Total depletion of Hg° in the upper troposphere-lower stratosphere

Our current understanding of atmospheric mercury lacks fundamental details. Gas phase elemental mercury (Hg degrees) was measured aboard the NASA DC-8 research aircraft during the Intercontinental Chemical Transport Experiment-Phase B (INTEX- B) conducted in spring 2006 over the North Pacific. Our data confirm efficient chemical cycling of Hg degrees in the tropopause region and show that it is strongly anti-correlated with O-3 there, yielding a tropospheric-stratospheric mixing curve of similar to-0.20 ppqv Hg degrees/ppbv O-3. In addition, we found frequent total depletion of Hg degrees in upper tropospheric (8-12 km) air masses impacted by stratospheric influence. When O-3 mixing ratios were > 300 ppbv, Hg degrees was rarely detected. It appears that in the tropopause and stratosphere Hg degrees is oxidized on the order of days followed by heterogeneous transformation to particulate mercury. These processes constitute an effective chemical sink for Hg degrees at the top of the troposphere, which may recycle mercury back to the gas phase as stratospheric aerosols are eventually dispersed into the troposphere.