1. Sensitivity of U.S. Surface Ozone to Isoprene Emissions & Chemistry: An Application of the 1°x1 ° North American Nested GEOS-CHEM Model GEOS-CHEM Model (v. 5-07-08) Arlene Fiore, Larry Horowitz, Drew Purves, Hiram Levy II, Mathew Evans, Yuxuan Wang, Qinbin Li and Bob Yantosca GEOS-CHEM Meeting April 5, 2005 Fiore et al., J. Geophys. Res., in press

2. GEOS-CHEM 1°x1 ° Evaluation: July 2001 1-5 p.m. Surface O 3 (ppbv) Mean Bias = 6±7 ppbv; r 2 = 0.40

3. Recent Changes in Biogenic VOC Emissions Isoprene [Purves et al., Global Change Biology, 2004] Sweetgum Invasion of Pine plantations  Substantial isoprene increases in southeastern USA largely driven by human land-use decisions  Land-use changes not presently considered in CTMs Ratio of mid-1990s to mid-1980s isoprene emissions

4. MOZART-2 … but not in MOZART-2 Change in July 1-5 p.m. surface O 3 Increases in isoprene emissions reduce surface O 3 in Southeastern US in GEOS-CHEM… ppbv GEOS-CHEM Little change (NO x -sensitive)

5. Isoprene nitrates Complications to Chemistry: Isoprene may also decrease surface O 3 in low-NO x, high isoprene settings O3O3 (slower ) Low-NO x, high-isoprene O3O3 Thought to occur in pre-industrial [Mickley et al., 2001]; and present-day tropical regions [von Kuhlmann et al., 2004] O3O3 OH RO 2 NO(very fast) NO 2 High-NO x ISOPRENE Isoprene does react directly with O 3 in our SE US GEIA simulation: O 3 +biogenics (10d) comparable to O 3 +HO x (16d), O 3 +h  -> OH (11d) Uncertainty #1 Emissions Uncertainty #2 ? Sink for NO x ?

6. #1: Uncertainty in Surface O 3 due to Isoprene Chemistry Change in July mean 1-5 p.m. surface O 3 (MOZART-2) When isoprene nitrates act as a NO x sink ppbv

7. GEIA GEIA: global inventory Purves et al., [2004] (based on FIA data; similar to BEIS-2) (10 11 molecules isoprene cm -2 s -1 ) #2: Uncertainty in Surface O 3 due to Isoprene Emissions 5.6 Tg C 2.8 Tg C (ppbv) Difference in July 1-5 p.m. surface O 3 (Purves–GEIA) July Anthrop. NO x emissions (10 11 molec cm -2 s -1 ) Higher isoprene decreases O 3 High-NO x regime July isoprene emissions

8. Understanding fate of isop. nitrates essential for predicting sign of response to changes in isoprene emissions With 12% yield of isoprene nitrates GEOS-CHEM: GEIA GEOS-CHEM: PurvesMOZART-2: GEIA ppbv MOZART-2 shows similar results to GEOS-CHEM if isoprene nitrates are a NO x sink Revisiting changes in July 1-5 p.m. surface O 3 (ppbv) due to isop emis changes from mid-1980s to mid-1990s

9. From Ugarte et al. (2003) Future scenario for conversion of agricultural lands to poplar plantations for biofuels from D. Purves Poplars are high-isoprene emitting trees  land-use decisions have implications for air quality High-NO x region

10. Better constrained isoprene emissions are needed to quantify: 1.isoprene contribution to E. U.S. surface O 3 2.how O 3 responds to both anthrop. and biogenic emission changes (deserves consideration in planning biofuel plantations)  Utility of satellite CH 2 O columns?  New inventories (MEGAN, BEIS-3) more accurate?  Constraints from ICARTT observations? Recent isoprene increases may have reduced surface O 3 in the SE  Does this regime actually exist?  Fate of organic nitrates produced during isoprene oxidation? Conclusions and Remaining Challenges

11. With PURVES Vertical slices through 34N: enhanced isoprene, CH 2 O, PAN at surface & upper trop for GEIA compared to Purves With GEIA  Insights from NASA INTEX-NA flights over SE US? 0 0.1.25 0.5 1. 5..1.2.5 1. 3. 5.0 0.1.25 0.5 0.8 1.00..05 0.1 0.2 0.5 2. Longitude Altitude

13. Trends in Anthropogenic Emissions: 1985 to 1995 CO  Large decreases in CO and VOC Emissions  Some local increases in NO x  Higher biogenic VOCs Net effect On O 3 ? from US EPA national emissions inventory database (http://www.epa.gov/air/data/neidb.html) VOCNO x Ratio of 1995 to 1985 Emissions

14. With GEIA Isoprene Emis With Purves et al. Isoprene Emis With BVOC Changes With Anthrop. Changes Change in Mean July Surface O 3 (ppbv; 1-5 p.m.) reflecting 1980s to 1990s emissions changes With Anthrop.+ BVOC Changes Changes in Anthropogenic NO x emissions dominate O 3 response But response depends upon choice of isoprene emission inventory Comparison with observed changes?

15. Model vs. Obs.: Change in July O 3 1980s to 1990s (ppbv; 1-5 p.m.) (1993-1997) – (1983-1987) Obs: EPA AIRS GEOS-CHEM: GEIA GEOS-CHEM: Purves Observed changes in O 3 are not explained by regional emission changes alone… Poor correlation (r 2 ~ 0) between observed and simulated changes with anthropogenic + biogenic emissions changes (ppbv)

16. ° High-NO x : O 3 as isop Change in July O 3 (ppbv; 1-5 p.m.) Isoprene reduced 25%NO x reduced 25% Low-NO x, high isop: O 3 as isop With GEIA With Purves Identify O 3 chemistry regime with precursor emissions reductions July Anthropogenic NO x Emissions (10 11 molec cm -2 s -1 ) Choice of isoprene inventory also critical for predicting O 3 response to changes in isoprene and anthropogenic NO x emissions strongly NO x -sensitive

17. Increasing Isoprene Decreases O 3 in Low-NO x, High-isoprene regions GEOS-CHEM base-case July 1-5 p.m. mean NO x ISOP Other model studies indicate SE US is near “maximum VOC capacity point”, beyond which VOCs suppress O 3 formation [Kang et al., 2003]. VOC Ozone NO x - saturated NO x - sensitive High-NO x Low-NO x “isoprene-saturated”??