1. Modelling the concentration and deposition of heavy metals in the UK
Tony Dore1, Massimo Vieno1, Ilia Ilyin2,
Heath Malcolm1, Heather Yorston1, Fiona Fordyce3, Mark Cave4,
Harry Harmens5,Małgorzata Werner6, Maciej Kryza6, Stefan Reis1
1 Centre for Ecology and Hydrology, Edinburgh, Scotland, UK
2 EMEP/MSC-East, Moscow, Russia
3 British Geological Survey, Edinburgh, UK
4 British Geological Survey, Nottingham, UK
5 Centre for Ecology and Hydrology, Bangor, Wales, UK
6 Department of Climatology and Atmosphere Protection, University of Wrocław, Poland.
Task Force on Modelling and Measurements
18 – 20 May 2016, Utrecht

2. Introduction
The UNECE protocol on heavy metals was signed in 1998, targeting the emissions of three key metals:
Lead (Pb), Cadmium (Cd) and Mercury (Hg – not considered in this study).
Pb is a multi-organ system toxicant that can cause neurological, cardiovascular, renal, gastrointestinal, haematological and reproductive effects. Main emissions sources are metal production and combustion of lubricants. Emissions have declined since introduction of unleaded petrol and decreased use of coal.
Cd is a toxic element for humans which can result in kidney and bone damage and is carcinogenic by inhalation. Main emissions sources are energy production, non-ferrous metal production and iron and steel manufacture. Emissions have declined due to a fall in coal combustion.
Estimates in the UK indicate large areas of forest where exceedance of critical loads occurs for Pb
UNECE protocol committed UK to reducing emissions of metals to below 1990 levels, phasing out leaded petrol and using Best Available Technology to reduce emissions from stationary sources.

3. Emission Trends for Pb and Cd from the UK
99% reduction in Pb emissions
between 1990 and 2010
Decline in emissions due to introduction of unleaded petrol, decline in heavy metals industry, reductions in coal combustion and improved technology
93% reduction in Cd emissions
between 1990 and 2008

4. Multi-pollutant Exchange model
FRAME The Fine Resolution Atmospheric
Multi-pollutant Exchange model
Simple Lagrangian model of the UK with 5 km grid resolution used primarily for nitrogen & sulphur deposition
Annually averaged meteorology: straight line trajectories with wind direction frequency weighting
Emissions of: Cd and Pb for the year 2012
Point source emissions treated with plume rise model
Particulate concentrations with four separate size categories
Wet removal using constant drizzle and a map of annual rainfall
Vegetation-specific resistance model for dry deposition
Vertical diffusion (33 layers) calculated using K-theory
Boundary conditions: European 50 km resolution simulation (EMEP grid)

5. 2012 FRAME Modelled concentration and deposition of Cd and Pb
Cd air concentration (ng m-3)
Cd dry deposition (g Ha-1)
Cd wet deposition (g Ha-1)
Pb air concentration (ng m-3)
Pb dry deposition (g Ha-1)
Pb wet deposition (g Ha-1)
Generally strong gradients showing decreases from south-east to north-west due to bothy UK emissions sources from urban areas in the south and import from European concentrations from south-east.
Wet deposition doesn’t appear to show up hill areas – Needs to be checked

6. UK Heavy Metal monitoring network
Bulk collectors used for precipitation chemistry – these could be subject to dry deposition.
Tipping bucket measurements of precipitation were not successful so precipitation chemistry collectors were used to calculate annual precipititation – this could lead to underestimate in measured precipitation

7. major under-estimates in modelled Cd and Pb concentration!
Comparison of model with monitoring data for 2012 using primary emissions only
major under-estimates in modelled Cd and Pb concentration!
Cd air concentration
Pb air concentration
NMB = -0.91
r = 0.94
NMB = -0.80
r = 0.91
Good correlation with measurements but major underestimate in modelled concentrations (note different scales on x-axis and y-axis). Using primary emissions only gives an 80% under-estimate in Cd concentrations and 90% under-estimate in Pb concentrations
Other metals have also shown similar under-estimates in modelled concentrations with NMB of -0.91, -0.71, -0.79, -0.64, -0.64, for As, Cr, Cu, Ni, Se, Zn
Dore, A.J., Hallsworth, S., McDonald, A.G., Werner, M., Kryza, M., Abbott, J., Nemitz, E., Dore, C.J., Malcolm, M., Vieno, M., Reis, S. and Fowler, D. (2014) Quantifying missing sources of heavy metals in the United Kingdom with an atmospheric transport model
Science of the Total Environment C,

8. The gap between the model and measurements of air concentrations
is evidence for missing / under-estimated emissions sources
RE-SUSPENSION
High concentrations of heavy metals in road-side dust can be re-suspended by vehicle turbulence
The surface dust layer on bare agricultural fields contains heavy metals which can be re-suspended by high winds
On a national scale res-suspension from bare soils will make a larger contribution to re-suspension but roadside dust could be important in urban areas.
We should also consider: Uncertainty in metal content of imported coal Missing fugitive emissions

9. Re-suspension calculations
Spatial estimates of re-suspended emissions of Cd and Pb for the UK for the year 2012 were calculated with the MSc-east model using dynamic meteorology to simulate the wind driven re-suspension of surface dust
These values suggest that re-suspension makes a higher contribution to total emissions than primary emissions sources
Emissions (Mg) Cd Pb
Primary
2.0 65
Re-suspended
3.5
233
Re-suspension of Pb is high relative to primary emissions due to the very rapid reduction in primary emissions, such that the ‘legacy’ of historic emissions is relatively more important

10. Comparison of model with monitoring data for 2012 with re-suspended emissions included
Cd air concentration
Cd wet deposition
NMB = -0.20
r = 0.66
Pb air concentration
Pb wet deposition
Inclusion of re-suspension estimates is effective in ‘closing the gap’ between measurements and modelled estimates which was present when only primary emissions were considered. Cd wet deposition shows considerable scatter – could dry deposition of particulate matter to the precipitation chemistry collectors be an issue?
NMB = -0.10
r = 0.60

11. Pb concentration in air Inter-comparison of preliminary results of high resolution (10 km) MSC-east simulation over UK with FRAME
Both models get reasonable agreement with measurements. Less scatter is evident for the MSC-east model due to its more complex representation of meteorology and dynamic linking of re-suspension with high wind speed events.
Note – some differences with the previous slide as 12 sites are used here but only 10 sites were used in my (FRAME) plot – this is being checked

12. Future work (in progress): Use UK data on metal concentrations in topsoil with high resolution meteorological simulations to generate spatially distributed re-suspension estimates Pb Cd
To date re-suspension estimates have used the low resolution European scale simulations (50 km?) and European data on soil concentrations. BGS have now supplied UK specific data on Pb and Cd concentration in soil.
Acknowledgement: Contains British Geological Survey materials © NERC 2016
Rawlins, B.G., McGrath, S.P., Scheib, A.J., Cave, M., Lister, T.R., Ingham, M., Gowing, C. & Carter. S
The Advanced Soil Geochemical Atlas of England and Wales. Keyworth: British Geological Survey.

13. Future work: Comparison of modelled metal deposition with measurements from moss samplingCd Pb
Slide shows site based measurements of metal concentrations in soils and the results of interpolation (part of wider European survey). Data for the UK is available only for 2005 as UK funding was stopped for this work.
Harmens et al. (2011). Mosses as biomonitors of atmospheric heavy metal deposition: spatial and temporal trends in Europe. Environmental Pollution 158:

14. Conclusion
Cd and Pb are a concern for both human health and the natural environment.
Atmospheric emissions of Cd and Pb from the UK have greatly reduced during recent years.
Metals are still present in the environment due to the legacy of higher historic emissions. Wind re-suspension of surface dust can make a more important contribution than primary emissions.
Measurement of heavy metal concentrations in the surface dust layer is required to better establish the contribution of re-suspension.
Simulations with a Eulerian model (using dynamic meteorology) can capture the episodic nature of re-suspension during high wind speed events.

15. Thank you for your attention !
Acknowledgement
Thank you for your attention !
Any questions?
Lake Morlich, the Cairngorms