Atmospheric Chemistry of Nonanal

During the Southern Oxidants Study 1999 field campaign at Dickson, TN, we conducted measurements of the n-aldehydes propanal, pentanal, hexanal, heptanal, octanal, and nonanal. Propanal and nonanal tended to have the largest concentrations, with afternoon maxima of ∼0.3 ppb. These aldehydes typically represented a significant fraction of the VOC reactivity defined as k OH[VOC]. However, this information is misleading with regard to the impact of these aldehydes on ozone formation, as their oxidation can represent a significant NO x sink. Motivated by the relatively large nonanal concentrations, we conducted a laboratory study of the products of the nonanal + OH reaction. The OH + nonanal reaction rate constant was determined via the relative rate technique and found to be 3.6 (±0.7) × 10-11 cm3 molecule-1 s-1. Under conditions of high [NO2]/[NO], we determined that 50 ± 6% of OH-nonanal reaction occurs via abstraction of the aldehydic H-atom through measurement of the peroxynonanyl nitrate yield. We also studied the production of organic nitrates from OH reaction with nonanal in the presence of NO. As expected, a major product (20% at large [NO]/[NO2]) of this reaction was 1-nitrooxy octane. We calculate that the branching ratio for 1-nitrooxy octane formation from peroxyoctyl radicals is 0.40 ± 0.05. On the basis of these measurements, we find that for more than 50% of the time OH reacts with nonanal (for midday summer conditions) an organic nitrate or PAN compound is formed, making this important atmospheric aldehyde an effective NO x sink.