# Presentation: 410287

## The Atmospheric Deposition of Mercury to the Great Lakes

- Dr. Mark Cohen
- Physical Scientist
- NOAA Air Resources Laboratory
- Silver Spring, Maryland

- Presentation at
**An Ecosystem Approach to the Health Effects**
**of Mercury in the Great Lakes Basin**
**February 26-27, 2003**
**Cleary International Conference Center**
**Windsor, Ontario**

**Notes:**
Thank you ___________, it is a pleasure to have the opportunity to participate in this meeting.

## Atmospheric Processes for Mercury

**Atmospheric Processes for Mercury**
**Key questions regarding atmospheric deposition**
**Methodological approaches to answer these questions**
**Preliminary results of an atmospheric modeling analysis**

**Notes:**
In my presentation, I am going to first go over our current understanding of the important atmospheric processes for mercury. Then I am going to discuss what I think are the key questions regarding atmospheric deposition of mercury to the Great Lakes, and some of the methods being used to develop answers to these questions. Finally, I will present summarize some preliminary results from a mercury modeling analysis I am conducting at the NOAA Air Resources Laboratory.

## Three “forms” of atmospheric mercury

**Notes:**
To begin, the atmospheric community generally classifies atmospheric mercury into three different “forms” –
First, there is elemental mercury...
Then there is Reactive Gaseous Mercury...
And finally there is particulate mercury...

## Slide 4

**Notes:**
First – direct anthropogenic emissions, from power plants, incinerators... 
different mixtures of the three types, depending on the source...
not as much known about emissions speciation as we need to know

Now, in the “dry atmosphere”, that in places where the relative humidity is fairly low (below 70% or so), you have these three forms of mercury...

And, we know that there are some gas-phase reactions that convert elemental mercury to RGM... these are relatively slow.

When the humidity gets higher, you have droplets and when the humidity gets very high, you can have clouds... In these cases, the chemistry is more complex... First, in these cases, the insoluble portions of particles are inside the droplets, and particulate mercury goes along with this... We are not sure how much of the mercury associated with particles dissolves into the aqueous phase in the droplet...

We know that RGM is very soluble, and a large percentage – sometimes an overwhelming percentage of it – goes into the droplet. And so I’ve shown the arrow here only going one way. Elemental mercury is much less soluble, and there can be a small amount of transfer into or out of the droplet, depending on the conditions (and on what happens next)

Once inside, we think that RGM can be transformed back to elemental mercury, by dissolved SO2 and the hydro-peroxyl radical HO2 (not H20, but HO2!)... there are questions about the rates of these reactions, and of their products. For example, recent work by Van Loon, Mader, and Scott at the University of Ottawa suggest that the product of the SO2 reaction is not dissolved elemental mercury, which would tend to evaporate from the droplet, due to its low solubility, but a complexed, much more soluble form of elemental mercury, that tends to stay in the droplet.

Second, elemental mercury that is dissolved in the droplet can be oxidized to dissolved Hg(II) – this is basically dissolved RGM – by ozone, hydrogen peroxide, chlorine, and hydroxl radical, and there may be other oxidation pathways. There are questions about the rates of these reactions. 

And, then, if it weren’t all complicated enough, dissolved RGM can adsorb to the insoluble core of the droplet – believed that its primarily being absorbed by the accessible soot portion of this material – or be desorbed from the soot... The direction of the transfer depends on how much is dissolved and how much is on the soot at any given time. If there is a LOT on the soot, there is a net desorption – if there is not much on the soot, there is a net absorption... And we think that this adsorbed material is somewhat different than “Hg(p)”, but there may be some overlap. All of these droplet processes are relatively fast, compared to the homogeneous gas phase processes mentioned earlier.

There is also another process that we know even less about, and this involves a heterogeneous process probably at the surface of ice particles in the atmosphere, and which probably involves bromine and maybe chlorine as well... This process is thought to be responsible for the artic mercury depletion events that are observed at polar sunrise in coastal regions of the Arctic and Antarctic... In these cases, the elemental mercury is quickly transformed to RGM and the RGM deposits relatively quickly to the surface. The deposition rate of elemental mercury is fairly small, so if this transformation didn’t take place, there would be much less deposition to the polar regions.  This process, or a variant of this process may be occurring elsewhere in the atmosphere... Measurements on mountain tops and airplanes have suggested that there is much more RGM present at high altitudes than we can probably explain, unless something like this heterogeneous process is occurring... 

There are a lot of uncertainties in the atmospheric chemistry of mercury still, and there still needs to be some additional work nailing down the rates and details of various processes... a lot of it is just plain old chemistry in a beaker, but there doesn’t seem to be enough support or interest in these areas...

And finally dry and wet deposition... all three forms of atmospheric mercury deposit in wet and dry deposition... Dry deposition is a very complex phenomena that depends on the local meteorology and nature of the surface (water, trees, buildings, frogs...)... But, in general, all things being equal for both wet and dry deposition, RGM tends to deposit the fastest, particulate mercury somewhat slower, and elemental mercury the slowest...

## Key questions regarding atmospheric deposition:

**1. How much is being deposited in each Lake?**

**2. How important is direct deposition to a given lake**
**relative to indirect loading via deposition to the**
**lake’s watershed?**

**3. How important is atmospheric deposition relative to other loading pathways (e.g., direct discharge to the Lake or its tributaries)**

**4. What is the relative importance of the contributions**
**from local, regional, national, continental, and global sources?**

**5. What is the relative importance of contributions from**
**different types of sources, e.g, coal fired utilities,**
**incinerators, natural emissions, etc.?**

***We need to know all these things to efficiently direct***
***action to reduce the contamination levels in a given lake.***

## Estimated atmospheric deposition of mercury to the Great Lakes and the Gulf of Maine

**Estimated atmospheric deposition of mercury to the Great Lakes and the Gulf of Maine**

**Notes:**
Regarding the first question, how much is being deposited... we have some idea...

Here is a set of model-estimated deposition totals for the Great Lakes, and the total for Lake Michigan estimated in the Lake Michigan Mass Balance Study – this result was a more measurement-based result... The model results are for 1996, and the Lake Michigan Mass Balance study was for the 1994-1995 period. The top graph shows the amounts being deposited to each lake, in kg/year, and the bottom graph shows the total flux of mercury, grams per km2 per year... Both of these estimates are for the deposition to the water surface of the Lake only... they do not include material deposited to the surrounding watershed, and subsequently delivered to the lake... The model results presented here are just preliminary, and I know there are a lot of uncertainties... And I’m pretty sure that there are some significant uncertainties in the Mass Balance estimates for Lake Michigan. So, take these numbers with a grain of salt... I suspect that we could be a factor of two or more off in these estimates at this time.

## For mercury, how important is atmospheric deposition relative to other loading pathways?

-  **We don’t have data for all the lakes**

**In general, insufficient measurements and modeling analysis to obtain accurate, timely estimates of this simple mass balance information for the Great Lakes**

** ****Many uncertainties in the existing estimates,**
**  ****e.g., significance of watershed processing**

**For mercury, how important is atmospheric deposition relative to other loading pathways?**

**Notes:**
Regarding the second question, in terms of how important atmospheric deposition is relative to other loading pathways, we have estimates for two of the Great Lakes... these estimates are for direct deposition to the surface of the lake... deposition to the watershed is not counted here, so the atmospheric contribution may be greater...

## Atmospheric deposition almost certainly

**plays a very significant**
**role in the mercury contamination**
**of the Great Lakes**

## Slide 9

**Notes:**
In terms of source-receptor relationships, it is instructive to look at a map of emissions surrounding the Great Lakes... 
These data from U.S. EPA and Environment Canada
The atmospheric pathway is complex... There is no pipeline that we can measure...
We want to know the geographic scale of the impacts... the relative importance of emissions at different length scales and directions from the lakes... 

## 1995 Global Hg Emissions Inventory, courtesy of Josef Pacyna, NILU, Norway (2001)

- 1995 Global Hg Emissions Inventory, courtesy of Josef Pacyna, NILU, Norway (2001)

**Notes:**
And, we can’t just consider only the U.S. and Canada, sources around the world may contribute, and in fact almost assuredly do... These data come from Josef Pacyna, of NILU, in Norway.

Preliminary estimates from a few groups using global mercury models, including Ashu Dastoor of Environment Canada, suggest that global sources may account for maybe 10-25% of the mercury being deposited in the Great Lakes, but much more work needs to be done in this area...

I will be present some detailed source-receptor estimates a little later when I show you some of my modeling results.

## Slide 11

**Notes:**
And, of course, we want to know what types of sources are the most important... Here is the U.S./Canadian emissions inventory for approximately 1996, broken down into a few basic categories – fuel combustion (largely coal), waste incineration, and manufacturing and other processes (including metallurgical processes).... I’ve shown here the estimated per capita emissions rates...

## Slide 12

**Notes:**
And of course, we have the estimates with more resolution... within each of these broad categories, there are a number of sub categories.. As we strive to limit the deposition of mercury to the Great Lakes, we want to know which types of sources are most important, as different strategies will be appropriate for different source types... 

## Model

**Model**
**evaluation**

**Notes:**
When we think about atmospheric deposition, there is a process starting at emissions, involving atmospheric transport and fate, there are atmospheric concentrations which we can try to measure along the way, from time to time at various places, and there is atmospheric deposition... 

## Model

**Model**
**evaluation**

**Notes:**
Conventional thinking has primarily focused on the ambient monitoring approach... Measurements are made, and the deposition to the a given lake is estimated... 

## Mercury Monitoring in Ambient Air

** ****Mercury Deposition Network:**
**  ****weekly *****wet *****deposition measurements;**
**  ****many sites in the U.S. and Canada;**
**  ****data are easily available through the web**

** ****CAMNET – Hg(0) ambient air concentrations**
**  ****at a number of  locations (Canada)**

** ****While several research programs measure**
**  ****RGM and Hg(p), there is no systematic network**
**  ****collecting publicly accessible data for these**
**  ****compounds, analogous to the MDN.**

** ****Unfortunate, because:**
** ****(1) they are important for dry deposition;**
** ****(2) they are needed for model evaluation**

** ****Mercury is not included in the**
**  ****Integrated Atmospheric Deposition**
**  ****Network (IADN)...  *****(but may be soon)***

**Notes:**
Let me try to briefly summarize the state of our mercury monitoring programs...

## We are generally not actually interested in the concentration or deposition at a single monitoring site...

***We are just using the few monitoring sites that we might have to give us a clue as to what the total impact might be...***

**We are interested in the deposition to an *****entire***** water body, or to a particular ecosystem **

**Notes:**
Now, it is important to remember that...

## There are large spatial variations in wet deposition

**There are large spatial variations in wet deposition**

- Source: M. Landis and G. Keeler, Atmospheric Mercury Deposition to
- Lake Michigan during the Lake Michigan Mass Balance Study.
- Environmental Science and Technology 36:4518-4524, 2002

**Notes:**
The problem with using measurements alone is that there are generally significant spatial (and temporal) and temporal variations in concentration and deposition, and you can’t measure everywhere all the time... So you can make measurements, but you’re never really sure if the measurements you have made can be scaled up to give an overall estimate of deposition to the ecosystem you are studying... 	

## There are large spatial variations in

**There are large spatial variations in**
**dry deposition and re-emission**

- Source: A. Vette, M. Landis and G. Keeler, Deposition and Emissions of
- Gaseous Mercury to and from Lake Michigan during the Lake Michigan
- Mass Balance Study (July, 1994-October 1995).
- Environmental Science and Technology 36:4525-4532, 2002

## Model

**Model**
**evaluation**

**Notes:**
and, with just measurements, you rarely can say much if anything about source-receptor relationships...

This is sometimes attempted, using receptor-based analyses like back trajectory analysis... This is a powerful tool, that can sometimes yield useful information, especially if there are large unknown sources...

However, you are never really sure where on the back-trajectory the important sources might have come from. And, as you have seen, the atmospheric chemistry of mercury is very complex, and it is very difficult (impossible?) to to a back trajectory analysis which includes these processes...  However, if one has no idea of the emissions inventory, this is certainly a good place to start.

## Model

**Model**
**evaluation**

**Notes:**
Potentially the most powerful way to analyze the atmospheric deposition pathway is to conduct comprehensive modeling:

Start with inventory, and model the fate and transport of emissions.

CLICK -- Use ambient monitoring to evaluate model.

Potential problems:
	Dependent on accuracy of emissions inventory
	Uncertainties in atmospheric fate processes

## Can’t reliably estimate the amount of deposition or source-receptor relationships using monitoring alone...

**Modeling can potentially give you these answers, but cannot be done credibly without using monitoring to ground-truth the results**

**Notes:**
I’m going to go out on a limb here, and say that... 

## Overall Project Goal

**Develop atmospheric mercury source-receptor information for the Great Lakes, the Gulf of Maine, and other selected receptors, **
**to estimate the *****amount of deposition***** and the relative contributions of different**

**source *****regions***
- (local, regional, national, continental, global)
**source *****categories***
- (e.g., coal combustion, waste incineration, etc.)

**...****to the atmospheric deposition to any given receptor**

## Overall Methodology

**Start with atmospheric mercury *****emissions  inventory***
**Perform *****atmospheric fate and transport modeling***** of these emissions (using a modified version of NOAA’s HSYPLIT model)**
**Keep track of *****source-receptor information***** during the modeling**
**Evaluate the modeling by *****comparison***** of the predictions *****against ambient monitoring data***
***If model is performing satisfactorily*****, report source-receptor results from the simulations**
**(Similar to earlier work with dioxin and atrazine)**

## Slide 24

**Notes:**
From a given source, material is emitted a certain location into the air, and then begins to be blown downwind – in whichever direction the wind is blowing at the time.

Several things begin to happen -- all at the same time...

First, as the material is carried along by the wind, it becomes progressively diluted by dispersion (in all three directions, i.e., up-down, in the direction of the wind, and in the direction perpendicular to the wind direction...) ... Thus, the concentrations and impacts will generally decrease with distance from the source (Note: very close to the source, where the plume hasn’t yet hit the ground, can have very little impact -- sometimes the safest place to be is right next to the stack!).

Also, mercury is transformed among several different types, as we’ve discussed, and there are important droplet, cloud, and ice interactions... -- i.e., into a new chemical, which may be more or less toxic -- by reactions (for example, with hydroxyl radical) or the direct impact of sunlight hitting the molecule.  Different chemical are more or less vulnerable to transformation by these phenomena, and, there can be significant diurnal variations as well. At night, there’s no sunlight and there’s very little hydroxyl radical, so, the rates of transformation can be substantially lower than in the daytime.  While there is some knowledge about these phenomena, there are substantial uncertainties for most pollutants.

Finally, there are deposition (and re-emission) phenomena. 

For this project, we are using a model developed at NOAA called HYSPLIT, and we are using meteorology data archived by NOAA for use with the model.

This is the same model that is used for emergency response situations, in the event of accidental releases of dangerous substances, and has been extensively tested and evaluated.

Advantages to using a model like HYSPLIT are that 

	Because of its emergency response role, it is designed to be very fast; thus we can simulate long time periods, e.g., an entire year, or even multiple years. For the Great Lakes, this is important, as one would like information on annual averages or even longer time periods -- not just for a few days...

	Also, it’s a type of model that is specially designed to preserve information about source-receptor relationships – it tries to keep track of the individual impacts of each source in the inventory, so that not only do you get the total deposition to the Lakes, but also the individual contributions from each source to the each Lake.



## Slide 25

**Notes:**
This is way we typically run the HYSPLIT model.  We do a run for one particular source location, and we release puffs periodically throughout the year (e.g., once every 7 hours).  We follow each puff and keep track of the dispersion and the fate processes (deposition, transformation, etc.).  The model uses the physical-chemical properties of the pollutant we are modeling, of course. By the end of the year, we have a good idea of the average impact that a source in this location would have on the receptors of interest (e.g., the Great Lakes).

## Slide 26

**Notes:**
So far, we’ve just included the U.S. and Canadian emissions... you’ve seen this map... and we are trying to determine the impact of each source in the inventory... (Data from U.S. EPA and Environment Canada)

## Transfer Coefficients for Hg are strongly influenced by the “type” of Hg emitted [Hg(II) has much greaterlocal impacts than Hg(0)]

- Transfer Coefficients for Hg are strongly influenced by the “type” of Hg emitted [Hg(II) has much greaterlocal impacts than Hg(0)]

## Slide 28

## Estimated atmospheric depositionof mercury to the Great Lakes

**Estimated atmospheric deposition****of mercury to the Great Lakes**

## Slide 30

## Slide 31

## Slide 32

## Contributions of different overall source sectors

**to the atmospheric deposition of mercury in 1996**
***per capita flux*****, i.e., nanograms of mercury**
**deposited per km2 of lake per person per year**

**“****fuels" = fuel combustion;**
**"incin." = waste incineration;**
**"metals" = metallurgcial processes**

## Slide 34

## Some concluding observations

**For the Great Lakes, atmospheric deposition**
**of mercury is almost certainly a very **
**significant loading pathway.**** **

***processes are complex, and there is still***
***much work to do to iron out the details...***

***Both monitoring and modeling must be used***
***together***** to answer the key questions we need**
**answered about atmospheric deposition**
**of mercury to the Great Lakes **

***magnitude ***
***relative importance***
***source-receptor relationships***

***Preliminary***** results for source receptor relationships**
**suggest the importance of coal-fired power plants,**
**over at least a regional and national scales**