The Oxidation Capacity of the Summertime Asian Monsoon Anticyclone - Airborne measurements of OH and HO2 radicals in the Upper Troposphere using Laser Induced Fluorescence Spectroscopy
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Abstract
The intense summertime heating over Southern Asia forms a strong convective transport system, which is capable of quickly uplifting South Asian polluted air masses into a quasi-stationary anticyclone that is situated within in the upper troposphere and lower stratosphere. This efficient transportation of Asian emissions during the summertime Asian monsoon has the potential to turn a regional-scale pollution problem into a global one. In July to August 2015, the High Altitude and Long Range aircraft (HALO) performed a series of 24 measurement flights that probed the western reaches of the summertime Asian monsoon anticyclone (AMA) up to altitudes of 15 km, as part of the Oxidation Mechanism Observations over Asia (OMO-Asia) airborne campaign. Measurements and budget calculations of OH and HO2 (HOX) are described in depth within this thesis in order to parameterize the self-cleaning capacity of AMA and characterize its impact on the atmospheric lifetimes of South Asian pollutants that it transports. For this study, a Laser-Induced Fluorescence – Fluorescence Assay by Gas Expansion (LIF-FAGE) instrument (HORUS) from the Max-Planck Institute for Chemistry, Mainz was developed and installed for use on board HALO. This was the first time that successful HOX measurements were performed by an airborne LIF-FAGE instrument capable of measuring atmospheric OH and its chemical background interferences (OH-CHEM) in-situ and in-real-time. This was achieved through effective characterization and operation of an Inlet Pre-Injector (IPI) system, which enabled the quantification of OH-CHEM and the correction of the OH measurements. Additionally as part of this PhD work, the All Pressure Altitude based Calibrator for HOX Experimentation (APACHE) was developed and utilized to resolve all known pressure dependent and independent terms affecting the HOX sensitivity of HORUS, and achieved an accuracy of 22.6 % (1σ).
The measured OH and HO2 concentrations are, on average, 31 % and 68 % higher respectively inside AMA when compared to the observed background upper troposphere. From the OH and HO2 budget analyses, the oxidative throughput of OH, i.e. the rate at which OH is produced and reacts with pollutants, was calculated to be, on average, ~ 52 % faster within AMA when compared to background upper Troposphere air masses. The average oxidative throughput rate of OH increased from ~ 4.3 (± 0.7) x105 molec. cm-3 s-1 outside to ~ 6.6 (± 0.9) x105 molec. cm-3 s-1 inside AMA, with over 80 % involving HOX recycling reactions. The main source of OH is the HO2 + NO channel, which increased from 3.2 (± 0.9) x105 molec. cm-3 s-1 outside AMA to 5.1 (± 1.4) x105 molec. cm-3 s-1 inside AMA. This 59.4 % increase in the production of OH via HO2 + NO is due the strong convectional transport processes that inject anthropogenic and lightning NOX into AMA, as supported by the observed 16 % higher NO mixing ratio inside AMA when compared to outside. Furthermore, the concentrations of CH4, CO and RO2 are also elevated inside AMA, due to the aforementioned convectional transport of polluted boundary layer air masses into AMA. This causes the cycling of OH to HO2 to accelerate by ~ 3.3 (± 0.93) x105 molec. cm-3 s-1 to a rate of ~ 5.5 (± 1.4) x105 molec. cm-3 s-1 inside AMA. The OH recycling probability (rOH) analysis showed that buffering processes are strengthened within AMA, whereby the local atmospheric oxidative capacity is maintained despite the increased pollutant loading. The injection of NOX enriched air masses into AMA increases the NOPR (Net Ozone Production Rate) by 91.7 %, and elevates O3 levels by 6 %. This enables the reaction rate of the HO2 + O3 channel to increase, and partially buffer rOH against the photochemical loss of NO that occurs in-between convection events.
Ultimately, AMA promotes cycling within HOX and buffers the rOH against pollutant loading, which in turn elevates overall HOX levels. This intensification of HOX chemistry inside AMA shortens the atmospheric lifetime of the transported South Asian pollutants by a factor of 1.52. This increased self-cleaning capacity of AMA limits the potential accumulation of pollutants before ejecting them out, thus moderates the global-scale impact of the transported South Asian emissions.