Size-Resolved Water-Soluble Ionic Composition of Ambient Particles in an Urban Area in Southern Poland

The ambient concentrations of PM-related anions ( Cl , 3 NO  , 4 SO  ) and cations (Na, , K, Ca, Mg), total and contained in the PM fractions, were investigated in a typical urban area within the Silesian Agglomeration. A DEKATI low pressure impactor (DLPI) was used to sample PM and separate it into 13 fractions. The PM concentrations were determined gravimetrically, the ion content of the PM water extracts—by means of ion chromatography (Herisau Metrohm AG ion chromatograph). In general, sulfate, nitrate, and ammonia had the greatest ambient concentrations. PM1 contained over 60% of the PM-related sulfate and nitrate mass and 90% of the ammonia mass. Also the majority of Na and Cl were bound onto fine particles. Instead, more of the PM-related K, Ca and Mg mass were in PM2.5-10 than in PM2.5. In the fine particles (sub-fractions of PM1.6) sulfate, nitrate and ammonia occur mainly as (NH4)2SO4 and NH4NO3. In the sub-fractions of PM1.6-10 sulfate and nitrate might also occur as K2SO4, CaSO4, Ca(NO3)2 or NaNO3. 4 NH 


Introduction
To assess the impact of atmospheric aerosol on the environment, including air quality, ecosystems, human health and climate change, it is necessary to know its concentration, chemical composition and mass size distribution of PM (ambient particulate matter) components [1][2][3][4][5][6][7][8].Knowledge of the mass size distribution of PM components is helpful in determining mechanisms of aerosol formation, as well as physical and chemical changes, it is subjected to on a given area [9][10][11][12][13].
Besides the obvious and relatively well-recognized relation between the content of various toxic compounds in ambient dust and human health [14][15][16][17], another example of a dust chemical composition impact on the environment, is the effect of some water-soluble inorganic compounds on the acidity and conductivity of aerosols.Under certain conditions, the water-soluble sulfur and nitrogen compounds contained in the dust, contribute to acidification of precipitation and/or deposition, whereas the deposition of particles rich in the water-soluble calcium, magnesium, potassium or sodium compounds, increases the alkalinity of the environment [18][19][20][21].
Sulfates, nitrates and ammonia are used to determine the share of secondary inorganic aerosol (SIA) in the mass of ambient dust.Oxidation of SO 2 in the air, then a binary nucleation of H 2 SO 4 -H 2 O and ternary H 2 SO 4 -H 2 O-NH 3 , results in the formation of dust particles, mostly smaller than 1 µm [19,21,27,28].These particles, together with nitrate (V) ammonium emerging in the analogous reaction of nitric acid (V) with ammonia, form SO  NH  , K + , Ca 2+ , Mg 2+ ) related to thirteen PM fractions in a typical urban area of southern Poland.Possible chemical composition of secondary inorganic aerosol in 13 dust fractions was also estimated.

Material and Methods
The site of experiment (Zabrze, Poland, Figure 1) is located in area representative of the air pollution conditions for the central part of Upper Silesia and it meets the criteria of urban background site (Directive 2008/50/EC).
Conditions at this point, characterize well dust concentration in residential areas exposed to municipal and industrial emissions in the Upper Silesia [29].

Results and Discussion
Samples have been collected from August to December 2008.Fourteen measurements were carried out and each lasted about a week.Dust was collected using a thirteen stage DEKATI low pressure impactor (DLPI) [13].
Masses of dust collected on aluminum substrates, were determined by weighing substrates before and after exposure, on a Mettler Toledo microbalance (accuracy 2 µg).Before weighing the substrates were kept in weighing room for 48 hours (temperature 20˚C ± 2˚C, relative air humidity 48% ± 5%).Concentrations of PM fractions were calculated by dividing each fraction's mass by the volume of air, from which it was collected.Dust samples were kept in a refrigerator in tight and lightproof containers until the analysis.
Most of ions' concentrations in Zabrze were comparable to concentrations noted between 1998-2008 in Europe.For example, concentration of sulfates in particulate matter in Zabrze, was comparable to the concentration recorded at two sites in Switzerland, suburban station in Menen (Belgium) and urban background station in Helsinki (Finland).Generally, higher concentrations than in Zabrze are listed in Asian countries [13,30,31].Concentration of Cl − associated with fine dust in Zabrze was extraordinarily high comparing to values recorded in other parts of Europe and similar to concentrations of chlorine in Menen and Melpitz, recorded in these cities during the winter season (Table 1).
Thirteen samples were fixed for chromatography analysis -for each fraction, a collective sample from 14 weeks was prepared.Samples were placed in ROTH extraction containers.For the extraction, 50 cm 3 of deionized water was added to each container and the containers were tightly capped to prevent leaking during the extraction.Extracts were then placed in an ultrasonic Sulfates, nitrates and ammonia associated with PM 1 , PM 2.5 and PM 10 , had the highest concentration of the eight analyzed ions in Zabrze (Tables 1 and 2).Average mass shares of  , with a maximum occurring between 0.4 -1 μm (Figure 2(a)), means that these ions are parts of the same compounds in the dust.The main mechanism of their formation are presumably the transformation processes of PM gaseous precursors occurring in the atmosphere.PM-related 4 had multimodal mass size distribution, with a one maximum occurring in the range of 0.4 -1 μm and two maxima between 1.6 -10 μm (Figure 2(b)).
On the areas where sea spray or sea water evaporation (marine aerosols) and road salt are main sources of sodium and chloride, ambient concentrations of Na + and Cl − related to PM 2.5-10 (coarse dust, ambient particles with aerodynamic diameters exceeding 2.5 and not greater than 10 µm) are generally higher than the concentrations of PM 1 -and PM 2.5 -related Na + and Cl − (Table 1).It is clear that in Zabrze, Na + and Cl − are related mostly with fine dust particles [26,32].PM 2.5 -related Na + and were respectively 80 and 85% of their total concentration in the air of Zabrze.The highest concentrations of PM-related Na + and , occurred in similar particle sizes range, as in the case of highest SO  , 3 and 4 concentrations (Table 2).Both, Na + and Cl − , were characterized by unimodal mass size distribution and its maximum occurred in the range of 0.4 -1 μm (Figures 2(a) and (b)).This indicates the anthropogenic origin of these ions (combustion processes).It is most likely that Na + and Cl  , occur in the dust mainly as a sodium chloride.The concentration of K + , Mg 2+ and Ca 2+ associated with each of 13 fractions, did not exceed 53 ng•m −3 (Table 1).Masses of these cations were distributed differently among PM fractions.More than 95% of the total mass of K + was concentrated in the PM 2.5 , over 25% of which were PM 0.26-0.4and PM 0.4-0.65.Distribution of Ca 2+ and Mg 2+ masses among 13 fractions was more variable, although the share of PM 2.5-10 -related ions' mass, was much bigger than their contribution in the fine dust particles amount, and was more than 50% of total mass of these ions in the Zabrze air.
Potassium and calcium were characterized by unimodal mass distribution with a maximum-as in the case of 2 4 SO  3 NO  , Na + and Cl − -in the range of 0.26 -0.65 μm (Figure 1(b)), whereas magnesium was determined with multimodal size mass distribution, without clearly dominant maximum.Highest potassium concentrations occurred for particles in the range of 0.17 -1 μm (Table 2).However, higher Mg 2+ and Ca 2+ concentrations occurred for particles with an aerodynamic diameter larger than 2.5 μm.Therefore, it seems that K + and Ca 2+ may be present in the compounds with , SO  NO  , Na + and Cl − ions, and their most probable source in Zabrze air are combustion processes.Relatively high proportion of Mg 2+ in the coarse fraction of particulate matter, proves that mechanical processes, including re-suspension of the soil and road dust could have had an influence on these ions concentration levels.
To assess the neutralizing capacity of occurring simultaneously in the air sulfates and nitrates by ammo-nium ion, neutralizing ratio (NR) was calculated for each fraction of particulate matter.NR is the ratio of 4 NH  concentration (in normal equivalent, neq•m −3 ) and the sum of and concentrations (in neq•m −3 )-Table 3.
For particles not greater than 1.6 µm, NR values ranged from ≈ 1 (PM 0.65-1 , PM 0.17-0.26, PM 0.108-0.17and PM 0.06-0.108 ) to 1.82 (PM 0.26-0.4).It means that the amount of 4 related to these dust fractions, was sufficient to neutralize sulfuric and nitric acid completely.This result also proves that ambient fine dust (PM 1.6 ) in Zabrze is alkaline (NR ≥ 1).(in neq•m −3 ) is satisfied.Therefore, the concentration of (NH 4 ) 2 SO 4 may be estimated from the formula: The concentration of (NH 4 ) 2 SO 4 associated with particles ≤ 1.6 µm, fit within the limits of 615.
(NH 4 ) 2 SO 4 and NH 4 NO 3 concentrations sum share, in a total SIA concentration for fractions of particles ≤ 1.6 µm, is shown in Table 3.For the fraction with ((NH 4 ) 2 SO 4 +NH 4 NO 3 )/SIA value exceeding 1, the share is overestimated.Still, stoichiometric calculations that have been carried out, show that these two compounds constitute the entirety of SIA in ambient particles not greater than 1.6 µm.The most probable distribution of (NH 4 ) 2 SO 4 and NH 4 NO 3 concentrations between the sum of these compounds concentrations were obtained for PM 0.65-1 and PM 0.4-0.65 , where the share of ((NH 4 ) 2 SO 4 + NH 4 NO 3 ) in the SIA did not exceed 100%.There are also these two fractions, in which the predominant part in the SIA takes ammonium sulfate, while the concentra-tions of these two dust fractions in the air are the highest of all 13 (Table 2).
NR for particles greater than 1.6 µm, was much smaller than 1 (Table 3).However, it doesn't mean that ions associated with these particles are not fully neutralized.The concentration sum ratio of anions to cations (Σ cations /Σ anions , in neq•m −3 ) is in the range of 1, for all frac-tions.
In all fractions of particles greater than 1.6 µm, the concentration ratios of and 4 (in neq•m −3 ) is considerably higher than 2.67.Also the relation SO  .Thus, there was not enough of 4 to form ammonium nitrate.Therefore, the (NH 4 ) 2 SO 4 concentration for particles greater than 1.6 µm, can be calculated from the for mula: The concentration of (NH 4 ) 2 SO 4 associated with particles greater than 1.6 µm, ranged from 3.67 ng•m −3 (PM 4.4-6.8 ) to 43.34 ng•m −3 for PM 1.6-2.5 .
It is impossible to determine precisely concentrations of all compounds constituting the secondary inorganic aerosol in Zabrze, still, estimating on the basis of stoichiometric relations.However, it can be shown that the amount of in the particles greater than 1.6 µm is enough to react the whole .
Using values listed in Table 3, it can be concluded that for PM >1.6 , there was not enough sulfate ion to complete reaction of calcium ions .

 
ex ''-SO 0      Therefore, it can be concluded, that the secondary inorganic aerosol in Zabrze, in the case of compounds occuring in particles greater than 1.6 µm, is mainly composed of ammonium sulfate, potassium sulfate and calcium sulfate.Certainly, there are also nitrates in these particles, however, in contrast to particles not greater than 1.6 µm, there is no ammonium nitrate but probably NaNO 3 and/or Ca(NO 3 ) 2 .

Conclusions
Most of ions' concentrations in Zabrze were comparable to concentrations presented in the literature.Generally, higher concentrations than in Zabrze are listed in Asian countries, this concerns particularly to , NO  NH  mass, was concentrated in particles with an aerodynamic diameter ≤ 1 micron.Na + and Cl − were also mostly associated with fine dust particles.Ions mentioned above, as well as K + and Ca 2+ , had similar mass size distributions, and generally, maxima of these distributions were in the same particle size ranges.This indicates the anthropogenic origin of seven of eight analyzed ions (combustion processes), associated with dust in Zabrze.
Relatively high proportion of Mg 2+ in the coarse fraction of particulate matter, proves that mechanical processes, including re-suspension of the soil and road dust could have had an influence on Mg 2+ concentration in the air.
In particles not greater than 1.6 µm, the amount of ammonium ion is sufficient to neutralize sulfuric and nitric acid, therefore, in dust precursors gas conversions, ammonium sulfate and nitrate are formed.In fractions of particles greater than 1.6 μm, the amount of ammonium ion is not sufficient to neutralize the nitric acid.Therefore, in these fractions, inorganic aerosol is composed of ammonium sulfate and other compounds, including K 2 SO 4 and CaSO 4 , and also NaNO 3 and/or Ca(NO 3 ) 2 .

Figure 1 .
Figure 1.Location of the sampling point.
concentrations in neq•m −3 ).It means that in these PM fractions,

4 2 4 SO
 NH  , K + , Mg2+ and Ca 2+ .Concentration of associated with fine dust in Zabrze was extraordinarily high, comparing to values recorded in other parts of the world.Cl Sulfates, nitrates and ammonium had the highest concentration of the eight analyzed ions in Zabrze.More than