How negative ions decrease

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Negative ions in the air decrease or seem to decrease by:

1. Neutralization with positive ions
2. Diffusion
3. Absorption by dust

1. Positive and negative ions are electrically cancelled out.
2. Negative ions widely spread in the air, resulting the number of negative ions per cc (i.e. ion density pcs/cc) decreases.
3. This is applicable to air ions only. After absorption, the mass of the combined object (i.e. negative ions and dust) is much more than ions themselves and it is regarded that they are no longer ions (small ions). This is because it is generally understood that negative ions have small mass. Negative ions neither disappear nor lose the electronic properties.