Numerical modeling of cloud chemistry effects on isocyanic acid (HNCO)

Isocyanic acid (HNCO), a product of some combustion processes, can potentially have negative human health effects. While gas phase HNCO loss processes are slow, HNCO loss in the aqueous phase is much faster. The fate of HNCO is studied for different cloud chemistry conditions using a zero-dimensional chemical box model. Exposure to clouds reduces HNCO concentrations substantially under typical cumulus cloud conditions, resulting in the chemical lifetime of HNCO dropping to ~2 h compared to clear-sky conditions of several years. The effect of clouds on HNCO is strongly dependent on the pH and temperature, with more HNCO hydrolyzed at lower pH (more acidic drops) and higher temperatures. Thus, HNCO is most efficiently removed by fog or low-level stratus clouds and least efficiently removed under middle to upper troposphere conditions where cumulonimbus and pyrocumulus clouds reside. Deliquesced aerosols may be highly efficient at reducing HNCO concentrations.