Shapes in submicron ammonium sulfate particles after long-term exposure on tree leaves

Assessing the effects of air pollutants, including aerosols, on trees is important for protecting forests in the future. This study determined the adsorption of particles on trees after 1- or 2-year long-term exposure (for 1 or 2 h/day) to submicron-scale ammonium sulfate (AS) particles using a field-emission scanning electron microscope (FE-SEM). Energy-dispersive X-ray spectroscopy (EDX) was also used to distinguish particles resulting from exposure from those present on the leaves under natural conditions prior to the 1- or 2-year exposure. We found submicron-sized AS particles were deposited on the leaf surfaces of four tree species after long-term exposure in a growth chamber < 70% humidity. These particles occurred as individual deposits without aggregation on the abaxial and adaxial surfaces. The particle shape deposited on the leaf surface in short-term (3–30 min) exposures in a growth chamber < 70% humidity was spherical with no corners, whereas that in long-term exposures was nonspherical flattened, angular, or irregular. Few micrometers was also observed, differing from 300 to 600 nm in diameter at exposure. These differences could be caused by the possibility that the particles have been deposited for a long time or that the humidity on the leaf surface has caused them to deliquescence and change shape after deposition. We hypothesized that these particle changes facilitate the uptake of AS into the leaf interior.