We present a new chemical mechanism for Hg0/HgI/HgII atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg0 by Br and OH, subsequent oxidation of HgI by ozone and radicals, respeciation of HgII in aerosols and cloud droplets, and speciated HgII photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg0 concentrations and HgII wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg0 to HgII. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII by OH. BrHgIIOH and HgII(OH)2, the initial HgII products of Hg0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of HgII to Hg0 takes place largely through photolysis of aqueous HgII–organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of HgI, and speciation and photoreduction of HgII in aerosols and clouds.

The dual fluorescence of 4-(dimethylamino)benzonitrile (DMABN) has been intensively studied in the last decades, but surprisingly there is not any detailed theoretical study of its photochemistry in polar solvents. In this work, we rationalize the different luminescent behavior of 4-aminobenzonitrile (ABN) and DMABN in acetonitrile by a computational study developed at the CASSCF/CASPT2 level and using the Polarized Continuum Model to reproduce the solvent environment. We present here the critical geometries, energies, and connections between the potential energy surfaces of the low-lying excited states: the locally excited state (LE) and several intramolecular charge transfer states (ICT). The computational results show that the topology of the potential energy surfaces (PES) does not change substantially when the effect of a polar solvent is included, in comparison with the gas phase. For DMABN, though, polar solvents stabilize preferentially the ICT states in such a way that the different interplay with the LE state induces strong qualitative changes in the photochemistry of this compound. Specifically, the planar ICT (PICT) species located on the S2 surface in the gas phase is, in acetonitrile, located on the S1 surface. that is, at the geometry of the PICT minimum, the LE state is higher in energy than the ICT. Now LE and PICT minima are practically degenerate and, given that both correspond to first excited state species, emission can take place from both of them. However, the twisted ICT (TICT) species is still the most favored thermodynamically so it is expected that this species would be preferentially populated. On the other hand, for ABN the equilibrium lies in favor of LE, as the TICT species was found at a much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN, even in polar solvents.

A study combining accurate quantum chemistry and full-dimensional quantum dynamics is presented to confirm the existence of an ultrafast radiationless decay channel from the charge-transfer state to the locally excited state in 4-aminobenzonitrile. This intramolecular charge-transfer pathway proceeds through a newly found planar conical intersection, and it is shown to be more efficient in the presence of acetonitrile than in the gas phase. Our results are consistent with recent experimental observations.