Caused by atmospheric electric fields, automotive exhaust gas, industrial smoke, etc.
1. Negative ions that naturally exist at or near the earths surface are usually created by radiant rays emitted from the core of the earth, cosmic rays and thunderbolts.
The same amount of positive ions are also produced during the process.
Only negative ions are carried to the sky by atmospheric electric fields stretching from the sky to the earth, which results in more positive ions at ground level.
2. Automotive exhaust gas and industrial smoke contain more positive ions than those negatively charged.
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