Mercury and Halogen Chemistry from Passively Degassing Volcanoes Deanna Donohoue, Luke Surl, Roland von Glasow

School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK School of Environmental Science

Passively degassing volcanoes are natural sources of trace gas species, such as

halogens and mercury. Recent field and modelling studies have suggested that these emissions have significant impacts on the regional atmospheric chemistry resulting in O3 depletion, a reduction in the OH radical lifetime, and mercury oxidation. Atmospheric processes influence the transport, deposition, and cycling of mercury species and an understanding of these processes is essential to evaluate the effects of volcanic mercury emissions on human health and the environment on both a global and regional scale. In an effort to better understand the effects of volcanic mercury on local, regional and global scales, we incorporated a reduced bromine/mercury mechanism into the Weather Research and Forecasting/Chemistry model (WRF-Chem) which includes both a detailed treatment of meteorological and chemical processes. We present preliminary model results which will help us to further our understanding of the plume chemistry and to evaluate the health and environmental impacts of mercury/halogen emissions from volcanic plumes on regional and global scales

Abstract WRF - an open-source numerical weather prediction (NWP) and atmospheric simulation system designed for both research and operational applications .

WRF-CHEM - the WRF model coupled with Chemistry. The model simulates the emission, transport, mixing, and chemical transformation of trace gases and aerosols simultaneously with the meteorology.

MOZBC- NCAR/ACD program that creates time-varying chemical boundary conditions for WRF-CHEM from MOZART-4 output

PREP-CHEM-SRC - comprehensive pre-processing tool to prepare emission fields of trace gases and aerosols for use in atmospheric chemistry transport models. Includes options for anthropogenic, biogenic, biomass burning, and volcanic emissions.

MISTRA – a 1D model including gas, aerosol and cloud droplet chemistry, as well as, a detailed microphysical module that calculates particle growth and treats feedbacks between radiation and particles. The evolution of the plume of a passively degassing volcano is explicitly described.

Models

Reduced Mechanism for WRF

Changes to full MISTRA model (von Glasow, 2010): Gas phase halogen chemistry reduced to include only bromine species Simplified heterogeneous Br chemistry mechanism Gas phase mercury/halogen chemistry reduced to include only bromine species No heterogeneous mercury chemistry

Difference between the full and reduced mechanism: The use of the simplified heterogeneous Br chemistry results in a delay in the bromine explosion and a reduction of bromine chemistry on the edges of the plume.

BrO

O3

SO2

RGM/TGM

Full MISTRA Mechanism

BrO

O3

SO2

RGM/TGM

Reduced Mechanism

Time since model start (hr) Time since model start (hr)

Layer

Heig

ht

(m)

Halogen and mercury chemistry included in reduced mechanism

The lack of heterogeneous mercury chemistry has little impacts as the primary mode of Hg reduction in the plume is reduction by SO2

MISTRA was used to develop a reduced mechanism for the bromine and mercury chemistry.

Summary/Future Work Developed WRF as a tool to investigate passively degassing volcanoes

Preliminary results show:

A simplified heterogeneous bromine chemistry can be used

The inclusion of heterogeneous mercury chemistry has little impact on the model

Significant ozone loss is observed in the plume, however the loss of ozone in the 3D model is significantly less than the 1D model

Depletion of the emitted Hg was observed, however no depletion of background Hg was observed. We are investigating whether this is due to a bias in the current model run or a result of the entrainment of background air

Future work

Further investigation of the mercury mechanism looking at resolutions of 3.3 km and 1 km.

Continue development of the reduced mechanism using both WRF and MISTRA

Compare model results with the field data collectedin July 2012 campaign on Mount Etna - data includes mercury (Oxford), BrO/SO2 ratios (Heidelberg), SO2 and other trace gases(Palermo), O3 and aerosol (UEA), and flight data including mercury species (HZG & CNR)

Estimate mercury deposition from volcanic emissions and compare to field observation

Base case for WRF and WRF-Chem Options for base case:

Microphysics = Lin (5 class including graupel) Long wave Radiation = RRTMG scheme Shortwave Radiation = Goddard scheme Surface layer = Monin-Obukhov(Janjic-Eta) scheme Surface Physics = NOAH land-surface model Boundary Layer = Mellor-Yamada-Janjic (Eta) Cumulus Physics = New Grell Scheme Chemistry Scheme= CBMZ KPP scheme Aerosol Scheme= MOSAIC 8 bin Photolysis Scheme = Fast-J Anthropogenic Emissions = from PREP-CHEM Biogenic Emissions = Guenther Scheme (calc. from land use) Initial/Boundary Conditions = from MOZART-4 Meteorological Conditions = NCEP Final Analyses (FNL) 1°×1°

CBM-Z/MOSAIC no volcano – Oct 19th 2009

Inputs for WRF (FNL data and topography)

“Bromine explosion”:

HBr (+acidity, aerosol surface) Br2

Br2 + hv 2 Br

Br + O3 BrO

Mixing with

ambient air O3

O3 destruction in plume;

particles provide reactive

surfaces

O

3 Br

HBr

HCl

hν

O3

O3

hot lava (~1100oC) O2 ≈ 10-11

air (~20oC )

O2 ≈ 21%

Initial Plume:

main gas

components

H2O, CO2, SO2..

no O3, rapid

cooling

~900oC

O2 ≈ 3%

“effective source

region”

after: Bobrowski et al., JGR, 2007; field observations and modelling

The initialization of the volcanic plume depends on the emissions of high temperature volcanic volatiles and the rapid mixing of ambient air in the crater and vents. We initialize the model by identifying the “effective source region”, the region where high-temperature volcanic volatiles are mixed with a certain volume fraction of air and where it is assumed that thermodynamic equilibrium holds for this mix of hot gases (Gerlach, 2004). As the addition of oxygen dramatically changes the composition in the effective source region a volcanic volatiles: air ratio of 85:15 was used in the WRF model.

Initialization of Volcanic Plume

Addition to WRF Br chemistry: Addition of reduced Hg mechanism

5 gas phase reactions no heterogeneous Hg chemistry

Addition of Hg species to volcanic emissions using PREP-Chem

Preliminary Results: Mercury chemistry appears to occur rapidly after emission Conversion of the emitted Hg to RGM was observed but no depletion of background air was observed

RGM/TGM

Mercury Chemistry

Preliminary Results Hg depletion due to BrO Heterogeneous Chemistry

No Heterogeneous Br Chemistry

RGM/TGM

Hg (ppqv) loss due to Heterogeneous Br Chemistry

Hg at 1.5 km Hg at 1.5 km

with Heterogeneous Br Chemistry

Bromine Chemistry Addition to WRF base case:

Addition of reduced bromine mechanism

15 gas phase reactions (including new photolysis to fast-J scheme) 2 reactions in the simplified heterogeneous Br mechanism

Volcanic emissions using PREP-Chem

Includes: SO2, ash, and Br species Br speciation from an 85:15 mix in effective source region. Emitted into single grid cell in WRF

Preliminary Results:

BrO production is similar to the values produced in reduced MISTRA mechanism, slightly lower O3 loss is observed in plume, up to 30%

Ozone depletion due to volcanic emissions Time series

By taking the difference between runs with and without volcanic emission we can see the loss of ozone due to volcanic emissions, here we see up to 30% loss. This less than ~100% destruction observed in the MISTRA model

BrO difference (ppbv)

BrO production from Hetero. Chemistry

Preliminary Results CBM-Z/MOSAIC with reduced MISTRA mechanism

and Volcanic emissions – Oct 21th 2010 09:00 57hrs after model start

O3 (ppbv)

BrO (ppbv)

O3 difference (ppbv)

The influence of the heterogeneous bromine chemistry can be observed by taking the difference of runs with and without the heterogeneous Br chemistry. Here we see the production of ~25 pptv within the plume

at 3.5 km

at 3.5 km

Acknowledgements We thank Saulo Freitas for helpful discussions and assistance with Prep-Chem and the addition of the halogen/SO2 ratio into the code and Oliver Wild for assistance with FAST-J. We also thank Linda Smoydzin and Martin Stuefer for useful discussions and advise on using WRF-CHEM.

References - Bobrowski, N., R. von Glasow, A. Aiuppa, S. Inguaggiato, I. Louban, O. W. Ibrahim, and U. Platt (2007), Reactive halogen

chemistry in volcanic plumes, J. Geophys. Res., 112, D06311, doi:10.1029/2006JD007206. - Fast J.D., W.I. Gustafson Jr., R.C. Easter, R.A. Zaveri, J.C. Barnard, E.G. Chapman, and G.A. Grell. (2006). Evolution of ozone,

particulates, and aerosol direct forcing in an urban area using a new fully-coupled meteorology, chemistry, and aerosol model. J. Geophys. Res., 111:D21305, doi:10.1029/2005JD006721.

- Freitas, S. R., Longo, K. M., Alonso, M. F., Pirre, M., Marecal, V., Grell, G., Stockler, R., Mello, R. F., Sánchez Gácita, M., 2011a. PREP CHEM-SRC 1.0: a pre-processor of trace gas and aerosol emission fields for regional and global atmospheric chemistry models. Geoscientific Model Development, 4, 419-433.

- Gerlach, T. M. (2004), Volcanic sources of trop, O3-depleting trace gases, Geochem Geophy Geos., 5, Q09007 - Grell G.A., S.E. Peckham, R. Schmitz, and S.A. McKeen, G. Frost, W.C. Skamarock, and B. Eder. (2005). Fully coupled 'online'

chemistry in the WRF model. Atmos. Environ., 39:6957-6976. - von Glasow, R. (2010), Atmos. chemistry in volcanic plumes, Proc. Natl. Acad. Sci. U. S. A., 107, 6594–6599. - von Glasow, R., von Kuhlmann, R., Lawrence, M. G. , Platt, U. And Crutzen, P. J. (2004) Impact of reactive bromine chemistry in

the troposphere Atmos. Chem. Phys., 4, 2481-2497

Here are two time series plot where we follow a single grid cell over time. This is a grid cell from the point of emission until it leaves the model boundaries.

We observe significant O3 loss close to emission source. The O3 levels increase to/near background as the plume ages.

The BrO/SO2 ratios show the expected diurnal behaviour and result in ratios with an order of magnitude of the ratios observed by DOAS