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

L. K. Whalley, M. A. Blitz, M. Desservettaz, P. W. Seakins, D. E. Heard
2013 Atmospheric Measurement Techniques Discussions  
M et al. (2 more authors) ( 2013) 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. Atmospheric Measurement Techniques, 6 1 instruments used for HO 2 detection to an interference from 2 RO 2 radicals and introducing a novel approach that 3 enables HO 2 and certain RO 2 types to be selectively 4 measured 5 6 Abstract 11
more » ... aboratory studies have revealed that alkene-derived RO 2 and longer-chain alkane-derived 12 RO 2 (>C 3 ) radicals rapidly convert to HO 2 and then to OH in the presence of NO in a 13 Fluorescence Assay by Gas Expansion (FAGE) detection cell (Fuchs et al., 2011). Three 14 different FAGE cells that have been used to make ambient measurements of OH and HO 2 in 15 the University of Leeds ground-based instrument have been assessed to determine the 16 sensitivity of each cell, when operating in HO 2 detection mode, to RO 2 radicals. The 17 sensitivity to this interference was found to be highly dependent on cell design and operating 18 parameters. Under the operating conditions employed during fieldwork undertaken in the 19 Borneo rainforest in 2008, an OH yield of 17% was experimentally determined for both 20 ethene-and isoprene-derived RO 2 radicals. The high pumping capacity of this system, 21 resulting in a short residence time in the cell, coupled with poor mixing of NO into the 22 ambient air-stream for the titration of HO 2 to OH effectively minimised this potential 23 interference. An OH yield of 46% was observed for ethene-derived RO 2 radicals when a 24 smaller detection cell was used, in which the mixing of NO into the ambient air was 25 improved and the cell residence times were much longer. For a newly developed RO x LIF 26 cell, used for detection of HO 2 and RO 2 radicals, when running in HO 2 mode an OH yield of 27 95% was observed for ethene-derived RO 2 radicals. 28 In experiments in which conditions ensured the conversion of RO 2 to OH was complete, the 1 yields of OH from a range of different RO 2 species agreed well with model predictions based 2 on the Master Chemical Mechanism version 3.2. For ethene and isoprene derived RO 2 3 species, the relative sensitivity of FAGE was found to be close to that for HO 2, with an OH 4 yield of 100% and 92% respectively. For the longer-chain or cyclic-alkane-derived RO 2 5 radicals (> C 3 ), model predicted OH yields were highly dependent upon temperature. A 6 model predicted OH yield of 74% at 298 K and 36% at 255 K were calculated for 7 cyclohexane derived RO 2 radicals, and an experimental yield of 38% was observed indicating 8 that the temperature within the cell was below ambient owing to the supersonic expansion of 9 the airstream in the low pressure cell. 10 These findings suggest that observations of HO 2 by some LIF instruments worldwide may be 11 higher than the true value if the instruments were sensitive to these RO 2 species. If this is the 12 case, it becomes necessary to compare atmospheric chemistry model simulations to HO 2 * 13 observations, where HO 2 * = [HO 2 ]+ i i [RO 2i ] and i is the mean fractional contribution of 14 the RO 2 species that interferes (RO 2i ). This methodology, however, relies on model 15 simulations of speciated RO 2 radicals, as instrumentation to make speciated RO 2 16 measurements does not currently exist. Here we present an approach that enables the 17 concentration of HO 2 and RO 2i to be selectively determined by varying the concentration of 18 NO injected into a FAGE cell. Measurements of [HO 2 ] and [RO 2i ] taken in London are 19 presented. 20
doi:10.5194/amtd-6-6249-2013 fatcat:6gwdz35pqrh7bekaqh6uol2twe