In this experiment, we propose to use lichen species diversity, overall types of lichen present, and relative abundance as indicators of SO2 levels. We plan to examine, collect, and identify lichens present on rocky outcroppings in the Hudson River in three different areas: urban, semi-urban/rural, and rural. We will then compare these data with measured levels of atmospheric sulfur dioxide. The objective is to determine whether there is a correlation between air quality (specifically sulfur dioxide levels) and the lichen species present, as well as the relative health of their populations.