Where does excess formic acid come from in the air?

Acid rain is usually associated with a “bad” environment. Sulfur and nitrogen oxides contained in the combustion products of coal, gas or oil products, falling into raindrops, are converted into solutions of the corresponding acids. Of course, for the time being, concentrated sulfuric acid will not drip on us from the sky, like on Venus, but there is nothing good from acid rains: they harm flora and fauna, acidify the soil and water bodies, spoil architectural structures. However, it was not a man who “invented” the acidification of the clouds – nature itself has successfully done it and is doing it for him with the help of formic acid.

A certain amount of this simplest organic acid is released into the air by plants and some animals, such as forest ants. But the amount of acid that is recorded in the earth’s atmosphere cannot be explained with the help of anthills and trees with bushes alone. Even taking into account the emissions from all factories, factories and steamships, we still have to find somewhere else 80-90% of the current volume of atmospheric formic acid. This means that acid is formed directly in the atmosphere as a result of chemical reactions, the whole question is – in which ones?

For a long time, it was believed that the source of formic acid is the gradual oxidation of formaldehyde in water droplets. Formaldehyde is formed as a result of the oxidation of various volatile organic substances, for example, isoprene, which is excreted in large quantities by plants. Part of the formed formic acid evaporates into the atmosphere from the surface of the drop, and the rest returns to the ground along with rain or snow. The problem is that the capacity of such an airborne chemical conveyor is categorically not enough to explain the large amount of formic acid, which is detected, for example, from satellites.

A group of scientists from the Juelich Research Center (Germany) have proposed a mechanism that helps explain where the “missing” acid comes from. They took a closer look at one chemical, methanediol, which is formed when formaldehyde reacts with ordinary water. This is a rather unstable compound that can easily fall apart back into formaldehyde and water, or be oxidized by something and turn into formic acid. It was believed that the lifetime of methanediol inside the droplet was not enough for it to somehow affect atmospheric chemistry, but, as the researchers write in Nature, it’s not like that at all.

It turned out that a significant fraction of the formed methanediol still manages to evaporate from the surface of the drop and oxidize to formic acid already in the atmosphere. If methanediol would oxidize inside the drop, then the further fate of the acid would depend on the fate of the drop: either it will evaporate and all its contents will be in the atmosphere, or it will fall to the ground as a precipitate, taking with it all the accumulated “chemistry”. The methanediol escaping from the droplet, then turning into formic acid and staying longer in the air, thereby increases the concentration of acid in the atmosphere. According to the researchers’ calculations, the “methanediol” mechanism produces approximately four times more formic acid than any other chemical process in the atmosphere. This amount lowers the pH of rainwater by three tenths, which is useful to consider when simulating various processes, for example, the acid balance of water bodies.

Source: Автономная некоммерческая организация "Редакция журнала «Наука и жизнь»" by www.nkj.ru.

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