Tropospheric ozone; kinetic and product studies

132 Pages·2012·3.96 MB·English

Tropospheric ozone and photochemical processing of hydrocarbons;
laboratory based kinetic and product studies.

Thesis submitted to the University of Manchester for the degree of Doctor
of Philosophy in the Faculty of Engineering and Physical Sciences

2012

Kimberley Edith Leather

School of Earth, Atmospheric and Environmental Sciences
2
Contents
Abstract ................................................................................................... 4
Declaration ................................................................................................................. 5
Copyright statement .................................................................................................. 6
Alternative format ...................................................................................................... 7
1 Introduction ...................................................................................... 9
1.1 Tropospheric ozone ......................................................................................... 9
1.2 Hydrocarbons in the atmosphere ................................................................. 11
1.2.1 Sources ................................................................................................. 11
1.2.2 Sinks ..................................................................................................... 12
1.3 Alkene ozonolysis; the importance of kinetics .............................................. 13
1.3.1 Structure Activity Relationships ........................................................... 15
1.4 Alkene ozonolysis reaction mechanism ........................................................ 16
1.5 The reaction of halogen oxides with peroxy radicals leading to ozone
depletion .................................................................................................. 20
1.5.1 Peroxy radical sources.......................................................................... 20
1.5.2 Halogen oxide sources ......................................................................... 20
1.5.3 Sinks via the XO + RO reaction leading to ozone depletion ............... 21
2
1.5.4 Existing XO + RO kinetics .................................................................... 23
2
1.6 Aims and objectives ....................................................................................... 24
2 Paper A ........................................................................................... 27
3 Paper B ........................................................................................... 29
4 Paper C ........................................................................................... 31
5 Paper D ........................................................................................... 33
6 Paper E ........................................................................................... 35
7 Conclusions and Future work .......................................................... 37
8 Acknowledgements ........................................................................ 42
9 References ...................................................................................... 43
Word count: 47,965

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Abstract
Laboratory based temperature-dependent kinetics and product yields for alkene
ozonolysis and the reaction of CH O with ClO and BrO have been measured via
3 2
chamber studies and a turbulent flow tube coupled to CIMS (Chemical Ionisation Mass
Spectrometry). In order to gain a better understanding of the fate of the products
formed during hydrocarbon oxidation and their subsequent impact on the ozone
budget (and so the oxidising capacity of the atmosphere) it is imperative to know the
rate at which these reactions proceed and to identify their product yields.

As tropospheric temperature varies, Arrhenius parameters were determined during the
ozonolysis of selected alkenes. The temperature dependent kinetic database was
extended and the activation energies for the ozonolysis of selected alkenes were
correlated with an existing SAR (Structure Activity Relationship). Given the myriad
organic species in the atmosphere, SARs are useful tools for the prediction of rate
coefficients. Inclusion of Arrhenius parameters into the SAR allows for prediction over a
range of temperatures, improving the conditions reflected in models. Achieving mass
balance for alkene ozonolysis has proven to be a difficult challenge considering the
numerous pathways of the Criegee Intermediate (CI). The product yield of formic acid –
an organic acid with significant atmospheric implications which is under predicted by
models – was determined as a function of relative humidity during ethene ozonolysis.
This reaction exhibited a strong water dependence which lead to the prediction of the
reaction rate of the CI with water which ranges between 1 × 10-12 – 1 × 10-15 cm3
molecule-1 s-1 and will therefore dominate its loss with respect to bimolecular processes
in the atmosphere.

Peroxy radicals, strongly influence the total oxidising capacity of the troposphere. The
reaction of peroxy radicals with halogen oxides is recognised to be responsible for
considerable ozone depletion in the atmosphere, exacerbated by reactive halogens (X,
XO) taking part in catalytic cycles. Arrhenius parameters were determined for ClO +
CH O and BrO + CH O . Temperature is an important parameter affecting rate,
3 2 3 2
exemplified here as the reaction involving ClO exhibited a positive temperature
dependence whereas for BrO a negative temperature dependence was evident. As a
consequence, the impact of ClO + CH O with respect to ozone loss is diminished.
3 2
Global modelling predicts a reduction in ozone loss by a factor of around 1.5 and
implicates regions such as clean marine environments rather than the polar
stratosphere. Conversely, a more pronounced temperature dependence for the
reaction of BrO with CH O placed particular importance on lower stratospheric
3 2
chemistry where the modelled CH O oxidation is doubled. The main products for this
3 2
reaction were identified to be HOBr and CH O . The decomposition of CH O could
2 2 2 2
enhance HO in the lower and middle stratosphere and contribute to a significant
x
source of HO in the upper troposphere. Bimolecular reaction of CH O with water
x 2 2
could also provide a none negligible source HC(O)OH in the upper troposphere.

Alkenes and peroxy radicals undergo chemical processing in the atmosphere whilst
acting as a source and sink of ozone and thus can impose detrimental effects on the
biosphere, climate and air quality of the Earth.
4
Declaration

No portion of the work referred to in the thesis has been submitted in support of an
application for another degree or qualification of this or any other university or other
institute of learning.
5
Copyright statement

The author of this thesis (including any appendices and/or schedules to this thesis)
owns certain copyright or related rights in it (the “Copyright”) and she has given The
University of Manchester certain rights to use such Copyright, including for
administrative purposes.

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may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as
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Intellectual Property and/or Reproductions.

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policy on presentation of Theses.
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Alternative format

Tropospheric ozone and photochemical processing of

hydrocarbons; laboratory based kinetic and product studies.

Chamber studies Flow tube studies

Research paper A: Temperature-
dependent ozonolysis kinetics of
selected alkenes in the gas
phase: an experimental and
Research paper D:
structure–activity relationship
Temperature and pressure
(SAR) study
dependence of the rate

coefficient for the reaction
between ClO and CH O in
3 2
How do
the gas-phase
heteroatomic
substituents affect
the rate of alkene
ozonolysis?

Is BrO analogous to ClO?
What are the product

yields of alkene
ozonolysis?

Research paper B:
Research paper E: A
Temperature-dependent
kinetic and product study
kinetics for the ozonolysis
of the reaction between
of selected chlorinated
BrO and CH O radicals as
3 2
alkenes in the gas phase
a function of temperature

using a Turbulent Flow
system coupled with
Chemical Ionisation Mass
Research paper C: Acid-yield
Spectrometry
measurements and product
studies of the gas-phase
ozonolysis of ethene as a
function of humidity using
Chemical Ionisation Mass
Spectrometry (CIMS)

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Introduction
1.1 Tropospheric ozone

The atmospheric implications of ozone relate human health, ecosystems and climate. It
was the infamous Los Angeles photochemical smog episode of 1945 that highlighted
the importance of tropospheric ozone (Haagen-Smit, 1951; Seinfeld 1998). Ozone is an
established pollutant, contributor to secondary organic aerosol (SOA) formation and a
greenhouse gas (Finlayson-Pitts, 2000; Science Policy Report, 2008).

Human health studies report acute and chronic effects owed to ozone exposure such as
reduced lung function, airway inflammation and increased asthma incidences (Science
Policy Report, 2008). An estimated 21400 deaths per year have been attributed to high
ozone levels (EEA Report, 2007). Ozone is a greenhouse gas (IPCC, 2007) though it can
also enhance global warming indirectly as reduced photosynthesis rates have been
correlated to elevated [O ], which may decrease the terrestrial carbon sink (Wittig,
3
2007).

In polluted regions, peaks of 200 ppb of ozone have been reported under warm and
sunny meteorological conditions (Science Policy Report, 2008). Ozone levels are
primarily dictated by chemical processes; however they are further affected by
stratosphere-troposphere exchange and dry deposition. The production of
tropospheric ozone includes both anthropogenic and biogenic sources formed through
reactions involving NO , NMHCs, CO and CH . Figure 1 illustrates tropospheric ozone
x 4
production pathways via hydrocarbon oxidation.

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O
3
HNO
3 HO hv
2
NO NO
2
OH 2
Hydrocarbons O
2 NO
HO HO
ROOH 2 RO HO 2 H O
2 2 2 2
NO
O
NO RO 2 RHO
2
O hv
2
O
3

Figure 1: Hydrocarbon oxidation; tropospheric ozone production.

Peroxy radicals are key intermediates in the oxidation of VOCs and are responsible for
the conversion of NO to NO . NO can then be photolysed, after which the odd oxygen
2 2
proceeds to form ozone (Figure 1). At intermediate NO levels (~70−1000 ppt NO )
x x
(Science Policy Report, 2008) associated with rural environments of industrialised
countries, a net ozone formation is observed, although the reaction between RO and
2
NO does not constitute the main sink of peroxy radicals (refer to Figure 1) and the cycle
can be termed ‘NO limited’. Here, peroxides (ROOH and H O ) are the main sink for
x 2 2
RO and HO though if NO were to increase, NO could compete for peroxy radicals to
2 2 x x
form NO and subsequently produce O . At high NO levels, indicative of urban areas,
2 3 x
reaction of NO with RO and HO become prevalent though if NO continues to increase
2 2 x
O formation will not rise indefinitely as the cycle now becomes ‘VOC limited’. OH
3
radicals are sequestered by NO in the presence of a third body, yielding HNO . If
2 3
however, elevated levels of VOCs compete for OH, high O episodes occur, though O is
3 3
also titrated by reaction of NO with O .
3

In ‘clean’ environments, such as the marine boundary layer, low NO levels exist (< 20
x
ppt). Under these conditions, a net O loss is apparent as the sink products ROOHand
3
H O become prominent at the expense of the reaction of RO + NO. Here, the net loss
2 2 2
is relatively small as ozone photolysis initiates the cycle. However, ozone loss can be
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