Atmospheric mercury concentrations at Storm Peak Laboratory in the Rocky Mountains: Evidence for long-range transport from Asia, boundary layer contributions, and plant mercury uptake

The goal of this study was to test if Asian long-range transport (ALRT) of pollutants leads to enhanced levels of atmospheric mercury in the Colorado Rocky Mountains in the United States. Storm Peak Laboratory, a high-elevation (3200 m asl) mountaintop research facility, is located on top of the Continental Divide ideally exposed to the prevailing westerly winds 1500 km inland from the Pacific where near-coastal stations previously measured significant atmospheric mercury enhancements due to ALRT. Measurements of gaseous elemental mercury (GEM) from October 2006 to May 2007 averaged 1.51 ± 0.12 ng m−3 at this laboratory, with minimum and maximum concentrations of 1.06 ng m−3 and 2.15 ng m−3, respectively. GEM showed diel patterns with peak concentrations during daytime and lowest concentrations during late night and early morning hours. In fall and winter, these patterns closely followed diel fluctuations of water vapor, aerosol number concentration, and ozone. The patterns are hence attributed to transitions of daytime boundary layer and nighttime air masses with regional representation as daytime surface heating causes air masses enhanced with air pollutants to rise from the valley floor to the laboratory. In spring, diel patterns of GEM and carbon monoxide (CO) occurred time-shifted in respect to water vapor, aerosol number, and ozone concentrations, indicating additional or different sources or sinks. A large GEM enhancement (2.15 ng m−3) was observed in early April and was associated with a pressure increase of almost 10 hPa and a decrease in water vapor concentration. CO levels increased correspondingly (213 ppbv) resulting in a GEM/CO enhancement ratio of 0.0061 ng GEM m−3/ppbv CO, which is in the range of previously observed ALRT. These patterns, along with 10-day HYSPLIT air mass trajectories and increased levels of coarse aerosols (i.e., 3–4 μm) typical of Asian dust events indicate the presence of Asian air masses at the laboratory. Further, springtime concentrations showed pronounced late-morning and afternoon decreases of GEM along with decreases of CO2 typical during the onset of photosynthetic activity of plants. We attribute GEM afternoon declines in spring to possible plant uptake of atmospheric mercury. We conclude that in spring, ALRT can be detected in the Rocky Mountains against local and regional patterns caused by slope flow and boundary layer upward mixing, and that plant activity may affect daytime GEM levels.