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Laboratory studies have revealed that alkene-derived RO2 andlonger chain alkane-derived RO2 (> C3) radicals rapidlyconvert to HO2 and then to OH in the presence of NO in a fluorescenceassay by gas expansion (FAGE) detection cell (Fuchs et al., 2011). Threedifferent FAGE cells that have been used to make ambient measurements of OHand HO2 in the University of Leeds ground-based instrument have beenassessed to determine the sensitivity of each cell, when operating inHO2 detection mode, to RO2 radicals. The sensitivity tothis interference was found to be highly dependent on cell design andoperating parameters. Under the operating conditions employed, duringfieldwork undertaken in the Borneo rainforest in 2008, an OH yield of 17%was experimentally determined for both ethene- and isoprene-derived RO2radicals. The high pumping capacity of this system, resulting in a shortresidence time in the cell, coupled with poor mixing of NO into the ambientair-stream for the titration of HO2 to OH effectively minimised thispotential interference. An OH yield of 46% was observed forethene-derived RO2 radicals when a smaller detection cell was used, inwhich the mixing of NO into the ambient air was improved and the cellresidence 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 RO2radicals, when running inHO2 mode. In experiments in which conditions ensured the conversion of RO2 to OHwere complete, the yields of OH from a range of different RO2 speciesagreed well with model predictions based on the Master Chemical Mechanismversion 3.2. For ethene and isoprene-derived RO2 species, the relativesensitivity of FAGE was found to be close to that for HO2, with an OHyield of 100% and 92%, respectively. For the longer chain or cyclicalkane-derived RO2 radicals (> C3), model predicted OHyields were highly dependent upon temperature. A model predicted OH yield of74% at 298 K and 36% at 255 K were calculated for cyclohexane-derivedRO2 radicals, and an experimental yield of 38% was observedindicating that the temperature within the cell was below ambient owing tothe supersonic expansion of the airstream in the low pressure cell. These findings suggest that observations of HO2 by some LIF instrumentsworldwide may be higher than the true value if the instruments were sensitiveto these RO2 species. If this is the case, it becomes necessary tocompare atmospheric chemistry model simulations to HO2* observations,where HO2* = [HO2] + Σi αi [RO2i], andαi is the mean fractional contribution of the RO2 species thatinterfere (RO2i). This methodology, however, relies on model simulationsof speciated RO2 radicals, as instrumentation to make speciated RO2measurements does not currently exist. Here we present an approach thatenables the concentration of HO2 and RO2i to be selectivelydetermined by varying the concentration of NO injected into a FAGE cell.Measurements of [HO2] and [RO2i] taken in London are presented

atmospheric measurement techniques

Reporting the sensitivity of laser-induced fluorescence instruments used for HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; detection to an interference from RO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; radicals and introducing a novel approach that enables HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and certain RO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; types to be selectively measured