Abstract
An atmospheric pressure/chemical ionization mass spectrometer (AP/CIMS) has been developed for continuous long-term measurements of atmospheric OH and H 2SO 4. The corresponding methods both involve chemical ionization of H 2SO 4 by NO 3 − ions with OH being first titrated by excess SO 2 to form equivalent concentrations of H 2SO 4 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 × 10 4 molecules cm −3 for H 2SO 4, and 54%, 48%, and 5 × 10 5 molecules cm −3 for OH measurements, respectively, based on 5 min signal integration.
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