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

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 K_H^eff 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 K_H of 26 ± 2 M atm⁻¹ is obtained, with an enthalpy of dissolution of −34 ± 2 kJ mol⁻¹, which translates to a K_H^eff 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 K_a = 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.