Chemical ionization mass spectrometer instrument for the measurement of tropospheric HO2 and RO2

Laboratory characterizations of the peroxy radical chemical ionization mass spectrometer (PerCIMS) instrument have been performed. The instrument functions by drawing ambient air through a 50-mum-diameter orifice into an inlet held at low pressure. Peroxy radicals (HO2 and RO2) within this air are detected by amplified chemical conversion into a unique ion (HSO4-) via the chemistry initiated by the addition of NO and SO2 to the inlet. HSO4- ions are then quantified by a quadrupole filter mass spectrometer. PerCIMS provides measurements of the sum of peroxy radicals, HO2 + RO2 (HOxROx mode), or the HO2 component only (HO2 mode), achieved through the control of concentration of NO and SO2 added to the instrument. The characterization and response of this instrument have been evaluated through modeling of inlet chemistry and laboratory experiments and have also been demonstrated through successful deployment during field campaigns. The performance of PerCIMS with respect to calibration pressure and relative humidity is reported, as are the sensitivities of the instrument to organic peroxy radicals with different hydrocarbon groups. These data show PerCIMS to be a practical field instrument for the fast and accurate evaluation of the concentration of peroxy radicals over a variety of atmospheric conditions. The estimated accuracy of the derived [HOxROx] concentrations is +/-35% (at the 95% confidence interval), while [HO2] measurements have accuracies of +/-41% (at the 95% confidence interval). Typical precision of measurements well above the detection limit is 10%, and typical detection limits are 1 x 107 radicals cm(7) for 15-s averaging times.