Impact of the South Asian monsoon outflow on atmospheric hydroperoxides in the upper troposphere

Abstract

During the OMO (Oxidation Mechanism Observation) mission, trace gas measurements were performed on board the HALO (High Altitude Long Range) research aircraft in summer 2015 in order to investigate the outflow of the South Asian summer monsoon and its influence on the composition of the Asian monsoon anticyclone (AMA) in the upper troposphere over the eastern Mediterranean and the Arabian Peninsula. This study focuses on in situ observations of hydrogen peroxide (H2O2obs) and organic hydroperoxides (ROOHobs) as well as their precursors and loss processes. Observations are compared to photostationary-state (PSS) calculations of H2O2PSS and extended by a separation of ROOHobs into methyl hydroperoxide (MHPPSS) and inferred unidentified hydroperoxide (UHPPSS) mixing ratios using PSS calculations. Measurements are also contrasted to simulations with the general circulation ECHAM–MESSy for Atmospheric Chemistry (EMAC) model. We observed enhanced mixing ratios of H2O2obs (45 %), MHPPSS (9 %), and UHPPSS (136 %) in the AMA relative to the northern hemispheric background. Highest concentrations for H2O2obs and MHPPSS of 211 and 152 ppbv, respectively, were found in the tropics outside the AMA, while for UHPPSS, with 208 pptv, highest concentrations were found within the AMA. In general, the observed concentrations are higher than steady-state calculations and EMAC simulations by a factor of 3 and 2, respectively. Especially in the AMA, EMAC underestimates the H2O2EMAC (medians: 71 pptv vs. 164 pptv) and ROOHEMAC (medians: 25 pptv vs. 278 pptv) mixing ratios. Longitudinal gradients indicate a pool of hydroperoxides towards the center of the AMA, most likely associated with upwind convection over India. This indicates main contributions of atmospheric transport to the local budgets of hydroperoxides along the flight track, explaining strong deviations from steady-state calculations which only account for local photochemistry. Underestimation of H2O2EMAC by approximately a factor of 2 in the Northern Hemisphere (NH) and the AMA and overestimation in the Southern Hemisphere (SH; factor 1.3) are most likely due to uncertainties in the scavenging efficiencies for individual hydroperoxides in deep convective transport to the upper troposphere, corroborated by a sensitivity study. It seems that the observed excess UHPPSS is excess MHP transported to the west from an upper tropospheric source related to convection in the summer monsoon over Southeast Asia.