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

摘要

A growing number of ambient measurements of isocyanic acid (HNCO) are beingmade, yet little is known about its fate in the atmosphere. To betterunderstand HNCO's loss processes and particularly its atmosphericpartitioning behaviour, we measure its effective Henry's Law coefficientK_Heff with a bubbler experiment using chemicalionization mass spectrometry as the gas phase analytical technique. Byconducting experiments at different pH values and temperature, a Henry's Lawcoefficient K_H of 26?±?2?M?atm−1 is obtained, with anenthalpy of dissolution of −34?±?2?kJ?mol−1, which translates toa K_Heff of 31?M?atm−1 at 298?K and at pH 3.Our approach also allows for the determination of HNCO's acid dissociationconstant, which we determine to be K_a?=?2.1?±?0.2??×??10−4?M at 298?K. Furthermore, by using ion chromatography to analyzeaqueous solution composition, we revisit the hydrolysis kinetics of HNCO atdifferent pH and temperature conditions. Three pH-dependent hydrolysismechanisms are in play and we determine the Arrhenius expressions for eachrate 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?1and 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 thereforeestimated to be 10?days to 28?years at pH values, liquid water contents, andtemperatures relevant to tropospheric clouds, years in oceans and months inhuman blood. In all, a better parameterized Henry's Law coefficient andhydrolysis rates of HNCO allow for more accurate predictions of itsconcentration in the atmosphere and consequently help define exposure ofthis toxic molecule.