Measurement of the Henry's Law Coefficient and First Order Loss Rate of Isocyanic Acid in Water Solutions

A recent study of biomass fires at the combustion facility of the U.S. Department of Agriculture (USDA) Fire Sciences Laboratory (FSL) in Missoula, MT has brought light to the importance of isocyanic acid (HNCO) in the troposphere. Negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS) was used to make first measurements of the Henry's Law solubility constant (H) and first order loss rate coefficient (k) of HNCO in aqueous solution as a function of pH. Preliminary results show that at a pH of 3, H was found to be a factor of ~4 lower than that estimated by a bond contribution model. Measurements of k were found to be a factor of ~70 lower than previous kinetic measurements. Solubility and aqueous-phase loss rate coefficients determined here correspond to atmospheric lifetimes of ~10-20 days for the loss of HNCO onto aerosol particles. Preliminary results of a similar experiment using electron impact ionization coupled to a time of flight mass spectrometer for detecting negative ions (NI-TOFMS) will also be discussed.