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

摘要

Laboratory studies have revealed that alkene-derived RO andlonger chain alkane-derived RO (> C) radicals rapidlyconvert to HO 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 HO in the University of Leeds ground-based instrument have beenassessed to determine the sensitivity of each cell, when operating inHO detection mode, to RO 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 ROradicals. 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 HO to OH effectively minimised thispotential interference. An OH yield of 46% was observed forethene-derived RO 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 ROLIF cell,used for detection of HO and RO radicals an OH yield of 95% was observed for ethene-derived ROradicals, when running inHO mode.In experiments in which conditions ensured the conversion of RO to OHwere complete, the yields of OH from a range of different RO speciesagreed well with model predictions based on the Master Chemical Mechanismversion 3.2. For ethene and isoprene-derived RO species, the relativesensitivity of FAGE was found to be close to that for HO, with an OHyield of 100% and 92%, respectively. For the longer chain or cyclicalkane-derived RO radicals (> C), 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-derivedRO 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 HO by some LIF instrumentsworldwide may be higher than the true value if the instruments were sensitiveto these RO species. If this is the case, it becomes necessary tocompare atmospheric chemistry model simulations to HO observations,where HO = [HO] + Σ α [RO], andα is the mean fractional contribution of the RO species thatinterfere (RO). This methodology, however, relies on model simulationsof speciated RO radicals, as instrumentation to make speciated ROmeasurements does not currently exist. Here we present an approach thatenables the concentration of HO and RO to be selectivelydetermined by varying the concentration of NO injected into a FAGE cell.Measurements of [HO] and [RO] taken in London are presented.