SILVER ANNIVERSARY International Conference on Heavy Metals in the Environment, 6-10 August, 2000; Ann Arbor, Michigan

 

Vertical Distribution of Mercury Compounds in the Atmosphere

 

K. Kvietkus and J. Sakalys

Atmospheric Pollution Research Laboratory, Institute of Physics, Gostauto 12, 2600 Vilnius, Lithuania, kvietkus@ktl.mii.lt

 

Abstract. A simple model based on previous studies and recent Hg measurements with an automated CVAAS Mercury Analyzer GARDIS has been proposed to describe the cycling of mercury in the atmosphere. The life-time for both gaseous and particulate mercury in the lower troposphere was estimated.

 

INTRODUCTION

 

In comparison with most of the other heavy metals, mercury and many of its compounds behave exceptionally due to their relatively high volatility. Total gaseous mercury (TGM) concentrations in air surpass the concentrations of particulate - phase mercury (Hgpart). Because of the small fraction of Hgpart in ambient air, only a small fraction of the total burden of mercury can be removed from the atmosphere by washout or rainout.

In the troposphere, mercury exists almost exclusively in a relatively insoluble gaseous Hg (0) state. It can be advected for long periods of time until chemically transformed to a soluble species or physically bound to the atmospheric aerosol.  Aqueous redox reactions are most important pathways for removal of elemental mercury from the regional atmosphere.  Ozone is considered to be the most important oxidant, while S03-2 and particulate matter (soot) is thought to control the reduction reactions.

The most likely deposition mechanism is via the aqueous-phase chemical conversion to Hg (II) with subsequent wet or dry deposition.  Evaporation of rain or cloud droplets containing mercury may produce secondary particulate mercury.  Any gaseous Hg (II) directly emitted to the atmosphere is expected to fast dry or wet deposit relatively to Hg (0).  The deposition velocity for Hg (II) is comparable to that of nitric acid (few cm/sec) while for Hg (0) it is evaluated to be 0.05 – 0.1 cm/sec (Fitzgerald et al., 1991).

 A low aqueous solubility and chemical reactivity of elemental mercury, which represents the major fraction of total gaseous mercury (TGM), are the major reasons for its long atmospheric life-time permitting long-range atmospheric transport to regions far from centers of anthropoghenic activity.

During the recent years, a better understanding of many environmental aspects of mercury has become available, as new sampling and analytical techniques were significantly improved. Up-to-date tasks of mercury cycling investigation in the atmosphere needed real time, high-time resolution atmospheric Hg measurements at environmental background levels. They could not be performed by conventional manual methods and required a highly stable, sensitive and automated mercury analysis system (Urba, 1999).

The main goal of the manuscript is estimation of the life-time for both gaseous and particulate mercury in the lower troposphere.

 

RESULTS and discussion

 

1. Evaluation of the TGM life-time from deposition data (Kvietkus et al., 1997; Urba, 1999).

 

The annual average Hg deposition rate from moss and precipitation data will be 15 μg/m2 y (σ = 1.1 μg/m2 y) of Hg or 4.94*10-4 ng/m2 s (σ = 0.33*10-4 ng/m2 s), respectively.

If we assume that the density of the total atmosphere is the same as near the ground, then the height of the total atmospheric layer will be 8 km. The average measured TGM concentration near the ground is 1.5 ng/m3  (σ = 0.1 ng/m3). If TGM concentration is the same in 1 m2 through the column height of 8 km, then the total amount of TGM in the column will be 12 μg/m2. If our calculations are correct, then the TGM life-time in the atmosphere is not longer than 12*365.25/15 = 290 days  (σ = 30 days). The above-obtained value is the maximum TGM life-time in the atmosphere. However, due to well-known processes of participation of TGM in various chemical reactions, adsorption on particles with following removal from the atmosphere, it should be shorter. Thus, it is evident that the TGM concentration decreases with the height. So, the real TGM life-time in the atmosphere should be shorter than 290 days.

 

2. Evaluation of the TGM life-time using a simple model (Kvietkus and Sakalys, 2000).

 

The source of Hg in the atmosphere is the earth’s surface including natural and anthropogenic sources. Mercury is deposited on the ground surface as a result of different processes in the atmosphere. So, it is evident that the Hg concentration, (ppb), decreases with the height as it was mentioned above.

If we assume that the atmospheric air is pressed through the total layer of the atmosphere up to 1 atm, then the height of the pressed total atmosphere will be h = 8 km. The pressed atmosphere height (h) can be obtained using the Barometric formula (p = po e-gz) and after integration the following expression is defined:

,                                                              (1)

where g is the coefficient of barometric formula.                       

The height x in the pressed atmosphere corresponds to the height z in the real atmosphere according the following formula:

 
 
                                                            .                                    (2)

The coefficient of turbulence in the pressed atmosphere can be written:

 ,                                               (3)

where K is the coefficient of turbulence in the real atmosphere.

For the pressed atmosphere, the following differential expression is designed:    

                             ,                                    (4)

where l is the TGM removal rate from the atmosphere; c is the TGM concentration.


Fig.1. Distribution of the TGM concentration in the atmosphere: a – real  concentration;

          b – concentration at pressure of 1 atm.

 

The emission (E) of mercury from the ground surface must be equal to the Hg removal rate from the atmosphere, when the equilibrium process is present:

                                                 .                                                          (5)

On the other hand, the Hg emission can be expressed through the coefficient of turbulent diffusion:

                                                .                                                   (6)

 

Using (4), (5) and (6) equations and the Bless method the TGM concentrations along a vertical profile were calculated and the coefficient l was evaluated, respectively.

 
 
Then, the TGM volumetric concentration will be as follows:

                                                .                                                              (7)

After the above-described calculations, the TGM removal rate from the atmosphere is equal:

λ = 4.5 * 10-8 s-1  (σ = 0.42 * 10-8 s-1).

 

The average life-time (τ = 1/ λ) of TGM is equal τ = 260 days  (σ = 25 days), respectively. The distribution along a vertical profile of calculated TGM concentrations is presented in Figure 1. The decreasing tendency of the TGM concentration with the height is evident.

The concentration ratio of TGM and particulate Hg can be expressed through the ratio of the average life-time of both near the ground:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Fig.2. Vertical profiles of measured TGM and particulate Hg in the lower troposphere (Kvietkus et al., 1986; Kvietkus, 1995).

 

,                                                                                    (8)

where cg and ca are average concentrations of TGM and particulate Hg in the atmosphere; tg  and ta  are average life-times of TGM and particulate Hg in the atmosphere.

From the data obtained in our previous study (Kvietkus, 1986), about 5% of particulate Hg exists in the atmosphere near the ground. If  tg=260 days, then  ta=16 days. From  calculations the ratio ca/cg is increasing with the height and the same can be seen from our previous measurements in Figure 2.

Measured and calculated values of Hgpart. do not coincidences in higher atmospheric layers. Our explanation is that TGM in higher levels can be adsorbed onto particles with following desorption in lower atmospheric levels. This process is unclear still.

 

Conclusions

The evaluated average life-time of TGM is 260 days and average life-time of particulate Hg is 16 days approximately. The obtained TGM life-time value has a global character because during that time mercury can travel around the globe once or twice. The ratio ca/cg is increasing with the height and the same has been obtained in our previous measurements.

 

References

Kvietkus K., Sakalys J., Belovas A. (1986), Atmospheric Physics (Vilnius), 11: 47-52. 

Kvietkus K. (1986), Atmospheric Physics (Vilnius), 11: 145-154. 

Fitzgerald W.F., Vandal G.M., Mason R. P. (1991), Water Air and Soil Pollution, 56: 745-767.

Kvietkus K. (1995), In: Proceedings of the 10th  World Clean Air Congress, Espoo, Finland, Atmospheric Pollution 2: 284- 287.

Kvietkus K., Urba A., Šakalys J., Čeburnis D. (1997), Atmospheric Physics (Vilnius), 19 (1): 41-46.

Urba A. (1999), Ph.D. Thesis, Institute of Physics, Vilnius.

Kvietkus K., Sakalys J. (2000). In: Proceedings of  EUROTRAC Symposium ’2000. Editors: P.M. Borrel and P. Borrel.  2000, WIT press, Southampton, accepted.