Sources, distribution and variability of airborne trace metals in La Plata City area, Argentina

Abstract

Airborne particulate trace metals have been measured bimonthly during day and night hours in four permanent stations located in residential, industrial and commercial sectors of La Plata City region, to characterize the sources and variability of atmospheric inputs. Airborne trace metal regional averages (Pb 64±62, Cu 30±27, Mn 26±20, Zn 273±227, Fe 1183±838, Ca 5343±3614, Mg 1472±967, Cr 4.3±2.4, Ni 3.2±3.5 and Cd 0.41±0.42 ng/m³) are comparable to the values reported for not grossly polluted cities and below the general trend described for urban particulates. Two- and three-way analysis of variance and variance components tests (P<0.05) were performed to assess the contribution of the diurnal (day vs. night), spatial (inter-station) and temporal (inter-month) components of variability. Trace metal concentrations followed the behavior of total suspended particles with higher concentrations during the day and at the Downtown station and lower at the Residential site. In general, spatial and temporal variations prevailed over diurnal differences. Spatial differences were clearly most significant for Pb, which presented higher values at the Downtown site reflecting the importance of motor exhaust inputs. In contrast, diurnal differences were more important for Mn due to increased dust resuspension during day hours. A seasonal trend with concentrations usually increasing during winter months and decreasing in spring–summer was also detected. Enrichment factors (EF) were calculated to evaluate anthropogenic versus natural element sources. High EF (21–376) were obtained for Pb, Zn, Cd and Cu reflecting the importance of anthropic inputs. The comparison with EF calculated for high-emitting vehicle particle emissions indicated that motor exhausts are the most important source of these elements in La Plata region. In contrast, the EF calculated for Mn, Cr, Ni, Ca and Mg were low (1.3–7.5) suggesting chiefly natural sources, i.e. soil-derived dusts.

Introduction

Man-induced mobilization of trace metals into the biosphere has become an important process in the global geochemical cycling of these elements (Nriagu and Pacyna, 1988). This effect is most evident in urban areas where several stationary and mobile sources (industrial activities, energy production, construction, urban waste treatment, vehicle exhausts) release large quantities of trace metals into the atmosphere, soils and aquatic ecosystems very often exceeding the natural emission rates. The atmosphere, in particular, is a key compartment in the global geochemical cycling of trace elements (Lantzy and Mackenzie, 1979). The main inputs of trace metals to the atmospheric cycle are strongly related to particle emission processes. For most of the toxic trace metals (e.g. Pb, Cd, Zn, Ni, Cu) anthropogenic inputs are more important than natural sources such as continental dust, volcanic dust and gas, sea spray, and biogenic particles (Bertine and Goldberg, 1971, Nriagu, 1979, Hriagu, 1989).

From a toxicological perspective, the airborne particulate material has important health implications, basically through the inhalation of small particles with diameters of 10 μm or less which can be absorbed in the alveolar region of the lung (Hileman, 1981). It is well established that these inhalable particles have higher concentrations of many trace elements, such as Pb, Cd, Zn, Cr, Ni, Mn and Cu (Natusch et al., 1974, Hlavay et al., 1992), and thus, are considered of major health concern. However, some sources have been reported to have emissions highly enriched in toxic metals without a significant increase in PM10 levels (Sweet et al., 1993). Thus, from a biogeochemical perspective, characterization of the total suspended fraction is relevant to identify the sources and variability of the airborne material.

In this study we determine the concentration of selected trace elements in airborne particles from La Plata region in order to: (1) assess the heavy metal concentration status of the region; (2) characterize the principal sources of atmospheric particulate trace metals; (3) characterize diurnal versus nocturnal trends; and (4) evaluate spatial and temporal variations.