Atmospheric Aerosols 


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.


Figure 1. Aerosol size distributions (volume distribution) showing a range of accumulation mode aerosol size distributions at smaller sizes (~0.3 μm diameter) and a range of coarse mode aerosol size distributions at the larger sizes (~ 10 μm). Further discussion of the aerosol size distributions is given in Porter and Clarke, 1997.

Hawaii Aerosols

Due to Hawaii's remote location in the Central Pacific, it typically experiences clean air with low aerosol loading which consist of coarse mode sea salt and accumulation mode sulfate aerosols. In clean marine air the sulfate accumulation mode concentrations are often 0.5 ug/m3. Sea salt concentrations depend on wind speed and for typical trade wind speeds of 7 m/s the aerosol mass is ~20 micrograms/m3. Examples of typical size distributions are shown in the next figure.

Example of typical aerosol size distributions near Hawaii for a aerosols in equilibrium at a relative humidity of 75%. In the free troposphere the accumulation mode concentration (red line) is small with the peak diameter shifted to small diameters. In the clean marine boundary layer the accumulation mode is small but occurs at slightly larger sizes (~0.35 um diameter). The coarse mode sea salt is shown for a wind speed at 7 m/s. The accumulation mode for a volcanic smog (vog) condition is shown (~12 ug/m3).

Further discussion 

Asian Dust and Pollution

Hawaii Volcanic Aerosols

Sea Salt

Vertical Distribution of Aerosols Near Hawaii

Biological Aerosols