The extremely dangerous sulfuric acid appears on the list of available possibilities. The atmosphere varies in a constant manner, and industry and some machines add the other necessary reaction products to promote the creation of sulfuric acid and others.
A plane is from other perspectives more than a method to transport people. The space in which it travels might seem an improvised lab and the motors create extreme high temperature thus another factor for the origin of new molecules.
One of the main elements to create sulfuric acid is sulfur dioxide, and while it's true that volcanic eruptions are a great source, in other areas the metallurgic industry and combustion vehicles are enough. Sulfur dioxide could be naturally oxidized giving sulfur trioxide, and then, with humidity or other kinds of water present in the atmosphere, the next possible reaction is the dreaded sulfuric acid.
But it's not the only one, also nitric acid if the aforementioned lab has the temperature source ready, and this time all the necessary atoms are present in the atmospheric air by themselves.