A study of the reaction between NaHCO3 and H: Apparent closure on the chemistry of mesospheric Na

The reaction between sodium bicarbonate (NaHCO3) and atomic H is the only l ikely route (apart possibly from daytime photolysis) for recycling this maj or sodium reservoir back to atomic Na in the upper mesosphere. The rate coe fficient was determined for the first time by reacting NaHCO3 in a known ex cess of H in a fast flow tube yielding k(227 K) = (1.7 +/- 0.6) x 10(-13) a nd k(307 K) = (5.2 +/- 2.5) x 10(-13) cm(3) molecule(-1) s(-1). These measu rements are very well described by transition state theory, using ab initio quantum calculations of the saddle points on the reaction potential energy surface. The resulting temperature dependence of the rate coefficient is t hen given by k(100-400 K) = 1.84 x 10(-13) T-0.78 exp -(1014(-96)(+130) / T ) cm(3) molecule(-1) s(-1). When k(T) is extrapolated to 180-220 K, there i s excellent agreement with the value predicted from atmospheric models of t he nighttime Na layer. Hence this measurement has, to a first order, demons trated closure of the neutral chemistry governing the underside of the Na l ayer above 80 km.