Houston Clear Houston Hazy Assessing PM 2 5

Houston Clear Houston Hazy Assessing PM 2. 5 Background Levels and Local Add-On Prepared by Bryan Lambeth, PE Field Operations Support Division Texas Commission on Environmental Quality For presentation at the National Air Quality Conference 2010

Causes of High PM 2. 5 • Regional and long-range transport – Haze, smoke, and/or dust already in the air coming into an area from distant sources – Cannot be controlled by local mitigation measures • Local primary and secondary sources – Local add-on of PM 2. 5 is increased by local air stagnation, limited vertical mixing of the air, and high relative humidity – Urban worst case is usually night-time winter stagnation with clear skies

Estimating Transport Contribution • Upwind monitors and monitors that are not downwind of significant local sources provide the best estimate of incoming background levels from transport • These monitors will usually have the lowest concentrations in the area • Thus for areas with adequate peripheral monitoring coverage, the area lowest or second lowest concentration can serve to estimate the contribution from transport on most days • The variation between the lowest and second lowest measurements may often indicate variability in the incoming background levels across an area with adequate monitoring coverage

Estimating Local Add-On • Once the incoming background level has been estimated, concentrations higher than this background can indicate either variability in the background levels and/or add-on from local sources • Subtracting the estimated background from a given measurement provides an estimate of impacts directly from local sources, but this estimate can be biased high when there is large spatial variability in the incoming background level • Where speciation data are available for both background add-on locations, the speciated components of local source impacts can also be evaluated by this method

Texas PM 2. 5 Sites

Background PM 2. 5 Sites Coastal Transport Regional Transport Conditional Transport

Southeast Texas PM 2. 5 Sites Coastal Transport Conditional Transport

North Central Texas PM 2. 5 Sites Conditional Transport

Central Texas PM 2. 5 Sites Regional Transport Conditional Transport

South Texas PM 2. 5 Sites Coastal Transport

Texas PM 2. 5 Annual Averages 2008 6. 8 7. 6 11. 5 11. 9 11. 2 15. 9 11. 1 8. 7 9. 9 5. 7 10. 6 10. 5 6. 1 9. 5 14. 0 12. 5 10. 5 8. 8 11. 3 10. 7 10. 3 12. 1 12. 0 Micrograms/cubic meter

Texas PM 2. 5 Background Averages 2008 6. 8 7. 6 8. 8 8. 0 9. 9 5. 7 6. 1 8. 6 7. 9 9. 5 9. 6 8. 7 8. 3 8. 8 Micrograms/cubic meter

Average of Daily Area Peak Local Add-On 2008 5. 3 4. 6 3. 8 3. 1 6. 9 6. 0 3. 3 3. 2 Micrograms/cubic meter

Highest Annual Local Add-On 2008 3. 9 3. 1 2. 7 2. 0 5. 3 4. 4 2. 2 2. 0 Micrograms/cubic meter

Highest Annual Local Percent Add-On 2008 32. 4% 26. 0% 25. 3% 18. 7% 37. 8% 31. 5% 20. 8% 16. 9% Micrograms/cubic meter

Conclusions • For the areas analyzed, transport appears to account for at least about 70 -80% of measured annual averages at sites with the greatest local source impacts • At most about 20 -30% of the annual average at analyzed sites with the worst local source impacts can be addressed by local control measures

Applications • Analysis of “but for” considerations in determining exceptional event days – This approach could be used to estimate whether a site would have exceeded the standard with a “normal” background level if the exceptional event had not occurred • Estimating how much local add-on must be reduced to meet standards • Model validation