¹ This nitric acid reaction is the way in which nitrate is "fixed" in the atmosphere by lightning discharge. Raindrops in a severe electrical storm are slightly acid because of dissolved nitric acid from this process. Of the nitrate that is fixed every year, a little over half comes from bacteria, a little over one third from industrial processes, and approximately 10% from atmospheric fixation during thunderstorms. Thunderstorms were an even more important means of chemical synthesis in the period before life arose, when there was no competition, conscious or unconscious, from living organisms.

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