Abstract
Abstract. Isocyanic acid (HNCO) is a chemical constituent suspected to be harmful to humans if ambient concentrations exceed ∼1 ppbv. HNCO is mainly emitted by combustion processes but is also inadvertently released by NOx mitigation measures in flue gas treatments. With increasing biomass burning and more widespread usage of catalytic converters in car engines, good prediction of HNCO atmospheric levels with global models is desirable. Little is known directly about the chemical loss processes of HNCO, which limits the implementation in global Earth system models. This study aims to close this knowledge gap by combining a theoretical kinetic study on the major oxidants reacting with HNCO with a global modelling study. The potential energy surfaces of the reactions of HNCO with OH and NO3 radicals, Cl atoms, and ozone were studied using high-level CCSD(T)/CBS(DTQ)//M06-2X/aug-cc-pVTZ quantum chemical methodologies, followed by transition state theory (TST) theoretical kinetic predictions of the rate coefficients at temperatures of 200–3000 K. It was found that the reactions are all slow in atmospheric conditions, with k(300K)≤7×10-16 cm3molecule-1s-1, and that product formation occurs predominantly by H abstraction; the predictions are in good agreement with earlier experimental work, where available. The reverse reactions of NCO radicals with H2O, HNO3, and HCl, of importance mostly in combustion, were also examined briefly. The findings are implemented into the atmospheric model EMAC (ECHAM/MESSy Atmospheric Chemistry) to estimate the importance of each chemical loss process on a global scale. The EMAC predictions confirm that the gas-phase chemical loss of HNCO is a negligible process, contributing less than 1 % and leaving heterogeneous losses as the major sinks. The removal of HNCO by clouds and precipitation contributes about 10 % of the total loss, while globally dry deposition is the main sink, accounting for ∼90 %. The global simulation also shows that due to its long chemical lifetime in the free troposphere, HNCO can be efficiently transported into the UTLS by deep convection events. Daily-average mixing ratios of ground-level HNCO are found to regularly exceed 1 ppbv in regions dominated by biomass burning events, but rarely exceed levels above 10 ppt in other areas of the troposphere, though locally instantaneous toxic levels are expected.
Highlights
The existence of isocyanic acid (HNCO) in the atmosphere has been established only recently (Roberts et al, 2011; Wentzell et al, 2013) despite its molecular structure and chemical synthesis being first discovered in the 19th century (Liebig and Wöhler, 1830)
Another aspect is the effect of redissociation of chemically activated adducts, which decreases the effective rate of HNCO loss
Though negligible at low temperature, we find that OH addition on the C atom of HNCO accounts for 7 % to 8 % of the reaction rate between 2000 and 3000 K, with other non-H-abstraction channels remaining negligible (< 0.1 %)
Summary
The existence of isocyanic acid (HNCO) in the atmosphere has been established only recently (Roberts et al, 2011; Wentzell et al, 2013) despite its molecular structure and chemical synthesis being first discovered in the 19th century (Liebig and Wöhler, 1830). HNCO emissions from traffic are originating mainly from usage of recent catalytic converters in the exhaust systems of gasoline-based (Brady et al, 2014) and diesel-based (Heeb et al, 2011) vehicles These converters are implemented to control the emission of primary pollutants such as hydrocarbons, carbon monoxide, particulate matter, and nitrogen oxides. The emissions and sources of HNCO have been focused on by many past studies, but there remain large uncertainties in our understanding of HNCO removal processes, especially in gas-phase chemistry This missing information on HNCO removal processes limits global models to predict HNCO with confidence. To alleviate the dearth of direct data and improve the representation of HNCO in global models, we first provide a theoretical analysis of the chemical reactions of HNCO with the dominant atmospheric oxidants: OH and NO3 radicals, Cl atoms, and O3 molecules, including the prediction of each rate coefficient at atmospheric conditions. The model is used to provide an estimate of the relative importance of primary and secondary HNCO sources
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