1. The Role of Background O 3 in the NAAQS Review George T. Wolff Air Improvement Resource, Inc.

2. History of Background O 3 1997 – 40 ppb based on U.S. measurements in clean air 2007 – All US air is contaminated. Must be modeled by zeroing out NA anthropogenic emissions = Policy Relevant Background =PRB = 15 – 35 ppb 2012 – USB = 20 – 45 ppb – CMAQ with US anthropogenics zeroed out – CAMx with source apportionment

3. CMAQ and Zeroing Out US Limitations – Not realistic – Alters atmospheric chemistry in non-linear fashion Better Alternative – Run unperturbed base case – Zero out all non US contributions and US biogenic emissions – Difference provides impact of USB

4. CAMx with Source Apportionment CAMx with APCA (Anthropogenic Precursor Culpability Assessment Tool) – Underestimates impact of USB – Assigns culpability to anthropogenic sources for control strategy development – Does not propagate effect of USB throughout US – Does not track source of “odd oxygen” – Need different tracking tool to determine impact of USB

5. How APCA Works VOC + NO x → O 3 (biogenic) (US anthropogenic) (US anthropogenic) O 3 + NO → NO 2 + sunlight → NO + O (Canada) (anthro) O + O 2 → O 3 (US anthro)

6. How APCA Works O 3 + VOC → aldehyde + sunshine → ∙OH (Canada) (US anthro) ∙OH + NO x + VOC →→→ 10 O 3 ( US anthropogenic) (US anthropogenic)

7. CAMx Need accounting procedure to track the sources of odd oxygen associated with USB across the country to determine impact of USB and ambient ozone

8. Even though USB estimates are biased low, they are significant

9. On days with MDA8 > 60 ppb

10. For All Days