Philip C. Swartzendruber

Observations of Reactive Gaseous Mercury in the Free Troposphere at the Mount Bachelor Observatory

August 2005. The mean mercury concentrations (at standard conditions) were 1.54 ng/m 3 (GEM), 5.2 pg/m 3 (PHg), and 43 pg/m 3 (RGM). RGM enhancements, up to 600 pg/m 3 , occurred at night and were linked to a diurnal pattern of upslope and downslope flows that mixed in boundary layer air during the day and free tropospheric air at night. During the night, RGM was inversely correlated (P < 0.0001) with CO (r = � 0.36), GEM (r = � 0.73), and H2 O( r =� 0.44), was positively correlated with ozone (r = 0.38), and could not be linked to recent anthropogenic emissions from local sources or long-range transport. Principal component analysis and a composite of change in RGM versus change in GEM during RGM enhancements indicate that a nearly quantitative shift in speciation is associated with increases in ozone and decreases in water vapor and CO. This argues that high concentrations of RGM are present in the free troposphere because of in situ oxidation of GEM to RGM. A global chemical transport model reproduces the RGM mean and diurnal pattern but underestimates the magnitude of the largest observed enhancements. Since the only modeled, in situ RGM production mechanisms are oxidation of GEM by ozone and OH, this implies that there are faster reaction rates or additional RGM production mechanisms in the free troposphere.

Vertical distribution of mercury, CO, ozone, and aerosol scattering coefficient in the Pacific Northwest during the spring 2006 INTEX-B campaign

[1] In the spring of 2006, we measured the vertical distribution of gaseous elemental mercury (GEM), CO, ozone, and aerosol scattering coefficient in the Pacific Northwest concurrent with NASA’s INTEX-B campaign. Seven profiles from the surface to 6 km were conducted from 12 April to 8 May along with one flight in the Seattle-Tacoma boundary layer. Ozone had a bimodal distribution with the lower mode occurring primarily in the mixed layer and the higher mode occurring in the free troposphere. In the free troposphere, the mixing ratios (1 � s) of GEM, CO, ozone, and aerosol scattering coefficient were 1.52 (0.165) ng/m 3 , 142 (14.9) ppbv, 78 (7.7) ppbv, and 3.0 (1.8) Mm � 1 , respectively. GEM and CO were correlated in the high ozone mode (r 2 = 0.30) but were uncorrelated in the lower mode (r 2 = 0.05). Three flights observed enhancements of GEM and CO with good correlations and with regression slopes (0.0067 (±0.0027) ng/m 3 /ppbv by ordinary least squares regression and 0.0097 (±0.0018) ng/m 3 /ppbv by reduced major axis regression) slightly higher than previous observations of enhancements due to Asian industrial long-range transport (LRT). The influence of Asian LRT is supported by back trajectories and a global chemical transport model. In the SeattleTacoma boundary layer flight, CO was uncorrelated with GEM, which reflects relatively weaker local GEM sources. On three flights, pockets of air were observed with strong inverse GEM-ozone and ozone-CO correlations (in contrast to all data), which is evidence of upper tropospheric/lower stratospheric (UTLS) influence. An extrapolation of the GEM-CO and GEM-ozone slopes suggests the UTLS can be depleted of GEM.