The reaction of isoprene with OH radicals has been shown to produce a different range of products depending on the fate of the peroxy radical (ISOPOO). In pristine or remote locations with low NO, ISOPOO can react with HO2 or RO2, or can isomerise. Reaction with HO2 leads to isoprene hydroxyhydroperoxides, which can then undergo further oxidation to isoprene epoxydiols. Subsequent reactive uptake onto aerosols leads to the formation of 2-methyltetrols or organosulfate. Under polluted conditions, ISOPOO can react with NO to form isoprene hydroxyl nitrate (IHN) or an alkoxy radical. The switching point between low and high NO chemistry depends on the reactivity of the system and is different in cities compared to remote regions. We carried out a detailed chemical characterisation of isoprene oxidation chemistry using observations made in Beijing during summer 2017 as part of the Air Pollution and Human Health program. Very low NO levels in the afternoon were routinely observed that could not be explained using current mechanistic understanding. The low levels of NO leads to the formation of IEPOX in this polluted environment and production of 2-methyl-tetrol-OS. Further reactions of this compound led to a range of products including some previously thought of as high NO tracers. Photochemical oxidation of IHN can also lead to the production of IEPOX. IHN were observed using an I-CIMS, with an average diurnal profile peak of 120 ppt at 12:00. We have carried out theoretical calculations of this reaction and find that the predicted branching ratio to IEPOX is higher than current estimates. Incorporating new branching ratios into a box model, constrained to measurements of OH, NO3, O3, isoprene and HO2, we estimate that OH oxidation of IHN contributes significantly to IEPOX production in Beijing. The measurements also indicate a significant role of NO3 chemistry in the formation of isoprene nitrates and nitrooxyorganosulfates in aerosol. Reactions of isoprene with NO3 are generally considered to be a night-time phenomenon, due to lower levels of NO and rapid photolysis of NO3 during the day. However, in Beijing, the low levels of NO in the afternoon result in the production of ~2 ppt NO3, accounting for up to 40 % of isoprene nitrate at 16:00. The observations indicate that biogenic-anthropogenic interactions in urban areas are extremely complex and that the use of specific compounds as tracers of “high” and “low” NO chemistry should be treated with caution.

Complex relationship of NOx on isoprene oxidation chemistry and secondary organic aerosol formation in polluted megacities

ACS Spring 2022

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