To Cut Daytime Smog, Attack It At Night
"During the night, ozone-forming nitrogen oxides undergo chemical reactions that transform them into nitric acid gas. This gas rapidly deposits on the surface, partially cleansing the air of key smogmaking ingredients. As Brown's colleague A.R. Ravishankara notes, this glimpse of a previously unknown process shows that "the nighttime chemistry is a new piece of the air-quality puzzle.... We need to find out more about when and where it is important." For example: Are there circumstances where it enhances smogmaking rather than moderating it? And what else is going on?"
Thanks to Chris Steins