Uptake of gas-phase NO_2 by water was studied in both the droplet train and bubble train flow reactors. Aqueous surface reaction of NO_2(g), reported previously in the literature, is not substantiated by these studies. The uptake of NO_2(g) is a function of Henry's law coefficient (H_(NO_2)) and the second-order NO_2(aq)-NO_2-(aq) hydrolysis reaction rate coefficient (k2), in the form H_(NO_2)(k2)~(1/2). The NO_2(aq)-NO_2(aq) hydrolysis rate coefficient is defined by: -d[NO_2(aq)]/dt = 2k_2[MO_2(aq)]~2. The coupled nature of the uptake makes it difficult to obtain reliable separate values for the two parameters H and k_2. Literature values for these parameters vary by as much as a factor of 5. With the bubble train apparatus it was possible to separate clearly the effects of solubility and reaction on NO_2 gas uptake. From these measurements and analysis of literature values, recommended values of these parameters at 293 K are H_NO_2) = (1.4 ± 0.2) * 10~(-2) M atm~(-1) and k_2 = (3.0 ± 0.9) * 10~7 M~(-1)s~(-1). At 276 K, our best estimates are H_(NO_2) = 2.3(+0.3-0.9) * 10~(-2) M atm~(-1) and k_2 = (2.2 ± 0.6) * 10~7 M~(-1) s~(-1).
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