Long-Term Variation in Speciated Mercury at Marine, Coastal, and Inland Sites in New England

A comprehensive analysis was conducted using long-term continuous measurements of elemental gaseous mercury (Hgο), reactive mercury (RGM), and particulate phase mercury (HgP) at a coastal (Thompson Farm, denoted as TF), marine (Appledore Island, denoted as AI), and elevated inland (Pac Monadnock, denoted as PM) monitoring site of the University of New Hampshire AIRMAP Observatory Network. Diurnal, seasonal, annual, and interannual variability in Hgο, RGM, and HgP from the three distinctly different environments were characterized and compared. The most pronounced diurnal, seasonal, annual variability in Hgο was found at TF and AI whereas at PM such variability was relatively dampened due to its being located in the free troposphere, and nighttime depletion at TF was attributed largely to dissolution of Hgο in dew water. It should be noted that the diurnal cycles of Hgο at TF and AI were of opposite phase in summer - daily maximum occurred in the afternoon at TF and at night on AI. This implies strong sinks of Hgο during daytime in the marine boundary layer, which is consistent with Hgο oxidation by halogen radicals in the marine environment reported previously. Annual maximum RGM levels were observed in spring at TF and AI, while most of RGM mixing ratios at PM were below the limit of detection. Mixing ratios of HgP at AI and TF were close in magnitude to RGM levels and were mostly below 1 ppqv, and annual maximum HgP mixing ratios occurred in winter and minimum in fall. Relationships between mercury of all forms and climate variables (e.g., temperature, wind speed, humidity, solar radiation, and precipitation) were examined. Correlations between Hgο/RGM/HgP and climate variables were largely obscure although a tendency of higher levels of RGM and HgP was observed in spring and summer under sunny, dry, and warm conditions. Subsets of measurement data demonstrated impacts of episodic dynamic processes as well as interannual variability of seasonal climate on ambient levels of mercury. To identify source types of mercury correlations between mercury of all forms and tracers of different sources (e.g., O3, CO, NOy, and SO2) were carefully examined for all seasons. Hgο-CO relationship was well defined for winters 2003 - 2008 at TF and changed to be rather scattered in winters 2009 and 2010. Higher levels of RGM were found together with enhancement in CO, NOy, and SO2 in plumes at TF and AI, whereas no similar relationships were observed for HgP. The primary mechanism controlling higher levels of RGM at TF and PM in spring was likely associated with photochemical processes whereas in winter it was primarily long range transport from the free troposphere at high latitudes.