Ion composition of coarse and fine particles in Iasi, north-eastern Romania: Implications for aerosols chemistry in the area

Atmospheric Environment 45 (2011) 906e916

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Ion composition of coarse and fine particles in Iasi, north-eastern Romania: Implications for aerosols chemistry in the area Cecilia Arsene a, *, Romeo Iulian Olariu a, Pavlos Zarmpas b, Maria Kanakidou b, Nikolaos Mihalopoulos b a b

Al.I Cuza University of Iasi, Faculty of Chemistry, Department of Chemistry, 11 Carol I, 700506 Iasi, Romania University of Crete, Department of Chemistry, Environmental Chemical Processes Laboratory, Voutes, 71003 Heraklion, Greece

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 August 2010 Received in revised form 2 November 2010 Accepted 10 November 2010

Atmospheric loadings of the aerosols coarse (particles of AED > 1.5 mm) and fine fractions (particles of AED < 1.5 mm) were determined in Iasi, north-eastern Romania from January 2007 to March 2008.
þ þ 2þ
2
þ and Mg2þ) were Concentrations of water soluble ions (SO2
4 , NO3 , Cl , C2O4 , NH4 , K , Na , Ca measured using ion chromatography (IC). In the coarse particles, calcium and carbonate are the main
þ
ionic constituents (∼65%), whereas in the fine particles SO2
4 , NO3 , Cl and NH4 are the most abundant. Temperature and relative humidity (RH) associated with increased concentrations of specific ions might be the main factors controlling the aerosol chemistry at the investigated site. From August 2007 to March 2008 high RH (as high as 80% for about 82% of the investigated period) was prevailing in Iasi and the collected particles were expected to have deliquesced and form an internal mixture. We found that in 2
fine particles ammonium nitrate (NH4NO3) is important especially under conditions of NHþ 4 /SO4 ratio higher than 1.5 and high RH (RH above deliquescence of NH4Cl, NH4NO3 and (NH4)2SO4). At the investigated site large ammonium artifacts may occur due to inter-particle interaction especially under favorable meteorological conditions. A methodology for estimating the artifact free ambient ammonium concentration is proposed for filter pack sampling data of deliquesced particles. Nitrate and sulfate ions in coarse particles are probably formed via reactions of nitric and sulfuric acid with calcium carbonate and sodium chloride which during specific seasons are abundant at the investigated site. In the fine mode sulfate concentration maximized during summer (due to enhanced photochemistry) and winter (due to high concentration of SO2 emitted from coal burning). Natural contributions, dust or sea-salt related, prevail mainly in the coarse particles. From May 2007 to August 2007, when air masses originated mainly from Black Sea, in the coarse particles an nss-Cl/Na ratio of 1.11 was measured. Elevated levels of chloride in fine particles have been attributed to waste burning in the proximity of the investigated site or to NaCl salt widely spread on roads during winter. Considering the importance of atmospheric aerosols, this study may constitute a reference point for Eastern Europe. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Aerosols Deliquesced particles Ionic composition Chemistry Iasi Romania

1. Introduction Atmospheric particulate matter (PM) is directly emitted into the air (primary PM) or generated in the atmosphere from precursor gases (secondary PM). PM is a component of air pollution that adversely affects human health (Spengler and Wilson, 1997), plays an important role in climate change (IPCC, 2007), alters biogeochemical cycles (Markaki et al., 2003) and chemistry of the atmosphere (Kouyoumdjian and Saliba, 2006), affects the visibility (Cheng and Tsai, 2000) and may contribute to the soiling of the monuments (Watt et al., 2008). Size distribution, microbes and * Corresponding author. Tel.: þ40 232 201354; fax: þ40 232 201313. E-mail address: [email protected] (C. Arsene). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.11.013

viruses concentration and chemical composition of the aerosols are directly related to various health impacts (Hetland et al., 2004). Small particles may contain numerous pathogens (Polymenakou et al., 2008) which degrade human, agricultural and ecosystem health. Long-range transboundary pollution transport (WHO, 2006a) contributes to the observed PM exceedances of the human exposure levels defined by the EU. According to the WHO (2006b) and the 2008/50/EC directive of the European Parliament and the Council of 21 May 2008 on ambient air quality and cleaner Europe the daily PM10 (particles of diameter smaller than 10 mm) values should be below 50 mg m
3 (which may only be exceeded for 35 days). For the PM2.5 (particles of diameter smaller than 2.5 mm) the same Directive addresses a limit of 17 mg m
3.

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

For these reasons PM10 concentrations and aerosol chemical composition data are now monitored in most EU countries (Avila et al., 1998; Bardouki et al., 2003; Kocak et al., 2007). To our knowledge, only few studies from Austria (Kocifaj et al., 2006), Hungary (Borbely-Kiss et al., 2004) and Slovenia (Tursic et al., 2006) refer to central to eastern European sites. Ammonia (NH3), sulfuric acid (H2SO4) and nitric acid (HNO3) are identified as the most abundant aerosol precursor gases. They originate from biogenic activity (NH3 emitted by farming and fertilization) or gas-phase photochemical processes (H2SO4 derives from gas phase oxidation of sulfur dioxide (SO2) and HNO3 from NOx (NO þ NO2)). Additional aerosol precursor gases include hydrochloric acid (HCl) and various condensable organic acids (R-COOH) (Metzger et al., 2006). In particular there is a lack of data on atmospheric particulate matter levels in Romania. Most probably the lack of standard regulations, the absence of serious efforts to control air pollution and the lack of information regarding the negative impacts to the population health are responsible for this. For the present study detailed aerosol mass concentration and chemical composition in the coarse and fine fractions of the particles have been made and analyzed at a populated urban site in the north-eastern Romania, Iasi region, from January 2007 to March

907

2008. These are the first data measurements, to our knowledge, in the region, for which gaps in data of aerosol concentrations and chemical composition have been identified by the WHO (2006a). The main purpose of this study is to determine the aerosol masses in the coarse and fine fractions, and the chemical composition of the investigated particles. Seasonal variation is also investigated and implications for the aerosol chemistry in the area are discussed. 2. Experimental 2.1. Aerosol sampling The sampling has been undertaken in the city of Iasi located in the north-eastern Romania. The site has been described by Arsene et al. (2007). The specific site is classified between a near city background (distance from large pollution sources 3e10 km) and an urban background (<2500 vehicles/day within a radius of 50 m). Sampling was performed at about 25 m above the ground twice a week, on weekend and working day between January 2007 and March 2008. The sampler was operating in a location free of any obstructing units (buildings or trees) and facing directly the town. The sampling time was nominally 24 hours with sampling starting at 6:00 UTC. A total of 106 aerosol

Fig. 1. Sectors contributions identified from classification of 2-day back trajectories of air masses ending at Iasi and representative backward trajectories of long and short range transport, Black Sea influence and African dust (shown trajectories correspond to sampling events).

908

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

samples were collected and analyzed. Meteorological information (temperature, relative humidity (RH) and wind speed) were accessed from the data base available for the nearest professional weather station located 1.5 km away from the sampling site (Iasi Airport North, Romania). The ambient aerosols were collected with Stacked Filter Units (SFUs) suitable for gravimetric measurements and ion analysis. The SFUs consist of 8.0 mm pore size 47-mm diameter Isopore polycarbonate filter mounted in front of a 0.4 mm pore size 47-mm diameter Isopore filter. Similar devices have been successfully used in other studies (Sciare et al., 2005; Kocak et al., 2007). The 24 h collected volume, as an average of the 106 sampling events, shows a variability of 35.96  1.45 m3. Following the suggestion from John et al. (1983) and Buser et al. (2007), at the flow rate of our system of 24.9  1.6 L min
1, the 50% cut-point diameter (D50) of the 8.0 mm pore size filter was estimated to be of the order on 1.5  0.2 mm aerodynamic equivalent diameter (AED). Considering the above mentioned aspects, in this paper, particles collected on the 8 mm pore size filters, with a diameter larger than 1.5 mm AED, will refer to the aerosol coarse fraction, while the particles collected on the 0.4 mm pore size filters will correspond to particles with a diameter below 1.5 mm AED. Following collection, the SFUs (protected in polyethylene bags) were transported from the sampling site to the laboratory (10 min). The filters were unloaded in a clean dust free room, and subsequently stored in Petri dishes. The filters were transported to the weighing room and equilibrated under constant temperature (22  C) and relative humidity (46% RH) for at least 24 h before weighing. During weighing static electricity problems were not faced. For the unloaded filters a similar procedure was followed. Mass of appropriate PM (coarse/fine) was calculated by the difference in weights for each filter samples. 2.2. Chemical analysis For the chemical analysis, all the steps in the analytical procedures were strictly quality-controlled to avoid any contamination of the samples. All the data regarding water soluble ions were derived from measurements performed by ion chromatography (IC). Each one of the collected filters, samples and blanks, was initially ultrasonically extracted for 45 min, after the addition of 15 ml of deionised water (resistivity ∼18 MU cm). Chloroform (100 mL) was added as a biocide in each extracted sample. The extract was then filtered through a 0.22 mm pore size Nylon filter which ended the preparative step of ion chromatographic analysis. The ions concentration was determined by the Dionex DX-500 ion chromatography system with the use of the AS4A-SC column along with the ASRS-II ultra suppressor for the analysis of water soluble anions 2
(chloride, Cl
, nitrate, NO
3 and sulphate, SO4 ). Organics as formate

2
(HCO2 ), acetate, (C2H3O2 ), oxalate, (C2O4 ), methane, sulphonate,


(CH3SO
3 , MS ), propionate (C3H5O2 ) and pyruvate (C3H3O3 ) were also analyzed. For the organics an AS11 analytical column was employed along with NaOH as eluent in gradient mode. For the water soluble cations, including sodium (Naþ), potassium (Kþ), 2þ 2þ ammonium (NHþ 4 ), magnesium (Mg ) and calcium (Ca ), a CS12SC column was used along with the CSRS ultra II suppressor. In both cases, anions and cations, a conductivity detector was used (DIONEX CD50). More IC details are as those given in Bardouki et al. (2003). The reproducibility of the measurements was better than 2%. The recovery of each ion was in the range of 92e105%. The limits of detection (defined as 3 times the standard deviation of blank measurements relative to the methods sensitivity) were about 1e5 mg L
1 both for the main analyzed anions and cations. Blanks were subtracted from each sample.

2.3. Air mass trajectories In order to identify the potential influence of different source regions on aerosol composition, air masses back-trajectories arriving at the sampling site were computed by the Hysplit Dispersion Model (Draxler and Rolph, 2003). 48 h (2 day) back trajectories arriving at the investigated site at 18:00 UTC have been computed. Back trajectories were evaluated at 500, 1000 and 2000 m above the sampling point, located at ground level. Trajectories at 500 and 1000 m were estimated as representative in this study. Representative trajectories are shown in Fig. 1.

3. Results and discussions 3.1. Coarse and fine fractions mass concentrations Levels of coarse (particles of AED > 1.5 mm) and fine (particles of AED < 1.5 mm) fractions mass concentrations at the investigated site and basic statistics (median, geometric mean, mean, minimum and maximum) are presented in Table 1. Table 2 reports the annual mean mass concentrations of total (lumped sum between coarse and fine fractions) and fine fractions at the investigated site versus values reported for 24 EU countries given by WHO (2006a) for the PM10 and PM2.5. The value reported in the present work for the coarse fraction, i.e. 38.3 mg m
3, is higher than the value reported by WHO (2006a/considering the EEA’s AirBase data base for 2002) for 24 EU countries at sites assigned as urban traffic (32.0 mg m
3 for PM10). The mass concentration of the fine fraction at our site, i.e. 10.5 mg m
3, is somewhat lower when compared with other EU urban background sites (15.0e30.0 mg m
3 for PM2.5). This is most probably a result either of the 50% cut-point diameter (D50) of the 8.0 mm pore size filter, estimated in our case to be of the order on 1.5  0.2 mm AED, or due to a smaller impact from anthropogenic pollution as the region is characterized by moderate population density and very little industrial activity. Fig. 2 shows the variation of the mass concentrations for coarse (2a) and fine (2b) fractions during the investigated period. Both coarse and fine fractions show similar profiles, and the shadowed areas in Fig. 2 are underlying long periods when coarse and fine fractions mass concentrations where larger than the limits settled by the WHO for PM10 and PM2.5 particles. Lower coarse fraction concentrations were recorded in the snowy/rainy seasons (January 2007eMay 2007). The slight increase in the coarse fraction mass concentrations from January 2008 to March 2008 may be a result of specific meteorological conditions (high RH under cloudy and clear sky, high wind speed and advection of air masses from ground level). However, in the present study the highest coarse fraction mass concentrations were determined during more windy and dry periods. The fine fraction, apart the period from August 2007 to October 2007 characterized by elevated mass concentrations,

Table 1 Levels of the coarse and fine fractions mass concentrations determined during the investigated period (n ¼ 106). Statistical parameter

Coarse fraction (mg m
3)

Fine fraction (mg m
3)

Week

Weekend

Annual

Week

Weekend

Annual

Median Geomean Mean Stdev Min Max

22.69 21.34 32.56 26.46 0.79 110.76

17.47 15.66 23.12 17.71 2.17 53.36

20.68 18.23 27.76 22.83 0.79 110.76

5.18 5.22 9.73 10.71 0.27 39.89

7.05 6.99 11.19 11.69 0.55 48.59

6.59 6.06 10.47 11.19 0.27 48.59

Note: coarse fraction e particles of AED > 1.5 mm and fine fractions e particles of AED < 1.5 mm.

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

909

Table 2 Annual arithmetic means of the total (lumped sum between coarse and fine) and fine fraction mass concentrations at the investigated site (n ¼ 106) and PM10 and PM2.5 levels at other European urban sites. Region/Site

Category

Period

Coarse related

Fine related

Reference

(mg m
3) Eastern Europe Iasi, Romaniaa 24 EU countriesb

2007e2008

38.3  25.4

10.5  11.2

This work

550 Urban areas (urban background) 550 Urban areas (urban traffic) 119 Urban areas (urban background) 119 Urban areas (urban traffic)

EEA’s AirBase data base for 2002

26.0

e

WHO (2006a)

EEA’s AirBase data base for 2002

32.0

e

EEA’s AirBase data base for 2002

e

15.0e20.0

EEA’s AirBase data base for 2002

e

20.0e30.0

For Iasi, Romania, are reported the total (coarse þ fine) and the fine fractions. For the 24 EU countries the data are related to PM10 and PM2.5.

showed similar trend over the investigated period. Lower mass concentrations in the coarse and fine fractions (<5 mg m
3) were observed especially in samples collected after raining events and this would be expected since precipitation efficiently removes the particles from the atmosphere. Exceeded coarse and fine fractions levels were observed in Iasi and these were caused both by anthropogenic and natural sources.

a

140 120

Concentration (μg m-3)

Total fraction (coarse and fine)

week weekend WHO limit for PM10 (2006)

100

(>50 μg m-3)

80 60

*

40 20 0

/0

07 01

/ 03

/2 0

07 01

/ 05

/2 0

07 01

/ 07

/2 0

07 01

/ 09

/2 0

07 01

/1 1

/2 0

07 01

/0 1

/2 0

08 01

/0 3

/2 0

08 01

/0 5

/2 0

08

Time

3.2. Seasonality of the different fractions of PM Fig. 3 shows monthly arithmetic mean concentrations of the coarse and fine fractions. Fig. 3 also includes standard deviations of all samples within the monthly averages (as a measure of the month-by-month variability and implicit of the seasonal variability). The coarse fraction indicates a possible seasonal variability with clear maxima during the warm season which can be due to the increased frequency of dust. The assumption is strengthened by the observation that the coarse fraction profile clearly follows that observed for calcium. The fine fraction presents also some seasonal features which are characterized by possible maxima during spring to summer and summer to fall periods. The slight difference in the seasonality between the two fractions may indicate contributions from various sources. The relative contribution of the fine toward the coarse particles shows clear peaks especially during coal/petroleum (winter) and vegetation (fall) burns. The fine/coarse fractions ratios indicate great

50

b

Fine fraction

week weekend

Concentration (μg m-3)

1.2

100

40 WHO limit for PM2.5 (2006)

* 30

(>17 μg m-3)

20

10

Coarse

Fine

1.0

80

0.8 60 0.6 40 0.4 20

0.2

0

0 1/0

1 /2

00

7

/0 01

3 /2

00

7

/0 01

5 /2

00

7

/0 01

7 /2

00

7

/0 01

9 /2

00

7

/1 01

1 /2

00

7

/0 01

1 /2

00

8

/0 01

3 /2

00

8

/0 01

5 /2

00

8

Time Fig. 2. Variation of the mass concentrations of coarse (2a) and fine (2b) fractions during the investigated period.

Fine/Coarse

Fine/Coarse

01

0 1/2

For example the sample that was collected from March 21 to March 22, 2007 (fine fraction mass concentration of 32.9 mg m
3) was highly influenced by transportation of African dust (the event marked in Fig. 2), an observation sustained also by the backward trajectory analysis. All other exceeded fine fraction mass concentrations are a result of more variable sources (soil, sea salt, combustion and biogenic sources).

μg m-3 (Coarse and fine fractions)

a b

Urban background

7 7 7 7 7 7 7 7 7 7 7 7 8 8 8 00 00 00 00 00 00 00 00 00 00 00 00 00 00 200 y 2 y 2 h 2 il 2 y 2 e 2 y 2 t 2 r 2 r 2 r 2 r 2 y 2 y 2 ar uar arc Apr Ma un Jul gus be obe be be uar uar arch u J r r m t m m n M M Au pte Oc ove ce Jan eb Ja F e b F N De Se

0.0

Period of the year Fig. 3. Monthly mean concentrations of the coarse and fine fractions and fine/coarse ratio during the investigated period.

910

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

variability amongst the months of the year and the ratios range from ∼0.10 to ∼1.0. This observation may suggest high variability in the aerosols chemical composition which is affected by various sources over the year (natural, anthropogenic). The PM2.5/PM10 ratios for urban background and/or traffic sites, as presented by WHO (2006a), range from 0.42 to 0.82. At the investigated site, as can be seen in Fig. 4a, the fine/coarse fractions ratios range from 0.82 (significant correlation at p < 0.05, r2 ¼ 0.96; cold periods) to 0.29 (significant correlation, r2 ¼ 0.93; warm periods). The fine/coarse fractions ratio of 0.11 is in close agreement with prevalence of high wind gust speeds over the specific months (confirmed by the meteorological information) and also due to more intense anthropogenic activities in the neighborhood of the sampling site (excavation) which would result in higher loading of coarse particles in the atmosphere. A pronounced increase in the fine/coarse ratio (0.82) occurs during cold seasons or during possible pollution events due to combustion processes. Such behavior might be attributed to the enhancement of secondary aerosols (sulfate and organic compounds) generation during coal/ petroleum burning (heating purpose) and human induced biomass burning. The previous mentioned processes are known to induce formation of fine anthropogenic particles. However, discrimination between dust and wood/coal burning contribution can be also seen from Fig. 4b. Distinct slopes are observed from winter (wood and coal burning prevalence, probably also fuel burning as elemental carbon in our work was estimated to bring contributions up to 10%

50

FF = 0.82xCF (r2 = 0.96) - coal and vegetation burning FF = 0.29xCC (r2 = 0.93) - dust over warm season FF = 0.11xCC (r2 = 0.49) - dust due to strong winds/gust

40

3.3. Role of inter-particle and gas-particle interactions in establishing fine and coarse fractions chemical composition During data analysis we observed two distinct behaviors in the chemical composition especially of the fine particles. One is associated with January 2007 to July 2007 (62 analyzed samples), called further period I and the other corresponds to August 2007 to March 2008 (44 analyzed samples), called further period II. High relative humidity (RH) was prevailing during period II of the investigation (on average RH ∼ 80%). We found that during this period, about 90% of the 24 h sampling time, the ambient RH was above deliquescence relative humidity (DRH, defined as the relative humidity at which deliquescence is completed) especially for the NH4Cl/H2O and NH4NO3/H2O systems (Martin, 2000). This observation may indicate favorable meteorological conditions for the existence of deliquesced particles and hence of NH4NO3(aq) and NH4Cl(aq). Most probably, deliquesced particles led to the formation of aqueous droplets a process which favors formation of internal mixture from externally mixed particles, which actually results in changes in the activities of semi-volatile species. In mixed collected particles the gas-particle equilibrium tends to re-establish by mass exchange between the gas and particulate phases (Pathak and Chan, 2005). If the meteorological conditions over period II were favorable to form deliquesced particles, then the following reactions could occur during inter-particle or gas-particle interactions:

-3

Fine fraction - FF (μg m )

a

in the fine fraction) to summer (dust prevalence) for the Ca/Kfine vs. Ca/Kcoarse ratio. Moreover, we should stress here out that the observed seasonal variation cannot be attributed solely to changes in source contribution, but also probably to variations of meteorological conditions.

RH>DRH


NH4 ClðsÞ4 NH4 ClðaqÞ#NHþ 4 ðaqÞ þ Cl ðaqÞ#NH3 ðgÞ

þHClðgÞ

30

RH>DRH

NH4 NO3 ðsÞ4 NH4 NO3 ðaqÞ#NHþ 4 ðaqÞ

20

þNO
3 ðaqÞ#NH3 ðgÞ þ HNO3 ðgÞ

10

0

b

0

20

40 60 Coarse fraction - CF (μg m-3)

12

80

100

2

Ca/Kfine = 0.05xCa/Kcoarse (r =0.98) warm season - dust Ca/Kfine = 0.02xCa/Kcoarse (r2=0.91) cold season coal and vegetation burning

10 8 Ca/Kfine

(R.1)

Hþ ðaqÞ þ Cl
ðaqÞ#HClðgÞ

(R.3)

Hþ ðaqÞ þ NO
3 ðaqÞ#HNO3 ðgÞ

(R.4)

Hþ ðaqÞ þ NH3 ðgÞ#NHþ 4 ðaqÞ

(R.5)

During sampling, reactions (R.1)e(R.5) may take place leading to important sampling artifacts. In investigation of atmospheric particles, sampling artifacts in sulfate, nitrate, chloride and ammonium concentrations relate with the ambient particulate 2
concentration ratio of NHþ 4 /SO4 (Pathak et al., 2004). Actually, reactions (R.1) and (R.2) would be responsible for the ammonium artifacts through ammonia concomitant evaporation with HNO3 and HCl while reaction (R.5) would involve absorption of ammonia on aqueous droplets. In a study performed with and without denuders, Pathak and Chan (2005) report that the possible loss in nitrate and chloride due to the reaction

6 4 2

Cl
þ HNO3 ðgÞ#NO
3 þ HClðgÞ

0 0

40

80

120

(R.2)

160

200

Ca/Kcoarse Fig. 4. Relationship between fine fraction vs. coarse fraction (4a) and Ca/Kfine vs. Ca/ Kcoarse (4b) for the investigated period.

(R.6)

is negligible in artifacting their level, especially at high concentrations. The same group of authors claims also that in ammonium rich environments reaction (R.5) may be responsible for large ammonium and acidity artifacts due to the neutralization of the acidity by ammonia.

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

h

i

NHþ 4 ðnegative artifact=lossesÞ

¼

h

NO
3

i measured

h i þ Cl


measured

This supposition is also sustained by the behavior observed for the
((SanionseScations) vs. ([NO
3 ] þ [Cl ])) pair, which by linear regression analysis give a ratio of 0.95 with a correlation coefficient (r2) of 0.98 (as determined from 44 samples of fine particles collected during period II). In this analysis (SanionseScations) is assigned as [NHþ 4 ](negative artifact/losses). For more details see also Section 3.4. We must here stress that during period II also meteorological conditions were of such nature that formation of fine-mode NH4NO3 or NH4Cl in the aqueous phase was favored. During period II RH > 80% were often prevailing even during day-time when temperature normally is rising and a decrease in RH is expected.

Although formation of NH4NO3(aq) and NH4Cl(aq) most probably occurred in the samples collected during period II, limited evaporation of HCl and HNO3 formed through reactions (R.1) and (R.2) is expected. Martin (2000), in a review article, gives the following deliquescence relative humidity for selected chemical systems at 298 K: NH4NO3/H2O with 60%, NH4Cl/H2O with 77% and (NH4)2SO4/ H2O with 80%. Actually, during period II measurements were often done at relative humidity above the deliquescence relative humidity (DRH) of NH4NO3 and NH4Cl and hence all nitrate and

a

500 Coarse fraction - raw data Coarse fraction - corrected

Σ cations (neq m-3)

400

300

200

100 Σcations = 1.00xΣanions - 3.63 (r2 = 0.98) estimated HCO3-/CO32- included in Σanions 0

-3

(Σanions − Σ cations) (neq m )

b

0

100

200 300 -3 Σ anions (neq m )

400

500

200 Fine fraction - ammonium rich particles 160

120

80

40

(Σanions− Σ cations) = 0.95x([NO3-] + [Cl-]) + 2.23 (r2 = 0.98)

0 0

40

80

120

160

200

[NO3-] + [Cl-] (neq m-3)

c

350 fine fraction - ammonium rich particles fine fraction - corrected fine fraction - ammonium poor particles

300 Σcations (neq m-3)

Pathak and Chan (2005) suggest that an ambient molar partic2
ulate [NHþ 4 ]/[SO4 ] ratio of 1.5 is a critical condition indicating sampling artifact characteristics of PM2.5. In an ammonium poor 2
atmosphere, when [NHþ 4 ]/[SO4 ] < 1.5, formation and evaporation of NH4NO3 or NH4Cl would be prevented. In this study, in the fine fraction, as determined from linear 2
regression analyses, during period I averaged NHþ 4 /SO4 ratio was 1.27 and during period II the same ratio was 1.73 (1.47 is the 2
averaged NHþ 4 /SO4 ratio over the entire period). We observed that 2
þ 2
in the fine mode ([NO
3 ]/[SO4 ]) vs. [NH4 ]/[SO4 ] was relatively þ 2
constant at [NH4 ]/[SO4 ] < 1.5 and it increases when [NHþ 4 ]/
[SO2
4 ] > 1.5 (all 106 events included in the analyses). The [Cl ]/ þ 2
[SO2
4 ] vs. [NH4 ]/[SO4 ] pair behaved in a similar way although above 1.5 the [Cl
]/[SO2
4 ] increases more dramatically when 2
compared to [NO
3 ]/[SO4 ]. These observations would imply uptake 2
of ambient ammonia to neutralize the acids until [NHþ 4 ]/[SO4 ] is equal to 1.5. Actually the existence of additional ammonia in the system would normally stabilize nitrate and chloride, as evidenced 2
2

by the increase in the [NO
3 ]/[SO4 ] and [Cl ]/[SO4 ] ratios as 2
]/[SO ] increases. [NHþ 4 4 In ammonium rich particles collected during period II, a significant number of samples showed very good correlation between nitrate in the particulate phase with the excess ammonium which þ 2
2
was calculated as [NHþ 4 ]excess ¼ ([NH4 ]/[SO4 ]
1.5)  [SO4 ]. In the present study 1.5 represents the approximation of the 1.47 2
averaged [NHþ 4 ]/[SO4 ] ratio and, hence, 1.5 would be considered as a critical value to differentiate between the ammonium rich and ammonium poor samples. The linear regression analysis applied to the pair (NHþ 4 excess, NO
3 ) determined in the ammonium rich data set, led to the þ equation [NO
3 ] ¼ 1.79[NH4 ]excess þ 1.70 with a correlation coeffi2 cient (r ) of 0.80. For the dependence [Cl
] ¼ f([NHþ 4 ]excess), the linear regression analysis led to a ratio [Cl
]/[NHþ 4 ]excess of 2.10 with a correlation coefficient of 0.79. These correlations would involve association between nitrate/chloride in the particulate phase and the excess ammonium in ambient aerosols. The amount of nitrate/ chloride remaining on the filters would be similar to the excess ammonium (ratio of 1) if reactions (R.3)e(R.5) were negligible (Pathak and Chan, 2005). In the present work, the excess ammonium is much lower than the nitrate/chloride on the filter and, under these circumstances we assume that reactions (R.1) and (R.2) would be mainly responsible for possible ammonium artifacts (negative artifacts, losses) while reaction (R.5) due to ammonia absorption on the acidic particles (positive artifacts, production) would be negligible. However, the contribution from a second scenario involving partial neutralization of NH3 by the atmospheric HCl and HNO3 is also possible. Based on the above mentioned observations we propose for a non-denuder system (as that used in the present study) artifacted ammonium (due to losses) to be calculated from the following equation:

911

250 200 150 100 2

Σcations = 1.02xΣanions - 1.72 (r = 0.99) 50

estimated

+ NH4

and

2HCO3 /CO3

included in Σcations and Σanions

0 0

50

100

150

200

250

300

350

-3

Σanions (neq m ) Fig. 5. Linear regressions in order to check for the ionic balance to be completed in the aerosols coarse (5a) and fine (5b,c) fractions.

912

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

chloride could be taken back in the aqueous phase. However, at the investigated site, losses in ammonium would be accounted by w45% due to NH4NO3 and ∼55% due to NH4Cl. 3.4. The ionic balance in the coarse and fine fractions The ionic balance was controlled both for the coarse and fine fractions. The fine fraction shows two distinct behaviors with clear changes in their chemical composition. The chemical composition for the fine particles, as determined in the present study, revealed a deficit of anions for period I and a deficit of cations for period II. In the coarse fraction for all samples, the arithmetic mean of Scations (83.4  76.9) and that of Sanions (30.7  30.6) (s) indicate an excess of 52.7 neq m
3 in cations. The arithmetic mean, as determined from 62 samples of the fine particles collected during period I, in Scations (14.1  7.8) and Sanions (12.7  6.0) (s) indicate an excess of 1.4 neq m
3 in cations. The fine particles collected during period II (44 samples), through the Scations (29.2  31.1) and Sanions (64.8  67.9) (s), indicate an excess of 35.6 neq m
3 in anions. The above mentioned observations are supported by the data presented in Fig. 5 which shows for both particle types the ratio between cations and anions before and after correction for the

missing anion/cation. Slopes higher than unity indicate anion deficit in the ionic balance. In the present work, both for coarse and fine particles, the species responsible for the anion deficit is 2
assumed to be in the form of HCO
3 /CO3 . Our assumption for the fine fraction is sustained by the fact that almost all samples (50 out of 62) collected during period I, showed the presence of Ca2þ and Mg2þ in their ionic composition (concentrations of these species during period I were about 50% higher than those determined 2
during period II). The estimation of HCO
3 /CO3 for both fractions was done accordingly to Arsene et al. (2007). For the coarse particles considering the indirectly estimated 2
HCO
3 /CO3 in the Sanions, through the linear regression analysis applied to the dependence Scations ¼ f(Sanions), we obtain a Scations/ Sanions ratio with a value of 1.00 and a correlation coefficient (r2) of 2
0.98 (Fig. 5a), which indeed proves that HCO
3 /CO3 could account for the missing anion. Fine particles were treated in full agreement with the two distinct behaviors which were observed. More details about the rationale for the corrections applied for the fine fractions are presented in Section 3.3. For period I correction was done in order to compensate for the missing anion (assumed to be in the form 2
HCO
3 /CO3 ). As during period II a strong cation deficit is prevailing, we propose the following rationale in order to identify the possible

Table 3 Mean, median concentrations (ng m
3) and the standard deviations (SD) of water soluble ions in the aerosols coarse and fine fractions in Iasi, north-eastern Romania (a total of 106 analyzed samples for each fraction). Chemical class

Ion

Coarse fraction Mean

Fine fraction Median

SD

ng m
3 Inorganic anions

Cl
NO
3 SO2
4 2
HCO
3 /CO3 (estimated)

Inorganic cations

Naþ NHþ 4 Kþ Mg2þ Ca2þ NHþ 4 (estimated)

Organic anions

HCOO
CH3COO
C2O2
4 C3H5O
2 C3H3O
3 CH3SO
3

Mean

Median

SD

ng m
3

1 2 1 2 1 2 1 2

215 476 431 1458 506 972 3508 3178

165 215 316 545 420 669 2408 1453

200 536 359 2275 413 1134 3130 4128

191 1600 211 1776 492 1874 201a e

81 1323 102 1334 350 1308 152a e

236 1094 237 1439 397 1803 174a e

1 2 1 2 1 2 1 2 1 2 1 2

145 312 46 198 15 34 119 60 1276 1236 e e

62 123 24 25 9 16 62 36 867 567 e e

188 385 74 380 19 50 183 64 1259 1632 e e

36 61 126 895 27 53 14 0 78 105 249b 985c

19 25 73 727 16 44 10 0 55 50 234b 790c

48 108 132 861 30 40 20 0 81 197 148b 798c

1 2 1 2 1 2 1 2 1 2 1 2

8 NM 20 NM 49 88 1 NM 1 NM 0.2 NM

5 NM 15 NM 42 53 0.4 NM 0.4 NM 0.0 NM

8 NM 18 NM 32 90 2 NM 1 NM 0.3 NM

7 NM 20 NM 35 109 12 NM 3 NM 5 NM

3 NM 15 NM 29 92 5 NM 2 NM 4 NM

8 NM 22 NM 23 68 13 NM 3 NM 3 NM

Note: coarse fraction e particles of AED > 1.5 mm and fine fractions e particles of AED < 1.5 mm. 1 90 analyzed samples collected during January 2007eMarch 2008. 2 16 extreme events: 21, 28 October 2007; 18, 21, 25 November 2007; 02, 16 December 2007; 20 January 2008; 06, 10, 13, 17 February 2008; 05, 09, 16 March 2008. NM, not measured. a Estimated from 62 samples collected during January 2007eJuly 2007. b Estimated from 28 samples out of a total of 44 samples collected during August 2007eMarch 2008. c Estimated from 16 samples out of a total of 44 samples collected during August 2007eMarch 2008.

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

missing species (supposed to be in the NHþ 4 form). In the present
work (SanionseScations) vs. ([NO
3 ] þ [Cl ]) showed a significant correlation (r2 ¼ 0.98) with a slope of 0.95 (Fig. 5b). Such significant correlation allows estimation of the missing ion from the measured
concentrations. In the fine fraction a significant NO
3 and Cl improvement in the ionic balance is achieved, with a ratio of Scations/Sanions of 1.02 and r2 ¼ 0.99 (Fig. 5c), when the identified missing anion (period I) and cation (period II) are considered in the total ionic balance. 3.5. Chemical composition and seasonality 3.5.1. Ion composition of the coarse and fine fractions Mean variation in the concentrations of the main identified ionic species is shown in Table 3. Data are presented in two stages: stage 1 corresponding to 90 analyzed samples during January 2007eMarch 2008 and stage 2 corresponding to 16 extreme events collected during August 2007eMarch 2008. These extreme events are separately presented here as in the statistical treatment of all data set they induce high variability in the measurements (standard deviation was used as a measure to obtain this information). Backward trajectory analyses revealed that their ionic composition is most probably affected by the advection (under 400 m with advection at 1000 m) of air masses traveling above large (long range transport) or short (local) continental areas. From Table 3 it can be easily observed that these 16 extreme events for some 2
þ species (Cl
, NO
3 , SO4 , NH4 ) were characterized by much higher concentration when compared to other 90 analyzed samples. With

a

+

K+ NH4 Na+ 2+ (0.4 %) (2.3 %) Mg (6.7 %) 2SO4 (6.7 %) (7.1 %) NO3 (6.0 %) Cl 2+ (3.8 %) Ca (34.0 %) Organics (1.9 %)


þ
regard to pollution associated ions (SO2
4 , NO3 , Cl , NH4 ) the concentrations determined at the investigated urban site were about 10 times lower than those reported for much larger urban areas (Wang et al., 2005). The relative contribution of the identified ions in the coarse and fine particles is presented in Fig. 6. In the coarse fraction, as shown in Fig. 6a, the main water soluble ions included Ca2þ and HCO
3
2þ (∼65%) while SO2
and Naþ contributed each with 4 , NO3 , Mg about 5 to 8%. In fine particles, apart period I where traces of HCO
3/ CO2
3 are present (Fig. 6b) in their chemical composition, during 2


period II species as NHþ 4 , SO4 , NO3 and Cl are mainly prevailing (Fig. 6c,d). In both particle types low amounts of Kþ and Mg2þ were detected.

3.5.2. Seasonal variation of ions Fig. 7 presents the seasonal variations for water-soluble ionic components in the aerosols coarse (Fig. 7a,b) and fine (Fig. 7c,d) fractions. Fig. 8 presents the seasonal variation for the [Cl
]/[Naþ] 2
(Fig. 8a) and [NO
3 ]/[SO4 ] (Fig. 8b) ratios. Data both in Figs. 7 and 8 are normalized with respect to the mass concentration of the coarse and fine particles. A total number of 106 samples are included in the analysis. Clear seasonal patterns were observed for all major ionic species in the coarse and fine fractions. 3.5.2.1. Selected ions behavior in the coarse fraction. In the coarse fraction Ca2þ and Mg2þ are tracers of soil dust. In the present work their seasonal distributions are governed by the atmospheric loading of the suspended particulate matter which is higher

2-

b

SO4 (27.0 %) NO3(5.1 %)

+

Na (8.7 %)

Cl(6.2 %) Organics (3.9 %)

NH4+ (15.9 %) K+ (3.3 %) Mg2+ (5.6 %)

HCO3(31.2 %)

(Coarse fraction, all period)

HCO3(8.1 %) 2+

Ca (16.2 %)

(Fine fraction, period I, 62 samples)

NO3(10.8 %)

c

913

d

NO3(12.2 %)

2-

SO4 19.5 %)

-

Cl (17.1 %)

+

Na (3.8 %)

Organics (3.7 %) +

NH4 (15.1 %) K+ 2+ 2+ (0.9 %)Mg Ca (1.7 %)(4.6 %)

NH4+

estimated (23.7 %)

(Fine fraction, period II, 28 samples)

SO42(16.5 %)

Cl(19.1 %)

Na+ (2.1 %)

Organics (2.5 %)

NH4+ (17.9 %)

+

K (0.7 %) Mg2+ Ca2+ (0.3 %) (2.7 %)

estimated NH4+ (26.2 %)

(Fine fraction, period II, 16 samples)

Fig. 6. Percentage distribution of the ionic composition in the aerosols coarse (6a) and fine (6b,c,d) fractions.

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C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

Fig. 7. Normalized anionic and cationic concentrations over the total mass concentrations of the coarse (7a,b) and fine (7c,d) fractions (for better clarity the normalized Kþ concentration in the coarse fraction is multiplied by 10).

especially during the warm season when less precipitation and prevalent high wind/gust speed occurs. Although with a minor contribution in the coarse particles, normalized Naþ shows higher concentrations during winter season indicating mainly contribution from anthropogenic sources (probably NaCl salt widely spread on roads during winter). In the coarse particles Kþ normalized concentration reveals also clear seasonal pattern with distinct maxima during the season characterized by intense wood burning. 2
Secondary pollution products in the form of NO
3 and SO4 present similar seasonal patterns with minima during summer and maxima during winter, a behavior which may suggest possible similar mechanistic chains responsible for their formation. 3.5.2.2. Selected ions behavior in the fine fraction. In fine particles Ca2þ shows an interesting pattern especially during January 2007 to July 2007 which actually would suggest lower concentration mass of the fine fraction but highly loaded in this element. Naþ normalized concentration is higher during summer and winter period. Kþ in the fine particles behave similarly as in the coarse fraction and this would actually suggest common sources. However, apart the maxima observed during cold season there is a maxima observed during June and July which are specific months characterized by vegetation burning. The ammonium normalized concentrations exhibited higher levels during period II. It shows all over the total investigated very good correlation with SO2
4 period.

In the fine fraction SO2
4 behave distinctly in agreement with contribution from secondary transformation through (photo) chemical reactions during warm season. According to suggestion from Mihalopoulos et al. (2007) observed submicron SO2
4 levels can be due to homogeneous (photochemical) gas-phase oxidation of SO2 to H2SO4, which is subsequently scavenged by aerosol particles. The increase during summer time in sulfate concentration may be due to the enhancement of the photochemistry under specific conditions (solar radiation intensity, humidity, temperature) (Yermakov et al., 2007) through the following reaction

H2 SO4 ðgÞ þ 2NH3 ðgÞ#ðNH4 Þ2 SO4 ðsÞ

(R.7)

Oxalate in fine particles presents similar behavior to that of sulfate. Its maximum during summer may suggest enhanced photochemistry of volatile organic compounds induced by high OH radical level. At the investigated site, NO
3 most probably is from NH4NO3 that is formed by the reaction of nitric acid, a product of the NOx gas phase oxidation, with NH3. Especially during period II, meteorology and abundance of various chemicals were of such nature that a shift from the gas phase of HNO3 to the particle phase of NH4NO3 was favored. The increase in the nitrate concentration during period II probably is also a result in enhanced photochemistry induced by the presence of higher concentration of the organic carbon fraction (data are not here presented) which could produce enough OH radical such that the reaction NO2 þ OH / HNO3(g) occurs (Vrekoussis et al., 2004).

C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

915

H2SO4 in the aqueous phases with formation of coarse mode nitrate and sulfate salts (see (R.8) and (R.9)). In the presence of relatively high concentrations of mineral dust, traced by CaCO3, reaction between CaCO3 and H2SO4 or HNO3 may occur via CaCO3 þ H2SO4 / CaSO4 þ H2O þ CO2

(R.10)

CaCO3 þ 2HNO3 / Ca(NO3)2 þ H2O þ CO2

(R.11)

which may also result in other salts formation (Kouyoumdjian and Saliba, 2006 and references therein). In the coarse particles collected during period II the linear regression analysis of the pair
2þ (Ca2þ, NO
3 ) leads to a molar concentration ratio [NO3 ]/[Ca ] of 1.97 (r2 ¼ 0.73) and this observation would imply existence of the Ca(NO3)2 association. If Ca(NO3)2 is formed, in deliquesced particles as those formed during period II, carbonates reactivity will represent a good chain for HNO3 transport in the aerosol system. 2
The [NO
3 ]/[SO4 ] ratio can be used as an indicator of the relative importance of mobile vs. stationary sources of sulfur and nitrogen in the atmosphere. As presented in Fig. 8b in fine particles the 2
seasonal variation of NO
3 /SO4 range from about 0.07 to 1.2 (mean 2
/SO value in winter-spring 2007 when 0.52). The lower NO
3 4 compared with winter-spring 2008 could be due to higher SO2 emission during the first period. Over summer 2007 lowering of the 2
NO
3 /SO4 ratio in the fine particles could be due to an increase in under specific conditions of high the formation rate of SO2
4 temperature and solar radiation intensity. In the coarse particles 2
NO
3 /SO4 average ratio is about 1.0 and its distribution do not reveal a clear seasonal trend. 4. Conclusions 2
Fig. 8. Seasonal pattern of the [Cl
]/[Naþ] and [NO
3 ]/[SO4 ] ratios estimated from normalized concentrations in the coarse and fine fractions.

3.5.2.3. Variability of other species both in the fine and coarse fractions. As Fig. 8a shows, in the fine particles higher [Cl
]/[Naþ] ratio during cold period would suggest that the major source of Cl
in Iasi might be from NaCl salt widely spread on roads during cold season. Its maximum observed for the coarse particles during the warm season may suggest another source. The chloride concentration originating from sea salt is defined as 1.174 the concentration of Naþ which constitute only a small percentage of the total concentration of chloride in the coarse particles (Kouyoumdjian and Saliba, 2006). The average [Cl
]/[Naþ] concentration ratio, as calculated from the samples collected from May 2007 to August 2007, is about 1.11  0.37 (s) and the obtained value may suggest also contribution from sources other than those prevailing in winter. Although Iasi is located far from the Black Sea, during summer sea-salt chloride contribution to the aerosol budget is not entirely excluded. Apart the cold season, Cl
concentrations peaked also during other months and the higher values can be attributed to the emission of HCl from the waste mass burning in incinerators. The high Cl
concentrations observed in fine particles especially during period II would imply limited losses of chloride via the following reactions

NaClðaqÞ þ HNO3 ðaqÞ#NaNO3 ðaq; sÞ þ HClðgÞ

(R.8)

NaClðaqÞ þ H2 SO4 ðaqÞ#Na2 SO4 ðaq; sÞ þ 2HClðgÞ

(R.9)

However, in the coarse particles chloride concentrations were lower than those of nitrate and sulfate over the investigated period which can be a result of the reaction between NaCl with HNO3 and

This work reports for the first time to our knowledge on variation of the ionic composition of the aerosols coarse and fine fractions at Iasi, north-eastern Romania. Concentrations of the major
2
þ
þ þ 2þ water soluble ions (SO2
4 , NO3 , Cl , C2O4 , Na , NH4 , K , Mg , 2þ Ca ) were measured for a total of 106 samples both in the fine and
coarse fractions. In the coarse fraction Ca2þ and CO2
3 /HCO3 are the major ions and existed mainly in the form CaCO3. NaCl salt may also

exist in the coarse particles. In the fine fraction SO2
4 , NO3 , Cl and represent major ions and existed mainly as (NH ) NHþ 4 4 2SO4, in the fine fraction exhibits high NH4NO3 and NH4Cl. SO2
4 concentrations in both summer and winter most probably due to the secondary transformation favored by high temperature and strong solar radiation during summer and higher concentrations of SO2 from coal burning and the lower removal rate in winter. Temperature and relative humidity seem to be among the main factors controlling the atmospheric aerosol chemistry. In the fine particles formation of NH4NO3 may occur at high relative humidity especially in environments characterized as ammonium rich media 2
(NHþ 4 /SO4 ratio higher than 1.5). As in the coarse particles CaCO3 seems to be abundant although NaCl may also occur, at high relative humidity enhancement in the reaction of NaCl and CaCO3 with nitric and sulfuric acid is expected which may result in an increase in the concentration of calcium nitrate and sulfate salts over the calcium and sodium chloride. Under these assumptions losses of Cl
from the filters by the Cl
depletion processes is diminished. CaCO3 represents also a chain for HNO3 recirculation. Acknowledgements This work was supported by the FP7-MERG-CT-2007 (Grant no. 203934, ICAARUS). The authors want to thank the two anonymous

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C. Arsene et al. / Atmospheric Environment 45 (2011) 906e916

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