Comparison of Isoprene Chemical Mechanisms with Chamber and Field Observations

The importance of global isoprene emissions has stimulated studies of oxidation mechanisms that follow the initial reaction of isoprene with atmospheric hydroxyl (OH). A key question involves the speed and pathways by which isoprene products isomerize. Some reactions in these mechanisms generate new hydroxyl, perhaps enough to recycle most hydroxyl. This research examines five different isoprene oxidation mechanisms using observations from a 2013 field study in an Alabama forest and a 2014 companion study in the Caltech Environmental Chamber. Model mechanisms and observations were compared for OH, hydroperoxyl (HO₂), and the isoprene oxidation products: isoprene hydroxyhydroperoxide (ISOPOOH), isoprene epoxydiol (IEPOX), hydroxyacetone, formaldehyde, methacrolein, and methyl vinyl ketone. Observed hydroxyl is generally simulated within uncertainties for both the chamber and the field studies, indicating that hydroxyl recycling is well captured by current model mechanisms, although two mechanisms are slightly better than the other three. When the observed and modeled uncertainties are considered, no currently accepted mechanism is clearly superior to the others for simulating isoprene products. For atmospheric conditions typical of forests-abundant isoprene and low nitric oxide-these model mechanisms produce concentrations of isoprene products that can be substantially different from observations and from each other. This result suggests both the common and different parts of the chemical mechanisms need to be reexamined, particularly by observing the later-generation products directly.