Bursera fagaroides bark as a bioindicator for air particle pollution using magnetic properties

Abstract

Shrub barks can accumulate airborne particles and seem to be suitable biological monitors for assessing air pollution. We studied the bark of the species Bursera fagaroides as a biomonitor of air pollution in an urban area of Mexico. Samples were collected in dry and rainy seasons during 2011 and 2012, in 28 sites which are exposed to magnetic pollutants emitted by different human activities (industrial point-sources and vehicular emissions). Environmental magnetism studies allow determining mineralogy, concentration and grain size of magnetic particles accumulated in barks, providing information on the sources and characteristics of their emitted particles. Magnetic properties of all samples were determined by environmental magnetism method. The highest values of magnetic concentration dependent parameters were observed in industrial and vehicular traffic sites. The mean values of saturation of isothermal remanent magnetization SIRM for dry seasons are two to threefold (SIRM = 0.8–1.2 × 10−3Am2kg−1) the mean value for rainy season (SIRM = 0.4 × 10−3Am2kg−1). The integrated magnetic analysis indicates the presence of ferrimagnetic minerals, mostly magnetite-like (remanent coercivity Hcr = 31.6–38.7 mT) particles of small sizes (1 – < 0.1 μm). Selected samples were observed by scanning electron microscopy, revealing the presence of such iron-rich particles hosted in species’ lenticels. These results demonstrate the usefulness of the species' cortex as a biomonitor of air pollution, with an affordable and immediate application, and raise new questions about what type of physiological mechanism is involved for trapping magnetic particles.