Development of abatement strategies and policies for air pollutants facilitated by the Master Chemical Mechanism
Submitting InstitutionUniversity of Leeds
Unit of AssessmentChemistry
Summary Impact TypePolitical
Research Subject Area(s)
Chemical Sciences: Theoretical and Computational Chemistry, Other Chemical Sciences
Summary of the impact
Air pollution is a major health concern and government policy driver.
Leeds researchers and colleagues have developed a detailed chemical
mechanism which describes reactions in the lower atmosphere leading to the
formation of ozone and secondary particulate matter, key air pollutants.
The so-called `master chemical mechanism' (MCM) is considered the `gold
standard' and has been used by the UK government and industry groups to
inform their position on EU legislation and by the US EPA to validate and
extend their regulatory models. The Hong Kong Environmental Protection
Department has used the MCM to identify key ozone precursors and provide
evidence for abatement strategies.
Ozone and particulate matter (PM) are important atmospheric pollutants.
Ozone is formed from the photo-oxidation, in the presence of nitrogen
oxides (NOx), of the large number of volatile organic compounds
(VOCs) that are emitted both naturally and from man-made sources.
Oxidation of VOCs also contributes substantially to PM formation. The master
chemical mechanism (MCM) describes these complex mechanisms
quantitatively. It is based on our current understanding of atmospheric
oxidation chemistry; is traceable to experimental measurements and
estimates of reaction rates and mechanisms; represents a synthesis of
current knowledge; and provides a web-based resource for atmospheric
chemistry modelling applications. Section 4 outlines the impact of the MCM
on the development of air quality policies in the UK and internationally.
2.1 MCM creation — synthesis of knowledge. The development of the
MCM was instigated by Dr R Derwent (Defra) in 1993 as the government
recognised that it required more understanding of how different organic
compounds in the atmosphere influence ozone formation. The project was
initiated following collaboration between Defra and Pilling on VOC
kinetics and ambient measurements of VOCs in cities. The MCM research was
co-led by Prof Mike Pilling (University of Leeds, 1989-) (mechanism
development and evaluation, website development) and Dr Mike Jenkin
(UKAEA/AEA Technology 1981-2001, then Imperial College 2001-2008, then
Atmospheric Chemistry Services, 2008-) (protocol and mechanism
development, and application).
The mechanisms are based on a protocol (e.g. (1) from 1997), developed on
the basis of a fundamental understanding of the detailed chemistry of
atmospheric oxidation, and incorporate reaction rate and mechanism data
from laboratory experiments. The project was funded by Defra (1993-2010),
with funding for specific applications and subsequent development being
secured through research grants (Grants: NERC(i), EU (ii)). The MCM is now
supported by the National Centre for Atmospheric Chemistry (Dr Andrew
Rickard), and may be accessed via a Leeds website (http://mcm.leeds.ac.uk/MCM/).
In addition to representation of the mechanism, together with rate
coefficients, the website provides output for incorporation into
2.2 MCM as a basis for policy modelling. Derwent, Jenkin and the
Leeds group also developed a predictive pollution model — the
photochemical trajectory model (PTM) — published in 1998 that combines
atmospheric transport and chemical reactions (2). The PTM models ozone
formation in an air parcel, driven by winds from central Europe to the UK,
and incorporates VOC and nitrogen oxide emissions from the surface. The
MCM is a key component of the model, allowing the different contributions
to ozone formation from specific VOCs to be quantified. A new VOC
characteristic — the photochemical ozone creation potential (POCP) -
provides a relative measure of the impact of different VOCs on ozone
2.3 MCM evaluation and application. The complexity of the MCM
demands direct experimental testing of its mechanistic predictions.
Pilling has led developments in the use of atmospheric simulation chambers
to evaluate the MCM in a series of European consortia (EXACT, 1999-2002;
EUROCHAMP-1, 2004-2009; EUROCHAMP-2, 2009-2013, Integrated Infrastructures
Initiative; grants ii) (3). Experimental field measurements, coupled with
the MCM, have also been used to develop and test our understanding of
atmospheric chemistry. The construction in Leeds and application in the
field of an instrument to measure OH and HO2 radicals by
Heard,(4) and modelling of their concentrations with the MCM (e.g. (5)
published in 2006) have been central to these developments.
2.4 MCM: a validation tool for simplified models. The MCM is a
very large mechanism that is too large and complex for use in global or
even regional atmospheric models that include detailed atmospheric
transport. The MCM is however widely used to validate models of
atmospheric pollution that typically incorporate simpler, less
fundamentally based chemical mechanisms. For example, in a collaboration
between Leeds and US scientists in 2005-7, the MCM was used to evaluate
the chemical mechanism in a 3D global model, to interpret satellite
measurements of formaldehyde and to evaluate emissions of anthropogenic
and biogenic VOCs in S E Asia (6).
Professor Mike Pilling, Professor of Physical Chemistry, 1989-2007 and
Research Professor 2013.
Professor Dwayne Heard, University Research Fellow, 1994-2002 then Reader,
Professor of Atmospheric Chemistry, 2004-.
Dr Sandra Saunders, postdoctoral researcher, 1991-2002.
Dr V Wagner, postdoctoral researcher, 1999 - 2001.
Dr Roberto Sommariva, graduate student, 2000-2004 and postdoctoral
Dr Claire Bloss, postdoctoral researcher, 2001-2003.
Dr Jenny Young (née Stanton), graduate student, 2002-2006 and postdoctoral
Competitively-awarded funding following peer review
i. NERC: (a) "Highly instrumented reactor for atmospheric chemistry,
HIRAC", NEC513493/1, PI: M. J. Pilling, £248k, 2004-2006; (b) "Integration
and co-development of the MCM and IUPAC databases and websites",
NE/E002668/1, PI: M. J. Pilling, £130k, 2006-2009; (c) "Laboratory
measurements of photochemical and kinetic processes of atmospheric
significance", RES20732, PI: M. J. Pilling, £118k, 2003-2006.
ii. EU: (a) EXACT, PI: M. J. Pilling, 1999-2002, €176k; (b) "Integration
of EUROpean Simulation CHAMbers for Investigating Atmospheric Processes"
(EUROCHAMP-1), PI: M. J. Pilling, 2004- 2009, €279k; (c) "Integration of
EUROpean Simulation CHAMbers for Investigating Atmospheric Processes"
(EUROCHAMP-2), PI: M. J. Pilling, 2009-2013, Integrated Infrastructures
Note: NERC and EU grants are awarded following extensive peer review on a
competitive basis, provided the proposed research meets stringent quality
References to the research
1. "The tropospheric degradation of volatile organic compounds: A
protocol for mechanism development", M. E. Jenkin, S. M. Saunders,
and M. J. Pilling, Atmospheric Environment, 1997, 31,
81 -104 (334 citations; Source: Scopus, 24/10/13) http://dx.doi.org/10.1016/S1352-2310(96)00105-7
2. "Photochemical ozone creation potentials for organic compounds in
northwest Europe calculated with a master chemical mechanism", R. G.
Derwent, M. E. Jenkin, S. M. Saunders and M. J. Pilling, Atmospheric
Environment 1998, 32, 2429-2441 (194 citations; Source: Scopus,
3. "Development of a detailed chemical mechanism (MCMv3.1) for the
atmospheric oxidation of aromatic hydrocarbons", C. Bloss, V. Wagner,
M. E. Jenkin, R. Volkamer, W. J. Bloss, J. D. Lee, D. E. Heard, K.
Wirtz, M. Martin-Reviejo, G. Rea, J. C. Wenger and M. J. Pilling,
Atmospheric Chemistry and Physics 2005, 5, 641-664. (143
citations; Source: Scopus, 24/10/13) http://dx.doi.org/10.5194/acp-5-641-2005
4. "Measurement of OH and HO2 in the troposphere", D. E. Heard
and M. J. Pilling, Chem. Rev., 2003, 103, 5163 -
5198 (145 citations; Source: Scopus, 24/10/13) http://dx.doi.org/10.1021/cr020522s
5. OH and HO2 chemistry during NAMBLEX: roles of oxygenates, halogen
oxides and heterogeneous uptake R. Sommariva, W. J. Bloss,
N. Brough, N. Carslaw, M. Flynn, A.-L. Haggerstone, D. E. Heard,
J. R. Hopkins, J. D. Lee, A. C. Lewis, G. McFiggans, P. S. Monks, S. A.
Penkett, M. J. Pilling, J. M. C. Plane, K. A. Read, A. Saiz-Lopez,
A. R. Rickard, and P. I. Williams, Atmospheric Chemistry and Physics,
2006, 6, 1135 - 1153 (42 citations; Source: Scopus, 24/10/13)) http://dx.doi.org/10.5194/acp-6-1135-2006
6. "Space-based formaldehyde measurements as constraints on volatile
organic compound emissions in east and south Asia and implications for o
zone", T.-M. Fu, D. J. Jacob, P. I. Palmer, K. Chance, Y. X. Wang, B.
Barletta, D. R. Blake, J. C. Stanton and M. J. Pilling, Journal
of Geophysical Research: Atmospheres 2007, 112, D06312. (74
citations; Source: Scopus, 24/10/13)) http://dx.doi.org/10.1029/2006JD007853
All Leeds researchers in bold. Citation data from web of
knowledge, accessed 03.09.2012. All papers are in internationally-leading
peer-reviewed journals and are hence ≥2*, but references 1-3 are
particularly highlighted by the UoA to demonstrate the quality of the
underpinning research. Reference 1 was awarded the Haagen-Smit Prize in
2010 as an "outstanding publication".
Details of the impact
According to a 2010 report of the House of Commons Environmental Audit
Committee "Poor air quality reduces the life expectancy of everyone in
the UK by an average of 7-8 months and up to 50,000 people a year die
prematurely because of it.' Defra estimates that poor health due to
air pollution costs the UK f07e£19 bn p.a. (http://www.defra.gov.uk/environment/quality/air/air-quality/eu/).
Health and Environment Alliance commented in 2013 "For the first time,
the Global Burden of Disease assessment has ranked an environmental
[health risk] factor [outdoor air pollution] among the more
widely discussed `life-style' risk factors, such as tobacco and alcohol"
(Lin et al, Lancet 2012, 380, 2224-60).
4.1 MCM informs policy. Action to manage and improve air quality
in the UK is largely driven by EU legislation. The 2008 ambient air
quality directive sets legally binding limits for concentrations
in outdoor air of major air pollutants that impact public health. The EU
also sets national emissions ceilings. VOC oxidation, to form ozone and
secondary particulate matter (PM), presents particular problems because of
its long range and therefore transnational nature. The Head of the Air
Quality Programme at Defra (Department for Environment, Food and Rural
Affairs) (2005-2010) (previously, Head of the Air Quality Science Unit,
Defra, 1993-2005) stated that "The MCM formed the core of the modelling
research to inform policy related to ozone in Defra. As part of the PTM
(Photochemical Trajectory Model), and as a benchmark against which other
mechanisms were evaluated, it played a crucial role in ensuring that the
UK's policy positions were founded on the basis of the best available
science, on EU Directives such as the National Emissions Ceilings
Directive and the Solvent Emissions Directive and the UNECE `Gothenburg
Protocol'. Over the period during which Defra supported the MCM it
became a world-wide benchmark for chemical mechanisms and we in Defra
were therefore able to feel great confidence in our international
negotiations having based our positions on such excellent science"
(A). A detailed review for Defra provides a discussion of the MCM and of
its application in policy related work (A review of the Master Chemical
Mechanism, prepared for Defra, July 2007). Developments between
2007-2009 can be found in a 2009 project report to Defra (B) that
outlines the use of the MCM to model the formation of secondary organic
particulate matter (termed "secondary organic aerosol" in the report).
4.2 MCM as the gold standard to benchmark atmospheric pollution
models. Because of the need to accommodate detailed atmospheric
transport, models of air pollution for policy purposes simplify the
chemistry and reduce the number of species involved to make the
atmospheric modelling more manageable. Because of its fundamental nature,
and the quality of its response to experimental evaluation, the MCM has
contributed substantially to policy applications through its use as a reference
mechanism that can be used to test the smaller, less fundamentally
based mechanisms that are used in atmospheric policy models. In their
report to Defra on modelling tools for policy applications (Review of
tools for modelling tropospheric ozone formation and assessing impacts
on human health & ecosystems, Report to Defra, November 2007),
Monks et al. stated: "the traceability of chemical schemes" (i.e.
mechanisms) "to an explicit basis is a more robust methodology than the
use of tuned generic schemes" (i.e. mechanisms not directly linked
to fundamental chemistry) "and the MCM should be used as a reference
benchmark for this process." The Head of the Air Quality Programme
at Defra (2005-2010) has stated that the impact of MCM has been sustained
since 2008: "The MCM has continued to provide a benchmark or standard
against which other, simpler, mechanisms are judged. Models using these
mechanisms are in continual use within Defra to inform and evaluate
policy, as demonstrated by the revision of the UNECE/CLRTAP 'Gothenburg'
Protocol in 2012 and the forthcoming negotiations on a revision of the
EU National Emissions Ceilings Directive later this year" (A). This
ongoing position was confirmed in 2013 by the current Head of the Air
Quality Science and Evidence Team at Defra (A).
A similar recommendation has also been made in the United States where
the Environmental Protection Agency now considers that the MCM "is the
`gold standard' of chemical mechanisms. Both of the chemical mechanisms
that US EPA recommends for regulatory actions since 2008 have drawn from
the mechanism sequences in MCM. If our mechanisms are consistent with
MCM, we are confident they will hold up to scrutiny" (C).
4.3 PTM/MCM informs policy
Section 2.2 outlined the importance of the MCM in a photochemical
trajectory model (PTM), which allowed the contribution of individual VOCs
to ozone formation in Europe and the UK to be quantified through
Photochemical Ozone Creation Potentials (POCPs). The PTM/MCM has been used
- by Defra to ensure that the UK's policy positions are founded on the
basis of the best available science. This included the evaluation of
policy strategies based on VOC reactivity, implications of multiday
ozone formation in Europe and the impacts of large VOC releases from
industrial plant (A).
- by the Hong Kong Environmental Protection Department to identify key
ozone precursors. "Policy debate on the environment has been
stimulated and informed by ... research evidence using the master
chemical mechanism (MCM) as the chemical mechanism within a
photochemical trajectory model (PTM). We are currently assessing the
potential of using the MCM/PTM model as a new tool in the development
of those policies" (D).
- by the European Solvents Industry group in 2009 (E) to highlight the
effectiveness of the Solvents Emissions Directive in reducing ozone
since 1990, while also establishing that further restrictions under the
2004/42/EC Directive (Paints) would not contribute to a significant
further ozone reduction in Europe.
- by Derwent et al. (F) to attribute improvements in peak episodic ozone
concentrations in the UK 1990-2010 to EU Air Quality Policies. They also
established that the upward trend in the annual mean daily maximum could
be attributed to intercontinental transport of pollutants, emphasising
the need for global policies to abate ozone, as discussed by the UNECE
Task force on Hemispheric Air Pollution (G).
Sources to corroborate the impact
A. Letter, Head of the Air Quality Programme at Defra (2005-2010)
(previously, Head of the Air Quality Science Unit, Defra, 1993-2005), 19th
April 2013, and confirmation from current Head of Air Quality
Science and Evidence Team at Defra, Sep 2013. Available on request from
B. "Modelling of Tropospheric Ozone: Project Summary Report", 2007-2009,
AEA, 2009. http://uk-air.defra.gov.uk/reports/cat05/1003151144_ED48749_Final_Report_tropospheric_ozone_AQ0704.pdf
C. Letter, Physical Scientist and CMAQ Atmospheric Chemistry Workgroup
Lead, US Environmental Protection Agency, April 26th 2013.
Available on request from HEI
D. Letter, Senior Environmental Protection Officer, Hong Kong
Environmental Protection Department. Available on request from HEI, 22nd
April 2013. Available on request from HEI.
E. "The Ozone Challenge", European Solvents VOC Coordination Group, ESIG,
Februrary 2009. http://www.esig.org/en/regulatory-information/ozone-modelling
F. "Ozone in Central England: the impact of 20 years of precursor
emission controls in Europe", R.G. Derwent, C. S. Witham, S. R. Utembe, M.
E. Jenkin and N. R. Passant, Environmental Science and Policy
2010, 13, 195-204. http://dx.doi.org/10.1016/j.envsci.2010.02.001
G. UNECE Task force on Hemispheric Air Pollution. Report. http://www.htap.org/index.htm