History of the Master Chemical Mechanism MCM and

History of the Master Chemical Mechanism (MCM) and its development protocols Mike Jenkin Centre for Environmental Policy m. jenkin@imperial. ac. uk

1993 – the conception of the MCM l University of Leeds Sam Saunders, Mike Pilling l AEA Technology Mike Jenkin, Colin Johnson l UK Meteorological Office Dick Derwent l Work commissioned by the Department of the Environment, Do. E (Air Quality Division), to improve the treatment of organic chemistry in ozone policy models

Chemical processing of ozone-precursor emissions inventory contains ca. 650 species VOC NOX emissions Ozone oxidation CO 2 H 2 O nitrate

Chemistry in Do. E ozone models in 1993 Photochemical Trajectory Model l l chemistry of 95 VOC represented although reasonably detailed, the chemistry did not reflect the current status of kinetic and mechanistic data, e. g. - no formation of organic nitrates from RO 2 + NO - RO 2 + HO 2 reactions not included (except for CH 3 O 2) - incomplete degradation of some VOC - - many VOC degraded via products known to be wrong (i. e. incorrect RO reactions applied) very limited representation of photolysis of organics

1993 -2007: Master Chemical Mechanism Philosophy l l l to use information on the kinetics and products of elementary reactions relevant to VOC oxidation to build up a rigorous explicit representation of the degradation mechanisms. - the resultant formation of ozone and other gas-phase secondary pollutants apply measured and evaluated parameters (e. g. rate coefficients; branching ratios) from the literature where possible. use analogy and ‘structure-reactivity correlations’ to define the other reactions and parameters.

Mechanism construction methodology Mechanism construction is broadly a two-stage process: 1. Development of a “mechanism construction protocol” 2. Application of the protocol to a series of emitted (primary) VOC to develop the mechanism/database

Mechanism development and protocol history 123 125 135 (1) Jenkin et al. (Atmos. Env. 31, 81, 1997): non-aromatic species (2) Report on DETR contract, EPG 1/3/70 (1998) : aromatic species (3) Saunders et al. (ACP, 3, 161, 2003): non-aromatic species (4) Jenkin et al. (ACP, 3, 181, 2003): aromatic species (5) Bloss et al. (ACP, 5, 641, 2005): aromatic species

MCM timeline 1996 MCM v 1 - 120 VOC; 7100 reactions; 2400 species 1999 MCM v 2 - 123 VOC; 11400 reactions; 3800 species 101 non-aromatic anthropogenic species 18 aromatics (provisional chemistry) 1 biogenic species (isoprene) 103 non-aromatic anthropogenic species 18 aromatics (extended provisional chemistry) 2 biogenic species (isoprene: a-pinene) 2002 MCM v 3 - 125 VOC; 12700 reactions; 4400 species 2004 MCM v 3. 1 - 135 VOC; 13500 reactions; 5900 species 104 non-aromatic anthropogenic species 18 aromatics (first rigorous representation) 3 biogenic species (isoprene: a-pinene: b-pinene) 114 non-aromatic anthropogenic species 18 aromatics (updated representation) 4 biogenic species (isoprene: a-pinene: b-pinene: MBO-232)

The Master Chemical Mechanism (1993 -2007) l l l l l Degradation of CH 4 and 134 non-methane VOC ca. 5, 900 chemical species ca. 13, 500 chemical reactions 22 alkanes (C 1 -C 12) 16 alkenes (C 2 -C 6) 2 dienes (C 4 -C 5) 2 monoterpenes (C 10) 1 alkyne (C 2) 18 aromatics (C 6 -C 11) 6 aldehydes (C 1 -C 5) l l l l 10 ketones (C 3 -C 6) 17 alcohols (C 1 -C 6) 10 ethers (C 2 -C 7) 8 esters (C 2 -C 6) 3 carboxylic acids (C 1 -C 3) 3 other oxygenates (C 3 -C 5) 17 halocarbons (C 1 -C 3) Species cover ca. 70% of the mass emissions in the UK National Atmospheric Emissions Inventory (anthropogenic) Includes isoprene, a-pinene, b-pinene and MBO-232 (biogenic)

MCM construction methodology

Flow diagram of main features of MCM protocols

Structure of the protocols l l l Initiation reactions OH, NO 3, photolysis Reactions of organic radicals reaction with O 2 Reactions of RO 2 intermediates reaction with NO, NO 2, NO 3, HO 2 and R’O 2 l Reactions of RO intermediates reaction with O 2, decomposition and isomerisation l Reactions of Criegee intermediates excited and stabilised l Removal of Cl atoms l Reactions of degradation products Approximate hierarchy of information sources 1) Experimental data (evaluated) 2) Experimental data (direct) 3) SARs (published) 4) SARs/analogy assumptions (defined in protocol) 5) Theoretical studies of specific structures Simplifications l Initiation criteria l Channel probability l Product degradation l RO 2 + RO 2/R’O 2

Free radical propagated reaction cycle O 3 hu O 2 NO 2 VOC O 2 NO OH HO 2 RO O 2 O 3 NO NO 2 hu carbonyl product O 2 reaction, decomposition or isomerisation NO 2 + hu → NO + O 2 (+M) → O 3 (+M)

Radical termination HNO 3 H 2 O 2 NO 2 VOC O 2 ROOH HO 2 RO HO 2 RO 2 NO NO NO 2 ROH + R-HO RO 2 NO 2 carbonyl product O 2 reaction, decomposition or isomerisation

Radical generation (or regeneration) through photolysis O 3 hu ROOH H 2 O 2 hu H 2 O NO 2 hu VOC O 2 hu carbonyls hu NO O 2 OH HO 2 RO O 2 NO NO 2 carbonyl product O 2 reaction, decomposition or isomerisation

OH-initiated degradation of methane (CH 4)

OH-initiated degradation of ethane (C 2 H 6)

OH-initiated degradation of 1, 3 -butadiene

Defining kinetic and mechanistic parameters NO 2 VOC or product O 2 NO OH HO 2 RO NO NO 2 carbonyl product(s) rxn with O 2, decomposition or isomerisation • OH + VOC/organic product • RO 2 + NO, NO 2, NO 3, HO 2, R’O 2 • RO O 2 reaction, decomp. , isom.

OH radical reactions Kinetics of OH + VOC/organic products l l Rate coefficients have been measured for several hundred organics Rate coefficients for ca. 4, 000 species need to be estimated (e. g. SAR method of Atkinson, 1994; Kwok and Atkinson, 1995) Product radical distribution of OH + VOC/organic product l Mainly inferred from SAR partial rate coefficients l Scheme simplification measures applied in some cases - minor channels (<5%) ignored - single representative channel for ≥ C 7 alkanes - so called ‘minor’ products (e. g. RONO 2; ROOH) degraded to regenerate existing species

RO 2 radical reactions Kinetics of RO 2 reactions l l l Reactions with NO, NO 2, NO 3, HO 2 and other peroxy radicals (R’O 2) are included in MCM There about 1000 RO 2 radicals in MCM Kinetic data are available for only ca. 20 RO 2 – parameters assigned to majority of reactions by analogy and structure reactivity correlations Product branching ratios l Multiple channels for reactions with NO, HO 2 and R’O 2 l Scheme simplification measures applied in some cases - RO 2 from ‘minor’ products react via single channel - RO 2 + R’O 2 reaction are necessarily parameterised (explicit chemistry for 1000 radicals would require 0. 5 million reactions!)

RO radical reactions reaction with O 2 decomposition isomerisation l l There about 1000 RO radicals in MCM Relative importance of these modes of reaction largely defined by SAR methods of Carter and Atkinson (1989) and Atkinson (1997)

Simplification measure oxygenated RO radicals – exclusive decomposition assumed

VOC/product initiation reactions l Reaction with OH – all VOC and oxygenated products l Reaction with O 3 – alkenes/dienes and unsaturated products l Reaction with NO 3 – alkenes/dienes, aldehydes and cresols l Photolysis – carbonyls, RONO 2, ROOH

Organic photolysis processes 26 photolysis processes defined 14 parameters also used to define photolysis rates for several thousand other species

• Laboratory studies Chamber data Website/ database • Theoretical and semiempirical methods e. g. rate coefficients, branching ratios, absorption spectra, quantum yields Detailed gas phase mechanism (i. e. MCM) Evaluation Parameterisation Fundamental parameters Scientific and policy modelling Reduced gas phase mechanisms Chemical mechansims Mechanism application