Photochemical sources of organic acids. 2. Formation of C5-C9 carboxylic acids from alkene ozonolysis under dry and humid conditions.

The dependence of organic acid generation by alkene ozonolysis on relative humidity, thermalized Criegee intermediate scavengers, and alkene structure is investigated. Carboxylic acids generated from the ozonolysis of 1-hexene, 1-octene, 1-decene, trans-3-octene, and 1-methylcyclohexene were analyzed as trimethylsilyl (TMS) derivatives. Experiments were performed under dry (relative humidity (RH) < 1%) and humid (RH = 65%) conditions with cyclohexane or n-butyl ether as an OH scavenger. Pentanoic acid is produced from 1-hexene and trans-3-octene with yields 8.5 +/- 2.6 and 5.0 +/- 1.5% under dry conditions and 5.1 +/- 1.5 and 2.8 +/- 0.8% under humid conditions, respectively. Heptanoic acid yields from 1-octene are 8.3 +/- 2.5 and 4.4 +/- 1.3% under dry and humid conditions, respectively. Ozonolysis of 1-methylcyclohexene produced six C5-C7 multifunctional carboxylic acids, with a total yield of 7%. Several other acids and aldehydes were also monitored and quantified. An additional set of experiments with added stabilized Criegee intermediate (SCI) scavengers was performed for 1-octene ozonolysis under dry conditions. The results indicate that SCIs and their reaction with water are minor contributors to acid formation in the atmosphere and suggest that many of the acids are formed directly.