Phase I—Hypothoses
Stomata are microscopic pores on the underside (abaxial) of the leaf. These allow air into and out of the leaf, which is how the plant takes in CO2 and lets out O2, and allows water vapor out in the process of transpiration. As air passes through the stomata, most of the airborne particles will not pass through the stomata but will rather land on the leaf’s outer surface. This is similar to a filter, where air is pulled through the filter by an air pump and the airborne particles deposit on the filter surface. IF this air flow is the major cause of particles depositing on the leaf, the result will be that the concentration of particles on the abaxial surface of the leaf will be higher than that of the top surface (adaxial) because the airflow through the stomata will be pulling more particles onto the bottom surface.
There is a certain amount of force needed for particles to stick to a surface. This amount is greater depending on the size of the particle. Because the airflow through the stomata is not very powerful, only the smaller particles will stick to the bottom surface. The particles on the top surface of the leaves will mainly be from the settling of dust. Because settled particles are mostly larger ones, those found on the top surface will be mostly larger. Therefore, analysis of the particle sizes on the leaves will show that particles on the tops of leaves are, on average, larger than those on the bottoms of leaves.
Because the particles deposited on the leaves are from the air, analysis of the particles will provide information about the particulate air pollution in the area sampled. The higher the concentration of PM in the area, the higher the concentration of particles on the leaf surface will be. The sizes and chemical compositions of the particles on the leaf surface wlll be representative of the airborne particles in the sampled area.
Urban areas tend to have the highest concentrations of airborne PM, because of greater numbers of motor vehicles and high amounts of activity. This is followed by suburban areas, and then rural areas, with decreasing amounts of PM [Abraham, M.E., 1998]. Therefore, if leaf sampling is accurate, particle concentrations on leaves collected from these three areas will decrease from urban to suburban and from suburban to rural.
Different types of leaves tend to have differences in several aspects of their surfaces. Some types of leaves have greater surface rigidity or roughness than other leaves, which may affect their stickiness or particle solubility. Stickier leaves would be better for collecting particles because more particles would stick to their surface. Therefore, some types of leaves may be better for use in this type of analysis than others.