Natural Background Conditions: Items for discussion with the Inter-RPO Monitoring/Data Analysis Workgroup Naresh Kumar EPRI 5 March 2004

2 Issues Identified in EPRI White Paper that are Pertinent to “Natural” Conditions Use of global models to estimate natural and background conditions How to go from annual average natural levels to 20% worst day natural conditions? Impact of refinements in the IMPROVE Equation

3 Use of GEOS-CHEM Model to Estimate Natural and Background PM concentrations Natural = No anthropogenic emissions anywhere in the world Background = No anthropogenic emissions in U.S. Findings from the model: –Natural levels of OCM in the West much higher (factor of 2-3) compared to EPA default, but for the East comparison was much better –Transboundary pollution can be a major component of background sulfate at IMPROVE sites (increase by a factor of 1.5 to 4) –Achieving the EPA default end point for 2064 is compromised because of the above results from the model

4 Natural Conditions for 20% Worst Days EPA Approach: 20% Worst (dV) = Annual Average (dV) σ σ = 2 dV (sites in West) and 3 dV (sites in East) Recommendations: The factor of 1.28 should be changed to 1.4 σ calculated by STI higher than that suggested in EPA guidance. Pat Ryan of STI to contact Roger Ames to sort out the differences

5 Refinements in IMPROVE Equation IMPROVE reconstruction equation overpredicts measured light scattering at low levels and underpredicts at high levels. STI found that using an OCM/OC factor of 2.0 and sulfate/nitrate efficiency of 2.5 m 2 /g fits IMPROVE data much better. Adopting OCM/OC factor of 2.0 and ammonium sulfate and nitrate scattering efficiency increasing with concentration predicts least amount of bias when comparing to measured light scattering.  Any changes in IMPROVE Equation will not only change the 2064 endpoint (in units of light extinction), but also change the slope of the glide path from

6 Adding a constant term to 2064 endpoint and current conditions also affects the slope As an example, take a hypothetical site located close to the coast –Reconstructed Bext for 2004 = 60 Mm -1 –EPA default natural Bext for 20% worst days = 25 Mm -1 –Bext due to sea-salt = 20 Mm -1 YearDV (no sea-salt) DV (with sea-salt)

7 Jury is still out on the issues raised! There is no consensus that global models like GEOS-CHEM be used for estimating 2064 endpoint –Although, more and more non-modelers agree that there is no other better alternative Should transboundary pollution be part of the 2064 endpoint? –U.S. has no control over overseas emissions –Transboundary impact can be revised every 10 years given the state of knowledge –This issue will become even more important after the first implementation period There seems to be consensus that the factor of 1.28 should be changed to 1.4, when estimating 20% worst day natural conditions from the annual average, though there are still disagreement on what the std. deviation should be.

8 Continued… There seems to be consensus that the 1.4 factor for OCM/OC ratio should be increased –Suggestions range from 1.6 to 2.0 (1.8 seems like a good mean until more is known) There seems to be no consensus on changing the sulfate/nitrate dry scattering efficiency –Data from special studies suggest that the value should be closer to 2.5 rather than 3.0 m 2 /g –This is also consistent with increasing the OCM/OC ratio –Data also suggest dry scattering efficiencies vary (increase) with concentration