1. Organic Aerosol: From Oxidation to Optical Depth IGAC Conference, Halifax, Canada July 14, 2010 Colette L. Heald Acknowledgements: Jesse Kroll (MIT), Jose Jimenez, Ken Docherty, Delphine Farmer, Pete DeCarlo, Allison Aiken (CU-Boulder, currently or former), Qi Chen & Scot Martin (Harvard), Paulo Artaxo (University of Sao Paulo) David Ridley (CSU), Easan Drury (NREL)

2. ORGANIC AEROSOL MAKES UP AN IMPORTANT FRACTION OF OBSERVED AEROSOL, YET MODELS STRUGGLE TO REPRODUCE THESE CONCENTRATIONS Globally makes up 25-75% of total fine aerosol at the surface (ignoring soot here) [Zhang et al., 2007] Sulfate Organics

3. WHY DON’T MODELS GET IT RIGHT…. Terpenes (gas-phase) PBAP Hydrocarbons (gas-phase & particulate) Uncertain Formation (Missing sources? Poorly understood processes?) Continuing Oxidation/Partitioning in the Atmosphere 10,000’s of (unidentified?) compounds with variable properties

4. A SIMPLIFIED DESCRIPTION OF ORGANIC AEROSOL COMPOSITION [Goldstein et al., 2008] Typically < 20% of OA mass can be identified [Williams et al., 2007]. Even if we could identify these species, global models couldn’t handle this complexity! 2D chromatogram of OA Hydrogen Carbon Oxygen Other (N, S, etc) Alternate: Look at bulk elemental composition of aerosol Need a framework to compare composition & track changes…

5. A FRAMEWORK FOR LOOKING AT CHANGES IN COMPOSITION: THE VAN KREVELEN DIAGRAM Example: Replace -CH 2 - with a carbonyl group -C(=O)-  Add 1O, lose 2H, slope = -2 Developed by Van Krevelen in 1950’s to describe oil formation Can be used to describe dynamic oxidation processes in the atmosphere: functionalization of an aliphatic carbon (-CH 2 -) shown here

6. LAB & FIELD ORGANIC AEROSOL LINE UP IN A VAN KREVELEN DIAGRAM! Surprisingly, despite complexity, MEAN aerosol composition changes during aging look like carboxylation! All measurements taken with the high resolution Aerosol Mass Spectrometer (HR-AMS)

7. EXAMPLES FROM THREE FIELD CAMPAIGNS… Riverside, California: dominated by urban sources Amazon basin: clean, low loadings, more oxygenated Mexico city (aircraft): regional sampling (clean & polluted) Photochemical clock shows moves “down” the line with aging. Some leveling off with long ages?

8. IMPLICATIONS 2.From a lab perspective: why does bulk OA “collapse” to this composition? What are the details of fragmentation & functionalization reactions in the atmosphere that result in net carboxylation? 1. From a modeling perspective: hope for a simple parameterization!  Need to understand aging timescale better (how fast do we move down the -1 slope?) … H:C O:C [Heald, Kroll et al., 2010]

9. A LARGE MISSING SOURCE OF ORGANIC AEROSOL? Goldstein and Galbally [2007] suggest that SOA source may be anywhere from 140-910 TgC/yr. Can total aerosol optical depth (AOD) measurements shed any light on the total budget of OA? For comparison, current model (GEOS-Chem) estimates total ~50 TgC/yr (~2/3 POA, 1/3 SOA), equivalent burden ~0.81 TgC TgC yr -1

10. SATELLITE OBSERVATIONS: ADVANTAGES & CHALLENGES ADVANTAGE: Global view of total atmospheric column CHALLENGE: AOD is an integrated measure of ALL aerosols – uncertainty on any single derived particle type will be high. Strategy: focus on continental AOD, use MISR as a global constraint Example of MISR/AERONET (MAM 2008)

11. IF ONLY AEROSOL IN THE ATMOSPHERE WAS OA, WHAT LOADING IS IMPLIED BY SATELLITE AOD? Calculate the “hypothetical” AOD implied by a constant 1  g/m 3 profile over the land, and see how we need to scale this locally to make up ENTIRE AOD reported by MISR. Inverted OA loading is 3.7 TgC over land. Assume a 6 days lifetime = 230 TgC/yr  extrapolate to include outflow ~460 TgC/yr. (middle of Goldstein & Galbally range) Inverted total MISR AOD: Equivalent uniform OA profile

12. A MORE REALISTIC POSSIBILITY: REMOVE CONTRIBUTIONS FROM DUST, BC, INORGANICS (assuming all the negative bias in the model is ONLY OA) Estimate that ~230 TgC/yr source is required to close the MISR-GEOS-Chem discrepancy. IF we remove N. Africa & the Middle East (dust) from this, total is reduced to ~180 TgC/yr If then account for more “reasonable” vertical profile, reduce to ~160 TgC/yr DJFJJA MISR GEOS-Chem* MISR- GEOS-Chem* *excluding OA

13. 910 460 230 47 180 If all AOD inverted for OA Existing GEOS-Chem sources If attribute all MISR “excess” AOD to OA If remove N. Africa & Middle East 140 Best estimate (“likely” vertical distribution) 160 Range estimated by: Goldstein and Galbally [2007] All units in TgC/yr HAVE WE REDUCED THE UNCERTAINTY ON THE OA BUDGET? [Heald et al., in prep] Suggests a MAXIMUM global source of ~160 TgC/yr of OA to the atmosphere. This is more than THREE TIMES what is currently included in global models…. BUT at the low end of Goldstein & Gallbally [2007] range. * Detailed uncertainty analysis suggests budget derived from AOD has ~80% uncertainty