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https://doi.org/10.5194/acp-2019-1138
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1138
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 03 Feb 2020

Submitted as: research article | 03 Feb 2020

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This preprint is currently under review for the journal ACP.

Chemical loss processes of isocyanic acid, HNCO, in the atmosphere

Simon Rosanka1, Giang H. T. Vu2, Hue M. T. Nguyen2, Tien V. Pham3, Umar Javed1, Domenico Taraborrelli1, and Luc Vereecken1 Simon Rosanka et al.
  • 1Institute for energy and climate research, Forschungszentrum Jülich Gmbh, Jülich, Germany
  • 2Faculty of Chemistry and Centrefor Computational Science, Hanoi National University of Education, Hanoi, Vietnam
  • 3School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam

Abstract. The impact of chemical loss processes of isocyanic acid was studied by a combined theoretical and modeling 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 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(300 K) ≤ 7 × 10−16 cm3 molecule−1 s−1; the predictions are in good agreement with earlier experimental work, where available. The reverse reactions of NCO radicals, of importance mostly in combustion, were also examined briefly. The global model confirms that gas phase chemical loss of HNCO is a negligible process, contributing less than 1 %. Removal of HNCO by clouds and precipitation is a larger sink, contributing for 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. Average daily concentrations of HNCO are found to rarely exceed levels considered potentially toxic, though locally instantaneous toxic levels are expected.the free troposphere, HNCO can be efficiently transported into the UTLS by deep convection events. Average daily concentrations of HNCO are found to rarely exceed levels considered potentially toxic, though locally instantaneous toxic levels are expected.

Simon Rosanka et al.

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Simon Rosanka et al.

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Short summary
Isocyanic acid, HNCO, is toxic chemical compound emitted to the atmosphere by biomass burning and by unwanted release in NOx mitigation systems in vehicles such as the AdBlue system. We have studied the loss processes of HNCO, finding that it is unreactive to most atmospheric oxidants, and thus has a long chemical lifetime. The main removal is then by deposition on surfaces, and transition to aqueous phase, such as clouds. The long lifetime also allows it to be transported to the stratosphere.
Isocyanic acid, HNCO, is toxic chemical compound emitted to the atmosphere by biomass burning...
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