Effect of OH radical scavengers on secondary organic aerosol formation from reactions of isoprene with ozone

Abstract

In order to understand the effect of OH radical scavengers on secondary organic aerosol formation, aerosol yields from the isoprene ozonolysis were measured in the presence of sufficient amounts of OH radical scavengers. Cyclohexane, CO, n-hexane, and diethyl ether were used as the OH radical scavengers. The aerosol yield was determined to be 0.002–0.023 for experiments without OH radical scavengers in the aerosol mass range 2–120 μg m−3. Similar aerosol yields were observed in experiments using cyclohexane. The aerosol yield observed with n-hexane was close to that observed without scavengers at 120 μg m−3, but this aerosol yield was slightly lower than those observed in reactions without scavengers in the range 3–83 μg m−3. The offline aerosol samples obtained in experiments with cyclohexane or n-hexane contained oxygenated hydrocarbons with six or more carbon atoms. Aerosol formation in experiments that used cyclohexane or n-hexane as the scavenger was enhanced. This was caused by the oxidation products of the OH radical scavengers, although the increase in the yield could not be quantified. The aerosol yields were 0.002–0.014 for experiments with CO and diethyl ether in the aerosol mass range 4–120 μg m−3. The reaction of CO with OH radicals forms HO2 radicals, whereas the reactions of cyclohexane, n-hexane, and diethyl ether, respectively, with OH radicals form organic peroxy (RO2) radicals. Present results show that the aerosol yield is independent of the HO2/RO2 ratio or that it decreases with increasing HO2/RO2 ratio. Since the HO2 concentration is much higher than the RO2 concentration in the atmosphere, the results obtained using CO in this study will be a good approximation of the aerosol yield from the ozonolysis of isoprene in the atmosphere.