OTAG Air Quality Analysis Workgroup

Weight-of-evidence Approach

• Use to synthesize all info when making OTAG policy decisions
  Uncertainties in all results
  "Bright lines" will never exist
  Look for directional corroboration from independent analyses to reduce uncertainty

• Consider strengths and weaknesses of each

Strengths and Weaknesses

• Strengths
  Air quality analysis

  • Based directly on measurements
  • Spatial coverage excellent for ozone
  • Includes non-episodes
  • Diverse analytical approaches, many provide similar results
  • "Sets stage" - provides baseline characterization of ozone problem
  Modeling

  • Incorporates chemistry, emissions, met
  • Covers entire domain (vert. too)
  • Allows "what if" studies
  • Provides forecasting ability

• Weaknesses
  Air quality analysis

  • Even measurements have uncertainty
  • Some spatial extrapolation
  • Includes non-episodes
  • No forecasting ability
  • Precursors spatial resolution poor
  Modeling

  • Emissions inventory uncertain
  • Uncertainties can be masked in results
  • Performance hard to evaluate everywhere
  • Local effects difficult to ascertain with regional grid resolution

Origins of Ozone - OTAG Domain

• VOC emissions - highest near urban areas
• NOx area emissions - highest near urban areas
• NOx point emissions - highest across central portion of OTAG domain

Pattern of Ozone

• Corners of domain at mostly at background levels
• General increasing trend from west to east across domain ( corresponds to prevailing direction for air transport)
• Highest peaks near urban areas
• Highest valleys" across central portion of domain
• Largest variability in urban areas
• Ozone events tend to last longer in South (typically several days) than in North (typically one to two days
• Exceedances of current standard - confined to major urban and near-urban
• Exceedances of proposed 8-hr standard- major and middle urban central OTAG domain

Ozone Transport

• Good news: hi transport = hi dispersion; can cause near source pollution dilution
• Bad news: stagnation followed by ventilation causes harmful transport of ozone and precursors: direction of transport dictates location and magnitude of impact

Ozone Transport - Lifetime & Extent

• Regional ozone - 1-2 day lifetime avg in OTAG domain
• Multiple analyses show extents of transport in OTAG domain ranging from 150 to 500+ miles
  Direct urban impact=150-200 miles
  Spatial coherence=400-500 miles
  Lifetime of 1.5 days=400 miles

Episode Characterization

• In progress

Control Implications

• OTAG domain is well-defined control region
• Weekday-weekend differences in ozone pattern show that emission changes do change ozone Exceedances
• Long-term ozone trends also show that many control efforts to date have reduced ozone levels and Exceedances
• Control of sources in central OTAG domain can provide greatest downwind benefit
• Because of ozone variability in urban areas, episodic controls may have greatest potential for success there

Model/Data Comparison

• Macro features of episodes replicated
• Some North/South bias
• Ozone aloft appears underpredicted
• Some precursor comparisons show poor agreement, especially for isoprene
• O3/NOx ratios show good agreement under NOx-limited conditions, supporting use of model to evaluate control strategies