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 Houston ClearHouston Hazy

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 and 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 Micrograms/cubic meter

Texas PM 2.5 Background Averages Micrograms/cubic meter 8.8

Average of Daily Area Peak Local Add-On Micrograms/cubic meter 3.2

Highest Annual Local Add-On Micrograms/cubic meter 2.0

37.8% 31.5% Highest Annual Local Percent Add-On % 26.0% 25.3% 18.7% 20.8% Micrograms/cubic meter 16.9%

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