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.
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).
Sea Salt
Vertical Distribution of Aerosols Near Hawaii
Biological Aerosols