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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 16, issue 2
Atmos. Chem. Phys., 16, 703–714, 2016
https://doi.org/10.5194/acp-16-703-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 703–714, 2016
https://doi.org/10.5194/acp-16-703-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 21 Jan 2016

Research article | 21 Jan 2016

Solubility and reactivity of HNCO in water: insights into HNCO's fate in the atmosphere

N. Borduas, B. Place, G. R. Wentworth, J. P. D. Abbatt, and J. G. Murphy N. Borduas et al.
  • Department of Chemistry, University of Toronto, Toronto, Ontario, Canada

Abstract. A growing number of ambient measurements of isocyanic acid (HNCO) are being made, yet little is known about its fate in the atmosphere. To better understand HNCO's loss processes and particularly its atmospheric partitioning behaviour, we measure its effective Henry's Law coefficient KHeff with a bubbler experiment using chemical ionization mass spectrometry as the gas phase analytical technique. By conducting experiments at different pH values and temperature, a Henry's Law coefficient KH of 26 ± 2 M atm−1 is obtained, with an enthalpy of dissolution of −34 ± 2 kJ mol−1, which translates to a KHeff of 31 M atm−1 at 298 K and at pH 3. Our approach also allows for the determination of HNCO's acid dissociation constant, which we determine to be Ka = 2.1 ± 0.2  ×  10−4 M at 298 K. Furthermore, by using ion chromatography to analyze aqueous solution composition, we revisit the hydrolysis kinetics of HNCO at different pH and temperature conditions. Three pH-dependent hydrolysis mechanisms are in play and we determine the Arrhenius expressions for each rate to be k1 = (4.4 ± 0.2)  ×  107 exp(−6000 ± 240∕T) M s−1, k2 = (8.9 ± 0.9)  ×  106  exp(−6770 ± 450∕T) s−1 and k3 =  (7.2 ± 1.5)  ×  108 exp(−10 900 ± 1400∕T) s−1, where k1 is for HNCO + H++ H2O  →  NH4++ CO2, k2 is for HNCO + H2O  →  NH3 + CO2 and k3 is for NCO+ 2 H2O  →  NH3+ HCO3. HNCO's lifetime against hydrolysis is therefore estimated to be 10 days to 28 years at pH values, liquid water contents, and temperatures relevant to tropospheric clouds, years in oceans and months in human blood. In all, a better parameterized Henry's Law coefficient and hydrolysis rates of HNCO allow for more accurate predictions of its concentration in the atmosphere and consequently help define exposure of this toxic molecule.

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Short summary
HNCO is a toxic molecule and can cause cardiovascular and cataract problems through protein carbamylation once inhaled. Recently reported ambient measurements of HNCO in North America raise concerns for human exposure. To better understand HNCO's loss processes and behaviour in the atmosphere, we provide thermochemical data on HNCO. The parameters allow for more accurate predictions of its lifetime in the atmosphere and consequently help define exposure of this toxic molecule.
HNCO is a toxic molecule and can cause cardiovascular and cataract problems through protein...
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