Intercomparison of peroxy radical measurements at a rural site using laser-induced fluorescence and Peroxy Radical Chemical Ionization Mass Spectrometer (PerCIMS) techniques

Two different instruments were deployed to measure hydroperoxy radical (HO₂) and total peroxy radicals during May and June 2002 at a rural site located at Rock Springs near State College, Pennsylvania. One instrument was the Penn State Ground-Based Tropospheric Hydrogen Oxides Sensor (GTHOS), which measured HO₂ by laser-induced fluorescence (LIF) of OH following the chemical conversion of HO₂ to OH by adding NO. The other was the Peroxy Radical Chemical Ionization Mass Spectrometer (PerCIMS) instrument that provided HO₂ (HO₂ mode) or total peroxy radicals (HO_xRO_x mode) measurements. The two instruments were compared by exchanging calibration sources and by side-by-side ambient measurements. Excellent agreement was obtained in the calibration comparisons with the slope of a correlation plot close to 1.0 and a small intercept. Daily HO₂ measurements from GTHOS and PerCIMS (in HO₂ mode) on 4 days show that diurnal variations normally agreed to within about 40%. A comparison of HO₂ measured by GTHOS and HO_xRO_x measured by PerCIMS shows good correlation for both average diurnal variation and individual diurnal cycles. On average, HO₂ from GTHOS was about 0.66 times the HO_xRO_x from PerCIMS, meaning that organic peroxy radicals account for about one third of the total peroxy radicals in this environment.