Thus, as raindrops fall to the ground, they pass through the dirty air and take in dirt. Therefore, what is contained in the raindrops varies from time to time. Sometimes it contains fine dust blown up by strong winds, and sometimes it contains salt from the sea. In the spring, there are also various kinds of pollen in the air.
Another recent global problem is air pollution caused by fumes from cars and factory engines. The smoke from cars and factories contains sulfur oxides and nitrogen oxides, which dissolve in the rain dust. When many of these substances are dissolved into the rain droplets, the rain becomes acid rain.
Acid rain is now a serious problem because it damages trees, kills fish in lakes, and makes concrete brittle.