PBT Monitoring Workshop An Analysis to Refine the PBT Monitoring Strategy Objectives (MDN)

1. PBT Monitoring WorkshopAn Analysis to Refine the PBT Monitoring Strategy Objectives (MDN) Sponsored by: EPA, USGS, CDC, NOAA, CEC at Sheraton Capital Center Hotel Raleigh, North Carolina April 22-24, 2002 presented by: Steven M. Bortnick

2. Acknowledgments PBT Monitoring Strategy Writing Team Members Russ Bullock, EPA Basil Coutant, Battelle Xiaoling Zhang, Battelle 12-3 A Wkshp.ppt

3. Goals of the Analysis • Compare mercury deposition network (MDN) to draft PBT monitoring strategy objectives • Refine draft objectives • Aid development of objectives and options • Provide example of model-monitor “integration” • Demonstrate value of “shared data system” 12-3 A Wkshp.ppt

4. Mercury Deposition Network (MDN) • Weekly concentrations of total mercury in precipitation • Information on spatial and seasonal trends in mercury deposited to sensitive receptors (e.g., surface waters) • Over 50 sites in operation during 2000 (sites anticipated to operate for a minimum of five years) • Managed at the NADP Coordination Office (many collaborating Federal, State, & Local agencies) 12-3 A Wkshp.ppt

5. Analysis Scope and Limitations • This analysis is NOT a critique or indictment of MDN. In contrast, the choice of MDN for analysis highlights its potential utility. • This analysis is still in draft form and therefore subject to change. It is presented for illustrative purposes. • This analysis relies on model predictions, treating them as some measure of truth. 12-3 A Wkshp.ppt

6. Draft PBT Monitoring Strategy:Goals, Objectives, and Issues • Goals: (1) Discern trends (2) Evaluate risk management actions • Objectives: • Collect data at geographic-climactic regions (“nested”) • Use representative, probabilistic sampling • Leverage existing programs • Issues: • How to target sub-populations using probability sampling • How to leverage programs not designed probabilistically 12-3 A Wkshp.ppt

7. Project Data (Hg Wet Deposition Case Study) • Wet deposition model predictions • Regional Lagrangian Model of Air Pollution (RELMAP) • Annual averages (μg/m2, 1989 MET, 1995 Emissions) • Constant degree grid (roughly 40*40 km2) • Provided by Russ Bullock • Locations of active MDN sites • Annual precipitation totals (inches, 2000, NCDC) • Population totals (1990 Census adjusted to 2000) 12-3 A Wkshp.ppt

9. Stratified Probability-Based Sampling • Why probability-based sampling? • Cheaper than a “census” • yields unbiased, representative estimates • Why stratify? • Target geo-regions • Improve national-level precision • Know region-specific precision • Convenient to administer • How best to stratify? 12-3 A Wkshp.ppt

19. Stratifying by Combined EPA Regions:monitoring objective impact on site allocation • Consider different population objectives: • Total U.S. land mass (lower 48 states in this case) • Land mass near major water bodies (approximated via precipitation) • Total number of people • Consider optimal site allocation in each case: • Proportional to product of variability & sub-population (stratum) total • Maximizes national-level precision • How does site allocation change depending on “population” objective? 12-3 A Wkshp.ppt

22. Optimal ‘Regional’ Allocation

23. In Summary … • Population objective impacts regional allocation, e.g., • Total population objective: ~17% of sites in EPA Region 9 • Total precipitation objective: ~6% of sites in EPA Region 9 • Population objective has little affect on national-level precision • Regional estimates are less precise than national estimates, e.g., ~13% CV nationally versus ~23% - 43% CV regionally under a 50-site network. 12-3 A Wkshp.ppt

24. Evaluate MDN Siting vs. Stratified Probability-Based Optimal Allocation • Bias • Percent bias [(MDN - Truth)/Truth]*100 • Region-specific, National • Precision • Percent CV (side-by-side) • Region-specific, National • Overall Accuracy (mean square error=variance+bias2) • Percent efficiency [(optimal / MDN)*100] • Region-specific, National 12-3 A Wkshp.ppt

31. Conclusions of the Analysis • MDN could be leveraged for PBT trends goal, but: • Must caveat any “national” interpretation or extrapolation due to lack of probability-based design. • At ~20% point-in-time CV, smaller or short-term trends would be difficult to discern as statistically significant. • MDN exemplifies value of “shared data system”: • 53 independent, distinct sites • 30 funding agencies and 34 operating agencies • In aggregate, provides national & geo-regional information 12-3 A Wkshp.ppt

32. Analysis Conclusions (cont’d) • Stratification is recommended for targeting geo-regions and improving national-level precision • Ecoregions and state clusters are only marginally better stratification variables than EPA Regions • Optimal allocation is most efficient for national-level precision, but region-specific concerns should not be ignored (e.g., consider targeted over-sampling) 12-3 A Wkshp.ppt

33. Analysis Conclusions (cont’d) • As expected, MDN over-samples the eastern U.S. compared to several potential PBT “population” objectives • Not as expected, eastern MDN sites (bulk of network) appear to be spatially distributed in a relatively unbiased manner • MDN remains critical for leveraging, but not ideal; so either … • re-consider PBT objectives (e.g., target high deposition areas); or • provide MDN options (e.g., more western sites) • Multi-media integration can be facilitated through careful consideration of objectives. 12-3 A Wkshp.ppt