Sensitivity of U.S. Surface Ozone to Isoprene Emissions & Chemistry: An Application of the 1°x1 °

1. Sensitivity of U.S. Surface Ozone to Isoprene Emissions & Chemistry: An Application of the 1°x1° North American Nested 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 O3 (ppbv) Mean Bias = 6±7 ppbv; r2 = 0.40

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

4. … but not in MOZART-2 MOZART-2 Increases in isoprene emissions reduce surface O3 in Southeastern US in GEOS-CHEM… Change in July 1-5 p.m. surface O3 GEOS-CHEM Little change (NOx-sensitive) ppbv

5. High-NOx O3 (very fast) NO NO2 ? RO2 OH Isoprene nitrates Sink for NOx? Uncertainty #1 ISOPRENE O3 Emissions O3 (slower ) Low-NOx, high-isoprene Uncertainty #2 Complications to Chemistry: Isoprene may also decrease surface O3 in low-NOx, high isoprene settings Thought to occur in pre-industrial [Mickley et al., 2001]; and present-day tropical regions [von Kuhlmann et al., 2004] Isoprene does react directly with O3 in our SE US GEIA simulation: O3+biogenics (10d) comparable to O3+HOx (16d), O3+hn -> OH (11d)

6. #1: Uncertainty in Surface O3 due to Isoprene Chemistry Change in July mean 1-5 p.m. surface O3 (MOZART-2) ppbv When isoprene nitrates act as a NOx sink

7. Difference in July 1-5 p.m. surface O3 (Purves–GEIA) (ppbv) #2: Uncertainty in Surface O3 due to Isoprene Emissions GEIA: global inventory July Anthrop. NOx emissions 5.6 Tg C (1011 molec cm-2 s-1) July isoprene emissions Purves et al., [2004] (based on FIA data; similar to BEIS-2) High-NOx regime 2.8 Tg C Higher isoprene decreases O3 GEIA (1011 molecules isoprene cm-2 s-1)

8. Revisiting changes in July 1-5 p.m. surface O3 (ppbv) due to isop emis changes from mid-1980s to mid-1990s With 12% yield of isoprene nitrates GEOS-CHEM: GEIA GEOS-CHEM: Purves MOZART-2: GEIA ppbv MOZART-2 shows similar results to GEOS-CHEM if isoprene nitrates are a NOx sink Understanding fate of isop. nitrates essential for predicting sign of response to changes in isoprene emissions

9. From Ugarte et al. (2003) Future scenario for conversion of agricultural lands to poplar plantations for biofuels High-NOx region Poplars are high-isoprene emitting trees  land-use decisions have implications for air quality from D. Purves

10. Conclusions and Remaining Challenges • Better constrained isoprene emissions are needed to quantify: • isoprene contribution to E. U.S. surface O3 • how O3 responds to both anthrop. and biogenic emission changes • (deserves consideration in planning biofuel plantations) • Utility of satellite CH2O columns? • New inventories (MEGAN, BEIS-3) more accurate? • Constraints from ICARTT observations? • Recent isoprene increases may have reduced surface O3 in the SE • Does this regime actually exist? • Fate of organic nitrates produced during isoprene oxidation?

11. Vertical slices through 34N: enhanced isoprene, CH2O, PAN at surface & upper trop for GEIA compared to Purves With PURVES Altitude With GEIA Longitude 0 0.1 .25 0.5 1. 5. .1 .2 .5 1. 3. 5. 0 0.1 .25 0.5 0.8 1.0 0. .05 0.1 0.2 0.5 2.  Insights from NASA INTEX-NA flights over SE US?

13. Trends in Anthropogenic Emissions: 1985 to 1995 from US EPA national emissions inventory database(http://www.epa.gov/air/data/neidb.html) CO VOC NOx Ratio of 1995 to 1985 Emissions • Large decreases in CO and VOC Emissions • Some local increases in NOx • Higher biogenic VOCs Net effect On O3?

14. Change in Mean July Surface O3 (ppbv; 1-5 p.m.) reflecting 1980s to 1990s emissions changes With Anthrop. Changes With Anthrop.+ BVOC Changes With BVOC Changes With GEIA Isoprene Emis With Purves et al. Isoprene Emis Changes in Anthropogenic NOx emissions dominate O3 response But response depends upon choice of isoprene emission inventory Comparison with observed changes?

15. Model vs. Obs.: Change in July O3 1980s to 1990s (ppbv; 1-5 p.m.) Obs: EPA AIRS GEOS-CHEM: GEIA GEOS-CHEM: Purves (1993-1997) – (1983-1987) with anthropogenic + biogenic emissions changes (ppbv) Poor correlation (r2 ~ 0) between observed and simulated changes Observed changes in O3 are not explained by regional emission changes alone…

16. Identify O3 chemistry regime with precursor emissions reductions Change in July O3 (ppbv; 1-5 p.m.) Isoprene reduced 25% NOx reduced 25% July Anthropogenic NOx Emissions With GEIA (1011 molec cm-2 s-1) ° Choice of isoprene inventory also critical for predicting O3 response to changes in isoprene and anthropogenic NOx emissions With Purves Low-NOx, high isop: O3 as isop High-NOx: O3 as isop strongly NOx-sensitive

17. NOx- saturated Ozone NOx- sensitive Low-NOx High-NOx VOC Other model studies indicate SE US is near “maximum VOC capacity point”, beyond which VOCs suppress O3 formation [Kang et al., 2003]. IncreasingIsoprene Decreases O3 in Low-NOx, High-isoprene regions GEOS-CHEM base-case July 1-5 p.m. mean NOx “isoprene-saturated”?? ISOP