Comparison of long-term trends and seasonal variations of atmospheric mercury concentrations at the two European coastal monitoring stations Mace Head, Ireland, and Zingst, Germany

Abstract

Monitoring of total gaseous mercury (TGM) concentrations has been carried out at Mace Head on the west coast of Ireland and at Zingst on the southern shore-line of the Baltic Sea. We have used the continuous measurements to assess long-term trends and seasonal variations of TGM concentrations over the same time period. Between 1998 and 2004 the annually averaged TGM concentrations measured at Mace Head (1.72 ng m⁻³) and Zingst (1.66 ng m⁻³) remained fairly stable. At both stations we measured higher TGM levels during winter months and lower concentrations during summer months. We also observed an unexpected West to East gradient and found that the overall mean Mace Head TGM concentration was 0.06 ng m⁻³ higher than those of Zingst. For the January to June period, the Mace Head TGM values (6-year mean=1.75 ng m⁻³) are significantly elevated compared to the Zingst results (6-year mean=1.64 ng m⁻³). Since no local anthropogenic mercury sources exist near the Mace Head station, enhanced emission from the sea appears to provide the most probable explanation for the observed differences. Multiple regression analysis with the atmospheric mercury concentrations measured at Zingst were made with selected meteorological and air quality parameters (wind component, dew point, dust and NOx concentrations). The short-term variation in the TGM concentrations at Zingst shows a strong positive correlation with weather conditions and selected air quality parameters, making TGM a usable tracer of air masses originating from different source regions.

Introduction

Mercury is emitted into the atmosphere from a number of natural (Ferrara et al., 2000a, Sexauer Gustin et al., 1999) as well as anthropogenic sources (Pirrone et al., 2001). Even in remote areas like Scandinavia, deposition of mercury into rivers and lakes is of significant importance for the food chain due to high accumulation factors in aquatic biota (Brosset, 1987, Iverfeldt, 1991). This pollution problem is caused by long-range atmospheric transport and is not connected to local discharges (Petersen et al., 1995). The importance of atmospheric mercury is reflected in the fact that this hazardous air pollutant is on the priority list of a number of international conventions aimed at protection of the environment (e.g. Protection of the Baltic Sea: HELCOM, Protection of the North Sea and the North Atlantic: OSPAR, Protection of the Arctic Ecosystem: AMAP). Very recently, Pirrone and Wichmann-Fiebig (2003) have proclaimed that a mandatory monitoring network for total gaseous mercury (TGM) in ambient air and mercury in precipitation (wet deposition) should be established in the European Union. The results of an EU project on contamination of the environment by mercury indicate that European emissions of Hg from anthropogenic sources have decreased during the last decade by a factor of 2 (Pacyna et al., 2001). In contrast, no long-term trend was evident since 1995 from a total of six monitoring stations dispersed all across the northern hemisphere (Kim et al., 2005). However, with the reduction of the industrial point sources the diffuse mercury emissions and reemissions are of increasing interest on local and regional scales. Mercury evasion from seawater is considered to be one of the main natural sources of mercury released to the atmosphere (Ferrara et al., 2000b). Ferrara et al., 2000b observed that the mercury flux shows a dependence on the intensity of solar radiation, with minimum values during the winter period and maximum values during the summer. Long-term monitoring of atmospheric mercury concentrations may provide direct evidence of temporal trends, due to an atmospheric residence time of about 1 year (Slemr et al., 1995). Slemr et al. (2003) presented a first attempt to reconstruct the worldwide trend of atmospheric mercury concentrations from direct measurements since the late 1970s. The observed trend suggests that atmospheric mercury concentrations increased from the late 1970s to a peak in the 1980s, then decreased to a minimum in 1996, and have been nearly constant since.

Our paper describes the results of two long-term TGM concentration datasets measured at the coastal monitoring stations of Mace Head and Zingst. Fully automated mercury vapor analyzers, Tekran Model 2537 A were used simultaneously at the two sites. During a period of 7 years we measured TGM with a time resolution of 15 min. The resulting unique database is herein used for an investigation of the TGM long-term trend, a comparison of the spatial distribution and seasonal variation and a correlation and multiple regression of TGM with some selected meteorological and air quality parameters.