Abstract
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.
Highlights
The interest in studying HNCO was from a fundamental science perspective with research conducted on its structure, preparation and physical properties (Belson and Strachan, 1982) and on its theoretical rovibrational spectra (Mladenovicand Lewerenz, 2008)
Both theoretical and experimental data indicate that HNCO is the most stable CHNO isomer with a near-linear π -bond system (Hocking et al, 1975; Jones et al, 1950; Poppinger et al, 1977)
HNCO has been observed to cause protein carbamylation leading to cardiovascular disease, rheumatoid arthritis and cataracts (Beswick and Harding, 1984; Lee and Manning, 1973; Mydel et al, 2010; Wang et al, 2007)
Summary
The interest in studying HNCO was from a fundamental science perspective with research conducted on its structure, preparation and physical properties (Belson and Strachan, 1982) and on its theoretical rovibrational spectra (Mladenovicand Lewerenz, 2008). HNCO has been detected simultaneously in the gas phase and in cloud water in La Jolla, California (Zhao et al, 2014) From these studies, typical urban concentrations range from below detection limits to approximately 100 pptv, whereas concentrations as high as 1.2 ppbv, enough to be of health concern, have been measured in air Published by Copernicus Publications on behalf of the European Geosciences Union
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