Elemental concentration and source identification of PM10 and PM2.5 by SR-XRF in Córdoba City, Argentina

Abstract

24-h samplings of PM10 and PM2.5 have been carried out during the period July 2009–April 2010 at an urban and at a semi-urban site of Córdoba City (Argentina). The samples in the PM2.5 fraction weighted in the average 71 ± 21 μg m⁻³ and 67 ± 18 μg m⁻³ respectively, whereas the samples of the same sites in the PM10 fraction weighted 107 ± 31 μg m⁻³ and 101 ± 14 μg m⁻³. The chemical composition of aerosol particles was determined by synchrotron radiation X-ray fluorescence (SR-XRF). Elemental composition was different in the two fractions: in the finer one the presence of elements with crustal origin is reduced, while the anthropogenic elements, with a relevant environmental and health impact, appear to be increased. An important but unmeasured component is likely constituted by organic and elemental carbon compounds. Multivariate analysis (Positive Matrix Factorization) of the SR-XRF data resolved a number of components (factors) which, on the basis of their chemical compositions, were assigned physical meanings.

Introduction

In urban areas, one of the main pollutants of concern is PM10 (particulate matter ≤10 μm in aerodynamic diameter). Particles that fall into this diameter range have been implicated as contributing to the incidence and severity of respiratory diseases. Size and chemical composition are two of the principal parameters that affect the way in which those particles correlate with population health. PM10 can penetrate deeply into human lungs. In addition, PM2.5 (particulate matter ≤2.5 μm in aerodynamic diameter) can contain a high proportion of various toxic metals, organic compounds, etc. High levels of PM10 and PM2.5 have been shown to decrease pulmonary function and exacerbate respiratory problems in respiratory-compromised people, i.e., asthmatics. In addition, studies have linked respiratory-associated hospital admittance with levels of particulate matter at concentrations below the current standards. A strong association between the fine air particulate pollution and mortality rates in six U.S. cities has been also reported (Dockery and Pope, 1994). Therefore, the increasing evidence indicating that fine particulate matter in the atmosphere is responsible for adverse effects on humans led to the imposition of regulative restrictions on PM2.5 and PM10. Thus, The United States adopted the National Ambient Air Quality Standard (NAAQS), which sets two standards for 24-h average: a limit of 150 μg m⁻³ for PM10 and 35 μg m⁻³ for PM2.5. On other hand, the EU legislation for air quality established a 24-h limit value of 40 μg m⁻³ for PM10 and 25 μg m⁻³ for PM2.5.

The toxicity of the particles is associated not only to higher particle mass, but also to variations in particle size, shape, and chemical composition. Furthermore, many trace chemical species in particles occur in the very fine size fractions, which can reach alveolar regions in the lungs. The chemical elements derived from anthropogenic sources are usually present in the fine fraction (<2.5 μm) while those derived from natural sources are usually present in the coarse fraction. Suspended road dust and soil dust are other potential source of elements. Anthropogenic elements are originated from different sources. Those emitted during the burning of fossil fuels are V, Co, Pb, Ni, and Cr and are mostly associated with particles in the PM2.5 fraction, although some particles are also present in the coarse fraction. Cr, Cu, Mn, and Zn are released into the atmosphere by metallurgical industries, and traffic pollution involves a wide range of trace element emissions that includes Fe, Ba, Pb, Cu and Zn, which may be associated with the fine and coarse particles (Marcazzan et al., 2001 and Smichowski et al., 2004). Other highly specific traffic-related elements include Sb (brake-pads), Pt and Rh (catalytic converters) (Bocca et al., 2006).

To characterize bulk aerosol samples in urban air quality studies, it is important to know the elemental composition of the particulate matter. Identification of trace elements in the sampled air can add substantial information to pollution source apportionment studies, although they do not contribute significantly to emissions in terms of mass. Spatial and temporal resolution are also important, since they will allow for a more profound source assessment of the complex urban air mix (traffic, industry, biomass burning) and to assess the influence of the local meteorology. Aerosol concentrations vary according to atmospheric conditions. For example, in Córdoba, particulate matter concentration is usually higher in winter months than in the rest of the year, because of the lack of rain and the persistent temperature inversions. During a campaign carried out by the City government in the period 1995–2001, this pollutant was the only one that has exceeded several times the 24-h standard. All the episodes were during winter time (Olcese and Toselli, 1997, Olcese and Toselli, 1998). Unfortunately these measurements have been stopped in 2001 and none additional study on particulate composition has been performed. This is aggravated by the fact that no additional air quality monitoring is currently underway by any governmental agency.

Elements of special interest are S, Si, K, Al, Ba, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, and Pb, since they may serve as fingerprints for different sources of particulate matter. One of the problems is that aerosol filter sampling with subsequent analysis is generally reaching its limit by the minimum quantity of material that has to be collected for subsequent analysis. Here, filter collection times were 24 h.

To achieve the abatement of aerosol pollution, it is necessary to identify sources of particulate matter pollution for which control measures may be possible. The method of source apportionment usually involves compositional analysis of aerosols. Plasma based methods such as inductively coupled plasma optical emission spectrometry (ICP-OES), and inductively coupled plasma-mass spectrometry (ICP-MS) are the most used techniques for routine aerosol multi-elemental characterization (Smichowski et al., 2004, Suzuki, 2006). However, due to the complex nature of the aerosol samples and the low concentrations involved, synchrotron radiation X-ray fluorescence (SR-XRF) analysis was utilized for this study. The technique satisfies the requirements of being sensitive, element-specific, and accurate and has proved to be a powerful tool for the elemental analysis of ambient air samples (Bukowiecki et al., 2008). SR-XRF is applicable to major, minor and traces constituents of atmospheric aerosols and is becoming an important tool in atmospheric chemistry (Cliff et al., 2003). Within the present work, bulk aerosol samples collected in Córdoba, were quantitatively analyzed by SR-XRF and their origins were identified using the multivariate factor analysis described in Ogulei et al. (2005). One of the main advantages of this technique, compared to conventional XRF, resides on the fact that the count accumulation interval per individual sample spot is substantially shorter (Bukowiecki et al., 2008). The detection limits achieved with SR-XRF experiments are in the range of ng m⁻³ of ambient air, allowing the determination of major and minor components of the filters without additional sample treatment.

Hence, the goals of this work are to quantify the elements present in the PM10 and PM2.5 of Córdoba, to correlate them with possible sources, to study seasonal variations on the particulate matter composition and to compare current levels in Córdoba City with other cities of the World.