Chemical ionization mass spectrometer for long-term measurements of atmospheric OH and H2SO4

Abstract An atmospheric pressure/chemical ionization mass spectrometer (AP/CIMS) has been developed for continuous long-term measurements of atmospheric OH and H2SO4. The corresponding methods both involve chemical ionization of H2SO4 by NO3− ions with OH being first titrated by excess SO2 to form equivalent concentrations of H2SO4 in the system. The chemical ionization mass spectrometry (CIMS) system has been operated since April 1998 at the Meteorological Observatory Hohenpeissenberg, a mountain research station of the German Weather Service in South Germany. A technical description of the apparatus is presented followed by a detailed estimate of uncertainties in calibration and ambient air measurements resulting from changes in instrumental and/or ambient parameters. Examples from both calibration runs and ambient air measurements are shown. For the present system and operating conditions accuracy, precision, and detection limit are estimated to be 39%, 30%, and 3 × 104 molecules cm−3 for H2SO4, and 54%, 48%, and 5 × 105 molecules cm−3 for OH measurements, respectively, based on 5 min signal integration.