A Four Carbon Organonitrate as a Significant Product of Secondary Isoprene Chemistry

Oxidation of isoprene by nitrate radicals (NO(3)) or by hydroxyl radicals (OH) under high NO(x) conditions forms a substantial amount of organonitrates (ONs). ONs impact NO(x) concentrations and consequently ozone formation while also contributing to secondary organic aerosol. Here we show that the ONs with the chemical formula C(4)H(7)NO(5) are a significant fraction of isoprene‐derived ONs, based on chamber experiments and ambient measurements from different sites around the globe. From chamber experiments we found that C(4)H(7)NO(5) isomers contribute 5%–17% of all measured ONs formed during nighttime and constitute more than 40% of the measured ONs after further daytime oxidation. In ambient measurements C(4)H(7)NO(5) isomers usually dominate both nighttime and daytime, implying a long residence time compared to C(5) ONs which are removed more rapidly. We propose potential nighttime sources and secondary formation pathways, and test them using a box model with an updated isoprene oxidation scheme.