Atmospheric chemical loss processes of isocyanic acid (HNCO): a combined theoretical kinetic and global modelling study
Simon Rosanka is a Postdoc at the Institute of Energy and Climate Research (Troposphere) at the Forschungszentrum Jülich. He participated as a PhD Candidate in the ESM Summer School in 2019 in Bad Aibling. In this article, he presents his study about isocyanic acid (HNCO), which is mainly emitted by biomass burning and causes adverse health effects for humans, and analyses the importance of atmospheric HNCO loss processes on a global scale.
Isocyanic acid (HNCO) is an atmospheric chemical constituent that is linked to protein carbamylation, which causes adverse health effects for humans such as rheumatoid arthritis, cardiovascular diseases, and cataracts if ambient levels exceed 1 ppb mixing ratio. It is mainly emitted by combustion processes like biomass burning 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, a good prediction of HNCO atmospheric levels with global models is desirable.
In this study, we, therefore, analyse the importance of atmospheric HNCO loss processes on a global scale. This is achieved by first studying the potential energy surfaces of HNCO reacting with OH and NO3 radicals, Cl atoms, and O3 using high-level quantum chemical methodologies. Afterwards, theoretical kinetic predictions of the rate coefficients at temperatures ranging from 200 to 3000 K are performed using transition state theory. Finally, these findings are implemented into the global numerical chemistry and climate simulation system ECHAM/MESSy (EMAC, https://www.messy-interface.org/), which describes tropospheric and middle-atmosphere processes and their interaction with oceans, land, and human influences. EMAC's gas-phase oxidation mechanism is extended to include formamide as an additional chemical source of HNCO and the mechanisms of methylamine, dimethylamine, and trimethylamine. Additionally, EMAC's standard aqueous-phase mechanism is extended to include HNCO and formamide. Within this study, two sensitivity simulations are performed using EMAC to account for the uncertainties in reported emission factors from biomass burning, its main source.
From the theoretical kinetics analysis, it can be concluded that the atmospheric reactions of HNCO are slow and the product formation occurs predominantly by H-abstraction, which is in good agreement with earlier experimental work. EMAC's predictions conform that the gas-phase chemical loss of HNCO is a negligible process, contributing less than 1 % to the total loss, leaving heterogeneous losses as major sinks. The removal of HNCO by clouds and precipitation contributes about 10 % to the total loss, while globally, dry deposition is the main sink, accounting for 90 %. These conclusions are robust against varying biomass burning emission factors. Daily-averaged mixing ratios of ground-level HNCO are found to regularly exceed 10 ppb in regions dominated by biomass burning events. Average daily concentrations in the order of 1 ppb are encountered more frequently, suggesting that local concentrations might peak to much higher values, e.g. in urban environments where road traffic emissions are highest or in the downwind plume of biomass burning events, and could impact regional air quality. Additionally, the global simulations show that due to its long chemical lifetime in the free troposphere, HNCO can be efficiently transported into the upper troposphere/lower stratosphere by deep convection events.
Rosanka, S., Vu, G. H. T., Nguyen, H. M. T., Pham, T. V., Javed, U., Taraborrelli, D., and Vereecken, L.: Atmospheric chemical loss processes of isocyanic acid (HNCO): a combined theoretical kinetic and global modelling study, Atmos. Chem. Phys., 20, 6671–6686, https://doi.org/10.5194/acp-20-6671-2020, 2020.