Interannual Variability in Baseline Ozone and Its Relationship to Surface Ozone in the Western U.S.

Baseline ozone refers to observed concentrations of tropospheric ozone at sites that have a negligible influence from local emissions. The Mount Bachelor Observatory (MBO) was established in 2004 to examine baseline air masses as they arrive to North America from the west. In May 2012, we observed an O₃ increase of 2.0–8.5 ppbv in monthly average maximum daily 8-hour average O₃ mixing ratio (MDA8 O₃) at MBO and numerous other sites in the western U.S. compared to previous years. This shift in the O₃ distribution had an impact on the number of exceedance days. We also observed a good correlation between daily MDA8 variations at MBO and at downwind sites. This suggests that under specific meteorological conditions, synoptic variation in O₃ at MBO can be observed at other surface sites in the western U.S. At MBO, the elevated O₃ concentrations in May 2012 are associated with low CO values and low water vapor values, consistent with transport from the upper troposphere/lower stratosphere (UT/LS). Furthermore, the Real-time Air Quality Modeling System (RAQMS) analyses indicate that a large flux of O₃ from the UT/LS in May 2012 contributed to the observed enhanced O₃ across the western U.S. Our results suggest that a network of mountaintop observations, LiDAR and satellite observations of O₃ could provide key data on daily and interannual variations in baseline O₃

Fast Time Resolution Oxidized Mercury Measurements during the Reno Atmospheric Mercury Intercomparison Experiment (RAMIX)

The Reno Atmospheric Mercury Intercomparison Experiment (RAMIX) was carried out from 22 August to 16 September, 2011 in Reno, NV to evaluate the performance of new and existing methods to measure atmospheric mercury (Hg). Measurements were made using a common sampling manifold to which controlled concentrations of Hg species, including gaseous elemental mercury (GEM) and HgBr₂ (a surrogate gaseous oxidized mercury (GOM) compound), and potential interferents were added. We present an analysis of Hg measurements made using the University of Washington’s Detector for Oxidized Hg Species (DOHGS), focusing on tests of GEM and HgBr₂ spike recovery, the potential for interference from ozone (O₃) and water vapor (WV), and temporal variability of ambient reactive mercury (RM). The mean GEM and HgBr₂ spike recoveries measured with the DOHGS were 95% and 66%, respectively. The DOHGS responded linearly to HgBr₂. We found no evidence that elevated O₃ interfered in the DOHGS RM measurements. A reduction in RM collection and retention efficiencies at very high ambient WV mixing ratios is possible. Comparisons between the DOHGS and participating Hg instruments demonstrate good agreement for GEM and large discrepancies for RM. The results suggest that existing GOM measurements are biased low