2 nd December 2004 MCM meeting Leeds 1.Aromatic hydrocarbon oxidation 2. Uncertainty analysis Mike Pilling University of Leeds, UK

2 MCM meeting Leeds 2 nd December 2004 Oxidation mechanisms initiated by OH Most NMHCs: –Abstraction or addition followed by addition of O 2 to form peroxy radical. –Peroxy radical reacts with NO to form an alkoxy, which reacts to form HO 2. Reaction with NO regenerates OH. –NO  NO 2 from peroxy reactions  O 3 Aromatics: –HO-aromatic-O 2 is short lived, regenerating reactants. Adduct is too short lived to react with NO under normal atmospheric conditions. –Intermediates are generally short lived cf parent

3 MCM meeting Leeds 2 nd December 2004 HO 2 Products Phenol + OH/O 2 + O 2 NO NO 2 HO 2 O2O2 Peroxide Bicyclic NO NO 2 O2O2 HO 2  -Hydroxy Peroxy HO 2 Products Oxepin Oxidation Routes for Toluene HO 2 O2O2 Epoxy-Oxy Also routes to: Benzaldehyde via abstraction MeBenzoquinone via 1,4 addition MCM1: Peroxide Bicyclic Route, predominantly producing butenedial + methylglyoxal MCM2: Oxepin Route, predominantly producing muconaldehyde type products MCM3.1 – latest mechanism

4 MCM meeting Leeds 2 nd December 2004 Mechanism for toluene oxidation Reviewed up to ~2000 by Calvert et al. Main contributions to mechanism from the groups of Atkinson, Becker and Seinfeld ~6% via abstraction from –CH 3 Rest via HO-toluene-O 2 adduct: –~12% via retention of aromatic ring –Addition of O 2 to form bridged bicyclic compound that leads to ring opening. –Role of NO not clear Mass balance only ~50%

5 MCM meeting Leeds 2 nd December 2004 Glyoxal, formed from the formation of the bicyclic compound and ring opening is a primary product only. (Volkamer et al, J Phys Chem, 2001, 105, 7865) Measurements of substituted phenol yields and demonstration of the need to work at low NO x (Volkamer et al, PCCP, 2002, 4, 1598) Subsequent key results from EUPHORE: Spectroscopic expts using DOAS

6 MCM meeting Leeds 2 nd December 2004 EXACT consortium Effects of oxidation of aromatic compounds in the troposphere (EU, Framework 5) Kinetics of elementary reactions in initial stages (Bordeaux, Hanover) Kinetics and mechanisms of secondary chemistry (Cork, Wuppertal) Development and testing of overall mechanism. Design of EUPHORE experiments. (Leeds, Imperial College) Synthesis of intermediates (Newcastle) Secondary aerosol formation (Wuppertal, Imperial College) Photochemical chamber studies at EUPHORE (Valencia, Wuppertal, Cork) Coordination (Leeds)

7 MCM meeting Leeds 2 nd December 2004 Chamber measurements at Wuppertal and Cork on kinetics of intermediates NO 3 reactions, relative to 2,3 dimethyl 3 butene (17, 15, 10)x cm 3 s -1 tolualdehyde k OH a,b k NO3 b,c Ortho20.8   0.5 Meta22.3   0.4 para20.9   0.4 a : cm 3 molecule -1 s -1 vs butyl ether and 1,2,4 trimethylbenzene b : 2s errors c : cm 3 molecule -1 s -1 vs tetrahydrofuran Also extensive measurements of products in reactions of intermediates, especially of hydroxyarenes (Olariu et al) Photolysis rates and mechanisms for  –dicarbonyls (Thuener et al)

8 MCM meeting Leeds 2 nd December 2004 Toluene Oxidation Routes in MCMv3.1 Little ring opening along phenol route Successive addition of OH, NO 3. Leads to formation of nitrophenols Low ozone formation route Ring opening routes are most active photochemically and dominate ozone formation

9 MCM meeting Leeds 2 nd December 2004 CEAM Labs, Valencia FTIR: Aromatics, O3, HCO2H,HCHO, HNO3 Absorption spectroscopy: O3 Chemiluminescence: NO DOAS:NO2, Glyoxal LIF: OH, HO2 GC-ECD: PAN, Methylglyoxal, PAN GC-FID: Aromatics HPLC/UV: Cresols, Benzaldehyde CO-Monitor: CO 2D-GC:carbonyls PFBHA: intermediates Derivatisation: Oxepin (Triazolin) Glyoxal, Methylglyoxal (Diaminobenzol) Filterradiometer:J(NO2) SMPS: Particle size distribution

10 MCM meeting Leeds 2 nd December 2004 Maleic anhydride Angelica lactone /oxopentanal unknown Benzaldehyde Me-benzoquinone Toluene Quantitative GCxGC of Toluene Oxidation Products Difficult to detect appropriate amounts of coproducts of glyoxal and Meglyoxal Measurements also made by GC/ECD

11 MCM meeting Leeds 2 nd December 2004 Test of [OH] LIF calibration Inferred [OH] Guggenheim (1926) k' = ln (c 2 / c 1 ) / (t 2 - t 1 ) [OH] HC = (k' - k dil ) / k OH+HC Hydrocarbon concentration Measured [OH] (LIF)

12 MCM meeting Leeds 2 nd December 2004 Test of HO 2 calibration HO 2 concentration evolution in the dark and second-order decay analysis From decay analysis k(HO 2 +HO 2 ) = 3.0 x molecule -1 cm 3 s -1 Literature (JPL 97-4) k(HO 2 +HO 2 ) = 2.8 x molecule -1 cm 3 s -1 HCHO photolysis No NOx

13 MCM meeting Leeds 2 nd December 2004 Toluene Oxidation Routes in MCMv3.1 Little ring opening along phenol route Successive addition of OH, NO 3. Leads to formation of nitrophenols Low ozone formation route Ring opening routes are most active photochemically and dominate ozone formation

14 MCM meeting Leeds 2 nd December 2004 Design of chamber experiments

15 MCM meeting Leeds 2 nd December 2004 Comparison of MCM3.1 to Toluene Chamber Experiment (27/09/01) Conclusions: - Ozone overpredicted but OH is too low. Need early OH source that doesn’t produce O 3 - NO 2 is not rapidly enough - Co-products of glyoxal/ Me glyoxal not detected in sufficient conc n

16 MCM meeting Leeds 2 nd December 2004  -dicarbonyls. Photolysis (NO = 0) and ‘photosmog’ experiments (with NO)(Cork, Valencia measurements) photolysis photosmog

17 MCM meeting Leeds 2 nd December 2004 MCM v3.1 photolysis mechanism vs photolysis observations

18 MCM meeting Leeds 2 nd December 2004 Searching for an OH production route Alkyl peroxy radicals isomerise / dissociate to from OH only at high T Modification of the peroxy can lead to low T production of OH: e.g. CH 3 CO + O 2 → CH 3 CO 3 * → OH + CH 2 CO 2 CH 3 CO 3 * + M → CH 3 CO 3 Can such routes operate in aromatic chemistry?

19 MCM meeting Leeds 2 nd December 2004 EXACT-1 : Attempts to improve the model performance by including an NO 2 aerosol sink /HONO source and an early source of OH Alternative mechanisms are also feasible, e.g. Volkamer, O 3 + furanones

20 MCM meeting Leeds 2 nd December 2004 Current status of aromatic mechanisms Mechanism underestimates total radical production rates by a factor of ~2 at short and long times. At the same time, mechanism overestimates O 3 formation – need route to radical formation that doesn’t give NO to NO 2 conversion. NOx removed from system more rapidly than mechanism indicates Identification of glyoxal co-products (  dicarbonyls) via GCxGC and GC/ECD with synthesis of targets – yields very low. Photochemistry and photosmog experiments on  dicarbonyls are incompatible in terms of radical yields – need new chemistry. Further extensive experiments on other aromatics – benzene, p-xylene, 1,3,5 trimethyl benzene, hydroxy aromatics, using both EUPHORE and small chambers for kinetics. Provide detailed mechanistic and kinetic data, but problems remain. Need new detailed experiments, e.g. by laser flash photolysis or discharge flow on targeted intermediates: – OH and HO 2 formation –O 3 +  -dicarbonyls / furanones Improved detection methods for glyoxal co-products

21 MCM meeting Leeds 2 nd December 2004 Comparison of ethene measurements and simulations Ethene experiments used to refine the auxiliary chamber mechanism 2  measurement uncertainty (grey bands) 2  uncertainties from Monte-Carlo simulations (error bars)

22 MCM meeting Leeds 2 nd December 2004

23 MCM meeting Leeds 2 nd December 2004 Uncertainty contributions, ethene, low and high NOx

24 MCM meeting Leeds 2 nd December 2004 Morris method – (MOAT analysis), high NO x Effects of individual rate constants on peak O 3 concentration