Airborne measurement and interpretation of peroxy radical concentrations with a focus on the oxidation mechanisms in the Asian free troposphere

Abstract

Hydroperoxyl (HO2) and organic peroxy (RO2, where R stands for any organic group) radicals are highly reactive molecules produced in the oxidation of many compounds in the troposphere. They participate in the catalytic cycle producing or destroying ozone (O3) in the troposphere. Thus, HO2 and RO2 measurements provide unique information about the chemical processing of an air mass. Over the last decades, the understanding of the role of HO2 and RO2 in the chemical processes in the planetary boundary layer (PBL) has improved through ground-based in-situ measurements. However, the number of unequivocal measurements of peroxy radicals in the free troposphere is still quite limited. Measurements from airborne platforms offer a unique opportunity to measure HO2 and RO2 together with other relevant trace gases to test and improve the understanding of their chemistry in the free troposphere. During this doctoral study, an extensive set of airborne RO2* (RO2* = HO2 + ∑RO2, where RO2 represents the organic peroxy radicals reacting with NO to produce NO2) measurements in the PBL and free troposphere was acquired, analysed and interpreted. The RO2* measurements were made using the Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS) developed at the Institut für Umweltphysik (IUP) of the University of Bremen. PeRCEAS has successfully deployed onboard the High Altitude LOng range research aircraft (HALO) in three research campaigns: the Oxidation Mechanism Observations (OMO) Asia and the Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales (EMeRGe) field missions in Europe and Asia. The PeRCEAS instrument was characterised and calibrated under atmospherically representative conditions in the laboratory to assure data quality, reproducibility, accuracy and to define optimal operating conditions for the airborne measurements. PeRCEAS successfully measured RO2* in 33 HALO flights. RO2* mixing ratios of up to 120 pmole mole-1 were measured in air masses having different origins, chemical compositions and physical conditions in Europe and Asia. The RO2* measurements, the simultaneous measurements of other relevant trace gases, aerosol concentration, photolysis frequencies and other meteorological parameters were synergistically analysed to identify the chemical processes controlling the amount of RO2*. From the analysis, it was found that RO2* is primarily produced following the photolysis of ozone (O3), formaldehyde (HCHO), glyoxal (CHOCHO), and nitrous acid (HONO) in the air masses investigated. The estimate for the contribution of O3 photolysis to RO2* production rate is > 40 % in the PBL and < 40% in the free troposphere. This reduction is explained by the decrease in the water vapour concentration ([H2O]) as a function of altitude. Subsequently, the RO2* mixing ratios in the air masses measured during the EMeRGe in Asia and Europe campaigns were calculated assuming a photostationary steady-state (PSS) for RO2*. The RO2* production from precursor photolysis, the loss through HO2 – HO2, RO2 – RO2 and HO2 – RO2 reactions, the hydroxyl radical (OH) and organic oxy-radicals (RO) loss during the radical interconversion, and HO2 uptake on aerosol were considered for the calculation of RO2*. The calculations were constrained by the simultaneous measurements of photolysis frequencies, trace gas concentrations and aerosol particle number concentrations onboard HALO. Case studies confirmed the validity of the PSS assumption for air masses having different chemical compositions under different physical conditions. The RO2* calculated are generally in excellent agreement with the RO2* measurements. An experimental budget analysis was performed to estimate the main loss processes of RO2* by introducing the RO2* measurements in the PSS equation. Except for the measurements inside pollution plumes with NO > 800 pmole mole-1 or aerosol particle number concentration > 800 particles cm-3, the HO2 – RO2 and HO2 – HO2 were the dominant RO2* loss process during both EMeRGe Asia and Europe. The RO2* losses through HO2 uptake on aerosol were higher in the pollution outflows measured in Asia than in Europe. This is attributed to the higher aerosol concentrations observed in the air masses probed during EMeRGe in Asia. The contribution from the HO2 uptake on aerosol increases up to 60 % for an assumed aerosol uptake coefficient of 0.24 inside pollution plumes in Asia, where the aerosol particle number concentration is > 1000 particles cm-3. In Europe, the OH – NOx reactions were the dominant RO2* loss process in the pollution outflow. This finding is explained by the EMeRGe in Europe measurements being typically closer to anthropogenic emissions sources than in Asia, except for the case study of Taipei and Manila.