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
- 1School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
- 2National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
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 ROxLIF 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] + Σi αi [RO2i], and αi 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.