Characterisation of individual ambient aerosol particles retained on taxus needles washed with water

Abstract

Scanning electron microscopy interfaced with automated image analysis and X-ray energy spectrometry (SAX) was used to characterise the size and elemental composition of ambient aerosol particles retained on Taxus needles washed with water. Washing with water for 45 min removed 96% of the particle mass from field-sampled Taxus needles, but the number of particles was reduced by only 63%. To address the removal of ‘recently deposited’ particles, Taxus seedlings were grown with twigs isolated in a ‘particle-free’ environment, and then were exposed for four days at an industrial site. Washing of these needles with water for 60 min removed 97% of the particle mass and reduced the number of particles on the needles by 85%. The percentage distribution of particles in 29 chemical classes, defined on the basis of the relative intensity of X-rays collected from individual particles for up to 27 elements (Z > 10), changed dramatically after washing due to effective removal of the ‘most-soluble’ classes of particles. Particle population percentage distributions were remarkably similar for washed field needles, washed industrial site (short-term exposure) needles, and unwashed field Taxus needles collected at the industrial site after rainfall events. On an elemental mass basis, sodium, phosphorus, sulphur, chlorine, and calcium were removed most efficiently; iron was removed least efficiently. Sample calculations demonstrate the importance of particle retention on washed needles, particularly in the determination of trace element concentrations in washed plant tissue.