Nitrogen oxides are naturally caused by combustion (and volcanism), so it makes sense to me to have it. It's dependent only on the heat of the burn so wood and

Nitrogen oxides are naturally caused by combustion (and volcanism), so it makes sense to me to have it. It's dependent only on the heat of the burn so wood and coal fires create quite a lot. Coal also produces a lot of sulfur oxides. Hydrogen-heavy fuels like light oil and natural gas are the only fuels that burn cool enough to not produce significant amounts of acid.