Background Aerosol in the United States: Natural Sources and Transboundary Pollution

1. Background Aerosol in the United States:Natural Sources and Transboundary Pollution Naresh Kumar, Ph.D. EPRI, Palo Alto, CA Presented at MANE-VU/MARAMA 2004 Science Meeting January 28, 2004 Baltimore, MD

2. Outline • “Natural Visibility” as defined by EPA for implementation of Regional Haze Rule • Study by Dr. Daniel Jacob and his group at Harvard • Results from the above study • Taken from presentation given by Dr. Jacob at the RPO meeting in November 2003 • Future work

3. Issues with EPA’s Definition of “Natural Visibility” Conditions • Based on Trijonis (1990) • Trijonis used best information available at that time and provided uncertainty bounds of 2-3 on his estimates • Lot more information available since then • Incumbent upon scientific community to use new information along with more analysis to come up with better estimates of natural levels • Should transboundary pollution be included in definition of 2064 endpoint? • U.S. has no control over overseas emissions • Transboundary impact likely to go up • This issue will become even more important in second and successive implementation periods

4. Issues with EPA’s Definition of “Natural Visibility” Conditions (Contd.) • OCM/OC ratio • Currently, IMPROVE uses a factor of 1.4 to go from Organic Carbon measurements to Organic Carbon Mass • Turpin and Lim (2001) suggest a factor of 2.1 for aged aerosols • This will have significant effect for the 2064 endpoint as OC is the largest component • IMPROVE equation ignores sea-salt • Important for coastal sites • Sea-salt starts absorbing water at lower RH than does ammonium sulfate, but both behave similarly between RH of 60-90 percent

5. Issues with EPA’s Definition of “Natural Visibility” Conditions (Contd.) • Fire and dust events • These events occur mostly in spring and summer, so their impact on “natural levels” of soil, EC and OC during those periods may be disproportionately higher • The natural value of 0.5 mg/m3 proposed by Trijonis should account for some of the impact from wildfires on annual basis, but need to determine impact by season

6. Issues with EPA’s Definition of “Natural Visibility” Conditions (Contd.) • Current approach for estimating natural visibility for 20% worst days • 20% Worst (dv) = Annual Average (dv) + 1.28 σ • σ = 2 dv (sites in West) and 3 dv (sites in East) • Problems with this approach: • 1.28 corresponds to 90th percentile for a “normal” distribution, but average of 20% worst days occurs at 92nd percentile giving a factor of 1.4 • Work done by Sonoma Technology, Inc. (STI), using actual data scaled to “natural levels”, suggests using a factor of 1.5 and σ of 3.0 for West and 3.5 for East

7. Effect of STI’s Approach on 20% Worst Natural Conditions The natural condition values (for 20% worst days) estimated by the STI approach are 1.2 to 2.0 dv greater than those determined by EPA’s default method

8. Work done by Dr. Jacob and Co-workers • Use of a global chemistry model to estimate natural and background levels of aerosols to answer the following questions: • How good are the “default estimated natural PM concentrations” proposed by EPA as 2064 endpoint for application of the Regional Haze Rule? • To what extent does transboundary pollution compromise achievability of natural PM concentrations?

9. 2004-2018 Emission Reductions for the RHRare Strongly Sensitive to Choice of 2064 Endpoint Conceptual calculation for mean western U.S. conditions, assuming linear relationship between emissions and bext Desired trend in visibility Phase 1 48% Required % decrease of U.S. anthropogenic emissions 30%

10. Strategy for Quantifying Background PM Concentrations in United States Start from best a priori estimates of natural and anthropogenic PM sources Improved emission estimates Simulate PM concentrations with GEOS-CHEM global model Evaluate with observations from IMPROVE, CASTNET, other networks Conduct sensitivity simulations Quantify natural aerosol concentrations Quantify transboundary pollution

11. GEOS-CHEM Global Chemical Transport Model(http://www-as.harvard.edu/chemistry/trop/geos) • Driven by NASA/GEOS-3 assimilated meteorological data with 2ox2.5o horizontal resolution, 48 levels in vertical • Carbonaceous aerosol simulation (OC, EC) for 1998 Park, R.J., D.J. Jacob, M. Chin, and R.V. Martin, Sources of carbonaceous aerosols over the United States: implications for natural visibility,J. Geophys. Res.., 108, 4355, 2003. • Coupled oxidant –sulfate-nitrate-ammonium simulation for 2001 (also 1998) Park, R.J., D.J. Jacob, et al., Natural and transboundary influences on ammonium-sulfate-nitrate aerosols in the United States: implications for visibility, J. Geophys. Res., manuscript submitted

12. Carbonaceous Aerosol Simulation Best a priori sources (1998) ELEMENTAL CARBON (EC) ORGANIC CARBON (OC) GLOBAL 22 Tg yr-1 130 Tg yr-1 UNITED STATES 0.66 Tg yr-1 2.7 Tg yr-1

13. Biomass Burning Emissions In GEOS-CHEM:Climatology from Duncan, Logan, et al. [JGR 2002] scaled to 1998using TOMS and ATSR satellite data Inter-annual variability derived from TOMS Aerosol Index [Duncan et al., 2002] • 1998 fires from ATSR satellite data: • Apr-May fires in Mexico • Jul-Sep fires in U.S/Canada

14. Seasonal Variation of OC and EC Constrain OC/EC sources using simulation of monthly mean IMPROVE observations and model tracers of individual source types. biomass burning IMPROVE vegetation model tracers (additive) model fossil fuel biofuel

15. Annual Mean OC AND EC (1998): GEOS-CHEM vs. IMPROVE (45 sites) • High OC in southeast U.S.: vegetation • High OC in Mexico, Canada: fires

16. Least-squares Fit of Model to Observations Generates Optimized a Posteriori Sources Fossil fuel h15% Biofuel h65% Biomass burning i17% Biogenic h11%

17. Carbonaceous Aerosol in the U.S.:Contributions from Natural Sources and Transboundary Pollution Annual regional means from GEOS-CHEM standard and sensitivity simulations • We find that EPA default natural concentrations are too low by factors of 2-3 except for OC in eastern U.S. – quantifying fire influences is critical • Transboundary pollution influences are relatively small except for EC from Canada/Mexico

18. H2SO4-HNO3-NH3-H2O Aerosol simulation GEOS-CHEM emissions (2001) UNITED STATES GLOBAL 78 8.3 Sulfur, Tg S yr-1 2.8 55 Ammonia, Tg N yr-1 43 7.4 NOx, Tg N yr-1

19. Annual Mean Sulfate (2001): GEOS-CHEM vs. IMPROVE (141 sites)

20. Sulfate atIMPROVE, CASTNET, NADP (deposition) Sites:model vs. observed for different seasons High correlation, no significant model bias except NADP summer

21. Annual Mean Ammonium (2001): GEOS-CHEM vs. CASTNET (79 sites) (no ammonium data at IMPROVE sites)

22. Annual Mean Nitrate (2001): GEOS-CHEM vs. CASTNET (79 sites)

23. Ammonium Nitrate Nitrate Ammonium and Nitrate at CASTNET and IMPROVE Sites:model vs. observed for different seasons • High bias for NH4+ in fall: • error in seasonal variation • of livestock emissions • High bias for NO3-, esp. in summer/fall, results from bias on [SO42-]-2[NH4+]

24. Sulfate-Nitrate-Ammonium Aerosol in the U.S.: Contributions from Natural Sources and Transboundary Pollution Annual regional means from GEOS-CHEM standard and sensitivity simulations • Achievability of EPA default estimates is compromised by transboundary pollution influences • Transboundary pollution influence from Asia is comparable in magnitude to that from Canada + Mexico

25. Background and natural levels using the GEOS-CHEM model compared to EPA’s default estimates of natural levels GEOS-CHEM OCM concentrations for sites in the West higher by a factor of 2 to 3 relative to the EPA default approach The incorporation of transboundary emissions increases sulfate by 50% to 400% in class I areas relative to the EPA default approach

26. Model Evaluation with Asian Outflow Observations of Sulfate from Trace-p Aircraft Campaign (Mar-Apr 2001) obs model Observations from Jordan et al. [[JGR 2003]

27. Intercontinental Transport of Asian and North American Anthropogenic Sulfate As determined from GEOS-CHEM 2001 sensitivity simulations with these sources shut off Asian anthropogenic sulfate N. American anthropogenic sulfate

28. Future Work proposed by Dr. Jacob • Correct known model flaws and increase model resolution • Overestimate of OC in the northwest • Overestimate of ammonium in fall • Increasing the resolution to 1ox1o should improve spatial resolution of coastal and topographical effects • Provide model results in form most pertinent to the RHR • Calculation of natural and background levels for 20% worst days • Extract the baseline and natural PM concentrations at individual IMPROVE sites rather than just providing numbers for the east and the west

29. Future Work (Contd.) • Improve confidence in baseline PM estimates and provide uncertainty ranges • Evaluation of soil simulation • Better estimates of fire emissions • Assess the impact of global changes in climate and emissions on baseline PM concentrations in the U.S. • Conduct 2050 simulation to examine (separately and in concert) the effects of climate change and the effects of changes in emissions • Estimate natural concentrations of PM at all IMPROVE sites for 2050 conditions

30. Final Thoughts • RPOs, EPA, industry should leverage resources to better define the 2064 endpoint for RHR • Global models are the best tools at present to synthesize all the data and provide estimates for that endpoint • Data analysis and global modeling groups need to work together in this effort • Global models can also provide boundary conditions to the regional models that would be used for developing implementation plans