Among the classifications of aerosols, there was a method to consider both Aerosol Optical Depth (AOD) and aerosol size distribution information (FF). The interrelationship between AOD and FF can be used to distinguish the three main air pollution aerosols: dust aerosols, sea-salt aerosols, and pollution aerosols. First, looking at the FF and AOD values of dust aerosols, they mainly have features of high AOD (>0.3) and low FF (<0.65). Second, pollution aerosols are observed in high FF (>0.65) and wide AOD ranges. Most of the last sea-salt aerosols showed AOD (<0.3) values with a lower AOD of 0.1 or less, and FF was larger than dust aerosols and lower than pollution aerosols.
To sum up, the FF value is in order of dust < sea-salt < pollution aerosols, and the AOD value is in order of
sea-salt < dust ≤ pollution aerosols. Since aerosols effectively reflect solar radiation with higher AOD and FF values, the most effective aerosols are pollution aerosols.
Types of pollution aerosols are divided into primary particulate matter directly emitted from sources such as cars, factories and chimneys, gas pollutants emitted into the atmosphere, and secondary particulate matter (nitrate, sulfate, ammonium, etc.). Then which of these substances is the most effective?
Most aerosols reflect sunlight, but some absorb sunlight, mainly depending on the composition and color of the particles. Generally, bright or translucent particles reflect light and dark particles absorb sunlight. First, aerosols consisting of sulfate and nitric acid reflect almost all radiation. Therefore, soluble ions such as sulfate and nitrate have high scattering coefficient values. Considering the definition of SSA, (overall AOD - absorption AOD)/(overall AOD), sulfate and nitrate would have the value close to 1, which have large amount of sacattering AOD. In addition, brightly colored particles also tend to produce bright, long-lived clouds in aerosol indirect effects that change Earth's albedo by creating clouds.
In conclusion, pollution aerosols with large FF and AOD values may be the most effective aerosols. Furthermore, among pollution aerosols, sulfate and nitrate particles have the closest SSA value to 1, which effectively causes light scattering. Thus, we selected sulfate and nitrate for the most effective aerosols to use as artifical curtains.