Edition: Urbino Questions 3
Research 1
Results fromACCENT
Research in the ACCENT project on Atmospheric Composition Change has a strong focus on
ozone in the troposphere and the boundary layer. The troposphere is the lowest layer in the
atmosphere where all weather events are taking place. The boundary layer is about the lowest
1-2 km of air close to the ground, just the air we are breathing every day.
Figure 1: The location of the boundary layer in the troposphere. Image: NOAA
Ozone is not healthy. In high concentrations it is really harmful, for our respiratory tract and
also for vegetation. An important reason for ozone formation is that emissions of nitrogen
oxides (NO and NO2) are released into the air. In urban areas nitrogen oxides come primarily
from road traffic and industry, on the oceans also from ships. But they have also natural
sources like vegetation fires and lightning.
In the engines of cars, trucks and busses it is so hot that atmospheric nitrogen (N2) is oxidised.
Catalytic converters helped a lot to reduce the emissions and to decrease ozone peak values,
but it is by far not yet solved. However, also the increase of methane and the concentration of
carbon monoxide (CO) and hydrocarbons in the atmosphere have impacts on the formation of
background ozone.
Figure 2: Recent ozone trends from a range of northern hemisphere ground-based stations.
Source: ACCENT, Answers to the Urbino Questions
Figure 3: Mace Head (source: Mace Head Observatory)
Figure 4: Zugspitze, author: Maximilian Dörrbecker, Wikipedia
The mountain peak of Zugspitze (German Alps) and Mace Head (observatory at a lonely
place of the Irish coast) are two stations in Europe, where the air is usually clean and less
influenced by cities. But the ozone level has been increasing here over the two recent decades.
This is partially because emissions of ozone precursors from human sources have been
increasing globally in the second half of the 20th century. They are transported over long
distances, for example as peroxyacetyl nitrate (PAN). Now nitrogen oxides are decreasing in
some industrialised countries, but a lot of ozone is still imported. We spoke about this and the
transport in ACCENT magazine No 8: Background ozone and long distance transport of
nitrogen oxides
http://www.atmosphere.mpg.de/enid/5og.html
But the figure also shows: Ozone production is a part in complex atmospheric chemistry and
can vary from year to year and station to station.
In rural areas in United Kingdom concentrations were slightly decreasing.
Figure 5:
Time series of the maximum 8-hourly mean ozone concentrations monitored at a
selection of long-running rural EMEP background sites in the United Kingdom between 1990
and 2003. Source: Derwent, 2006
Relevant amounts of nitrogen oxides are still formed in urban areas and reduction strategies
are less successful than for other air pollutants. The partial reduction leads to the effect, that
less ozone is destroyed by reactions with NO in the city, but there is still enough NO and NO2
to drive ozone formation. Therefore ozone increases in the city centres. Only drastic
reductions would help to avoid this, but this is not yet achieved.
Figure 6: Local ozone consists of background ozone which can be due to chemicals imported
from other continents like PAN and local production, which is to a major extent coming from
traffic and emitted in the cities as NO or NO2. NO reduces ozone in the cities by reaction to
NO2. NO2 forms ozone again under certain conditions. See also:
http://www.atmosphere.mpg.de/enid/23c.html
Therefore ACCENT scientists say in the answers to the Urbino Questions:
“The effect on exposure of the population to ozone will be complex. Although one may
initially be reducing the overall concentration field of ozone in rural areas by reducing
precursor emissions, the increases within cities may actually increase the exposure to ozone of
more of the population.”
The conclusion is: ozone peak values are decreasing, while background ozone is on the way
to reach less healthy mixing ratios of 35-40ppb in particular in the sunny and warm European
South. This is also in a regime where ozone can have negative impacts on plant growth. It is
not expected that there will be strong changes in the next 20 – 30 years.
Figure 6: Target, information and alarm values for ozone in concentrations [µg/m3] or mixing
ratios [ppb]
In the part “Research 2” of this edition we will see, that not all ozone problems are imported
in Europe. Cleaning technologies for cars like particle filters can even be sources of more
nitrogen dioxide in the urban areas.
Research 2
Ozone and particle filters
We are concerned about air pollution in our cities. Particulate matter coming from emissions
of vehicles in busy streets, from tyre abrasion and other sources is proven to have negative
consequences for our health.
Although it is not really well known how high this risk is and although other health risk as
smoking, unhealthy nourishment or too much alcohol have clearly worse impacts, we would
like to reduce the risk. One possible measure is the installation of particle filters. This is most
necessary for diesel cars, because they emit most of the particles.
Figure 1: Worldwide deaths in 2000 attributable to selected leading risk factors. Source:
WHO 2002
However, it is not always possible to tackle all problems at once. A research effort in
Copenhagen tells us why diesel particle filters can increase the risk of ozone smog again.
The study took place at Jagtvej, a busy street in Copenhagen. Traffic has been observed and
counted there on normal weekdays. At the same time NO2 emissions have been measured.
The same sort of analysis has also been made on week-end nights, when the traffic is
dominated by taxis.
Figure 2: Jagtvej is a busy street in Copenhagen.
The taxis are equipped with diesel particles filters (DPF) of the newer generation, so called
continuous regeneration traps (CRT, see article: background). The filters are efficient in
particle reduction. Therefore, they help to reduce the health risk arising from particles for
people living in the vicinity of busy streets. But for the continuous regeneration of the filter
more nitrogen dioxide has to be generated than in a car without DPF. The consequences can
be directly observed in the streets:
The taxis emit more nitrogen oxides than other diesel cars. And direct emissions of NO2
contribute with 40% to their total nitrogen oxide emissions. For the normal average traffic
direct NO2 only contributes with 16%. The major fraction is NO, which reduces the ozone
level in the city centres. In the years 1995 -2006 the direct NO2 contribution to nitrogen
oxides from road traffic increased from approximately 5 % to 18 %.
From the long term trend of four stations in Denmark it can be seen that in the 1990th the total
amount of nitrogen oxides (NOx = NO + NO2) has been reduced thanks to the introduction of
catalysts. But the observed amount of NO2 in the air has hardly changed. This means that the
share of direct NO2 emissions must have been strongly increasing.
Figure 3: Trends in the NO2 emissions (above) and total nitrogen oxide emissions (below) at
different urban stations in Denmark. Source: Danish Nat. Envir. Research Inst., Technical
Report No. 623, 2007
The study of taxi emissions shows that vehicles with diesel particle filters have a high
relevance in this balance.
The National Environmental Research Institute (NERI) in Denmark also modelled different
scenarios for the future. How much direct NO2 would be emitted by a car fleet equipped with
present standard diesel particle filters, a larger fraction of diesel vehicles and new
technologies like Selected Catalytic Reduction (SCR), which would be available for the wider
market in the near future.
Figure 4: Modelled direct NO2 emissions under different scenarios for the technical
equipment of the car fleet. Source: NERI, 2007
The results show, that it is hardly possible to reduce direct NO2 emissions below the direct
emissions of the present car fleet. Most likely direct NO2 emissions are going to increase. In
this case the envisioned future emission targets in the European Union cannot be met with the
present standard technologies.
Background 1
Diesel engines and particle filters
Speaking about car exhaust, we have to consider basically three sorts of emissions:
1) carbon dioxide
2) gases from fuel or the combustion process
3) particles
Carbon dioxide (CO2) is the greenhouse gas, which contributes most to human induced
global warming. It is not an immediate problem for our health but for the climate system.
The amount of CO2 emitted depends on the amount of fuel needed and burnt by the car.
There is no way to reduce the emitted amount of carbon dioxide by catalytic converters or
other cleaning technologies. The only way to reduce is to have a more efficient engine, a
smaller and lighter car with a low air resistance and to avoid stop and go as well as high
speeds on the highway while driving.
Gaseous emissions may come from evaporated or not completely burnt fuel. We call them
volatile organic compounds VOC, consisting of hydrocarbons or oxidised hydrocarbons.
Other gases are products of the combustion process: nitrogen oxides (NO and NO2 = NOx)
and carbon monoxide (CO). They are not healthy and they cause also indirect problems
because they cause ozone smog in particular in summer.
Particles are formed in larger amounts in diesel engines, for example when the car accelerates.
They consist of soot and not burnt compounds in the fuel condensing on soot or tiny sulphate
particles. Particles are a larger problem in diesel engines, because the engines inject varying
amounts of fuel into always the same amount of hot air. If the mixture is oversaturated with
fuel, particles form easily.
The amount of gaseous emissions, VOC, CO and NOx, is strongly reduced if a catalytic
converter is used in a gasoline car. This is now standard in most cars in the industrialised
countries. In diesel cars such emissions are lower.
Diesel engines are also more efficient and therefore cause less carbon dioxide emissions than
a comparable gasoline car, in particular on long distances.
But diesel engines emit a lot of particles. Therefore, in the future, particle filters may become
obligatory in many countries. There are several types of diesel particle filters (DPF) available,
which are based on different technologies:
Particle filter technologies
CRT (Continuous Regenerating Trap) and DPX (filters with a noble metal layer) work based
on the so-called “wall flow principle”. In this process, the exhaust gas goes into a number of
dead-end channels. The unburned carbon deposits itself in the channel and the gas portion of
the exhaust passes through the porous walls, which have a defined pore structure.
Alternatively the “flow through principle” can be used. Flow through filters consist of metal
fibers or films the gas passes along and particles adhere on the walls. An electrical current can
be passed through the monolith to heat the core for regeneration purposes. Or it is regenerated
with the CRT technology by hot gas.
Flow through filters have usually the disadvantage that in particular the finer particles pass
more easily. Therefore they are less favourable.
Regeneration systems
For the regeneration (= cleaning) of a wall flow system the deposited soot and particles have
to be burnt at high temperatures of more than 600°C. This is much if we consider, that the
temperature can go down to about 200°C while driving in cities. The regeneration temperature
can be reduced to 500 – 550°C with the help of an additive. This temperatures is
automatically given on the highway or can be generated by an additional injection of fuel. But
additives are complex in less comfortable in the handling.
A solution for continuous soot removal is the “continuous regenerating trap” (CRT), which
are often used in buses. In these emission reduction systems, the exhaust gas passes an
oxidising catalytic converter. Here, CO and organic compounds are nearly completely
oxidised. Such emissions are anyway minor in diesel engines. Nitrogen compounds are
primarily oxidised to NO2, because the NO2 shall react with the soot forming CO2 at a
temperature between 200°C and 450°C.
A disadvantage of the systems is, that NO2 is usually produced in advance. The NO2 fraction
in the exhaust gas may be in the range of 5% before the catalytic system and 20% behind. The
system reduces particle emissions at the cost of higher NO2 emissions, which in turn promote
the formation of ozone smog.
So called Selective Catalytic Reduction systems (SRC catalysts) might become a solution for
this problem. SCR uses a reductant that is added to the exhaust gas in front of the catalyst.
The most common way for the SCR-reaction is the use of ammonia that can be generated
onboard by bringing urea into the hot exhaust gas stream. By this means nitrogen oxide
emissions can be significantly reduced. But also this technology is complicated and the
reduction rate depends on the conditions. Diesel particle filters and SCR can be combined.
Activities in the classroom and in your town
Background:
There is a strong links between traffic in the streets, the local weather situation and ozone
levels in the towns and the suburbs of the town. If your school is close to a station of the local
air quality network,
We encourage you to investigate this phenomenon in the streets next to your school.
Material and conditions:
Ideal conditions would be given, if the school would have one or even more mobile ozone
detectors available.
Since this is usually not the case, please check if your school is located in a larger city and if
there is a measurement point of the local air quality network installed. Such points often look
like a small metal building or box on the street as for construction working and have some
instruments on top. (see image)
Their location can often be found out checking the website of the local air quality
superveillance maintained by the local, regional or state administration. Please check also, if
the data automatically generated in this station can be accessed online in the Internet.
Alternatively, please ask the local officers of the air quality surveillance for assistance and
data.
If you are used to work in geography with GIS systems, you may develop a small map of the
traffic density for different times of the day. And use this in a GIS software.
An example for free GIS softwar is SAGA GIS: www.saga-gis.org
For this you need geo-referenced maps from your region. If you do not have access to them,
you may work for example with Google Earth.
If a “field experiment” is not possible during the classes, you may also work with data from
stations in your region and analyse them in the classroom as shown in the example below.
Objective:
Investigate, if and how the local ozone production is related to the traffic next to the station or
in the region of the station. If an ozone detector is available in your school, you can measure
ozone yourself.
It is recommended to choose a sunny day in spring, summer or early autumn.
Task:
Measure the traffic in a street or several streets next to one or two ozone measurements points.
For this purpose, count cars at different times of the day and compare with ozone values.
What has to be considered?
If the measurement point for ozone is next to a busy street it is likely that local ozone is
depleted by the NO emissions from traffic. Such stations are suitable for the parallel analysis
of nitrogen oxide data and comparison of data from traffic counting.
If the measurement point for ozone is in a housing area or a suburb it is likely that the role of
local traffic is minor. If you choose places for traffic counting, look also for major streets in
the vicinity.
Please consider the wind direction. Choose places for the traffic counting which are located in
a way that the wind blows from the street where you count cars to the measurement point..
Example of zone and nitrogen oxide measurements
In the following diagrams we show air quality measurements from different stations in the
same region in Germany for the days May 17th + 18th 2007. We compare three different places,
one in the vicinity to a busy street (Koblenz), one in an commercial and industrial area out of
next to a mid size town (Neuwied) and one in the forest (Neuhäusel).
We can see that ozone is clearly going down in the city and next to the town of Neuwied
during the night. This is not the case at the station in the forest. Therefore we can conclude,
that ozone is more subject to depleting reactions in cities and in the urban environment.
We can see, that NO levels are higher in the city (Koblenz) than around the city (Neuwied).
Since NO reacts with ozone, the ozone concentration rises later in Koblenz than in Neuwied,
where less NO is available. At the station in the forest, NO is not present.
Questions for the classes
1) Ozone diurnal pattern
a) Which differences do you see about in the concentration changes of ozone between a
station in or next to a town and a station in the forest? Describe in 4-8 sentences!
b) Why does ozone levels rise later in the morning next to busy streets? Mention the role of
nitrogen oxide (NO)!
c) Nitrate radicals (NO3) are stable in the night. They form if ozone reacts with NO2. Which
consequences has this for the diurnal cycle of ozone concentrations in urban areas? What
happens in the forest?
2) Ozone formation and transport
a) What are typical concentrations [µg/m3] or mixing ratios [ppb] for background ozone?
b) Before catalytic converters have been invented, ozone peak values were higher on sunny
days. Why?
c) The ozone values at the forest station are higher than the values in or next to a town. Can
you explain why?
3) Particle filters
a) If more NO2 is emitted by cars with particle filters, what are the consequences?
b) Can the consequences be better observed in the city centre or in the suburbs?
c) Which problems do expert see when even stricter guidelines come into force for all
emissions of cars in Europe (nitrogen oxides, particles, carbon dioxide)? Are the present
technologies sufficient in order to guarantee that pollutants in urban areas do not exceed limit
values?
Further material:
Text ozone smog on ESPERE
http://www.atmosphere.mpg.de/enid/23c.html
Worksheet on ESPERE
http://www.atmosphere.mpg.de/enid/3z4.html
http://www.atmosphere.mpg.de/enid/3z5.html
Koblenz Friedrich Ebert Straße: Bild Peter Jansen, licenced for Panoramio.com
Luftmessnetz:
http://www.luft-rlp.de/aktuell/messnetz/aufbau/
Image: measurement station:
http://www.sh-landtag.de/plenumonline/juni2005/texte/10_23_statistik_landw_feinstaub.htm
Land Schleswig-Holstein
Information about CRT:
http://www.umweltbundesamt.de/verkehr/techemissmm/technik/crt/crt.htm
Filter Image BMW:
http://www.bmw.com/com/en/insights/technology/technology_guide/articles/diesel_particle_f
ilter.html
Zur SCR Technologie
Diss. Andreas Lacroix
(e-mail vermutlich: alacroixka@aol.com )Sonst Prof. Müller, Kaiserslautern, fragen.
WHO health risk table:
http://www.who.int/dietphysicalactivity/publications/facts/en/gsfs_ppt_rf.pdf