HUMAN INFLUENCE ON OZONE IN THE GLOBAL TROPOSPHERE

1. HUMAN INFLUENCE ON OZONE IN THE GLOBAL TROPOSPHERE Daniel J. Jacob • Climate response from increasing ozone • What is background ozone in the U.S.? • Mapping of ozone precursor emissions from space

2. INTERCONTINENTAL TRANSPORT vs. HEMISPHERIC POLLUTION For ozone, background enhancement by hemispheric pollution is critical issue; for PM, intercontinental transport events are more important Tropopause HEMISPHERIC POLLUTION BACKGROUND Mixing Free troposphere “Direct” intercontinental transport PBL boundary layer advection WCB subsidence N. America Europe Asia

3. QUANTIFYING OZONE BACKGROUND IN U.S. - current round of revision of NAAQS - “Background” = ozone present in absence of N American anthrop. emissions • What background concentrations should be used to assess risk? • Do frequent occurrences of high background compromise achievability of NAAQS, or preclude the imposition of a tougher NAAQS? • Fiore et al. [JGR 2002, GRL 2002], using GEOS-CHEM: • Mean present-day ozone background in U.S. is 20-35 ppbv • Background includes 5-10 ppbv enhancement from anthropogenic emissions outside North America, with equal contributions from CH4 and NOx emissions • Background decreases to 10-15 ppbv during pollution episodes (>80 ppbv O3) • Lefohn et al. [JGR 2001], using obs stats at remote sites and backtrajectories: • Frequent occurrences of ozone above 50-60 ppbv observed at remote sites in the U.S. in spring do not appear to be associated with regional production  Suggests current 25-45 ppbv EPA background definition is inadequate  Implies that NAAQS may be unattainable via domestic emissions reductions

4. GEOS-CHEM SENSITIVITY SIMULATIONS FOR OZONE SOURCE ATTRIBUTION • Standard simulation…..2o x2.5o GEOS-CHEM, 48 sigma levels 2001 • Background………………no anthrop. NOx, CO, NMVOC emissions from N. America • Natural O3 level………….no anthrop. NOx, CO, NMVOC emissions globally; CH4 = 700 ppbv • Stratospheric…………….tagged O3 tracer simulation Regional Pollution =Standard –Background Hemispheric Pollution =Background–Natural O3 level Conduct 2001 ozone season simulation, evaluate with stats from CASTNET sites

5. * CASTNet sites Model at CASTNet Model entire region Background Natural O3 level Stratospheric + Seasonal cycle in mean afternoon (1-5 p.m.) ozone in surface air, 2001 Stratospheric ozone: 0-10 ppbv, highest in spring Natural ozone: 15-25 ppbv Hemispheric pollution enhancement: 5-15 ppbv, highest in spring Fiore et al., JGR in prep.

6. * CASTNet sites Model Background Natural O3 level Stratospheric + } Regional pollution D = } D = Hemispheric pollution Ozone Time Series at CASTNet stations in spring 2001 1-5 pm daily data Model reproduces structure; regional pollution is #1 factor, hemispheric pollution also significant Fiore et al., JGR in prep.

7. * CASTNet sites Model Background Natural O3 level Stratospheric Continental lower troposphere + } Regional pollution D = High-O3 “Haywood County” event in North Carolina in spring 2000 } D = Hemispheric pollution APR-MAY 2000 APR-MAY 2001 Events are explained in model by regional pollution, not transport from stratosphere Fiore et al., JGR in prep.

8. * CASTNet sites Model Background Natural O3 level Stratospheric + Cumulative probability distributions for daily mean afternoon O3 at CASTNET sites, spring-summer 2001 Apr-May Jul-Aug 11 remote sites (western U.S.) 34 polluted sites (eastern U.S.) Regional pollution Background and pollution increments can be quantified along probability distribution & for different seasons; background is never > 50 ppbv except perhaps under exceptional circumstances Fiore et al., JGR in prep.