Chemical ionisation mass spectrometer for measurements of OH and Peroxy radical concentrations in moderately polluted atmospheres

摘要

A new version of an atmospheric pressure chemical ionisation mass spectrometer has been developed for ground based in situ atmospheric measurements of OH and total peroxy (HO_2 + organic peroxy) radicals. Based on the previously developed principle of chemical conversion of OH radicals to H_2SO_4 in reaction with SO_2 and detection of H_2SO_4 using an ion molecule reaction with NO_3~-, the new instrument is equipped with a turbulent chemical conversion reactor allowing for measurements in moderately polluted atmosphere at NO concentrations up to several ppb. Unlike other similar devices, where the primary NO_3~- ions are produced using radioactive ion sources, the new instrument is equipped with a specially developed corona discharge ion source. According to laboratory measurements, the overall accuracy and detection limits are estimated to be, respectively, 25% and 2 × 10~5 molecule cm'3 for OH and 30% and 1 × 10~5 molecule cm~(-3) for HO_2 at 10 min integration times. The detection limit for measurements of OH radicals under polluted conditions is 5 × 10~5 molecules cm~(-3) at 10 min integration times. Examples of ambient air measurements during a field campaign near Paris in July 2007 are presented demonstrating the capability of the new instrument, although with reduced performance due to the employment of non isotopic SO_2.