Objectives
The main objective is to understand the dynamic behaviour of atmospheric Hg species resulting in the deposition of oxidized fractions of Hg and re-emission of volatile inorganic and organic Hg species. These fluxes are largely governed by complex chemical redox reactions and biogeochemical processes where Hg is transformed between different oxidation states with different chemical and physical properties.
Description of Work
WP1 strategy includes complementary field and laboratory investigations of Hg redox reactions in atmospheric and aqueous (sea water, ice, cloud water) environments.
Task 1.1: ESR1 will compare commercial atmospheric Hg speciation instruments (Tekran) with improved cation exchange membrane techniques for reactive Hg (RM) sampling at three coastal polar stations. The Hg isotope composition of RM and GEM will be measured during a secondment to CNRS to provide additional information on oxidation mechanisms and on poorly understood polar GEM seasonality. The novel observations will be used during modelling secondments to HZG and MIT to improve GEM oxidation and reduction parameterization in 1D box models of the polar marine boundary layer (MBL) and in state-of-the-art 3D regional and global Hg models (WP5 and 6).
Task 1.2: ESR2 will specifically address photolytically driven redox reactions in the aqueous and gas phases by performing laboratory kinetic experiments. The ESR will perform photo-incubations (using a Xe lamp solar simulator with and wavelength cut-off filters) of Hg in seawater, rainwater, cloud water, aerosols, and of GOM in the gas phase. Complementary field-based experiments under natural sunlight will be conducted to validate laboratory studies (ESRs 1, 4, 8, 9). Photoreduction and photo-oxidation rates will be determined from Hg product and reactant concentration analysis. Photochemistry mechanisms will be studied from the Hg stable isotope composition of products and reactants during secondments at CNRS and JSI laboratories. The observations will be directly used, during ESR2 modelling secondments to Hereon and Harvard, to improve gas and aqueous phase Hg redox mechanisms and kinetics in 3D regional and global Hg models (WP5 and 6).
Expected Results
ESR1
Traceable and robust methodology for the quantification of reactive Hg (including stable isotopes) with spatial and temporal variability (paper 1). Measurements of Hg species concentrations and isotopic ratios and oxidants (BrO, NOx, O3) in polar regions where high oxidation rates are observed. An improved understanding of re-emission sources (snow, sea-ice, ocean) and their role on Hg atmospheric budget using 1D atmospheric models (paper 2). Improved understanding of of re-emissions sources on Hg budget at a global scale in 3D models. Improvement of reactions schemes and parametrization of GEM oxidation using the last version of models (i.e. GEOSChem 3D). Sensitivity testing and validation using GEM data base that are available worldwide (GMOS network, AMNET) (paper 3).
ESR2
(i) Experimental kinetic rate constants for reduction reactions in the aqueous phase for Hg(II) complexes in seawater, rainwater, cloud water, aerosols, and of GOM in the gas phase under different wavelength regions (using solar simulator) (ii) validation of laboratory experiments with complementary field based experiments, and (iii) mechanistic understanding of photochemical reactions from the Hg stable isotope composition of products and reactants and (iv) improved representation of aqueous and gaseous redox processes in 3D regional and global Hg models (3 papers).
Deliverables
| Del. | Deliverable Title – Science deliverables | WP | Lead | Type | Diss. | Due |
| D1.1 | ESR1: 3 papers on (i) traceable and robust methodology for the quantification of GOM and GEM, (ii) improved understanding of global GOM variability, (iii) Improved reaction scheme of Hg oxidation using novel models and databases | 1-6 | UGA | R | PU | 31 Oct 2024 |
| D1.2 | ESR2: 3 papers on (i) aqueous Hg photochemistry (ii) gas phase Hg photochemistry (iii) model validated atmospheric redox chemistry | 1-6 | JSI | R | PU | 31 Oct 2024 |
Milestones
| No | Title | WP(s) | Lead Benef. | Due Date | Means of Verification |
| 1 | Validated and calibrated methodology for GOM and GEM developed by ESR10 | 1, 4 | JSI | M14 | Secondments at PSA, VSL, JSI
Completed for ESRs1 and 2 |
| 4 | New field observations on atmospheric Hg speciation | 1, 5 | UGA | M26 | Field survey completed and data quality validated, ESR1 |
| 5 | New kinetic data for redox chemistry available | 1, 5 | JSI | M48 | Reactions verified and reported (ESR2) |
| 7 | Traceable calibration sources for Hg(0) and Hg(II) | 1, 4 | JSI | M30 | Report on validation and calibration |
| 22 | Regional high-resolution model study on impact of red-ox chemistry | 1, 5 | Hereon | M48 | Modelling and field data comparison for validation |
| 24 | PhD thesis submitted | 1-6 | JSI | M48 | All beneficiaries: PhD thesis approved by (co)supervisor |