Measurements of atmospheric mercury species at a German rural background site from 2009 to 2011-methods and results

Measurements of mercury species started in 2009 at the air pollution monitoring site 'Waldhof' of the German Federal Environmental Agency. Waldhof (52 degrees 48'N, 10 degrees 45'E) is a rural background site located in the northern German lowlands in a flat terrain, 100 km south-east of Hamburg. The temporally highly resolved measurements of total gaseous mercury (TGM), gaseous oxidised mercury (GOM), particle-bound mercury (PBMPM2.5, with particulate matter of a diameter of <= 2.5 mu m) and gaseous elemental mercury (GEM) cover the period from 2009 to 2011. The complete measurement procedure turned out to be well applicable to detect GOM and PBMPM2.5 levels in the range of 0.4 to 65 pg m(-3). As the linearity of the analyser was proven to be constant over orders of magnitude, even larger concentrations can be measured accurately. The 3-year median concentration of GEM is found to be 1.61 ng m(-3), representing typical northern hemispheric background concentrations. With 6.3 pg m(-3), the 3-year average concentration of PBMPM2.5 is found to be approximately six times higher than the 3-year average GOM concentration. During winter the PBMPM2.5 concentration is on average twice as high as the PBMPM2.5 summer concentration, whereas the GOM concentration shows no clear seasonality. However, on a comparatively low level, a significant diurnal cycle is shown for GOM concentrations. This cycle is most likely related to photochemical oxidation mechanisms. Comparison with selected North American long-term mercury speciation datasets shows that the Waldhof 3-year median speciated mercury data represent typical rural background values.