Summary of EPAs 8hr Ozone Episode Selection Criteria

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Summary of EPAs 8-hr OzoneEpisode Selection
Criteria

• T. W. Tesche
• Alpine Geophysics, LLC
• 20 April 2005

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EPA Episode Selection Criteria

• EPAs (2005) primary criteria
• Choose mix of episodes reflecting variety of
  meteorological conditions corresponding to
  observed 8-hr daily maxima exceeding 84 ppb (use
  of CART methods recommended)
• Model periods where 8-hr daily max. is close to
  4th highest 8-hr daily max. at each monitor whose
  DV gt 85 ppb.
• Choose episodes with intensive data bases if
  possible.
• Choose sufficient number of days to enable RRF
  test to be performed on several days at each
  monitor violating the NAAQS

These four criteria may often conflict with one
another. (EPA, 2005, pg. 58)
Guidance on the Use of Models and Other Analyses
in Attainment Demonstrations for the 8-hour Ozone
NAAQS, draft final guidance, 17 February 2005.
Office of Air Quality Planning and Standards,
Research Triangle Park, NC.
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• EPAs (2005) secondary criteria
• Consider previously-modeled episodes if model has
  been demonstrated to performance properly and
  EPAs four primary criteria are met
• Choose episodes from current 3-year DV period
• Choose episodes close to NAAQS on as many days
  and sites as possible
• Consider including weekend days if DVs gt 84 ppb
• If several nonattainment areas need to be modeled
  simultaneously, optimize primary secondary
  criteria

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• EPAs (2005) guidance also notes

Due to increased computer speeds, it is now
prudent to recommend modeling relatively long
time periods. At a minimum, modeling episodes
which cover full synoptic cycles is desirable.
Depending on the area and the time of year, a
synoptic cycle may be anywhere from 5-15 days.
Modeling even longer time periods of up to a
full ozone season may simplify the episode
selection process and provide a rich database
with which to apply the modeled attainment test.

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Extent to Which Current Episode Contains
Near-DV Modeling Days at 20 of 23 HGB Exceedance
Monitors
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Aug-Sept 00 Episode Assessment

• Episode days within 10 ppb of DV (i.e.,
  near-DV days)
• Range from 0 to 6 days, depending on monitor
• Some monitors (CLTA, GALC, HNWA) have no
  near-DV days
• Most monitors have 1 to 4 near-DV days
• HRM-7 and HRM-10 have high of 6 near-DV days
• Model performance for Aug-Sept 00 episode
• Adequate in some respects for examining 8-hr
  attainment
• Insufficient number of near-DV days covering
  all exceedance monitors
• Existing performance problems need to be
  addressed.
• If EPA guidance is to be followed
• Additional episodes are needed, possibly one or
  more summer long periods.
• Episodes should be selected following methodology
  laid out in EPA (2005).