A novel technique for the humidity dependent calibration of hypoiodous acid (HOI) and iodine (I₂)

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.