Quantification of the depletion of ozone in the plume of Mount Etna

Volcanoes are an important source of inorganic halogen species into the atmosphere. Chemical processing of these species generates oxidised, highly reactive, halogen species which catalyse considerable O₃ destruction within volcanic plumes. A campaign of ground-based in situ O₃, SO₂ and meteorology measurements was undertaken at the summit of Mount Etna volcano in July/August 2012. At the same time, spectroscopic measurements were made of BrO and SO₂ columns in the plume downwind. <br><br> Depletions of ozone were seen at all in-plume measurement locations, with average O₃ depletions ranging from 11–35 nmol mol⁻¹ (15–45%). Atmospheric processing times of the plume were estimated to be between 1 and 4 min. A 1-D numerical model of early plume evolution was also used. It was found that in the early plume O₃ was destroyed at an approximately constant rate relative to an inert plume tracer. This is ascribed to reactive halogen chemistry, and the data suggests the majority of the reactive halogen that destroys O₃ in the early plume is generated within the crater, including a substantial proportion generated in a high-temperature "effective source region" immediately after emission. The model could approximately reproduce the main measured features of the ozone chemistry. Model results show a strong dependence of the near-vent bromine chemistry on the presence or absence of volcanic NO_x emissions and suggest that near-vent ozone measurements can be used as a qualitative indicator of NO_x emission

Temperature and Pressure Dependent Rate Coefficients for the Reaction of Hg with Cl and the Reaction of Cl with Cl: A Pulsed Laser Photolysis−Pulsed Laser Induced Fluorescence Study

A pulsed laser photolysis−pulsed laser induced fluorescence technique has been employed to study the recombination of mercury and chlorine atoms, Hg + Cl + M → HgCl + M (1), and the self-reaction of chlorine atoms, Cl + Cl + M → Cl2 + M (2). Rate coefficients were determined as a function of pressure (200−600 Torr) and temperature (243−293 K) in N2 buffer gas and as a function of pressure (200−600 Torr) in He buffer gas at room temperature. For reaction (1) kinetic measurements were obtained under conditions in which either mercury or chlorine atoms were the reactant in excess concentration while simultaneously monitoring the concentration of both reactants. An Arrhenius expression of (2.2 ± 0.5) × 10-32 exp{(680 ± 400)(1/ T − 1/298)} cm6 molecule-2 s-1 was determined for the third-order recombination rate coefficient in nitrogen buffer gas. The effective second-order rate coefficient for reaction 1 under atmospheric conditions is much smaller than prior determinations using relative rate techniques. For reaction (2) we obtain an Arrhenius expression of (8.4 ± 2.3) × 10-33 exp{(850 ± 470)(1/ T − 1/298)} cm6 molecule-2 s-1 for the third-order recombination rate coefficient in nitrogen buffer gas. The rate coefficients are reported with a 2σ error of precision only; however, due to the uncertainty in the determination of absolute chlorine atom concentrations we conservatively estimate an uncertainty of ±50% in the rate coefficients. For both reactions the observed pressure, temperature, and buffer gas dependencies are consistent with the expected behavior for three-body recombination

Programmable Thermal Dissociation of Reactive Gaseous Mercury, a Potential Approach to Chemical Speciation: Results from a Field Study

Programmable Thermal Dissociation (PTD) has been used to investigate the chemical speciation of Reactive Gaseous Mercury (RGM, Hg2+). RGM was collected on denuders and analyzed using PTD. The technique was tested in a field campaign at a coal-fired power plant in Pensacola, Florida. Stack gas samples were collected from ducts located after the electrostatic precipitator and prior to entering the stack. An airship was used to sample from the stack plume, downwind of the stack exit. The PTD profiles from these samples were compared with PTD profiles of HgCl2. Comparison of stack and in-plume samples suggest that the chemical speciation are the same and that it is possible to track a specific chemical form of RGM from the stack and follow its evolution in the stack plume. Comparison of the measured plume RGM with the amount calculated from in-stack measurements and the measured plume dilution suggest that the stack and plume RGM concentrations are consistent with dilution. The PTD profiles of the stack and plume samples are consistent with HgCl2 being the chemical form of the sampled RGM. Comparison with literature PTD profiles of reference mercury compounds suggests no other likely candidates for the speciation of RGM

A Reply to Keith Schofield: Comment on Ernest et al. Programmable Thermal Dissociation of Reactive Gaseous Mercury, a Potential Approach to Chemical Speciation: Results from a Field Study. Atmosphere 2014, 5, 575–596

Dr. Schofield [1] states that:[...