Ultrafine Metal Concentration in Atmospheric Aerosols in Urban Gwangju , Korea

PM10, PM5, and ultrafine (< 0.132 μm) mass concentrations, and metals (As, Be, Ca, Cd, Fe, K, Mn, Ni, Pb, Sb, Se, and Zn) in ultrafine particles were determined in urban Gwangju, Korea during the sampling periods of 4/2/2007-4/20/2007 in spring, 8/2/2007-9/12/2007 in summer, 11/19/200712/2/2007 in fall, and 1/16/2008-2/3/2008 in winter. Data showed that PM10 mass concentration was the highest in spring due to the contribution of long-range transported and fugitive dust particles, whereas mass concentration of ultrafine particles had no seasonal variation and was not significantly affected by dust particles. Enrichment factor (EF) for each metal and Principal Component Analysis (PCA) among ultrafine metals were conducted to evaluate effects of anthropogenic and natural sources on ultrafine metals and to determine association among metals. We found that Fe, Ni, Zn, Sb, and K exhibited relatively higher fraction in ultrafine size and had higher EF values (i.e., anthropogenic). Results from wind-dependent metal concentrations suggested that Zn and Ni in ultrafine particles originated from metallurgical sources from a nearby industrial complex. We also found that during an Asian dust event, Ca concentration increased most significantly among ultrafine metals.


INTRODUCTION
, exhaust emission from vehicles, industrial processes, and waste incineration can be anthropogenic sources for metal aerosols in the ambient atmosphere (Wang et al., 2008;Lin et al., 2008;Srivastava et al., 2008).
Erosion, surface dusts, volcanic activity, oceans, and forest fires can contribute as well (Karanasiou et al., 2007;Zhang et al., Combustion of fossil fuels and wood, natural metal aerosols (Buerki et al., 1989;2008).High concentration and/or long exposure of metals may cause toxic effects on human health, even though they constitute a small fraction of PM (Berggren et al., 1990;Devries et al., 1996).
Determination of metals composition of inhalable particles is important in determining their potential impact on human health (Allen et al., 2001).
Depending on emission sources, rates of wet and dry deposition, and physical/chemical transformation, concentration and size distributions of metal aerosols will vary (Gu et al., 2008).Longrange transport of aerosols will also affect concentration and size distribution of metals (Xu et al., 2008).Typically, particles in the accumulation mode have a long residence time and can be transported over a long distance affecting remote regions from sources.Therefore, size-resolved metal concentration will provide information on the toxicity level of metals, as well as on transport behavior in the ambient atmosphere and on inhalation characteristics of the human respiratory system.Recently, ultrafine particles in the ambient atmosphere have been of particular interest because they provide a high surface area-to-volume ratio, leading to higher toxicity and reactivity (Dockery et al., 1994;Peters et al., 1997;Oberdörster, 2000).However, there has been limited information on size-resolved metal concentration, especially in ultrafine fraction.
In this study, chemical speciation of Teflon filters with a 37 mm diameter and 2 μm pore size (TefloTM, Gelman Laboratory, USA) with particle collection efficiency of 99.7% for 0.3 μm-sized particles were used for collection of particles (Kodavanti et al., 2005).Flow rates were checked before and after the sampling period (~ 24 hours) and did not significantly deviate from the initial value (9 L/min for the impactor and 5 L/min for the minivol sampler).The filters were weighed before and after the sampling using a microbalance (SartoriusTM, MC-5, USA)  Choi et al., 2001;Kim et al., 2002;Senlin et al., 2007).However, we observed no significant variation of ultrafine particle mass concentration.Previously, particles substantially increased in summer in this area due to a strong photochemical activity (Park et al., 2008).Our PM data suggest that even when ultrafine particles exist in large number in summer, their contribution to PM 10 mass is not significant.Enrichments Factors (EFs) of metals can be used to evaluate the effects of anthropogenic and natural sources on metals, relative to the earth's crustal abundances.

Metal concentrations
EFs of metals (M) were calculated by using Fe as a reference for crustal material and crustal fractions for the metals, given by: EF= [M/Fe] air / [M/Fe] crust . (1) The EF should be much higher than a unit suggesting that these metals may originate from combustion-related anthropogenic sources.Cd has a high EF (> 700), but a low ultrafine fraction, suggesting that this metal might not be related to combustion sources.
We also conducted Principal Component Analysis (PCA) for ultrafine metals and their results are summarized in Table 1, which excludes the Asian dust event.The PCA is usually used to identify the patterns of correlations among observed elements (variables), and to reduce such large data sets into a small number of principal components (PCs) without losing significant information from the origin of the data.We conducted Varimax (orthogonal) rotation to determine principal components having an eigenvalue larger than 1.With the PCA method, we are able to infer whether metals come from a similar source or not.As, Be, Park et al., Aerosol and Air Quality Research, Vol. 8, No. 4, pp. 411-422, 2008

with 1 Fig. 1 .
Fig. 1.Map of the sampling site including possible local sources.
Metal concentrations in PM 10 averaged over the whole sampling period are shown in Fig. 4. Based on the field blank experiments, concentration of metals in the field blanks were less than 0-5 ng/m 3 .The most abundant elements in the PM 10 were Ca (1484.58 ng/m 3 ), Fe (691.67 ng/m 3 ), K (680.73 ng/m 3 ), and Zn (170.72 ng/m 3 ) followed by Pb, Mn, and Sb in 60-10 ng/m 3 .Concentrations of Be, Cd, and Se were below 5 ng/m 3 .The Ni concentration exceeded the PM 10 limit (~ 5 ng/m 3 ) established by the European Union.Metal concentrations in ultrafine particles are also included in Fig. 4. We observed that ultrafine fraction over PM 10 for Ni, Sb, Zn, Fe, Ca, and K metals was greater than 15%.Anthropogenic sources such as combustion of fossil fuels and biofuels, exhaust emission from vehicles, and industrial processes are responsible for metal aerosols in ultrafine fraction, rather than natural sources such as erosion and surface dusts that would contribute to particles present in the larger particle size range (Espinosa et al., 2001).Our sampling site is usually affected by multiple sources, such as Hanam industrial activities, traffic from a nearby highway, residential heating, and biomass burning.During the sampling period, western, southwestern, and south winds were dominant at our sampling site.The southwestern wind typically passed over the industrial complex before reaching our sampling site.To find possible emission sources for ultrafine metals, we examined their wind-direction dependent concentrations.As shown in Fig. 5, ultrafine Ni and Zn metals had a high concentration during days with prevailing southwest wind, while Fe and K did not have such dependence, suggesting that ultrafine Ni and Zn metals might be transported from the Hanam industrial complex (~ 4.2 km away).
to consider sources of metals as anthropogenic origin.EFs of ultrafine metals as a function of ultrafine fraction are shown in Fig.6.Se and Sb have the highest EF (~ 7000), which were not included in Fig.6for clarity.Ni, Pb, and Zn have high EFs (> 400) and ultrafine fractions (> 0.10),

Fig. 2 .
Fig. 2. Daily average of PM 10 , PM 5 , and ultrafine particle mass concentrations over the whole sampling period.

Fig. 6 .
Fig. 6.Enrichment Factor (EF) of ultrafine metals as a function of ultrafine fraction in PM 10 .