Combined Experimental and Computational Kinetics Studies for the Atmospherically Important BrHg Radical Reacting with NO and O2

We report on the first experimental determination of the absolute rate constant of the reaction of BrHg + NO in N-2 bath gas using a laser photolysis-laser-induced fluorescence (LP-LIF) system. The rate constant of the reaction of BrHg + NO is determined to be 7.0(-0.9)(+1.3) x 10(-12) cm(3) molecule(-1) s(-1) over 50-700 Torr and 315-353 K. The absence of a pressure or temperature dependence suggests that this reaction leads mainly to mercury reduction (Hg + BrNO) rather than mercury oxidation (BrHgNO). Our theoretical calculations using NEVPT2 energies on density functional theory (DFT) geometries are consistent with a barrierless reaction to form Hg + BrNO. The equilibrium constants and the rate constants of the reaction BrHg + O-2 (sic) BrHgOO are computed theoretically because they are too low to be measured in the LP-LIF system. Molecular oxygen quenches the LIF signal of BrHg with a large rate constant of (1.7 +/- 0.1) x 10(-10) cm(3) molecule(-1) s(-1). Thus, different techniques that capture the absolute [BrHg((X) over tilde)] would be advantageous for kinetics studies of BrHg reactions in the presence of O-2. The computed equilibrium constant suggests a non-negligible upper limit of the fraction of BrHg stored as BrHgOO (up to 0.5) at low-temperature conditions, e.g., in the upper troposphere and in polar winters at ground level. Preliminary results indicate that BrHgOO behaves like HOO or organic peroxy radicals in reactions with atmospheric radicals.

Automated summary

This study reports the experimental determination of the absolute rate constant for the reaction of BrHg with NO. The results suggest that the reaction leads to mercury reduction rather than mercury oxidation. Additionally, molecular oxygen significantly quenches the LIF signal of BrHg.