Scheme 1.

IHNs are formed through a small, but important, pathway ($\alpha$) present in the reaction of isoprene and OH (+${\text{O}}_2$) in the presence of NO. The dominant pathway ($1-\alpha$) forms ${\text{NO}}_2$ and promotes ozone production. The formation of the two IHN isomers shown here (1,2-IHN and 4,3-IHN; dashed boxes) represent more than 90% of the IHN produced at atmospherically relevant ${\text{RO}}_2$ lifetimes (17). We note that the $\alpha$ to 1,2-IHN ($14\pm 3\%$) is very similar to the $\alpha$ to 4,3-IHN ($13\pm 3\%$) (18). Once formed, 1,2-IHN and 4,3-IHN can undergo deposition, oxidation, or incorporation into aerosol, where they can hydrolyze. The branching between IHN-loss pathways directly affects isoprene’s impact on NO_x and oxidant levels. IHN-loss pathways that result in NO_x recycling are highlighted in blue, while those that result in the permanent loss of NO_x are red.