Principal Research Results

Annual Changes in Wet Deposition Fluxes of Trace Metals andScavenging of Mercury by Precipitation in an Urban Area
BackgroundRisks caused by the intake of dietary toxic trace substances which are bioaccumulated through the food chain after their deposition to the ground, are important for the assessment of the human health risks related to atmospheric trace substances. For the evaluation of those risks, quantification of atmospheric deposition of trace substances is primarily required. Atmospheric deposition occurs through both wet and dry processes, which are due to the precipitation and the deposition of gases and particles, respectively. InJapan, deposition of trace substances including mercury has been not monitored over long periods. Hence, levels and annual changes
in their deposition fluxes remain unclear.

Objectives
To clarify annual changes in wet deposition fluxes of mercury (Hg), cadmium (Cd), copper (Cu), manganese (Mn) and lead(Pb) and scavenging of mercury by precipitation on the basis of the observations for 4 years (2000 - 2003) in Komae City, Tokyo(Fig. 1).

Principal Results
1. Annual changes in wet deposition fluxes of trace metals and factors affecting their annual changes
Annual wet deposition fluxes of Hg, Cd, Pb and Zn showed a declining trend that corresponds to changes in atmosphericconcentration. In particular, for Hg and Pb, a large reduction wet deposition flux was observed (Fig. 2). According to the previous studies based on lead isotope ratios and trace metal concentrations in the atmosphere, Hg, Cd, Pb and Zn in the atmosphere originate
significantly if not predominantly from municipal solid waste (MSW) incinerators located around the study site. It was found that theemissions of Hg and particles from MSW incinerators in the Tokyo area (23 wards) have decreased annually (Fig. 3). Thus, the reduction in wet deposition fluxes of Hg, Cd, Pb and Zn is due to the decrease in their emissions from MSW incinerators by an improved
flue gas control system in recent years ＊ 1.
2. Scavenging of particulate mercury and gaseous mercury by precipitation
The contribution of the scavenging of gaseous and particulate Hg by precipitation to the bulk Hg wet deposition flux was evaluated on the basis of scavenging ratios for trace metals (= concentration in precipitation/concentration in air)＊ 2. The results indicated that approximately 30 - 60 % of the Hg scavenged by precipitation originated from particulate Hg, which is similar to the contribution from gaseous Hg (Table 1). This suggests that both the scavenging of atmospheric particulate and gaseous Hg by precipitation
plays an important role in Hg wet deposition.

Future Developments
Bioaccumulation of trace metals such as Hg in fishes via the deposition to aquatic regions is an important pathway for theassessment of their human health risks. In future studies, wet and dry deposition of trace metals will be measured in Tokyo Bay, which
is selected as a study aquatic region.
Main Researcher: Masahiro Sakata, Ph.D,
Senior Research Scientist, Atmospheric Environment Sector, Environmental Science Research Laboratory
Reference
M. Sakata and K. Marumoto, 2004, Annual changes in wet deposition fluxes of trace metals and scavenging of mercury by precipitation in an urban area, CRIEPI Report No. T03015 (in Japanese)＊1：Since MSW incinerators are not the major emission sources for Cu and Mn, the reduction in their emissions did not contribute significantly tothe decrease in the atmospheric concentrations and wet deposition fluxes.
＊ 2：Wet deposition flux of particulate Hg was calculated, assuming that the scavenging ratio of particulate Hg is same as the mean value of othertrace metals which exist entirely in particulate form.

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C. Harmonization of energy and environment
N
● 195

● 600

● 900

● 195

● 600

● 210

10 km

★
Stud y site

● 900

● 450
● 600

Fig.1 Location of Komae City, Tokyo and municipal solid waste (MSW) incinerators within 10 kmof the study site (Number: capacity (tons d-1) of each incinerator)
Many large MSW incinerators are located around the study site (9 sites within 10 km).
840

1.68Hg

31.3

6Pb
30

1.2

20

0.8

10

0.4

006
4.36

4

21.0

16.5

3.76

4

15.0
2.47

202000

2001

2003

2002

2.67

2000

2001

2002

22003

0Fig.2 Changes in monthly and annual wet deposition fluxes of mercury and leadWet deposition fluxes of Hg and Pb showed an annually declining trend, repeating aseasonal change that wet deposition fluxes decrease during winter when the precipitation
amount decreases. Annual reduction rates of wet deposition fluxes from 2000 to 2003
were 16 % for both metals.
125500
Pa rticles

400

100
75

300
20050Hg

1000251997

1998

2000
1999
Fiscal year

2001

2002

0

Total emissions of Hg and particles
from MSW incinerators (17 – 20facilities) in Tokyo area were estimatedon the basis of the published annual
average concentrations in flue gases.
The emissions of Hg and particles in2001 decreased to approximately 1/3and 1/5 of those in 1997, respectively.

Fig.3 Annual changes in emissions of Hg and particles from municipal solid waste incineratorsin the Tokyo area (23 wards) in the fiscal years of 1997 – 2002
Table1 Contribution of the scavenging of particulate Hg (Hg(p)) by precipitation to the bulk Hg deposition
YearScavenging ratiosof trace metals

Wet deposition ofHg(p),μgm-2Contributionof Hg(p), %2000

112 ± 32

12.8482002

170 ± 49

7.358

2003

122 ± 15

4.53235

The scavenging ratios of tracemetals
except for Hg are similar to each other. Thewet deposition flux of Hg(p) calculatedusing their scavenging ratios corresponds toapproximately 32 – 58 % of the bulk Hg
wet deposition flux measured.