Nitric acid is a nasty chemical. One of those with the skull and bones logo as part of the EU classification 'very toxic' and to be handled with care in a laboratory setting. Two villages in Belgium have now first hand experience in dealing with a whole nitric acid cloud after a chemical tank accident. (nieuwsblad.be link) What to do? Run for it and evacuate! Nobody got hurt so we can be relaxed about it. The culprit was a manure processing plant, the amount of nitric acid spilled was 15,000 liters. Another news report mentions that the chemical tank had failed because the acid was eating away metal parts. It is also suggested the tank was very new and that sulfuric acid is more commonly used in this industry.
A good thing nitric acid quickly colors yellow in secondary oxidation processes otherwise it is not just lethal and yellow but lethal and colorless. And why did the cloud form? Nitric acid is known to fume, a process in which nitric acid vapor acts as an nucleation site for water condensation, according to this report.
But what does nitric acid have to do with manure in the first place? Acids are added in general to bind ammonia that is toxic and explosive. You can research it if you have to. Berg and Helleman did so back in 2008 (link). They monitored 50 Kg of manure and the effect of nitric acid for 100 days at different pH levels. By just increasing the acidity the acid also kills the biological processes responsible for forming not only ammonia but also methane. Their conclusion: do not use nitric acid but lactic acid! Lactic acid does a better job preventing nitrous oxide emissions and the amount of methane emissions is also much smaller.
02-04: article updated