Atmospheric Measurement Techniques Measurements of tropospheric HO2 and RO2 by oxygen dilution modulation and chemical ionization mass spectrometry

Abstract. An improved method for the measurement of hy-droperoxy radicals (HO2) and organic peroxy radicals (RO2, where R is any organic group) has been developed that combines two previous chemical conversion/chemical ion-ization mass spectrometry (CIMS) peroxy radical measure-ment techniques. Applicable to both ground-based and air-craft platforms, the method provides good separation be-tween HO2 and RO2, and frequent measurement capabil-ity with observations of both HO2 and HO2 + RO2 amounts each minute. These improvements allow for analyses of measured [HO2]/[HO2 + RO2] ratios on timescales relevant to tropospheric photochemistry. By varying both [NO] and [O2] simultaneously in the chemical conversion region of the PeRCIMS (Peroxy Radical CIMS) inlet, the method ex-ploits the changing conversion efficiency of RO2 to HO