UNECE – Convention on Long Range Transboundary Air Pollution
Cooperative Programme for Monitoring and Evaluation of the Long-range Transmission
of Air Pollutants in Europe (EMEP)
Paper for the
Task Force on Integrated Assessment Modelling
(Twenty-sixth session, Brussels, 14-16 May 2001)
Integrating the environmental effects of Particulate
Matter in emission control strategies
Jean-Marc BRIGNON
Guy LANDRIEU
INERIS
Institut National de l’Environnement Industriel et des Risques – France
May 2001
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 1/14
The Protocol to Abate Acidification, Eutrophication, and Ground-level Ozone was adopted in
Gothenburg in December 1999. It has been recognised that the Protocol will need future
revision, and that future strategies need to integrate the health effects of particulate matter.
The objective of this paper is to give some views of the effects of particulate matter, especially
the environmental effects.
First, the paper briefly reviews recent scientific literature showing that particulate matter is
involved in important effects such as climate change, visibility impairment, and
photochemical smog. Then we explain why we regard as important to integrate all these
environmental effects of particulate matter in the assessment.
1. The effects of particulate matter on climate change
Particulate matter can have an effect on global warming in many different ways, depending
among others on the chemical composition of the aerosol. These effects are said to be direct
when they are related to the scattering and/or absorbing properties of UV and infrared
radiation by the aerosol. They are said to be indirect when they involve the action of aerosols
on other actors of the radiative balance of earth, such as clouds.
Direct effects of particulate matter
Aerosols can both absorb and backscatter a fraction of incoming solar radiation, emit infrared
radiation and absorb infrared radiation emitted by the Earth [Jacobson, M.Z. 1998]. Their
actual behaviour depends on their chemical composition, their state of mixing with other
aerosols, ambient humidity,…
The dependence on the chemical composition is very complex and not yet fully understood.
However, it could be very schematically summarised as follows :
Sulphate aerosols mainly scatter incoming solar radiation back to space [Dickerson,
1997], while they are only weakly absorbing infrared radiation emitted by the earth.
These two properties result in a cooling effect.
Nitrates aerosols may have a similar behaviour, but few global studies are available.
Organic carbon particulate matter, because of incoming UV backscattering properties,
has a cooling effect [Dickerson, 1997 ; Jacobson M.C. 2000].
Elemental carbon (or soot) has a warming effect, due to its solar radiation absorption
properties [Jacobson, M.Z., 2001].
The effect of mineral dusts is uncertain, and they are known to both scatter incident
UV and absorb infrared radiation. The overall global effect might be week [Miller and
Tegen, 1998].
Indirect effects of particulate matter
The indirect effects of aerosols are often thought to be the most important source of
uncertainty in climate change predictions. Uncertainty comes mainly from gaps in knowledge
in emissions ; spatial and temporal distribution and chemical composition of the global
aerosol and clouds [Baltenspenger, 1998] ; the influence of relative humidity (among others).
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 2/14
The main actions of aerosols on clouds include the increase of cloud albedo1 (trough droplets
or ice crystals modifications and the extension of cloud cover), and the extension of cloud
lifetime2 (along with rain inhibition). Until now, most studies of the indirect effects have
focused on sulphates. Organic carbon has been less studied, and it is thought that indirect
positive or negative effects can be encountered, depending on the organic chemicals and local
conditions involved [Facchini, 2001].
These effects are thought to result in a net cooling, and this view is reflected by the IPCC
synthesis on radiative forcings such as shown on Figure 1. It can be seen from this Figure that
the aerosol effects on climate are uncertain and probably very important.
Aviation-induced effects on high altitude cirrus clouds are also related to aerosols, especially
black carbon.
Figure 1 (from [IPCC, 2001].
The picture of the impact of black carbon aerosols on climate might be changing
Quite recently, uncertainties and new highlights on the mechanisms of both direct and indirect
effects of soot aerosols have been uncovered, that underlined more than ever the potentially
important impact of these aerosols in global warming.
Concerning the indirect effects, Ackerman [Ackerman, 2000], using observations from the
INDOEX experiment, concludes that soot aerosols can reduce the cloud cover (because the
absorption of solar radiation by soot causes the heating and evaporation of cloud droplets).
This effect then reduces the reflection of incoming solar UV. This warming, often referred to
1
Also referred to as the « first indirect effect ». The albedo is an index of the UV scattering efficiency (ratio of
scattered flux to incident flux).
2
Also referred to as the « second indirect effect »
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 3/14
as a “semi-direct” effect, even if it could not compensate the direct cooling effects of sulphate
aerosols [Lohmann and Feichter, 2001] could however reduce them substantially.
Investigations of aerosol/cloud interactions mainly focused until now on lower-altitude and
warmer clouds. The effects of aerosols (especially black carbon emitted by aircrafts) on highaltitude cirrus clouds are poorly understood. Some authors think however that the indirect
effect of aerosols on cirrus3 results in a net warming which could be substantial [Noone,
2001].
Concerning the direct effects, Jacobson [Jacobson M.Z., 2001], following himself other
studies reviewed in [Haywood and Boucher, 2000], states that, contrary to what was
previously admitted, black carbon aerosol is predominantly present in the form of an internal
mixture4 with sulphate aerosol particles. This changes the radiative properties of black carbon
aerosols. The calculations of Jacobson lead to the conclusion that black carbon could be the
second most important component of global warming after CO2 et before CH4 in terms of
direct forcing. Black carbon aerosols alone could nearly balance the net cooling effect of other
antrophogenic aerosols constituents5.
The state of mixing between black carbon and organic carbon has analogous implications, as
discussed in [Liousse et al., 1998].
These facts imply that black carbon aerosol control might be of great importance in terms of
climate change. Penner [Penner, 1998], using a global coupled circulation/chemistry model,
finds that in the northern hemisphere, forcing by fossil fuel emissions of black carbon and
associated organic carbon is mainly associated with areas of high domestic coal use and diesel
fuel use. According to this study, the warming effect of carbonaceous aerosols seems
particularly relevant for Europe6, compared to other regions of the globe as suggested by
Figure 2, showing an example of the spatial repartition of the direct positive forcing of
carbonaceous aerosols from fossil fuel burning.
3
Contrary to what occurs for lower altitude-clouds, the global extension of the optically thin cirrus cloud cover
would result in a warming effect (because optically thin cirrus clouds act as greenhouse gases).
4
In an external mixture, each particle only contains one substance, whereas in an internal mixture, particles can
be made up of several substances (and in this case, particles of the same size are generally assumed to have the
same composition).
5
The internal mixture in supported by observations over the Pacific and the North-Atalntic Oceans reported in
[Jacobson, M.Z., 2001] and by results of the TARFOX experiment in the USA, as reported in the review by
[Haywood and Boucher, 2000].
6
Over Europe, this positive forcing could nearly offset the direct effect of sulphates, as appears in [Haywood and
Boucher, 2000]
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 4/14
Figure 2. An example of the spatial repartition of the direct positive forcing of carbonaceous aerosols from
fossil fuel burning, from [Haywood and Boucher, 2000] themselves quoting [Penner et al.1998] and [Grant et
al. 1999].The darker parts of the picture refer to areas of more intense positive forcings.
Such views on the importance of black carbon are indirectly supported by Hansen [Hansen,
2000], who calls for an alternative scenario for global warming. For the author, strategies for
global warming must also target non-CO2 pollutants. Among these non-CO2 air pollutants, the
author states that black carbon aerosol is contributing significantly to global warming. He
estimates that including black carbon in the strategy would also be beneficial to air pollution,
especially in terms of aerosols and tropospheric ozone concentrations. Conversely, regional
reductions in black carbon aerosol would have beneficial consequences regarding global
warming.
2. The effects of particulate matter on hydrological cycles.
As mentioned above, particulate matter reduces cloud droplets diameter, causing less droplets
to reach the critical diameter for the formation of rain. This “local” effect of rain suppression
by urban and industrial air pollution, especially ash particles, is reported among other sources
in [Rosenfeld, 2000]. Particulate Matter has the ability to modify the local hydrological
regimes.
Possible regional effects of dust aerosols on local climate and precipitation have been
modeled by [Miller and Tegen, 2000]. In contrast to South-Asia and Mid and South Africa,
Europe appears to be relatively unaffected in this study.
At a global scale, Satheesh and Ramanathan [Satheesh and Ramanathan, 2000] argue that the
heating of the atmosphere above the Indian Ocean by soot aerosol may decrease the intensity
of the hydrological cycle over the ocean and have regional climate effects. These effects on
hydrology might in turn cause impacts on air quality, but few studies seem to be available on
this topic.
3. The effects of particulate matter on visibility.
Reduced visibility is generally experienced trough the occurrence of regional haze. Haze
reduces the distances how far we can see, and degrades the colour, clarity and contrast of the
scenes. This visibility impairment occurs as a result of the scattering and absorption of light
by air pollution, including particles and gases.
Several parameters are used to characterise visibility. The more intuitive parameter is the
visual range V (in km), that is the maximum distance at which an average individual could
distinguish an ideal black object silhouetted against the horizon sky. V is easily related to the
light extinction coefficient of the atmosphere (b, in km-1), which is a measurable parameter,
by the relation V = 3.912 / b. The haziness (expressed in “deciview”), is the analogous of
decibel in sound intensity, and is defined as 10 x ln(b/0.01)7.
Both primary and secondary particulate matter are highly responsible for visibility reduction.
7
This implies that haziness is near zero for a pristine atmosphere, and increases as visibility is degraded.
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 5/14
In the USA, the sulphate aerosol is generally the main contributor to haze. Organic carbon
aerosol also sometimes stands at the first row. Nitrate aerosol, elemental carbon, and crustal
material are also involved [Jacobson M.C., 2000],[USEPA, 2001]. Humidity enhances
visibility reduction by sulphate, because sulphate aerosol accumulates water and grows in
size, becoming this way more efficient at scattering light.
It is estimated that in many parts of the USA, current visibility is about 70% reduced
compared to visibility in natural condition. The visual range, which is currently typically
about 20 km, was about 100 km before the industrial revolution.
Visibility, as affecting national parks and wilderness areas is thought to have an important
social and economic impact. In a report to the US Congress [Croote, 1999] an evaluation is
given of the benefits resulting from the Clean Air Act. The results show that the benefit
estimated for the improvement of visibility (57 billions of 1990 $) is of the same order of
magnitude (twice higher) than the benefits from avoided crop damages by near-ground ozone
reduction.
A specific program to tackle visibility problems has been launched in the USA, called the
“Regional Haze Program”. Its objective is to restore normal unaffected visibility in the natural
parks within 60 years. Important ancillary benefits of the program are also expected in terms
of a better visibility in regions with higher population density.
In Europe, it seems that very few is known about visibility impairment and its social costs.
This apparent lack of concern is surprising, considering the global spatial distribution of haze
on the globe given in [Husar, 2000] and reproduced on Figure 3. These data allow for some
comparison between the US case and Europe. On the basis of daily-averaged visibility data
during the period 1994-1998, visibility reduction appears to be significantly higher in Europe
than in the United States for all of the four periods of the year studied by the authors.
A study by [Holland and King, 1999] however, using US data from “Willingness To Pay”
studies, evaluated the annual benefits through improved visibility, in the framework of various
scenarios for a cost-effectiveness study on the project of a National Emissions Ceilings in the
European Union. The benefits were of the same order of magnitude as benefits for agriculture
(roughly 2 to 3 times lower) and substantially higher than benefits for improved forest
productivity and effects on materials. The report concluded that the reliability of benefits
transfer from US studies is questionable, given the lack of concern and studies over this topic
in Europe.
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 6/14
Fig.3 : Global extinction coefficient maps, from [Husar, 2000]
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 7/14
4. The effects of particulate matter on air quality
In addition to their direct health effects on populations, aerosols modify air quality by acting
on other air pollutants.
There are various ways by which particulate matter interacts with photochemical atmospheric
activity, among which .
1)
Aerosols impact on the energy of radiative fluxes, and this way affect the photolytic
reactions in the atmosphere. UV absorbing aerosols (soot, mineral dust) decrease
photolysis rates. Scattering aerosols, like sulphates, have an opposite effect.
2)
There are numerous and complex interactions between secondary PM and ozone
chemistry, as illustrated by Figure 3.
Of particular importance is that secondary organic aerosol is formed from precursor
VOC gases, in the presence of oxidising agents like ozone and NOx [Dusek, 2000]
Among these interactions, heterogeneous chemistry occurring between gases and
particulate matter has also an influence on tropospheric ozone :
 Aerosols can store and later release gaseous species that arise in tropospheric
ozone formation.
 They can impact on chemical reactions in the atmosphere because catalysis or
inhibition of chemical reactions can take place at their surface. For instance, their
is evidence of significant influence of aerosols on global budgets of NOx and
ozone
A very complete review of the current knowledge and associated gaps about
heterogeneous chemistry is available in [Jacob, 2000].
3)
Aerosols impact ozone formation via ambient temperature modifications that affect the
kinetics of some reaction pathways.
In order to assess the overall impact of the ambient aerosol on tropospheric ozone formation,
several authors have used numerical models in which photochemical reactions are coupled
with a representation of aerosols. [Dickerson, 1997] and [He and Carmichael, 1999], working
on aerosol-rich conditions, respectively in the United States and in China, both find that the
rural aerosol (generally rich in scattering particulate matter like sulphate) tends to increase
near-ground ozone concentrations and that the presence of absorbing aerosols tends to inhibit
near-ground ozone formation. The effect of ambient aerosol during a severe smog episode for
the region of the East Coast of the USA, is, according to [Dickerson, 1997], an increase of one
third of the amount of ozone produced during this episode. [Jacobson M. Z., 1998], working
on an episode in Los Angeles with a model coupling gases and aerosol species, found that
ambient aerosols could have decreased near-surface ozone by 5 to 8 % during the period of
the study. A substantial decrease in near-ground ozone was also found for Mexico-city by
[Castro et al., 2000].
The photochemical effects assessed in these studies affect not only the urban site but also have
a regional impact, because pollution plumes from cities and industrial regions can travel large
distances and because, in the Los Angeles and Mexico city cases, large quantities of unreacted
ozone precursor gaseous species can be advected over theses distances and cause remote air
quality impacts.
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 8/14
Regional effects of ozone / aerosol heterogeneous interaction are found in several studies
reviewed in [Jacob, 2000]. Several found that decreases in ozone concentrations up to 25%
are encountered that are due to the take-up of ozone precursors by aerosols.
5. The need to consider environmental effects in the definition and assessment of
strategies.
Particulate matter participates to a variety of health and environmental problems :
acidification, eutrophication, photochemical pollution, direct health effects, visibility, climate
change and perturbation of the hydrological cycle.
This wide range of effects calls for a careful assessment of all implications of the strategies for
primary particulates and secondary precursors emissions reduction. Until now, for the
extension of the Gothenburg Protocol major emphasis has been given to the health effects of
particulate matter. In view of the globality of the “PM problem”, a strategy with a broader
perspective on various environmental effects of particulates seems to be adapted. The
following issues could be considered :
PM Strategies and Climate change
If soot aerosol is confirmed in the near future to be the major non-CO2 greenhouse air
pollutant, this fact will have to be taken into account in any regional strategy aiming at
reducing ambient aerosol concentrations.
PM Strategies and visibility impairment
Consideration of other PM-related effects such as visibility could have direct implication on
the emissions reduction strategy. A strategy only addressing human exposure to PM (mainly
in urban regions) could bring insufficient results for diminishing the haze over the most
valued natural regions of Europe. More importantly, in a study in Los Angeles, [Kleeman and
Cass, 1999] find that applying only primary particulate matter controls could cause an increase
in visibility impairment, because the secondary material would be redistributed on particulate
matter having such diameters that they scatter light more efficiently.
PM Strategies and photochemical air pollution : integrating PM and ozone strategies
The examination of particulate matter involvement in tropospheric photochemistry highlights
the multiple connections between particulate matter and other pollutants, especially
tropospheric ozone and its precursors8 (see Figure 3). Therefore, there are contrasted possible
PM response pathways to reductions in SO2, NOx and VOC emissions. Taking the example of
NOx, possible impacts on PM of emission reductions are shown on Figure 4 : positive or
negative impact on either sulphate or nitrate aerosol could occur.
Some of these theoretical responses are reported in local studies : the study reported in
[Dennis, 2000], in which a reduction of NOx is shown to increase sulphate aerosol
concentration in eastern north America, because NOx reduction changes some of the
photochemical pathways involved in SO2 oxidation; and in another study [Kuhns, 2000],
evidence is given that under certain conditions, at a more local scale (the Treasure Valley in
8
We will not focus here on toxicological synergy between particulate matter and ozone. Some information on the
subject can be found for instance in [Elder, 2000]
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 9/14
Idaho, USA), NOx emission reductions can lead to an increase in nitrate aerosol
concentrations.
This relatively high coupling between particulate matter and ozone and its precursors should urge
to evaluate PM-related effects response to current strategies addressing ozone, and to evaluate
ozone-related effects response to future choices in PM and its precursors reduction strategies.
Integrating the two strategies is the way that has been chosen recently by the North American
Research Strategy for Tropospheric Ozone (NARSTO) ([NARSTO, 2000] and [Hidy, 2000]).
Figure 3 from [Hidy et al., 2000] : Diagram illustrating some tropospheric pathways linking
oxidant and secondary PM formation.
It also appears as a consequence of the variety of its environmental effects that particulate
matter can hardly be considered as a single pollutant. Since different aerosols have
considerably different and even opposite environmental effects, it seems adequate to consider
them separately when elaborating strategies.
As a first step, a schematic and tentative overview of the different behaviours of each aerosol
type regarding different effects can be given as in the following Table 1. This Table obviously
needs much further validation and investigation, but the main idea is to illustrate that, unless
all aerosol effects and all aerosol types are considered, the design of a satisfying strategy for
controlling particulate matter in Europe could only be hardly achievable.
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 10/14
Figure 4 adapted from [Hidy et al, 2000]
Climate Climate Visibility
direct indirect
effect
effects
Elemental
carbon
(soot)
Organic
carbon
Sulphate
Nitrate
Hydrology
and local
climate
Tropospheric
Ozone
-
-
-
-
+?
+
?
-
-?
-
+
?
0
+
?
?
?
-?
?
?
?
+?
Mineral
dust
Table 1. A qualitative and tentative assessment of the effects of different types of particulate
matter relatively to various environmental effects.
UNECE CLRTAP / EMEP – TFIAM May 2001 – page 11/14
Another important implication for the assessment of strategies is that it might be desirable to
evaluate the effects of each control measure on the chemical composition of the emissions so
as to be able to capture the overall effect of the control measure.
For example, it might prove useful to evaluate, for the sectors and for each sector for the
control measures as detailed in [Lükewille, 2000], the impact of the control measure in the
sector on the different environmental effects of particulate matter. This would require to take
account of the chemical composition along the size distribution of the parituclate matter
emitted in each sector, and of the efficiency of the control measures for different size and
composition of PM.
It is likely that far from all necessary data would be available, but a qualitative assessment
could be tried, at least in a simplified way.
The motivations and interests for such a study would be :
to avoid undesired consequences of control strategies, such as possible increases in
visibility impairment from primary PM controls, possible negative impacts on local air
quality,…
to fully take into account of all benefits from PM reductions strategies, especially in
terms of climate change and visibility improvements.
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