Why is the rain dirty?

This question should be answered from two aspects, one is the process of precipitation formation and the other is the process of precipitation falling. Because these two processes directly determine whether the rain is dirty or not.

Particles that form precipitation are called precipitation particles, and liquid precipitation in a narrow sense is raindrops, which are formed by the growth process of cloud droplets.

CCN, cloud condensation nucleus, also known as atmospheric condensation nucleus. Water vapor in the atmosphere can condense on it, forming suspended particles of small water droplets, usually called condensation nuclei. Because particles suspended in the atmosphere can play the role of condensation nuclei to varying degrees, atmospheric condensation nuclei and atmospheric aerosol particles are actually synonyms.

Atmospheric condensation nuclei are composed of solid substances, solution droplets or a mixture of the two, and their chemical composition is very complex. The most common compounds are chlorine, nitrogen, carbon, magnesium, sodium, calcium and so on.

For clean areas (islands and continental shelves affected by oceanography, areas with less human industrial activities, such as the Arctic and Antarctic), CCN concentration is low, condensed nucleus composition is single, air is clean, and raindrops are slightly clean.

For polluted areas, the concentration of CCN in continental air mass is very high, coupled with the wet deposition process of pollutants, and finally there are raindrops formed by various inorganic salts, organic matter, dust and microorganisms. Such rain is obviously dirty.

It can be seen that pollutant particles, as condensation nuclei, will be wrapped by raindrops during rainfall and brought to the ground through the cleaning effect of precipitation particles. Therefore, clean precipitation must ensure the cleanliness of the air to a great extent.