Worldwide, the air is becoming cleaner. This trend started in Europe and North America in the 1980s and is now also visible in China. Cleaner air is good news for health, but it also affects the weather and climate. There is less cloud cover and fog and more sunshine.
Particulate Matter or Aerosols
Particulate matter is a notorious component of air pollution. It consists of very small solid or liquid floating particles, also known as aerosols. Some are directly emitted as particles into the air, while others are formed by chemical reactions of various gases in the atmosphere. The most well-known of these reactions is the formation of sulfate particles through the oxidation of sulfur dioxide. Aerosols can also have a natural origin, such as sea salt, desert sand, or pollen.
Aerosols Block Sunlight
Aerosols influence the weather and climate by preventing incoming sunlight from reaching the Earths surface. This can occur in two ways, directly and indirectly. Directly, by reflecting or absorbing sunlight. In the first case (reflection), some of the sunlight is reflected back into space. In the second case (absorption), instead of warming the Earths surface, the atmosphere is warmed. In both cases, less sunlight reaches the Earths surface. Since most aerosols reflect sunlight (soot particles are the exception: they absorb sunlight), the direct effect is cooling.
Aerosols Influence Cloud Formation
The indirect effect occurs through clouds and is therefore also called aerosol-cloud interaction. Aerosols influence the formation of clouds. Clouds consist of small water droplets and/or ice crystals. These form when the air cools and water vapor condenses or freezes. You can observe this effect on cold winter days when the moisture in your exhaled breath condenses and forms a breath cloud.
Droplets, however, need an aerosol particle as a nucleus to which the water molecules can cling. Without a nucleus, there is no cloud. If there are few aerosols that can serve as nuclei, only a few, but large droplets will form. If there are many aerosols, the cloud will consist of many small droplets.
Whether a cloud consists of large or small droplets makes a significant difference. Large droplets are heavy and fall as precipitation sooner. Small droplets, on the other hand, remain suspended in the air longer. Thus, the cloud lasts longer. Moreover, clouds with a large number of small droplets reflect more sunlight than clouds consisting of a smaller number of large droplets.
More aerosols therefore lead to more and whiter clouds. More, because the clouds rain out less quickly and can thus exist longer. And whiter, because they consist of a larger number of small droplets. As a result, less solar radiation reaches the Earths surface. The indirect effect of aerosols also has a cooling effect on the climate.
Fog on New Years Eve
How the amount of aerosols affects condensation is often clearly visible on a cold and windless New Years Eve. When setting off New Years fireworks, large amounts of particulate matter are generated (according to estimates, this accounts for about 5% of the total national particulate matter emissions in a year). Just after midnight, the amount of aerosols suddenly increases (this can be seen in the animations of dust radar). Depending on the weather conditions, many small droplets can then form, resulting in fog.
An example is the New Years Eve of 2007/2008. For this night, image 1 shows the maximum visibility, relative humidity, and the presence of fog in De Bilt. Just after midnight, visibility drops from over a kilometer to less than 200 meters – fog has formed. In the eastern parts of the Netherlands, visibility that night was even less than 50 meters.
