Measurement of interferences associated with the detection of the hydroperoxy radical in the atmosphere using laser-induced fluorescence
One technique used to measure concentrations of the hydroperoxy radical (HO 2) in the atmosphere involves chemically converting it to OH by addition of NO and subsequent detection of OH. However, some organic peroxy radicals (RO 2) can also be rapidly converted to HO 2 (and subsequently OH) in the presence of NO, interfering with measurements of ambient HO 2 radical concentrations. This interference must be characterized for each instrument to determine to what extent various RO 2 radicals interfere with measurements of HO 2 and to assess the impact of this interference on past measurements. The efficiency of RO 2-to-HO 2 conversion for the Indiana University laser-induced fluorescence–fluorescence assay by gas expansion (IU-FAGE) instrument was measured for a variety of RO 2 radicals. Known quantities of OH and HO 2 radicals were produced from the photolysis of water vapor at 184.9 nm, and RO 2 radicals were produced by the reaction of several volatile organic compounds (VOCs) with OH. The conversion efficiency of RO 2 radicals to HO 2 was measured when NO was added to the sampling cell for conditions employed during several previous field campaigns. For these conditions, approximately 80 % of alkene-derived RO 2 radicals and 20 % of alkane-derived RO 2 radicals were converted to HO 2. Based on these measurements, interferences from various RO 2 radicals contributed to approximately 35 % of the measured HO 2 signal during the Mexico City Metropolitan Area (MCMA) 2006 campaign (MCMA-2006), where the measured VOCs consisted of a mixture of saturated and unsaturated species. However, this interference can contribute more significantly to the measured HO 2 signal in forested environments dominated by unsaturated biogenic emissions such as isoprene.