# Presentation: 445458

## Source-attribution for atmospheric mercury deposition:Where does the mercury in mercury deposition come from?

**Dr. Mark Cohen**
**NOAA Air Resources Laboratory**
**Silver Spring, Maryland, **
***mark.cohen@noaa.gov***
_***http://www.arl.noaa.gov/ss/transport/cohen.html***_

**Presentation given via speakerphone to a meeting of the **
**Mercury Working Group, Office of Air Quality,**
**Indiana Department of Environmental Management (IDEM)**
**April 21, 2005**

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

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

## 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...

## modeling the fate and transportof atmosphericmercury

## Atmospheric Fate Processes for Hg

**Atmospheric Fate Processes for Hg**

**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...

## Slide 6

**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 7

**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).

## some illustrative modelingresults

## The impact of any given mercury emissions source on any receptor is highly variable, both in space and in time 

***The impact of any given mercury emissions source on any receptor is highly variable, both in space and in time***** **

**Think about the weather and then add in all the chemistry and physics of mercury’s interactions with the weather**

## The impact of any given mercury emissions source on any receptor is highly dependent on the “type” of mercury emitted 

**Elemental mercury - Hg****0**** - is not readily dry or wet deposited, and its conversion to ionic Hg or Hg(p) is relatively slow**
**Particulate mercury – Hg(p) - is moderately susceptible to dry and wet deposition**
**Ionic mercury – also called Reactive Gaseous Mercury or RGM – is very easily dry and wet deposited**
***Conversion of RGM to Hg******0****** in plumes*****?**

## 0.1o x 0.1o subgrid 

- 0.1o x 0.1o subgrid
- for
- near-field analysis

- source
- location

## Slide 12

## Slide 13

## Slide 14

## Slide 15

## Estimated Speciation Profile for 1999 U.S.Atmospheric Anthropogenic Mercury Emissions 

**Very uncertain for most sources**

## Each type of source has a very different emissions speciation profile

**Each type of source has a very different emissions speciation profile**
**Even within a given source type, there can be big differences – depending on process type, fuels and raw materials, pollution control equipment, etc.**

##  Estimated 1999 U.S. Atmospheric Anthropogenic Mercury Emissions

## Emissions of Ionic Mercury (RGM) from Different AnthropogenicSource Sectors in Great Lakes States and Provinces (~1999-2000)[Total RGM emissions = 13.4 metric tons/year]

- Emissions of Ionic Mercury (RGM) from Different AnthropogenicSource Sectors in Great Lakes States and Provinces (~1999-2000)[Total RGM emissions = 13.4 metric tons/year]

## There can be large local and regional impacts

***Hypothesized rapid reduction of Hg(II) in plumes? ***
***If true, then dramatic impact on modeling results...***

***There can be large local and regional impacts***

## At the same time, medium to long range transport can’t be ignored

***At the same time, medium to long range transport can’t be ignored***

## There are a lot of sources...

***There are a lot of sources...***

**Large spatial and temporal variations**
**Each source emits mercury forms in different proportions**
**A lot of different sources can contribute significant amounts of mercury through atmospheric deposition to any given receptor**

## Geographic Distribution of Largest Anthropogenic Mercury Emissions Sources in the U.S. (1999) and Canada (2000)

**Geographic Distribution of Largest Anthropogenic Mercury Emissions Sources in the U.S. (1999) and Canada (2000)**

## Slide 24

## Getting the source-apportionment information we all want is difficult

***Getting the source-apportionment information we all want is difficult***

**With measurements alone, generally impossible**
**Coupling measurements with back-trajectory analyses yields only a little information**
**Comprehensive fate and transport modeling – “forward” from emissions to deposition – holds the promise of generating detailed source-receptor information**

## There are a lot of uncertainties in current comprehensive fate and transport models... Nevertheless, many models seem to be performing reasonably well, i.e., are able to explain a lot of what we see

***There are a lot of uncertainties in current comprehensive fate and transport models... Nevertheless, many models seem to be performing reasonably well, i.e., are able to explain a lot of what we see***

## somesource-receptorresults for the Great Lakes

## Note: results in this paper are for 1996; 

**Cohen, M., Artz, R., Draxler, R., Miller, P., Poissant, L., Niemi, D., Ratte, D., Deslauriers, M., Duval, R., Laurin, R., Slotnick, J., Nettesheim, T., McDonald, J.****   “Modeling the Atmospheric Transport and Deposition of Mercury to the Great Lakes.” *****Environmental Research***** 95(3), 247-265, 2004.**
**Note: Volume 95(3) is a Special Issue: "An Ecosystem Approach to Health Effects of Mercury in the St. Lawrence Great Lakes", edited by David O. Carpenter.**

- Note: results in this paper are for 1996;
- Results shown in the following slides have been updated to 1999

## Emissions sources which are among the top-25 model-estimated contributors to one or more of the Great Lakes

**Emissions sources which are among the top-25 model-estimated contributors to one or more of the Great Lakes**

## Geographical Distribution of 1999 Direct Deposition Contributions to Lake Michigan

**Geographical Distribution of 1999 Direct Deposition Contributions to Lake Michigan**

## Emissions and Deposition Contributions from Different Distance Ranges Away From Lake Michigan

## Top 25 Contributors to 1999 Hg Deposition Directly to Lake Michigan

## source-attribution –the “big picture”

## Natural vs. 

**Natural vs. **
**anthropogenic**
**mercury?**
**Studies show that anthropogenic activities have **
**typically increased bioavailable Hg  concentrations in ecosystems by a**
**factor of 2 – 10**** **

## Hg flux / pre-industrial Hg flux 

**Average mercury accumulation rate relative to pre-industrial (1800-1850)**
**accumulation rate in five lakes in Northern Alaska (based on sediment cores)**
- from Fitzgerald et al. (2005), “Modern and Historic Atmospheric Mercury Fluxes in Northern Alaska: Global Sources and Arctic Depletion” *Environ Sci Tech* **39**, 557-568

**Hg flux / pre-industrial Hg flux **

**another example of atmospheric deposition flux increasing due to anthropogenic emissions**

## What is the relative importance of global, national, regional, and local sources?

## ?

**?**

- Data used by EPA to support recent Clean Air Mercury Rule

## the Seigneur et al. (2004) paper shows that, for regions where deposition is high, the impact of local/regional sources is the main cause for these elevated concentrations. 

**the Seigneur et al. (2004) paper shows that, for regions where deposition is high, the impact of local/regional sources is the main cause for these elevated concentrations.**** **

***(slide courtesy of Rob Mason, Univ. of CT)***

- [Seigneur et al., (2004), “Global Source Attribution for Mercury Deposition in the United States”, *ES&T ***38**, 555-569.]

- 1999 Hg Emissions

## Results from the EPA REMSAD Mercury model

***Source: slide developed by Anne Pope for the Hg Roundtable conference call April 21, 2005***

**Percent non-US for 1998**

## Modeled vs. Measured Wet Deposition at Mercury Deposition Network Site MD_13 during 1996

**Modeled vs. Measured Wet Deposition at Mercury Deposition Network Site MD_13 during 1996**

**HYSPLIT modeling has shown that**
**in areas of significant local and regional anthropogenic sources,**
**ambient measurements can be explained reasonably well by considering only these local and regional anthropogenic sources.**

## Source-apportionment answers depend a lot on where you are

**the deposition may be relatively low,**
**but what deposition there is may largely come from natural and global sources**
_**For areas **__***with large emissions sources***_
**the deposition will be higher**
**and be more strongly influenced by these large emissions sources...**

**Source-apportionment answers depend a lot on **_**where you are**_

## What is the relative importance of global, national, regional, and local sources?Possible answers are emerging as our understanding improves, but there is no scientific consensus yet...

## Conclusions