Experimental determination of Henry's law constants of difluoromethane (HFC-32) and the salting-out effects in aqueous salt solutions relevant to seawater

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Abstract

Gas-to-water equilibrium coefficients, <i>K</i><sub>eq</sub><sup><i>S</i></sup> (in M&amp;thinsp;atm<sup>&amp;minus;1</sup>) of difluoromethane (CH<sub>2</sub>F<sub>2</sub>), a hydrofluorocarbon refrigerant (HFC-32), in aqueous salt solutions relevant to seawater were determined over a temperature (<i>T</i>) range from 276 to 313&amp;thinsp;K and a salinity (<i>S</i>) range up to 51&amp;thinsp;‰ by means of an inert-gas stripping method. From the van't Hoff equation, the <i>K</i><sub>eq</sub><sup>S</sup> value in water, which corresponds to the Henry's law constant (<i>K</i><sub>H</sub>), at 298&amp;thinsp;K was determined to be 0.064 M&amp;thinsp;atm<sup>&amp;minus;1</sup>. The salinity dependence of <i>K</i><sub>eq</sub><sup><i>S</i></sup> (the salting-out effect), ln(<i>K</i><sub>H</sub>/<i>K</i><sub>eq</sub><sup><i>S</i></sup>), did not obey the Sechenov equation but was represented by an equation involving <i>S</i> and <i>S</i><sup>0.5</sup>. Overall, the <i>K</i><sub>eq</sub><sup><i>S</i></sup>(<i>T</i>) value was expressed by ln(<i>K</i><sub>eq</sub><sup><i>S</i></sup>(<i>T</i>))&amp;thinsp;=&amp;thinsp;&amp;minus;49.7122&amp;thinsp;+&amp;thinsp;77.7018&amp;thinsp;&amp;times;&amp;thinsp;(100/<i>T</i>)&amp;thinsp;+&amp;thinsp;19.1379&amp;thinsp;&amp;times;&amp;thinsp;ln(<i>T</i>/100)&amp;thinsp;+&amp;thinsp;[&amp;minus;0.2261&amp;thinsp;+&amp;thinsp;0.5176&amp;thinsp;&amp;times;&amp;thinsp;(<i>T</i>/100)]&amp;thinsp;&amp;times;&amp;thinsp;<i>S</i><sup>0.5</sup>&amp;thinsp;+&amp;thinsp;[0.0362&amp;thinsp;&amp;minus;&amp;thinsp;0.1046&amp;thinsp;&amp;times;&amp;thinsp;(<i>T</i>/100)]&amp;thinsp;&amp;times;&amp;thinsp;<i>S</i>. By using this equation in a lower tropospheric semi-hemisphere (30°&amp;thinsp;S&amp;ndash;90°&amp;thinsp;S) of the Advanced Global Atmospheric Gases Experiment (AGAGE) 12-box model, we estimated that 1 to 5&amp;thinsp;% of the atmospheric burden of CH<sub>2</sub>F<sub>2</sub> resided in the ocean mixed layer and that this percentage was at least 5&amp;thinsp;% in the winter; dissolution of CH<sub>2</sub>F<sub>2</sub> in the ocean may partially influence estimates of CH<sub>2</sub>F<sub>2</sub> emissions from long-term observational data of atmospheric CH<sub>2</sub>F<sub>2</sub> concentrations.

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Kutsuna, S. (2017). Experimental determination of Henry’s law constants of difluoromethane (HFC-32) and the salting-out effects in aqueous salt solutions relevant to seawater. Atmospheric Chemistry and Physics, 17(12), 7495–7507. https://doi.org/10.5194/acp-17-7495-2017

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