Elevated Methyl Hydroperoxide in Urban Air Suggests Sources from Heterogeneous Chemistry

Methyl hydroperoxide (CH₃OOH, MHP) is produced in the atmosphere by the CH₃O₂+HO₂ reaction, competing with the CH₃O₂+NO reaction in the oxidation cascade of volatile organic compounds (VOCs). It is an important sink and reservoir of hydrogen oxide (HO_x) radicals that drive atmospheric oxidant chemistry. KORUS-AQ aircraft vertical profiles over Seoul in May-June 2016 with an extensive chemical payload show MHP concentrations exceeding 1 ppbv in the urban planetary boundary layer (PBL). This is five times higher than simulated by the GEOS-Chem atmospheric chemistry model where MHP formation is suppressed by high NO from fuel combustion. By contrast, the model successfully simulates MHP observations in the background free troposphere above the PBL where the main CH₃O₂ source is from methane oxidation and MHP formation is determined by the HO₂/NO ratio. Observed MHP is highest in the residual PBL in the early morning at high relative humidity and high organic aerosol concentrations, suggesting missing aqueous-phase cloud chemistry in current mechanisms possibly involving peroxyacetyl radicals (CH₃C(O)OO). This could have important implications for oxidant chemistry in urban air and in convective outflow.