Application of Time-of-Flight Aerosol Mass Spectrometry for the Online Measurement of Gaseous Molecular Iodine

Here we present a new application of a time-of-flight aerosol mass spectrometer (TOF-AMS) for the measurement of atmospheric trace gases in real-time. Usually, TOF-AMS instruments are not sensitive to gas-phase species due to the aerodynamic particle focusing inlet system which reduces the gas phase species by a factor of about 10⁷ relative to the particle phase. This efficient removal of the gas phase and the resulting high relative enrichment of particles is one reason for the very high sensitivity of TOF-AMS instruments for particle phase compounds (detection limits in the sub-μg/m³-range for online measurements with 1 min integration time), which allows application of the instruments even under clean atmospheric conditions. Here we use artificially generated particles as sampling probes to transfer selected atmospheric trace gases into the particle phase before entering the AMS (gaseous compound trapping in artificially generated particles-AMS, GTRAP-AMS). The sampling probe particles are mixed with the gaseous analytes upstream of the TOF-AMS in a 0.5 L flow tube. As an exemplary application of the method, the measurement of trace levels of gaseous molecular iodine is demonstrated. α-Cyclodextrin (α-CD/NH₄Br) particles are used as selective sampling probes to transfer molecular iodine into the AMS. A detection limit in the subparts-per-billion (sub-ppb) range was achieved. The method was compared to a recently developed off-line method that combines denuder sampling of gaseous I₂ and gas chromatography/mass spectrometry (GC/MS) analysis. To demonstrate the usability of the method, temporally resolved I₂ emission profiles from a brown algae species (<i>Laminaria saccharina</i>) under exposure of ambient ozone levels were investigated. Total I₂ release rates of 36.5 pmol min^–1 grams fresh weight (gFW)⁻¹ at 100 pbb O₃ and 33.4 pmol min^–1 gFW^–1 at 50 ppb O₃ were obtained within the first hour of ozone exposure