New instrument for measuring atmospheric concentrations of non-OH oxidants of SO2

In addition to the hydroxyl radical (OH), also other oxidants of sulphur dioxide (SO2) can play a substantial role in the production of atmospheric sulphuric acid (H2SO4). Some of these non-OH oxidants are stabilized Criegee intermediates (sCIs) formed in the ozonolysis of alkenes. This paper introduces an FR-CI-APi-TOF instrument which measures the total concentration of all non-OH oxidants (X) reacting with SO2 at a reasonable rate. The instrument consists of a flow reactor (FR) and a chemical ionisation (CI) atmospheric pressure interface (APi) time of flight (TOF) mass spectrometer. The first field measurements at a boreal forest site indicated that the summer concentration, production rate and apparent lifetime of X were (0.5-8.0) x 10(5) cm(-3), (0.3-1.6) x 10(6) cm(-3) s(-1) and 0.1-1.8 s, respectively. The estimated concentration and production rate of sCIs formed in the ozonolysis of monoterpenes were substantially lower, possibly indicating the presence of sCIs from other alkenes. Further instrument development is needed to reduce the uncertainties in FR-CI-APi-TOF measurements.