Developing Secondary Organic Aerosol (SOA) Code for the MCM David Johnson (Mike Jenkin and Steve Utembe) Department of Environmental Science and Technology, Imperial College London, Silwood Park, Ascot, SL5 7PY MCM Developer and User Workshop, Leeds, Thursday/Friday 2/3 December 2004

Introduction  Effects of particulate matter in the atmosphere  Contribution that organic material makes to total particulate burden  SOA from atmospheric oxidation of biogenic volatile organic compounds (VOCs) and anthropogenic aromatic hydrocarbons  Gas-phase degradation mechanisms  Transfer of material from gas- to condensed organic-phase

Formation of SOA and Oxidation Mechanisms  VOC  higher polarity, lower volatility products  SOA material  Generally only for VOC ≥ C 6 aromatic hydrocarbons (e.g. toluene, ethylbenzene, xylenes) terpenic biogenic hydrocarbons (e.g.  -pinene,  -pinene) Oxidation mechanisms for large VOCs are very complex  Master Chemical Mechanism (MCM) e.g. for toluene (methylbenzene) : 268 species, 754 reactions ca. 120 species with Tb > 450 K

Gas-to-particle partitioning of organic material  Pankow absorption model  Transfer of material represented as a dynamic equilibrium POH (g) POH (abs) [POH (abs) ]/[POH (g) ] = k in [POH (g) ] / k out M O = K p M O

Odum et al. “two-product”-model i = 1,2 At small M 0 Y  M 0  K p,i As M 0   Y   i

MCM v3.1-“many product”-model  Pankow absorption model  e.g. for toluene, ca. 150 species with Tb > 450 K  Transfer of material represented as a dynamic equilibrium POH (g) POH (abs) [POH (abs) ]/[POH (g) ] = k in [POH (g) ] / k out [OA] = K p [OA] need to estimate saturated vapour pressures for 150 species

How to estimate K p,i for a large number of species? Use the MCM (Accord) Database and ChemDraw for Excel 1.Convert to unique SMILES strings.

How to estimate K p for a large number of species? Use the MCM (Accord) Database and ChemDraw for Excel 2. “Eliminate” radical species (replace “[O]” with “Z”)

How to estimate K p for a large number of species? Use the MCM (Accord) Database and ChemDraw for Excel 3. Sub-structure search for oxynitro-compounds (organic nitrates)

Use the MCM (Accord) Database and ChemDraw for Excel 4. Molecular property calculations using Chem Office for Excel How to estimate K p for a large number of species?

Use the MCM (Accord) Database and ChemDraw for Excel 4. Molecular property calculations using Chem Office for Excel How to estimate K p for a large number of species?

“Deployment” of K p values “on-line” vs. “off-line” k in = 6.2  s -1 k out = k in / K p Average MW of Absorbing organic condensed-phase species

Smog Chamber Aerosol Data e.g. for toluene data from EXACT (Effects of the oXidation of Aromatic Compounds in the Troposphere) All partitioning coefficients  27.5

Condensed-Phase Chemistry (Association Reactions) peroxyhemiacetal Tobias and Ziemann

Simplified Peroxyhemiacetal Chemistry ROOH + HC(=O)R’  ROOC(OH)R’H adduct forming chemistry included

Simplified Peroxyhemiacetal Chemistry ROOH + HC(=O)R’  ROOC(OH)R’H  27.5 (original model)  9.9 (association chemistry)

Effect of NO x on Toluene SOA

Effect of NO x on Benzene SOA

Effect of NO x on p-Xylene SOA

Effect of NO x on Mesitylene SOA

Effect of NO x on Toluene SOA low-NO x mid-NO x high-NO x

Other toluene SOA mass concentration data and the role of NO RO 2. + NO  RO. + NO 2 HO 2. + NO  OH + NO 2 RO 2. + HO 2.  ROOH ROOH  adduct

Comparing datasets of toluene SOA yields  point of reference = SOA yield at 50  g m -3 aerosol loading NO x -free, limiting yield

SOA Forming Propensity of other Aromatics 1,3,5-trimethylbenzene toluene 1,2,4-trimethylbenzene

Can we relate variations in SOA yield to differences in gas-phase chemistry?  Unsaturated aldehydes are reactive in terms of indicated association chemistry

Variations in SOA yield (Aromatics)

Extension to the entire MCM  124 parent VOCs (MCM v3)  ca chemical reactions  ca chemical species  ca closed-shell species with T b (estimated) > 450 K need to define 2000 new species, 2000 phase-equilibria  Which are the most important components of simulated SOA?  Which are the most important SOA precursors? Partitioning coefficients?

 (pseudo-) Lagrangian, well-mixed boundary layer, chemical box model  Background anthropogenic and biogenic emissions throughout (NAEI)  Enhanced anthropogenic emissions for 3 hours  Idealised “trajectory”  Primary emitted OA  Background organic aerosol Preliminary (Box) Modelling A B Day 0 Day x hour 03X 1 10 Anthropogenic Emission factor

Preliminary (Box) Model Simulations Scaling factor = 50 Background organic aerosol Secondary organic aerosol Primary organic aerosol

Preliminary (Box) Model Simulations  Which are the most important SOA precursors?  Which are the most prevalent SOA components?

Conclusions and Further Work  Gas-aerosol partitioning (equilibrium) coefficients have been estimated for ca species within the MCM v3.1.  Validation simulations have been performed using measured SOA data for the photooxidation of (  -pinene) benzene, toluene, p-xylene and mesitylene.  These simulations strongly imply the key role of condensed organic- phase association chemistry.  Simulations further suggest the important role of ROOH.  effect of NO x concentration. To do:-  Model-measurement comparisons – scaling factors?; NO x ?

Speciation of simulated SOA for low-NO x conditions  Simulated aerosol material dominated by five species:-  In aerosol peroxyhemiacetal formation ROOH + HC(=O)R’  ROOC(OH)R’H

Peroxyhemiacetal forming chemistry

Amended Catechol (1,2-dihydroxybenzene) Chemistry Similar to MCM v3.1a