mercury and halogen chemistry from passively … and halogen chemistry from passively degassing...
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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
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