Applications of cavity ring-down Sspectroscopy in environmental physics: Rayleigh scattering cross-sections and airborne measurements of peroxy radicals

Abstract

This doctoral thesis aims at the improvement of the airborne measurement of peroxy radicals by focusing on two applications of cavity ring-down spectroscopy (CRDS): the determination of the EM radiation extinction cross-section of gas molecules, and the determination of gas molecule concentrations with the knowledge of the EM radiation extinction and the EM radiation extinction cross-sections. The experimental part of the work is based on the use of the Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS) instrument which combines the peroxy radical chemical amplification (PERCA), for the amplified conversion of peroxy radicals in NO2, with a sensitive NO₂ detection by CRDS. For the first CRDS application, three setups were used for the determination of Rayleigh scattering cross-sections σ_(Rayl.)of atmospheric gases at 408 nm. Thus, the EM radiation extinction by N₂, O₂, Ar, CO, CO₂, N₂O, CH₄, and synthetic air (SA) was measured using step pressure changes and pressure ramps in optical cavities. The experimentally determined σ_(Rayl.) were compared with calculated values based on literature refractive index (n) and King correction factors. The calculated and measured σ_(Rayl.) agreed within 0.6 %, 2.4 %, 1.2 %, 2.2 %, and 1.5 % for CO₂, N₂, O₂, SA, and Ar, respectively, at ~ 408 nm. The measured σ_(Rayl.) for N₂O and CH₄ confirmed the improved accuracy of the most recently published determinations of their refractive indexes. The CO σ_(Rayl.) experimentally determined for the first time in this work at 408 nm, is 4.1 % higher than the calculated value resulting from extrapolation, suggesting the need for improved knowledge of the refractive index and King correction factors in the blue spectral region. The second CRDS application is a pre-requisite for the acquisition of airborne peroxy radical data and involves the deployment of PeRCEAS on an airborne platform for the measurement of the total sum of peroxy radicals which react with NO (RO_2^*) during the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional and Global scales) project. The retrieved RO_2^* were then compared with the results of four atmospheric models and with calculations based on photostationary steady state (PSS) assumptions. The box model and the PSS calculations are constrained to the airborne measurements. Three case studies/scenarios were investigated: 1) close to Rome and Manila's major population centres, 2) long-range transported air masses from megacities in the Western Pacific, and 3) over a very populated area in Taiwan during different take-off and landing scenarios. Generally, models agree with measurements within their uncertainties, but they face difficulties with short-term variations. Notably, in Rome, models underestimated measurements by up to 80% at lower altitudes close to the boundary layer, while in Manila the box model underestimated 30% of the observations. In the Taiwan cases, the models showcased a 2 to 5 times underestimation in capturing short-term RO_2^* variability in more polluted, aerosol-rich conditions. A re-evaluation method, adjusting the ratio of different peroxy radicals in the RO_2^* calculations, reduced differences between the box model and PSS results by up to 40%. Overestimations by the PSS calculations were linked to high NO levels, organic nitrate formation, and aerosol-induced radical losses unaccounted for in current calculations. The findings underscore the capabilities of the models investigated and highlight the necessity for the speciation of radical measurements to improve the understanding of the short-term variability of RO_2^* in complex polluted areas.