Isoprene Photooxidation via the Hydroperoxyl Pathway

Abstract

Photooxidation of isoprene has a large influence on the oxidation capacity of the atmosphere and is a significant source of secondary organic material (SOM) of atmospheric particles. A quantitative understanding of isoprene photooxidation mechanism, in particular in the clean atmosphere, is important but challenging. This thesis presents laboratory and field studies of isoprene photooxidation via the hydroperoxyl (HO2) pathway, believed to be the most important reaction pathway of isoprene-derived peroxy radicals under clean conditions, by using a switchable-reagent-ion (NO+; H3O+) time-of-flight mass spectrometer (SRI-TOF-MS). Isoprene photooxidation via the HO2 pathway was investigated in a continuous-flow chamber. Production yields of methyl vinyl ketone (MVK) and methacrolein (MACR) pathway were determined as (3.8 ± 1.3)% and (2.5 ± 0.9)%, respectively, at <2% RH at 25 oC. Production of MVK and MACR via the HO2 pathway implies concomitant production of hydroxyl ((6.3± 2.1)%) and hydroperoxyl ((6.3± 2.1)%) radicals. The experiments also revealed an unexpected instrument bias that isoprene-derived hydroperoxides (ISOPOOH), the major isoprene oxidation products via the HO2 pathway, were detected as the same product ions as the MVK and MACR. This finding implies that scientific conclusions based on previous ambient measurements of MVK and MACR under clean conditions using similar techniques need to be revisited. As a follow-up study, preliminary analysis of the ambient measurements in the Amazon Basin showed that ISOPOOH isomers contributed to (36±15)% of the total concentration of MVK, MACR, and ISOPOOH isomers under clean conditions. The value is useful for re-evaluation of previous measurements and also defines range of possible anthropogenic influence on isoprene chemistry in the region. SOM production from isoprene photooxidation via the HO2 pathway was also investigated. Isoprene photooxidation was separated from SOM production by using two continuous-flow reactors connected in series and operated at steady state. Relative importance of various intermediates was directly compared from the drop of their gas-phase signals. Isoprene epoxydiols (IEPOX) had a much larger contribution to SOM production than other previously suggested isoprene SOM precursor, ISOPOOH isomers and isoprene, for a wide range of aciddity The IEPOX contribution to isoprene SOM production was further quantified. IEPOX isomers lost from the gas phase accounted for (46 ±11)% of the produced SOM mass concentration. The IEPOX isomers comprised (59±21)% (molecular count) of the loss of monitored gas-phase species. The implication is that for the investigated reaction conditions IEPOX pathway accounted for half of the SOM mass concentration.