the roles of different emission sources in the summer 2003 european pollution episode with a...

7/29/2019 The roles of different emission sources in the summer 2003 European pollution episode with a particular focus on ozone: implications for the Clean Air for Eur

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SCHOOL OF GEOGRAPHY, EARTH AND ENVIRONMENTAL SCIENCES

MSc Air Pollution Management and Control

Causes and Effects of Air Pollution Essay

COVERSHEET

To be used for electronic submission through Turnitin

STUDENT ID

949072

Markers Initials _______________________

Mark _______________________________


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The roles of different emission sources in the summer 2003European pollution episode with a particular focus on ozone:

implications for the Clean Air for Europe strategy

IntroductionIn the summer of 2003, Europe experienced a long lasting and spatially extensive episode

of high ozone and particulate matter (PM) pollution, typical of a summer photochemical

episode (Lee et al., 2006). There was a strong heatwave in the first two weeks of August,

with the highest temperatures witnessed since 1500 (Luterbacher et al., 2004). 68% of

European Union (EU) monitoring stations reported an exceedance of the 180 g m-3 EU

information threshold for ozone and at all European monitoring sites the average number of

hourly exceedances of the 180 g m-3 information threshold limit for ozone was higher than

in the previous 12 years (figure 1) (Fiala et al., 2003). The population weighted mean PM10

concentration for the first two weeks of August 2003 was 29 g m-3 compared to 16 g m-3

for the corresponding period in 2002 (Stedman , 2004).

Impacts of the episode

These high reported concentrations were estimated to have significant effects on human

health, with 400-600 deaths in Netherlands and 423-769 deaths in the U.K. directly

attributable to high PM10 and ozone concentrations (Fischer et al., 2004, Stedman, 2004). At

concentrations of 120 g m-3 ozone can decrease lung function and above 180 g m-3 there is

a risk to human health of particularly sensitive groups of population from brief exposure

(Solberg et al., 2008). High ozone concentrations also have significant effects on vegetation

and crops also, with grassland productivity decreasing by 20% with ozone concentrations

only 1.5 times ambient levels (Volk et al., 2006).

This essay will focus on the roles of different emission sources on ozone

concentrations in particular, which can be difficult to establish as it is a secondary pollutant.


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Figure 1 Figure showing the number of exceedances of the one hour mean ozone

concentration of 180 g m-3, which is the EU threshold value for the information of the

public, at rural and urban background sites from April to August 2003 (from Fiala et al 2003)


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Main body

Ozone is a secondary pollutant in the troposphere, produced mainly by the photochemical

reaction of nitrogen oxides (NOx) with sunlight, aided by organic peroxy radicals producedby the oxidation of Volatile Organic Compounds (VOCs) (Sillman, 1999; Jacobson, 2002)

(figure 2). Figure 3 shows the basic chemical reaction mechanism for the production of

ozone. The key reaction is the photolysis of NO2 to create NO and the free oxygen atom (

, as the reaction of this free oxygen atom with oxygen in the air is the only known reaction

to create ozone (Jacobson, 2002). Another key reaction is that of peroxy radicals (RO2) with

NO to produce NO2. This is important as this NO2 then undergoes photolysis to produce the

free oxygen atom. The peroxy radicals are produced by the chemical breakdown of VOCs,explaining why VOCs are so important in ozone production.

Figure 2 A schematic of the chemistry of ozone production in the troposphere (RSR, 2008).

Figure 3 The basic chemical reaction mechanism for the production of ozone (Jacobson,

2002)


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VOCs

Sources

VOCs are key precursors to ozone production, with both anthropogenic and biogenicsources. The approximate emission source profiles for the U.K. in 2003 are given in table 1

(Hayman et al., 2006), showing that solvent usage is the most important source of VOCs into

the atmosphere with 29% of total emissions coming from that source. It also shows that

natural sources of VOCs contribute significantly to total VOC emissions (13%) which will be

discussed later. Road transport is also significant with 15% of total VOC emissions but this

has come down significantly compared to the 20 years prior due to the introduction of

catalytic converters for new vehicles (EEA, 2007).

Table 1 Table showing the 2003 UK Annual VOC Emission Estimate by emission source(adapted from Hayman et al., 2006)

Source 2003 UK Annual VOC Emission

Estimate (NAEI) (ktonne per annum)

% of Total

Emissions

Solvent Usage 390 29%

Road Transport 211 15%

Industrial Processes 184 14%

Fossil Fuel Extraction and

Distribution

277 20%

Domestic Combustion 36 3%

Major Point Sources 9 0%

Other 79 6%

Natural 178 13%

Total 1364 100%


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VOC Species Reactivities

VOCs constitute a wide range of chemicals, with the more reactive and abundant

VOCs being more important as ozone precursors, as they are more readily broken down to

form free radicals. Methane although relatively abundant is the least reactive of the VOCs, so

despite being important for global ozone concentrations it is unimportant for pollution

episodes (Jacobson, 2002). The lifetimes of some important VOCs are given in table 2 with

respect to reactions with different gases (Jacobson, 2002). Some of the more reactive VOCs

such as ethene and toluene are characterised by a carbon-carbon (C=C) double bond (figure

4). When this C=C bond is broken it produces radicals via a variety of chemical reactions that

react with NOx to form ozone.

Table 2 Lifetimes of some important VOCs with respect to reactions with different gases thatare known to break them down in the atmosphere (from Jacobson, 2002)


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Isoprene Ethene

Toluene M-Xylene

Figure 4 Chemical stick diagrams for the 4 of the most important VOCs for ozone

production; Isoprene, Ethene, Toluene and M-Xylene each possessing (a) carbon bond(s).

High Emissions in the 2003 episode

As discussed VOCs are highly important for ozone production however their role in

the 2003 episode may have been heightened further. A key reason for this is that evaporative

emissions of VOCs increase exponentially with increases in temperature around temperatures

typical of a European summer (Fiala et al., 2003; Lee et al., 2006). Lee et al (2006) measured

VOC concentrations at Essex, England in August and defined a heatwave period (5-11

August), where there was slow moving air over the United Kingdom and when the highest

temperatures were recorded. They found that the concentration of ethane was on average

1000 pptv higher and the concentration of ethyne was 200-400 pptv during this heatwave

period. They also found that the vast majority of 65 VOCs measured had elevated

concentrations during this period. The evidence of higher VOC emissions during theheatwave can be explained in part by the higher temperatures but also because of the


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increased photochemistry that occurred during this period. There was a high level of peroxy

radicals also recorded during the episode which is indicative of local photochemical activity

and their levels tended to correlate with peaks in ozone concentrations (Lee et al., 2006).

Total VOC reactivity to the hydroxyl (OH) radical approximately doubled during the episode

(Lee et al., 2006). This is important as the hydroxyl radical is important in breaking up VOCs

to produce the radicals that form ozone.

Lee et al (2006) also measured the VOC reactivity as a product of the average

concentration of each species with its rate coefficient with OH. Some of the most important

VOCs are shown in table 3. The table shows that formaldehyde and acetaldehyde, both with a

wide range of natural and anthropogenic sources were highly important to VOC reactivity, as

acetaldehyde contributed 25% in total to VOC reactivity. Isoprene, a biogenic VOC that is

discussed later was also shown to be important. Chemicals such as ethane and propane had

high concentrations relative to the above chemicals but low reactivities due to their low rate

coefficient with OH. The main source of toluene is from solvents and the main source of

ethene and propene is from road transport (Zheng et al., 2009). Table 4 shows that emissions

from road transport were particularly important as they were emitted in large quantities and

had high incremental reactivities, higher than chemical industry or Other solvent use, in

August 2003 (increased ozone production by incremental increase in emissions) (Hayman etal., 2006).

Table 3 Reactivities of different important VOCs for ozone production as measured duringthe heatwave period in Writtle, Essex, England. The chemicals in blue at the bottom of thetable had high concentrations but low reactivities due to their low rate coefficient with OH(from Lee et al., 2006)

Chemical Reactivity (s-1)

Ethene 0.26

Propene 0.22

Isoprene 1.33

Toluene 0.15

Acetaldehyde 1.67

Formaldehyde 1.15

Ethane 0.03

Propane < 0.01

Acetone 0.04


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Table 4 The percentage contribution to emissions, incremental reactivity and the contributionto ozone production for different VOC source categories along a 5 day trajectory in August2003, for the top 20 ozone-forming VOCs (from Hayman et al., 2006).

Natural Isoprene Emissions

One important source was the natural emission of the biogenic VOC isoprene from

vegetation (Lee et al., 2006). Isoprene may react with hydroxyl radicals in the atmosphere to

form peroxy radicals, which can react with NOx to produce ozone (Hewitt et al., 2011).

Isoprene has a close relationship with temperature with emissions increasingly

eponentially up to 30 C and reaching a maximum at 40 C (figure 5) (Lee et al., 2006).

Emissions also increase with increased solar radiation (Guenther et al., 1993). It is estimated

that the contribution of isoprene emissions to ozone concentrations for the episode was up to

40 g m-3 (20% of total) in some areas (figure 6) (Solberg et al., 2008). Isoprene emissions

were double the normal summer average in 2003 as measured in one forested site in France,

due to the high temperatures and increased solar radiation. Lee et al (2006) estimated that theisoprene contribution to ozone production went up from 14% to 20% during the heatwave


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period (5-11 August) and Hayman et al (2006) found the contribution of biogenic VOCs to

ozone production to be around 33% during the heatwave period. This is compared to a typical

contribution of 20% over the summer. It was also found that a hypothetical 1% increase in

biogenic emissions increased ozone concentrations more than a 1% increase in anthropogenic

emissions (Hayman et al., 2006), likely because of the short lifetime of isoprene and its

strong non-linear relationship with temperature. Because isoprene is highly reactive with the

OH radical (table 2) this means that it produces radicals readily when emitted and because it

is also emitted at high concentrations in warm and sunny conditions, overall it is a highly

important ozone precursor.

Figure 5Graph to showing the relationship between temperature and isoprene concentrations

as measured using a Gas Chromatography Flame Ionization Detector between 1 and 31

August 2003 (from Lee et al., 2006)


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Figure 6Image showing the EMEP unified model estimates of the contribution of isoprene to

maximum ozone concentrations on 8 August 2003 (from Solberg et al., 2008)

Forest fires

Forest fires produce a host of air pollutants such as PM and ammonia (NH3), as well

as the primary pollutants used for ozone formation including carbon monoxide (CO), CH4,

NOx and VOCs (Wiedinmyer et al., 2006; Wu et al., 2006). The 2003 fire season in Portugal

was the worst for 23 years with 5.6% (355,976 ha) of forest area burned up to 20 August

(figure 7) (EC, 2003), and these fires contributed to 40-55% of primary organic carbon

content in the Aveiro region of Portugal (Pio et al., 2008). A backward Lagrangian dispersion

model was used to show that these fires could have been responsible for the peaks in ozone

concentrations witnessed around 7 August, as the polluted air advected over to northern

Europe from Portugal around this time (Solberg et al., 2008). The presence of such a high

concentration of aerosols may have inhibited photolysis rates by 10-30% however, lessening

the impact of forest fires on ozone concentrations slightly (Hodzic et al., 2007).


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Figure 7An image taken from the Terra satellite (4 August 2003) which shows the extensive

fires over Portugal. Image courtesy of MODIS Rapid Response Project at NASA/GSFC.

(from Solberg et al., 2008)

Reduced dry deposition

Although not an actual emission source the effect of dry deposition was considerable

on ozone concentrations. The dry conditions of summer 2003 meant plants closed their

stomata so less ozone would be removed by plants (RSR, 2008). This dry deposition is the

dominant removal mechanism of ozone (Solberg et al., 2008). Ozone has a long lifetime in

the atmosphere and without being removed it could transported over thousands of kilometres

over most of Europe causing the observed wide scale high pollution witnessed over Europe.

NOx/VOC relationship

It is also important to understand the relationship between NOx and VOC

concentrations as this has significant effects on ozone concentrations (Jacobson, 2002). The

isopleth shown in figure 8 helps explain this relationship. It is shown that at high NOx orVOC concentrations, increasing the concentration of the other pollutant increases ozone


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concentrations. However at low NOX or VOC concentrations, increasing the concentration of

the other pollutant has little effect on ozone concentrations. If the VOC:NOx ratio is lower

than 8:1 then this means that reducing VOC concentrations is more effective in controlling

ozone concentrations (Jacobson, 2002). This isopleth is important for regulators and data

from Hayman et al (2006) found that the 2003 pollution episode was dominated by VOC-

limited conditions and that a 10% decrease in NOx concentrations may have increased ozone

concentrations. However NOx emissions, dominated by emissions from road transport and

industrial combustion processes, are particularly important in NOx-limited regimes (i.e.

forests) where there are high natural VOC concentrations (COM, 2004; Jonson et al., 2006).

Figure 9 finally summarises the main emission sources and pollutants for the episode.

Figure 8 An isopleth showing peak ozone mixing ratios as a result of different mixing ratios

of NOx and VOCs (ROG). The black line shows a line where the VOC:NOx ratio is 8:1.

(from Jacobson, 2002).


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Clean Air for Europe (CAFE) strategy

The Clean Air for Europe (CAFE) strategy is part of the CAFE programme to establish a

long-term, integrated strategy to tackle air pollution and to protect against its effects on

human health and the environment (European Union, 2006). The 2003 pollution episode wasa long lasting and spatially extensive episode of high ozone and particulate matter (PM)

pollution that affected most of Europe and led to thousands of air pollution related deaths and

significant effects on crops and vegetation (Fischer et al., 2004; Stedman, 2004; Keller et al.,

2007). As part of the CAFE programme then it is inherent that a strategy is developed that

tackles the causes of such episodes to protect human health and the environment.

The CAFE programme is limited to developing a strategy to tackle anthropogenic sources

of the key ozone precursors however it is evident that natural factors played an important role

in the observed ozone concentrations. The warm temperatures during the episode, for

example, affected the emissions of the biogenic precursors and was a cause of the forest fires

(EC, 2003). The Commission recognised this and wrote that if natural emissions are

monitored and recognised as a natural source then they do not count towards exceedance of

threshold values (COM, 2005).It is also predicted that such warm temperatures will become

more common in Europe with predicted climate change, therefore significantly increasing the

frequency of pollution episodes in the future (Schaer et al., 2004). Developing climatelegislation and policy is beyond the scope of the CAFE programme however and it is limited

Figure 9 Schematic diagram showing the key emission sources and the pollutants

emitted from these sources for the summer 2003 pollution episode

Forest Fires

Vegetation emissions

Road Transport

Solvent use

Emission source Emitted pollutant

NOx

VOCs

CO


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to offering recommendations and warnings about the effects of future climate change on the

frequency of high pollution episodes.

Addressing VOC emissions

Anthropogenic emissions of VOCs are an area that can be addressed to reduce the

severity of the high ozone concentrations experienced. VOC emissions have fallen steadily

by 32% in western and central Europe from 1991-2002 (Vestreng et al., 2004) (figure 10),

mainly due to the introduction of catalytic converters for new vehicles (EEA, 2007).

Although this progress is positive it was evidently not enough to stop episodic high ozone

concentrations as experienced in 2003. One area that has been addressed as a result of the

2003 pollution episode is the emissions from solvent use, which produce important VOCs for

ozone production such as toluene (Zheng et al., 2009). Directive 2004/42/EC was introduced

in April 2004 limiting emissions of VOCs from organic solvents, amending the previous

directive 1999/13/EC (COM, 2005). This has been successful in reducing emissions of VOCs

from solvents with an estimated 30% emission reduction in the last 8 years (Dick Derwent,

personal communication).


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Figure 10EEA.32 (32 member countries in the European Environment Agency) emissions of

total ozone precursors, and of precursors subject to targets (NMVOC = Non Methane

Volatile Organic Compounds) from 19902004 Emissions of ozone precursor gases were

reduced by 36 % across the EEA.32 between 1990 and 2004 (from EEA, 2007).

Road Transport

Hayman et al (2006) have shown that the largest contribution to episodic peak ozone

concentrations between 2000 and 2010 will be from cars; particularly without catalytic

converters (table 5). In order to tackle this, the commission proposed, cutting emissions from

heavy duty vehicles and a retrofitting and scrapping scheme for older vehicles without

catalytic converters as they produce disproportionate amounts of pollution (COM, 2005).

Hayman et al (2006) also modelled the emission source categories that would produce the

greatest contribution to episodic peak ozone concentrations in 2020 based on DEFRA

Emission projections (table 6). It is estimated that there will be no cars without catalysts

contributing to ozone formation and that road transport will no longer be the main

anthropogenic source contributing to episodic ozone concentrations.


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Table 5A model prediction of VOC emission source contributions to episodic peak ozoneconcentrations between 2000 and 2010 based on DEFRA VOC emission projections (fromHayman et al., 2006)

Table 6A model prediction of VOC emission source contributions to episodic peak ozoneconcentrations in 2020 based on DEFRA VOC emission projections (from Hayman et al.,2006)

Transboundary Impacts

The 2003 episode was further proof that ozone is a regional-scale pollutant and it was

recognised that measures to control emissions had to be taken at the Community level (COM,

2004). It has been shown that locally limited emissions reductions have little effect on

reducing ozone concentrations (Smeets and Beck, 2001).


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Conclusions

Overall the 2003 pollution episode was particularly important as lots of important

discoveries were made as to how ozone concentrations behave under these heatwave

conditions. It is predicted that heatwaves such as the 2003 episode will become more frequent

later in the 21st Century because of warming due to climate change (Schaer et al., 2004) so

understanding the reasons for high ozone concentrations in these episodes and the appropriate

sources is important. It is predicted that the effects of climate change will mean that

exceedances of EU limits will occur, disregarding unrealistically large emissions reductions,

however the severity of these exceedances can be managed reasonably well with reductions

in VOC emissions as proposed and as has been witnessed with the reductions in peak ozone

concentrations annually over recent years, for example by the introduction of catalytic

converters to cars (EEA, 2007).

Word Count: 2499

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