Sources and Regional Transport of Mercury to the Northeast: Insights from Air Observations at VT99

Eric K. Miller1, Rich Poirot2, Jamie Shanley3 and Mim Pendelton4

At VT99 GEM concentrations range from 0.81 to 5.58 ng/m3, averaging 1.45 ng/m3. RGM concentrations range from 0 to 132.5 pg/m3, averaging 3.56 pg/m3. HGP ranges from 0 to 121 pg/m3, averaging 11.50 pg/m3. The concentrations of all three species are dependent on meteorological conditions but in different ways for RGM and HGP than for GEM. There are significant differences in concentrations based on surface wetness state (dry, moist, or wet), time of year, time of day, and in response to different atmospheric conditions. Of particular note are the correlation of RGM concentrations with relative humidity, the correlation of HGP concentrations with water-vapor mixing ratio, and the tendency for GEM concentrations to rise after first insolation of a moist surface at dawn or after precipitation. The dependence of RGM concentrations on RH may reflect the tendency for that species to be readily scavenged by moist aerosols at moderate RH. The dependence of HGP on the water vapor mixing ratio may relate to HGP source regions and accompanying seasonal variations in water vapor. The concentrations of the three mercury species exhibit strong seasonal patterns that are slightly out of phase with each other. GEM concentrations peak in winter and spring with an early fall minimum, HGP concentrations peak in late winter and RGM concentrations peak in spring. The wintertime peak in HGP may be due, in part, to increased local combustion for home heating. However, trajectory analysis also indicates major out-of-region sources contribute to the observed HGP signal. The spring peak in RGM is likely due to a combination of factors including favored trajectories over major EGU RGM sources and relatively low atmospheric moisture levels at a time when leaves are off of trees along the favored trajectories. As soon as leaves emerge in late spring and early summer, the surface area for dry-deposition removal along the transport pathway increases by a factor of 3 to 4. Atmospheric moisture and relative humidity increase as well, allowing more scavenging by particles and ultimately cloud and rain droplets. RGM and HGP concentrations exhibited two distinct temporal patterns that we interpret as driven by either 1) atmospheric mixing processes in conjunction with the balance between deposition and formation reactions and 2) regional transport episodes. The relatively long and continuous record of high-temporal resolution measurements at VT99 permits unique analysis opportunities for understanding the atmospheric chemistry and regional transport of mercury.

1 Ecosystems Research Group, Ltd.
2 Vermont DEC, Air Pollution Control Division
4 Vermont Monitoring Cooperative