Size Distribution of Atmospheric Particulate Mercury in Marine and Continental Atmospheres

A study was conducted to determine the size distribution of particulate mercury at both a marine site, Appledore Island, and a continental site, Thompson Farm. Cascade impactors were used to separate 10 size fractions of particulate mercury using Teflon filters including a pre-filter and backup filter to collect large (>10μm) and fine particulates (<0.4μm). A complete set of data was established including measurements for two consecutive summers, winter, and spring. In summer 2009, particulate mercury was found mainly (70%) in the coarse fraction, composed mainly of sea salt size particles between 1-6 μm at both Appledore Island and Thompson Farm. In the winter, particulate mercury was dominated by fine particles <1μm. The spring showed a mixture of fine and coarse particles. The results for summer 2010 are very similar to the spring results with mercury in the both fine and coarse fractions. Direct comparisons were made between the marine and continental sites in summer 2010 as two campaigns were run simultaneously. It was shown that Appledore Island had two times more total particulate mercury than Thompson Farm, but the trend in size distribution was the same suggesting a similar source of air parcels. Occasionally, Thompson Farm is influenced by marine air, while Appledore Island is influenced by continental air about 30% of the time in summer. Backward trajectories showed that the source air from summer 2009 is completely different from the source of air parcels during summer 2010. This may explain the difference in the size distributions between the two years. It was shown that weather, sources, and other factors directly influenced the distribution and amount of particulate mercury. Another campaign was run to test the daytime and nighttime cycling of particulate mercury at Appledore Island. The results show that the size distribution is the same for day and nighttime aerosols; however, the daytime produces about two times more particulate mercury than nighttime. The two impactors performed well with each other during the sensitivity test running two impactors simultaneously showing the same size distribution and total mercury amount. Understanding the size distribution of particulate mercury in both the marine and continental sites facilitates understanding differences in the phase partitioning and cycling in the two atmospheres.