New insights into urban isoprene oxidation chemistry and impacts on air quality

Isoprene is the dominant non-methane volatile organic compound (VOC) emitted into the atmosphere globally. Within urban areas, there can be significant emissions of isoprene due to urban green spaces and planting, which can have important atmospheric chemistry impacts on ozone and secondary organic aerosol. The main loss route of isoprene is reaction with OH radicals, which leads to the formation of a hydroxyperoxy radical intermediate (ISOPO₂). In clean or “low NO” environments, ISOPO₂ predominantly reacts with HO₂ radicals to form isoprene hydroxyhydroperoxides (ISOPOOH), which can be further oxidized by OH radicals to produce isoprene epoxydiols (IEPOX), accompanied by OH recycling. In more polluted “high NO” environments, ISOPO₂ can react with NO to form MACR and MVK as the main reaction products and isoprene hydroxynitrates (IHN) with a yield of 0.04-0.15.

 

During a period of field observations during summer 2017 in Beijing, China, we observed in-situ formation of gas- and aerosol-phase oxidation products that are usually associated with low-NO “rainforest-like” atmospheric oxidation pathways. IEPOX and ISOPOOH concentrations measured by I-CIMS peaked during the afternoon, with an associated increase in particulate methyltetrol organosulfates via heterogenous reaction of IEPOX with sulfate aerosol. High levels of ozone scavenged NO, with concentrations decreasing to less than 1 ppb in the afternoon, and less than 0.1 ppb on some days. Box model simulations, using the Master Chemical Mechanism, suggest that during the morning high-NO chemistry predominates (95 %) but in the afternoon low-NO chemistry plays a greater role (30 %) in VOC oxidation in Beijing, with implications for the formation of highly oxidised molecules and SOA. Additional measurements in other urban areas (Manchester, Guangzhou) indicate that low-NO isoprene oxidation products are often observed in more polluted environments. 

 

In addition, we combined quantum calculations and box model simulations, to determine that the oxidation of isoprene hydroxynitrates (IHN) can be an alternative, NO-driven pathway leading to the formation of IEPOX in urban areas. Theoretical calculations indicated that the currently acknowledged yield of IEPOX from the IHN reaction with OH might be underestimated. The updated chemistry was incorporated into a box model using the full isoprene oxidation scheme from the MCMv3.3.1. For a steady state concentration of 1 ppb isoprene, the cross-over point at which the IEPOX production from IHN equals that from ISOPOOH, occurs at NO ~1 ppb using the IEPOX yields from this work (varies with [HO₂]). The model was then constrained to the measurements from Beijing to demonstrate the relative contributions of the IHN and ISOPOOH pathways to IEPOX formation. We show that the oxidation of IHN contributed to more than 50 % of IEPOX formation in the morning and early afternoon.

 

Our observations show that classifying specific isoprene oxidation products in both gas and particle phase as tracers for the NO regime needs to be carefully considered. The results improve our understanding of the NO_x dependence of isoprene oxidation chemistry in polluted areas, where anthropogenic emissions can significantly impact biogenic SOA formation.