Phase II—Hypothoses

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 airborne particles in the sampled area will be correlated with those of the particles on the leaf surface

Urban areas tend to have the highest concentrations of airborne PM, because of greater motor vehicle traffic and other sources of PM. This is followed by suburban areas, and then rural areas. 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.

Smaller particles can travel very far before depositing. Therefore, small particles from the city will travel out to the rural locations and deposit on the leaves there, while the larger particles, which settle faster, deposit in the suburban areas closer to the city and in the city itself. Therefore, the average sizes will be smallest in the rural area, and largest in the suburban area. The urban leaf average will be in between because it has a wider range of sizes than either rural or suburban.

One of the main concerns with leaf sampling is the effect of rain on the particles on the leaves. Rain has been shown to remove particles of certain types and sizes from leaf surfaces, as well as to form new, "salt-type" particles on the leaves. Therefore, it is important to know what effects rain has on analysis. I am hypothesizing that leaves collected after the first rain following a dry period, as well as those rinsed artificially, will have lower particle concentrations, smaller average particle size, and will show changes in the average elemental compositions of particles.

One of the most important parts of determining the ability of leaves to act as PM samplers is the comparison of leaves to the proven method of sampling, which is the use of aerosol filters. Although the filters that will be used in this experiment were collected at different times than the leaves, they will show the same differences between rural, suburban, and urban areas as the leaf samples, in both size data and elemental composition.