Measurement of Peroxy Radicals using Laser-Induced Fluorescence Technique

Peroxy radicals are produced during the photochemical degradation of volatile organic compounds (VOCs) in the atmosphere. They are part of the catalytic radical cycle initiated by hydroxyl radicals (OH) in which hydrocarbons are oxidized leading to the removal of pollutants. The reaction of peroxy radicals with nitrogen monoxide (NO) in conjunction with the photochemical cycling of nitrogen dioxide (NO$_{2}$) and nitrogen monoxide leads to the formation of ozone (O$_{3}$). This process is the major photochemical source for ozone in the troposphere concerning the local as well as the global ozone budget. A new method for the measurement of atmospheric hydroperoxy and organic peroxy radical concentrations (HO$_{2}$ and RO$_{2}$) was developed using the successive conversion of RO$_{2}$ and HO$_{2}$ to OH radicals which are detected by laser-induced fluorescence (LIF). The detection system consists of two differentially pumped chambers. Ambient air is sampled through a nozzle into the first chamber in which the pressure is reduced from atmospheric pressure to 25 $\textit{hPa}$. An excess of NO and CO is added downstream of the inlet, leading to the conversion of RO$_{2}$ to HO$_{2}$ radicals. The pressure is further reduced in the second chamber to 3.5 $\textit{hPa}$. HO$_{2}$ is transformed to OH by the reaction with a further excess of NO. The detection of OH radicals is achieved by time delayed gated photon counting after resonant excitation of the OH-fluorescence at 308$\textit{nm}$ (A$^{2}$Σ$^{+}$ - X$^{2}$Π). The sensitivity of the system is calibrated using a radical source in which OH and HO$_{2}$ radicals are produced by water photolysis. In order to calibrate the RO$_{2}$ sensitivity, a hydrocarbon is mixed to the calibration gas. OH reacts quantitatively with the hydrocarbon resulting in the formation of RO$_{2}$ radicals. The typical detection limit of the LIF system for peroxy radicals is 2·10$^{6}$ $\textit{cm}^{−3}$ to 7·10$^{6}$ $\textit{cm}^{−3}$ (0.1 $\textit{pptv}$ to 0.3 $\textit{pptv}$) for an integration time of 30 $\textit{s}$ and for a signal-to-noise-ratio of two. The estimated accuracy is 10% which is mainly determined by the uncertainty of the calibration. Unlike in peroxy radical amplifier (PERCA) instruments which are used for measuring HO$_{2}$+RO$_{2}$-radical concentrations, only a weak dependence of the sensitivity on the water vapor content in the sampled air is found which can be explained by humidity dependent quenching of the OH fluorescence. The instrument was characterized in laboratory measurements. The results agree with analytic and numerical calculations investigating the reaction kinetics of the RO$_{2}$ conversion process. Measurements of peroxy radicals were validated by simultaneous measurements of the new LIF instrument and an established measurement technique (Matrix Isolation and Electron Spin Resonance, MIESR) in experiments at the atmosphere simulation chamber SAPHIR. The SAPHIR chamber allows to investigate chemical processes under controlled atmospheric conditions. First ambient air measurements of peroxy radicals were performed during the international field campaign HOxCOMP in July 2005 showing reasonable diurnal profiles