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. Isocyanic acid (HNCO) is a chemical constituent suspected to be harmful tohumans if ambient concentrations exceed ∼1  ppbv. HNCO ismainly emitted by combustion processes but is also inadvertently releasedby NOx mitigation measures in flue gas treatments. With increasingbiomass burning and more widespread usage of catalytic converters in carengines, good prediction of HNCO atmospheric levels with global models isdesirable. Little is known directly about the chemical loss processes of HNCO,which limits the implementation in global Earth system models. This studyaims to close this knowledge gap by combining a theoretical kinetic study onthe major oxidants reacting with HNCO with a global modelling study. Thepotential energy surfaces of the reactions of HNCO with OH and NO3 radicals, Cl atoms, and ozone were studied using high-levelCCSD(T)/CBS(DTQ)//M06-2X/aug-cc-pVTZ quantum chemical methodologies,followed by transition state theory (TST) theoretical kinetic predictions of the rate coefficients attemperatures of 200–3000  K . It was found that the reactions are all slow inatmospheric conditions, with k ( 300 K ) ≤ 7 × 10 - 16 cm 3 molecule - 1 s - 1 , and that product formation occurs predominantly byH abstraction; the predictions are in good agreement with earlierexperimental work, where available. The reverse reactions of NCO radicalswith H2O , HNO3 , and HCl, of importance mostly in combustion, werealso examined briefly. The findings are implemented into the atmospheric model EMAC (ECHAM/MESSy Atmospheric Chemistry) to estimate theimportance of each chemical loss process on a global scale. The EMACpredictions confirm that the gas-phase chemical loss of HNCO is a negligibleprocess, contributing less than 1 % and leaving heterogeneous losses as themajor sinks. The removal of HNCO by clouds and precipitation contributesabout 10 % of the total loss, while globally dry deposition is the mainsink, accounting for ∼90  %. The global simulation alsoshows that due to its long chemical lifetime in the free troposphere, HNCOcan be efficiently transported into the UTLS by deep convection events.Daily-average mixing ratios of ground-level HNCO are found to regularlyexceed 1 ppbv in regions dominated by biomass burning events, but rarelyexceed levels above 10 ppt in other areas of the troposphere, though locallyinstantaneous toxic levels are expected.

Atmospheric chemical loss processes of isocyanic acid (HNCO): a combined theoretical kinetic and global modelling study