Exceptional Events:Complexity for Ozone

2009-02-25WESTAR Exceptional Events Meeting

Scott Bohning, EPA Region 9

Ozone Chemistry

NO + O3 O2 + NO2

O

1.NO-NO2 cycle by itselfdoes not build up O3.

2.VOC pathway createsNO2 without O3 destruction, so O3 builds up. OH is keyin making VOC radicals.

hsunlight

HNO3OH

4.Low VOC/NOxfavors OH sink.

“NOx disbenefit”:NOx reduction

frees OH to makemore RO2, enhancing

O3 creation.

RH (hydrocarbons/VOC)

3.High VOC/NOx favorsperoxides, instead ofO3 cycle; more sowith NOx reductions.

ROOR'

RO2 RO

VOC radical(peroxy)

very fast

O3

h

H2O

100'sof

reac-tions

nitric acid;rapid depositionperoxides, incl. H2O2

O2

<420nm

Coarse Particulate Matter

Wind speed abovesaltation 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

• 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

Fire Additional Complexity

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’ eventcan 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)

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)

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"

Types of Evidence or Analysis, 1 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)

Types of Evidence or Analysis, 2 of 2

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