A novel technique for the humidity dependent calibration of hypoiodous acid (HOI) and iodine (I 2 )

Abstract. Hypoiodous acid (HOI) and molecular iodine (I2) are important precursors of reactive gaseous iodine, which plays an important role in the oxidative capacity of the atmosphere and in aerosol formation in the marine boundary layer. HOI and I2 are emitted from the ocean surface and recycled on atmospheric aerosol via heterogeneous chemistry. Measurements of these molecules, which are typically present in the marine boundary layer at the low-to-sub part per trillion (ppt) level, are sparse, in part due to difficulties in quantification with a lack of appropriate instrumentation and calibration techniques. A novel calibration technique is developed for HOI via generation from I2 hydrolysis and then 1:1 conversion of HOI back to I2 through a NaI trap, allowing the sensitivity of HOI to be calculated relative to I2, which is readily calibrated using a permeation tube system. Using this calibration method, we describe the use of a reduced pressure high resolution chemical ionisation mass spectrometer (CIMS) to characterise the sensitivities of HOI and I2 over a range of humidities representative of the marine boundary layer and to measure these molecules in the field. At humidities of over 50 % RH, the CIMS sensitivity of I2 is humidity independent whereas HOI exhibits a slight negative humidity dependence. The effect of inlet interactions on HOI and I2 signals is investigated, with HOI observed to convert to I2. The implications of these inlet interactions and humidity sensitivities for future ambient measurement configurations are discussed.