Impact of the South Asian monsoon outflow on atmospheric hydroperoxides in the upper troposphere
<p>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 Asi...
| Main Authors: | , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-11-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/20/12655/2020/acp-20-12655-2020.pdf |
| Summary: | <p>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 (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><msup><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">obs</mi></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="4a06e359a69817294d02680fc6493a00"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-12655-2020-ie00001.svg" width="41pt" height="16pt" src="acp-20-12655-2020-ie00001.png"/></svg:svg></span></span>) and organic hydroperoxides
(ROOH<span class="inline-formula"><sup>obs</sup></span>) as well as their precursors and loss processes. Observations
are compared to photostationary-state (PSS) calculations of
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><msup><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">PSS</mi></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="44pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="c1a737167874f2ccbf08988e07d07f25"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-12655-2020-ie00002.svg" width="44pt" height="16pt" src="acp-20-12655-2020-ie00002.png"/></svg:svg></span></span> and extended by a separation of ROOH<span class="inline-formula"><sup>obs</sup></span> into
methyl hydroperoxide (MHP<span class="inline-formula"><sup>PSS</sup></span>) and inferred unidentified
hydroperoxide (UHP<span class="inline-formula"><sup>PSS</sup></span>) 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><msup><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">obs</mi></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="53a3a26b1b065e9a05624882beaeba09"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-12655-2020-ie00003.svg" width="41pt" height="16pt" src="acp-20-12655-2020-ie00003.png"/></svg:svg></span></span> (45 %), MHP<span class="inline-formula"><sup>PSS</sup></span> (9 %),
and UHP<span class="inline-formula"><sup>PSS</sup></span> (136 %) in the AMA relative to the northern hemispheric
background. Highest concentrations for <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><msup><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">obs</mi></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="df8512609a48642df3c45d21b56e6efd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-12655-2020-ie00004.svg" width="41pt" height="16pt" src="acp-20-12655-2020-ie00004.png"/></svg:svg></span></span> and
MHP<span class="inline-formula"><sup>PSS</sup></span> of 211 and 152 ppb<span class="inline-formula"><sub>v</sub></span>, respectively, were found in
the tropics outside the AMA, while for UHP<span class="inline-formula"><sup>PSS</sup></span>, with 208 ppt<span class="inline-formula"><sub>v</sub></span>,
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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><msup><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">EMAC</mi></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a0a7f6cb6633bb76eb343de1d4fb0d0b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-12655-2020-ie00005.svg" width="51pt" height="16pt" src="acp-20-12655-2020-ie00005.png"/></svg:svg></span></span> (medians: 71 ppt<span class="inline-formula"><sub>v</sub></span> vs.
164 ppt<span class="inline-formula"><sub>v</sub></span>) and ROOH<span class="inline-formula"><sup>EMAC</sup></span> (medians: 25 ppt<span class="inline-formula"><sub>v</sub></span> vs. 278 ppt<span class="inline-formula"><sub>v</sub></span>)
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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><msup><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">EMAC</mi></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a2eb7195e9447141ea80e919613bb6b1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-12655-2020-ie00006.svg" width="51pt" height="16pt" src="acp-20-12655-2020-ie00006.png"/></svg:svg></span></span> 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 UHP<span class="inline-formula"><sup>PSS</sup></span> is excess MHP transported to the west from an upper
tropospheric source related to convection in the summer monsoon over
Southeast Asia.</p> |
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| ISSN: | 1680-7316 1680-7324 |