Assessing PM2.5 Background Levels and Local Add-On

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Assessing PM2.5Background Levelsand Local Add-On
Houston Clear
Houston Hazy

• Prepared by Bryan Lambeth, PE
• Field Operations Support Division
• Texas Commission on Environmental Quality
• For presentation at the
• National Air Quality Conference 2010

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Causes of High PM2.5

• 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 PM2.5 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

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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

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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

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Texas PM2.5 Sites
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Background PM2.5 Sites
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Southeast Texas PM2.5 Sites
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North Central Texas PM2.5 Sites
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Central Texas PM2.5 Sites
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South Texas PM2.5 Sites
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Texas PM2.5 Annual Averages 2008
6.8
7.6
11.5
11.9 11.2
9.9
15.9 11.1
8.7
9.9
5.7
10.5
10.6 10.5
14.0 12.5
9.5
6.1
8.8
10.7 10.3
11.3
12.1 12.0
Micrograms/cubic meter
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Texas PM2.5 Background Averages 2008
6.8
7.6
8.8 8.0
9.9
5.7
8.3
8.6 7.9
9.6 8.7
9.5
6.1
8.8
Micrograms/cubic meter
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Average of Daily Area Peak Local Add-On 2008
5.3 4.6
3.3
3.8 3.1
6.9 6.0
3.2
Micrograms/cubic meter
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Highest Annual Local Add-On 2008
3.9 3.1
2.2
2.7 2.0
5.3 4.4
2.0
Micrograms/cubic meter
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Highest Annual Local Percent Add-On 2008
32.4 26.0
20.8
25.3 18.7
37.8 31.5
16.9
Micrograms/cubic meter
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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

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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