Exceptional Events: Complexity for Ozone 2009-02-25 WESTAR Exceptional Events Meeting Scott Bohning, EPA Region 9

Ozone Chemistry hn O NO + O3 O2 + NO2 RO2 RO OH HNO3 ROOR' sunlight 1. NO-NO2 cycle by itself does not build up O3. hn l<420nm O very fast NO + O3 O2 + NO2 2. VOC pathway creates NO2 without O3 destruction, so O3 builds up. OH is key in making VOC radicals. 4. Low VOC/NOx favors OH sink. “NOx disbenefit”: NOx reduction frees OH to make more RO2, enhancing O3 creation. VOC radical (peroxy) 3. High VOC/NOx favors peroxides, instead of O3 cycle; more so with NOx reductions. RO2 100's of reac- tions RO hn O3 H2O O2 OH HNO3 ROOR' RH (hydrocarbons/VOC) nitric acid; rapid deposition peroxides, incl. H2O2

Coarse Particulate Matter Particulate emissions Wind speed above saltation threshold Particulate emissions

Ozone More Complex Than PM emission speciation complex and nonlinear chemistry temporal dimension: photochemical "aging" interaction with meteorology different types of episodes / meteorological regimes large variability long-range transport / regional background sparse ambient precursor data recirculation, PAN as NOx reservoir

Fire Additional Complexity uncertain emissions location, amount, speciation uncertain vertical rise changes meteorology itself relatively few studies of downwind effects except in boreal and tropics tends to increase ozone: precursor emissions NOy reservoir (PAN and HNO3) direct ozone formation tends to decrease ozone: decreased sunlight NOx scavenging of O3 VOC radical "sink" for NOx

Implications for Exceptional Events Rule (B) causality multiple episode types multiple ways to interact in a given episode type direct O3 vs. precursors; timing (C) concentration high ANY exceedance is already a high percentile; large fluctuations normal even a unique natural event might cause only moderate exceedance (D) no exceedance ‘but for’ event can be difficult due to complex causality; a range of possibilities: easy: O3 spike with no prior build-up, low temperatures, small local emissions medium: enhanced O3, but significantly different pattern than typical episode hard: typical O3 episode but with substantial increment potentially due to fire clauses B, C, D in 40 CFR § 50.14 (c)(3)(iii)

Types of Evidence or Analysis, 1 of 2 40 CFR § 50.14 (c)(3)(iii) clauses (B) because : "clear causal relationship" (C) concentration : "in excess of normal historical fluctuations" (D) delta : "no exceedance ... but for the event" Conceptual Model (B,C,D) narrative supported by evidence: how impact occurred, different from typical episode A. Did the fire exist? anecdotal, occurrence databases; dates, area, material, emissions B. Did the fire plume reach the area? 1. Anecdotal evidence (B,C,D) 2. Trajectory for transport path (B,D) 3. Remote sensing of path & presence -- satellite photos, aerosol, column O3 (B,C,D) 4. Signature pollutants levoglucosan, retene, PAH, CH3Cl, isotopes (B) 5. Altered pollutant amounts, ratios, patterns a. CO and PM -- levels, timing, pattern, composition, size distribution (B) b. Precursor composition and “age” oxygenated VOCs, radicals, NO2 timing, pattern (B,D) c. Ratios: CO/NOx, CO/PM10, O3/NOy, O3/CO (B,D) d. Spatial and temporal patterns: NO, NO2, O3 (B,D)

Types of Evidence or Analysis, 2 of 2 C. Did the fire create O3 NAAQS exceedance? 1. Compare similar episodes or days (B,C,D) 2. Ozone frequency distribution fit to distribution; confidence interval (B,C,D) 3. Regression model meteorology regression model(s) (B,C,D) 4. “Smog Production” algorithm compare to normal day max O3 (B,C,D) 5. Photochemical modeling Eulerian or Lagrangian photochemical model (B,C,D) Causality vs. Consistency Evidence may be consistent with fire impact, but not necessarily proof of fire as cause Case for causal connection strengthened by examining evidence that would be inconsistent with fire impact, ruling out other causes