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.
Global modelling study (GSM TIP) of the ionospheric effects of excited N