Correction to A Theoretical Study of the Oxidation of Hg0 to HgBr2 in

Addition/Correction pubs.acs.org/est

Correction to A Theoretical Study of the Oxidation of Hg0 to HgBr2 in the Troposphere M. E. Goodsite, J. M. C. Plane, and H. Skov Environ. Sci. Technol. 2004, 38 (6), 1772−1776; DOI: 10.1021/es034680s.

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t has recently been drawn to our attention by T. S. Dibble (SUNY-ESF) that an accurate spectroscopic determination of the HgBr bond energy, Do(Hg−Br), had been published prior to our study. Tellinghuisen and Ashmore (1983)1 recorded emission spectra of the B − X transition of HgBr, and obtained Do(Hg−Br) = 64.9 ± 1.9 kJ mol−1 by extrapolating the vibrational progression to the dissociation limit (the uncertainty arises because the highest vibrational level observed was at least 12% below the dissociation limit). The theoretical value of 63.8 kJ mol−1 calculated in our study (Table 1) is actually very close to this value. Dibble brought to our attention to two items to correct: First, the incorrect referencing of the bond energy in Table 1 of our paper. The experimental Hg−Br bond energy (at 0 K) of 74.9 ± 4 kJ mol−1 given in Table 1 was taken from the CRC Handbook (ref 29 in the paper). The bond energy listed in the Handbook is given at 298 K. We therefore extrapolated this to obtain the bond energy at 0 K using the vibrational frequency for HgBr(X) measured in ref 21. Therefore the footnote f in Table 1 should have listed both refs 21 and 29. Similarly for footnote g for the Hg−I bond energy. Second, is our failure to cite the Tellinghuisen and Ashmore1 measurement in our paper; however, since our study was published, a much higher level calculation of the bond energy using coupled cluster CCSD(T) theory obtained a value of 67.0 kJ mol−1.2 These values are somewhat lower than the old experimental value of 74.9 kJ mol−1 we used for the RRKM calculations on reactions 4 and −4. We have therefore repeated these calculations using Do(Hg−Br) = 66.3 kJ mol−1, which is an average of the spectroscopic value,1 the CCSD(T) value,2 and the currently recommended value from NIST of 66.9 kJ mol−1.3 With minimal changes to the parameters in the RRKM calculation (average energy for downward transitions ⟨ΔE⟩down = 300 cm−1 for N2 as the bath gas at 300 K with a temperature dependence T −0.6; σ = 3 Å and ε/kB = 250 K to describe the intermolecular potential between HgBr and N2), we obtain an excellent fit to the recent direct measurement of k4 by Donohoue et al.4 The third-order rate coefficient over the temperature range 200 − 300 K is calculated to be k(Hg + Br + N2 → HgBr + N2) = 1.5 × 10−32 (T/298 K)−1.76 cm6 molecule−2 s−1, and the rate coefficients for reactions 4 and −4 at 1 atm pressure are then

Using these revised expressions causes an increase in the estimated lifetime τ of Hg against oxidation to HgBr2. The summary sentences at the bottom of page 1775 in the paper should now read (old values of τ in parentheses). “During Hg depletion events [Br] is estimated to vary from 0.2 ppt, when τ will range from 61 to 257 (35 to 60) h, to 6 ppt, when τ will be only 1.9−2.3 (0.7−1.5) h. A typically observed 10 h lifetime of Hg would correspond to [Br] ∼ 1.5 (0.7) ppt at an “average” temperature of 245 K.” Thus, the conclusion of the paperthat the oxidation of Hg in the atmosphere is very sensitive both to temperature and the atomic Br concentration because of the instability of HgBr remains unchanged.

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ACKNOWLEDGMENTS We credit and thank Prof. T.S. Dibble (SUNY-ESF) for bringing the above to our attention, as well as participating in the following discussion as we formulated this correction.

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REFERENCES

(1) Tellinghuisen, J.; Ashmore, J. G. Chem. Phys. Lett. 1983, 102, 10. (2) Shepler, B. C.; Balabanov, N. B.; Peterson, K. A. J. Chem. Phys. 2007, 127, 164304. (3) http://webbook.nist.gov/chemistry/ (accessed March 4, 2012). (4) Donohoue, D. L.; Bauer, D.; Cossairt, B.; Hynes, A. J. J. Phys. Chem. A 2006, 110, 6623−6632.

k4(Hg + Br → HgBr, 200 − 300K) = 3.7 × 10−13(T /298K)−2.76 cm3molecule−1s−1 k −4(HgBr → Hg + Br, 200 − 300K) = 4.0 × 109exp( −7292/T )s−1 © 2012 American Chemical Society

Published: April 13, 2012 5262

dx.doi.org/10.1021/es301201c | Environ. Sci. Technol. 2012, 46, 5262−5262