Techniques for quantifying gaseous HOCl using atmospheric pressure ionization mass spectrometry

HOCl is speculated to be an important intermediate in mid-latitude marine b oundary layer chemistry and at high latitudes in spring-time when surface-l evel O-3 depletion occurs. However, techniques do not currently exist to me asure HOCl in the troposphere. We demonstrate that atmospheric pressure ion ization mass spectrometry (API-MS) is a highly sensitive and selective tech nique which has promise for HOCl measurements both in laboratory systems an d in held studies. While HOCl can be measured as the (HOCl . O-2)(-) adduct with air as the chemical ionization (CI) reagent gas, the intensity of thi s adduct is sensitive to the presence of acids and to the amount of water v apor, complicating its use for quantitative measurements. However, the addi tion of bromoform vapor to the corona discharge region forms bromide ions t hat attach to HOCl and allow its detection via the (HOCl . Br)- adduct. The ion-molecule chemistry associated with the use of air or bromoform as the CI reagent gas is discussed, with particular emphasis on the effects of rel ative humidity and gas phase acid concentrations. HOCl is quantified by mea suring the amount of Cl-2 formed by its heterogeneous reaction with condens ed-phase HCl/H2O. Detection limits are similar to 3 parts per billion (ppb) using air as the CI reagent gas and similar to 0.9 ppb using bromoform. Th e atmospheric implications of this work are discussed.