Impact of the South Asian Monsoon Outflow on Atmospheric Hydroperoxides
Abstract
Hydroperoxides play a dual role as tropospheric HOx (OH+HO2) reservoirs and sinks and thus have a strong influence on the oxidizing power of the atmosphere. Here we present airborne in situ observations of H2O2 and the sum of organic hydroperoxides (ROOH) obtained during the OMO (Oxidation Mechanism Observation) campaign in July/August 2015, investigating the Asian monsoon anticyclone (AMA) over the Arabian Peninsula. We present budgets for H2O2, MHP (methyl hydroperoxide) and non-MHP (NMHP) mixing ratios based on photostationary state (PSS) calculations using observations of HOx, CO, CH4 and photolysis frequencies.
In addition, observations of H2O2 and ROOH are compared to simulations with the EMAC global chemistry climate model. In general, we found good agreement between observations and model simulations of H2O2 and ROOH in background air masses, while significant discrepancies were found for ROOH under AMA influence. In the AMA measured organic hydroperoxides were strongly enhanced, which was not reproduced by the model. A comparison of measured H2O2 with PSS calculations based on observed HO2, OH and photolysis rates yields a good agreement ([H2O2]calc = (1.15 ± 0.05) · [H2O2]meas - (0.13± 0.02) ppb; R2=0.45), indicating that the assumption of PSS is fulfilled. Estimates of MHP mixing ratios based on PSS calculations show that the speciation of ROOH depends on the air mass origin. In air masses influenced by the AMA most of the measured ROOH cannot be accounted for by MHP. The median of the NMHP mixing ratios increases from 103 ppt in background air to 211 ppt under AMA influence, while neither measured H2O2 nor PSS-estimated MHP show significant differences between both air masses (159 ppt for H2O2 in the background, 163 ppt in the AMA outflow, and 77 ppt and 68 ppt for MHP respectively). A positive correlation of the deduced NMHP with acetone indicates that PAA (peracetic acid, a degradation product of acetone) is the most likely candidate to account for the NMHP increase in AMA air masses (R2 = 0.43). Convective enhancement of NMHP in the monsoon influenced air can be excluded due to the negative correlation with airmass age. Highest NMHP mixing ratios were found in processed air masses with low NO/NOy ratios, a marker for airmass age. This indicates that the excess NMHP is most likely of photochemical origin formed in the AMA.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A43M3282H
- Keywords:
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- 0317 Chemical kinetic and photochemical properties;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0345 Pollution: urban and regional;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0365 Troposphere: composition and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0368 Troposphere: constituent transport and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE