Department of Environmental Science and Engineering UNC, Chapel Hill Secondary Aerosol Formation from Gas and Particle Phase Reactions of Aromatic Hydrocarbons Richard Kamens and Di Hu Funded by the USEPA STAR program July 30, 2003 to July 29, 2006 Department of Environmental Science and Engineering UNC, Chapel Hill

The overall goal of this project is to represent new chemistry as a unified, multi-phase, chemical reaction mechanism that will explain the observed chemical phenomena and amounts of secondary organic aerosol that result from aromatics reacting in an urban atmosphere.

Current Approaches do not incorporate newly discovered particle phase heterogeneous reactions that lead to significant SOA formation A “next generation” chemical mechanistic approach is needed that captures the essence of the fundamental chemistry that leads to secondary aerosol formation

Volatile aromatic compounds comprise a significant part of the urban hydrocarbon mixture in the atmosphere, up to 45% in urban US and European locations If an alcohol happened to be present hemiacetal and acetal formation could also take place after acid catalysis. Note that the ultimate result in this case is conversion of the aldehyde carbonyl to hydroxy and alkoxy groups. In addition to hydration and polymerization, if alcohols are present hemiacetal and acetal formation may take place on the protonated carbonyls. If we take at look at the aldehyde SOA from the bag experiments in the IR region we are able to see evidence of these types of transformations.

Toluene, m- & p-xylenes, benzene and 1,2,4-trimethyl benzene, o-xylene and ethylbenzene make up 60-75% of this load. In the US, transportation sources contributed ~67% to the total aromatic emissions which range from 2.4 x 106 to 1.9 x 106 tons/year.

Laboratory studies show that gas phase reactions of aromatics and biogenics form a host of oxygenates   secondary organic aerosol material (SOA) hydroxy unsaturated dicarbonyls di and tri carboxylic acids Nitrated hydroxy carbonyls

Is there Atmospheric Evidence for SOA formation??

Turpin and co-workers In the LA area estimated on smoggy days {from OC /EC ratios}, that as much as 50 - 80% of the aerosol organic carbon comes from secondary aerosol formation (1984 and 1987 samples) On average, organic carbon can make up between 10-40% of the total fine TSP in the US

Spyros Pandis also recently looked at OC/EC ratios (Pittsburgh area) He estimates that SOA formation can account for 35-50% of the organic carbon

OC/EC Ratio and Photochemical Activity Pittsburgh, 2001

In the context of this work, how much of this SOA comes from aromatic emissions in to an airshed?

kinetic mechanism development outdoor chamber experiments Tolune, Overall Approach kinetic mechanism development outdoor chamber experiments Tolune, m-xylene 1,3 5 trimethyl benzenes Simulation of chamber experiments

kinetic mechanism development Overall Approach kinetic mechanism development Illustrate this with a simple reaction scheme of toluene

Now let me go back and explain in detail each of the reaction steps in the previous three slides. No, shoot me if I start….. If an alcohol happened to be present hemiacetal and acetal formation could also take place after acid catalysis. Note that the ultimate result in this case is conversion of the aldehyde carbonyl to hydroxy and alkoxy groups. In addition to hydration and polymerization, if alcohols are present hemiacetal and acetal formation may take place on the protonated carbonyls. If we take at look at the aldehyde SOA from the bag experiments in the IR region we are able to see evidence of these types of transformations.

* toluene O2 . . OH oxygen bridge rearrangement butenedial O=CH CH 3 OH + HO 2 + H O o-cresol benzaldehyde CH 3 OH 2 . NO +O H * O2 + toluene CH 3 H OH O . NO 2 +O rearrangement oxygen bridge OH H O . +O 2 CH 3 + methylglyoxal butenedial + HO ring cleavage radical

Pent-dione + OH 0.5 pent-rad +0.5 pent-oo pent-oo  XO2 + 0.5 GLY+ 0.5 MGLY + 0.5 CO + 0.5HO2 +0.25 OHoxybutal +0.25 but-tricarb pent-rad  Maleic + 1.5 XO2 + HO2 + HCHO  

Aromatic Aldehydes Ring Aldehydes Ring opening carbonyls Oxo acids OH-carbonyls

Edney and Keindienst et al, 2001

Historically, from a Modeling perspective Equilibrium Organic Gas-particle partitioning has provided a context for addressing SOA Formation If an alcohol happened to be present hemiacetal and acetal formation could also take place after acid catalysis. Note that the ultimate result in this case is conversion of the aldehyde carbonyl to hydroxy and alkoxy groups. In addition to hydration and polymerization, if alcohols are present hemiacetal and acetal formation may take place on the protonated carbonyls. If we take at look at the aldehyde SOA from the bag experiments in the IR region we are able to see evidence of these types of transformations.

Gas and particle phases can be linked via G/P partitioning Methyl glyoxal Gas phase reactions CH3-C-C=O 1Cgas + surf  1Cpart CH3-C-C=O particle

Kp = kon/koff kon koff [ igas] + [part] [ ipart] kon koff particle CH3-C-C=O O [ igas] + [part] [ ipart] Kp = kon/koff kon koff

Particle Phase reactions Polymerization reactions

Particle Phase Reactions + ozone + acid seeds  aerosols a-pinene

ESI-QTOF mass spectrum of SOA from reaction of a-pinene with ozone + acid seed aerosol.

Particle phase pinonaldehyde dimers from acid a-pinene +O3 M Na+ (ESI-QTOF Tolocka et al, 2003)

Particle Phase Reactions

GlyP + H2O ----> Gly2OHP Gly2OHP + H2O ----> Gly4OHP Gly4OHP + GlyAcidP ----> pre-Poly1 Pre-Poly1 + C4OHALD ----> Poly1

Particle Phase reactions Gas phase reactions C=O O cis-pinonaldhyde C=O O polymers particle

Particle Phase reactions Gas phase reactions C=O O cis-pinonaldhyde C=O O particle polymers

Particle Phase reactions Gas phase reactions C=O O cis-pinonaldhyde C=O O polymers

The Outdoor Chamber Reactor System

Hanging Teflon It took all of us to put up each panel of the Teflon walls.

Dual 270m3 chamber fine particle t 1/2 >17 h

Rates of polymerization. methylglyoxal +NOx chamber. experiments Rates of polymerization methylglyoxal +NOx chamber experiments these rates may be related to HNO3 gas phase and associated particle HNO3

Quantum yields. dicarbonyls. multifunctional carbonyls. Liu et al Quantum yields dicarbonyls multifunctional carbonyls Liu et al. 1999–adsorption cross sections

Pinonaldehyde quantum yields in natural sunlight kphototyis = S ( al fl Il ) By adding pinonaldehyde to the chamber in clear sunlight in the presence of an OH scavenger and measuring its rate of decay fl can be fit to the decay data assuming a shape with wave length similar to other aldehydes

Pinonaldehyde quantum yields in sunlight Normalized to one pinonaldehyde CH3CH2CH2=O CH2=O O pinonaldehyde CH3CH2=O

An Exploratory Chemical Model Toluene + propylene + NOx + Sunlight  gas phase prod. + SOA

hexadiene-dicarb. butene-dicarbonyl. pentene-dicarbonyl. benzaldehyde hexadiene-dicarb butene-dicarbonyl pentene-dicarbonyl benzaldehyde cresol maleic anhydride 43 AROMATIC reactions + Carbon 4

carbonyl-PAN C4-carbonyl-acid butane-tricarbonyl OH-oxobutanal polymer1

A second generation Model

A second generation Model

Analysis

PFBHA O-(2,3,4,5,6-pentafluorobenzyl) -hydroxylamine for carbonyl groups

PFBBr, Pentafluorobenzyl bromide derivatization for carboxylic and aromatic-OH 2 O 3 F B r P

BSTFA for hydroxyl, and/or carboxylic groups ( 3 )

BF3-CH3OH + BSTFA Derivatization Method Citramalic acid GC-ITMS analysis - electron impact ionization (EI) - methane chemical ionization (CI-methane) - tandem mass spectrometry (MS/MS)

What are some challenges? Photolysis reactions of the 2nd and 3rd products Quantum yields of product carbonylsParticle phase reactions Wall reactions of the products Integration of particle phase reactions GC-Ion-trap analysis ES-LCMS-MS

Aerosol sampling??? Presently methods to simultaneously measure gas and particle SVOCs have artifacts? We need an new generation of analytical techniques…