rain water is not pure H20. Pure H20 is not typically found in nature, only in analytic chemical laboratories and some factories. Carbon Dioxide from the atmosphere is absorbed by water molecules and and turns into carbonic acid, which is found in all rainwater. It is slightly acidic, but typically not harmful to most creatures/plants on Earth. Other chemicals can make a stronger acid in rainwater and thus be harmful.
The water in a raindrop is one of the cleanest sources of water available. Rainwater can absorb gases such as carbon dioxide, oxygen, nitrogen dioxide, and sulfur dioxide from the atmosphere. It can also capture soot and other microscopic particulates as it falls through the sky. Nevertheless, rainwater is almost 100% pure water before it reaches the ground.
Could the "microscopic particulates" account for the algae growth soon after?


