Aerosols are small particles suspended in the atmosphere. They can often be seen in a dark room in a streak of sunlight. Typical examples of aerosols include sea spray, smoke from fires, and dust. Aerosols typically exist in two modes commonly known as the accumulation mode and the coarse mode. The accumulation mode is formed when various atmospheric gasses condense to form new aerosol mass. These small particles coagulate in the atmosphere to form a final accumulation mode size distribution. It has been found that on average the peak diameter of the accumulation mode shifts to larger sizes as the concentration increases (Porter and Clarke, 1997). The coarse mode aerosols are created when wind blows either salt spray or dust from the earth’s surface. The size distribution of the coarse mode also tends to shift to larger sizes as their concentration increases. Figure 1 shows several cases of accumulation and coarse mode aerosol. Both the accumulation and coarse mode aerosol can be hygroscopic so that they are also holding water. Therefore the aerosol actual ambient size will depend on the relative humidity as well. As the relative humidity increases in a cloud, the aerosols continue to pick up water and grow into cloud droplets. Rain and gravitational settling are the most common way to remove aerosol from the atmosphere.