Organic Aerosol: From Oxidation to Optical Depth

IGAC Conference, Halifax, CanadaJuly 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)

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]SulfateOrganics

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

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

OxygenOther

(N, S, etc)

Alternate: Look at bulk elemental composition of aerosol

Need a framework to compare composition & track changes…

A FRAMEWORK FOR LOOKING AT CHANGES IN COMPOSITION: THE VAN KREVELEN DIAGRAM

Example: Replace -CH2- 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 (-CH2-) shown here

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)

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?

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]

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

SATELLITE OBSERVATIONS: ADVANTAGES & CHALLENGES

ADVANTAGE: Global view of total atmospheric columnCHALLENGE: 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)

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/m3 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

topz

0

AOD= α RH z M z dz

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

DJF JJA

MISR

GEOS-Chem*

MISR-GEOS-Chem*

*excluding OA

910

460

230

47

180

If all AOD inverted for OA

Existing GEOS-Chem sources

If attribute all MISR “excess” AOD to OAIf 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