Atmospheric pollutant monitoring by neutron activation analysis of epiphytic lichens

Abstract

The epiphytic lichen Canoparmelia texana was used to monitor atmospheric pollution in the São Paulo metropolitan area, SP, Brazil and to obtain distribution maps of element concentrations. From July 2003 to February 2004, lichens were collected from the bark of trees in 23 sites all near automatic monitoring stations belonging to the Environmental Protection Agency of the State of São Paulo and analyzed by instrumental neutron activation analysis. For the analyses, lichen samples were first cleaned by examining them under a stereomicroscopy to emove foreign materials, and then immersed in purified water for about 3-5 minutes. Then the samples were freeze-dried and ground to a powder using a vibratory micro mill. Samples and synthetic element standards were irradiated for long and short periods at the IEA-R1 nuclear research reactor under thermal neutron flux of about 5 x1012n cm-2s-1 . The induced gamma activities of the irradiated samples and standards were measured using a HGe detector coupled to a gamma ray spectrometer. Concentrations of the elements As, Ba, Br, Cl, Co, Cr, Cs, Fe, K, La, Mn, Mo, Na, Rb, Sb, Sc, Se, U and Zn were determined in the lichens and these results were submitted for sampling sites classification by cluster analysis. The cluster analysis confirmed the site groups of different levels of pollution due to industrial and vehicular emissions. In the distribution maps of element concentrations, higher concentrations of elements were observed in the vicinity of industries. High concentrations of elements found in lichens from the Santo André sites may be associated to the emissions from industries and a petrochemical complex located in this region. The highest concentration of Co found in lichens from São Miguel Paulista site is probably due to the emission from a metallurgical processing plant that produces this element. The accuracy and the precision of the results were evaluated by the analyses of certified reference materials IAEA 336 Lichen and INCT-MPH-2 Mixed Polish Herbs. Results obtained for these certified reference materials showed a good precision and a good agreement with the certified values for most elements.