10.5194/acp-16-8095-2016-supplement.


http://dx.doi.org/10.5194/acp-16-8095-2016-supplement


Acknowledgements. We acknowledge funding from the NationalScience Foundation under projects AGS-1050052, AGS-1052611,and AGS-1051338. Measurements at SPC were also funded by theEuropean Union FP7 project PEGASOS (FP7-ENV-2010/265148)and by the Regione Emilia Romagna (project SUPERSITODRG no. 428/10). The authors thank the European Union FP7ÉCLAIRE (FP7-ENV-2011/282910) project for funding theammonia measurements in Bologna, the Energy Research Centreof the Netherlands (ECN) for providing the MARGA instrumentat SPC, and C. DiMarco, M. Blom, S. Leeson, T. Hutchings,C. Braban, and L. Giulianelli for supporting the ammonia mea-surements. The authors gratefully acknowledge the NOAA AirResources Laboratory (ARL) for the provision of the HYS-PLIT transport and dispersion model and/or READY website(http://www.arl.noaa.gov/ready.html) used in this publication.

Edited by: R. McLaren

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