Reporting the sensitivity of laser-induced fluorescence instruments used for HO₂ detection to an interference from RO₂ radicals and introducing a novel approach that enables HO₂ and certain RO₂ types to be selectively measured

Abstract. Laboratory studies have revealed that alkene-derived RO₂ and longer chain alkane-derived RO₂ (> C₃) radicals rapidly convert to HO₂ and then to OH in the presence of NO in a fluorescence assay by gas expansion (FAGE) detection cell (Fuchs et al., 2011). Three different FAGE cells that have been used to make ambient measurements of OH and HO₂ in the University of Leeds ground-based instrument have been assessed to determine the sensitivity of each cell, when operating in HO₂ detection mode, to RO₂ radicals. The sensitivity to this interference was found to be highly dependent on cell design and operating parameters. Under the operating conditions employed, during fieldwork undertaken in the Borneo rainforest in 2008, an OH yield of 17% was experimentally determined for both ethene- and isoprene-derived RO₂ radicals. The high pumping capacity of this system, resulting in a short residence time in the cell, coupled with poor mixing of NO into the ambient air-stream for the titration of HO₂ to OH effectively minimised this potential interference. An OH yield of 46% was observed for ethene-derived RO₂ radicals when a smaller detection cell was used, in which the mixing of NO into the ambient air was improved and the cell residence times were much longer. For a newly developed RO_xLIF cell, used for detection of HO₂ and RO₂ radicals an OH yield of 95% was observed for ethene-derived RO₂ radicals, when running in HO₂ mode.

In experiments in which conditions ensured the conversion of RO₂ to OH were complete, the yields of OH from a range of different RO₂ species agreed well with model predictions based on the Master Chemical Mechanism version 3.2. For ethene and isoprene-derived RO₂ species, the relative sensitivity of FAGE was found to be close to that for HO₂, with an OH yield of 100% and 92%, respectively. For the longer chain or cyclic alkane-derived RO₂ radicals (> C₃), model predicted OH yields were highly dependent upon temperature. A model predicted OH yield of 74% at 298 K and 36% at 255 K were calculated for cyclohexane-derived RO₂ radicals, and an experimental yield of 38% was observed indicating that the temperature within the cell was below ambient owing to the supersonic expansion of the airstream in the low pressure cell.

These findings suggest that observations of HO₂ by some LIF instruments worldwide may be higher than the true value if the instruments were sensitive to these RO₂ species. If this is the case, it becomes necessary to compare atmospheric chemistry model simulations to HO₂^* observations, where HO₂^* = [HO₂] + Σ_i α_i [RO₂i], and α_i is the mean fractional contribution of the RO₂ species that interfere (RO₂i). This methodology, however, relies on model simulations of speciated RO₂ radicals, as instrumentation to make speciated RO₂ measurements does not currently exist. Here we present an approach that enables the concentration of HO₂ and RO₂i to be selectively determined by varying the concentration of NO injected into a FAGE cell. Measurements of [HO₂] and [RO₂i] taken in London are presented.