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DUP-POLPO
ESA Contract No 15564/01/I-LG
Case study collection
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POLPO result
Part II of III
Andrea Weiss,
Daniel Schaub,
Andrea Petritoli,
Paolo Bonasoni
2
26 September 2002
DUP-POLPO
Question list and case studies
August 2002
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Questions_cases.doc
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Distribution list:
POLPO Team
C. Zehner
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user
EMPA
user
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Summary
contents:
The case studies demonstrate the use and the limitations of GOME data for
monitoring atmospheric pollution. A question list is intended as a help for users which
want to use GOME for atmospheric pollution monitoring. It is shown which kind of
questions could be answered with help of the satellite GOME, and which not.
Examples are provided with case studies. This is part II of the DUP-POLPO result.
Authors:
Dr. Andrea K. Weiss
EMPA
Dipl. nat. Daniel Schaub
EMPA
PhD Andrea Petritoli
ISAC
Prof. Paolo Bonasoni
ISAC
CHANGE RECORD SHEET
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Homogenisation of POLPO documentation,
Case 6 update
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Table of Contents
1
INTRODUCTION ................................................................................................. 4
1.1
1.2
PURPOSE.................................................................................................................................... 4
DEFINITIONS AND ACRONYMS ................................................................................................... 5
2
QUESTION LIST........................................................................................................ 6
3
CASE LIST 12
3.1
3.2
3.3
3.4
3.5
3.6
4
CASE 1: CONVEYOR BELT AND THREE HOT SPOTS .................................................................... 12
CASE 2: POLLUTION TRANSPORT ACROSS EUROPE .................................................................. 18
CASE 3: HIGH NO2 AND CLOUDY NORTHERN EUROPE ............................................................ 25
CASE 4: OCCLUSION LIFTING POLLUTION ................................................................................ 32
CASE 5: STAGNANT AIR OVER CENTRAL EUROPE .................................................................... 40
CASE 6: POLLUTION SOURCE ATTRIBUTION ............................................................................. 43
REFERENCES ......................................................................................................... 64
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1 INTRODUCTION
1.1 Purpose
This report is a result of the project DUP-POLPO. DUP-POLPO stands for:
Pollution Hot Spot Monitoring from GOME - applied to the Po-basin
The aim of the project is to develop a prototype tool for applying space-borne measurements
of tropospheric NO2 for purposes of air pollution surveillance.
A question list is provided as a help for users who intend to apply GOME for atmospheric
pollution monitoring. It is shown which kind of questions could be answered with help of the
satellite GOME, and which not. According to the user’s questions, examples with detailed case
studies are provided. These case studies demonstrate the use and the limitations of GOME
data for monitoring atmospheric pollution.
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1.2 Definitions and acronyms
BOLAM
BOlogna Limited Area Model
DOAS
Differential Optical Absorption Spectroscopy
DUP
Data User Program
EMPA
Swiss Federal Laboratories for Materials Testing
and Research
GOME
Global Ozone Monitoring Experiment,
instrument on the ERS-2 satellite
ISAC
Institute for Atmosphere and Climate (previous ISAO)
NABEL
"Nationales Beobachtungsnetz für Luftfremdstoffe",
the Swiss National Air Pollution Monitoring Network
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2 Question list
with links to case studies
Do we see the emission regions?
Yes, the large scale emission regions show up especially pronounced in a summer
months average over Europe.
See Fig. 9 of the DUP-POLPO User Handbook [Weiss et al., 2002] for summer
means 2000 and 2001, respectively.
What is the resolution of the GOME data?
Every 1-2 days an overpass over a certain location in Europe takes place at around
10:15 local time (which is about 9:15 UTC at Switzerland). Three adjacent pixel of
each 40x320 km are read during a scan, the shorter scale is in the North-South
direction.
Can we estimate the emission strength?
Only qualitatively and relative to other regions or other times. Quantitative estimates
are not possible with the present tools, because mixing processes are ill-defined and
vary between geographical locations. In future, incorporating chemical and transport
modelling and cloud algorithms may bring advantages.
Can we observe cross-country transport?
Yes, with the limitations mentioned above (case 6).
easy case: The plume of a hot spot is spreading into adjacent areas. Employing wind
fields will allow verification for short time scales, at which wind fields can be assumed
constant.
hard case: (changing wind fields, complicated meteorological situations): Calculation
of back-trajectories is necessary to investigate the history of the air mass.
Do we see the days with worst pollution ?
yes: in case photo smog formed under clear sky, in stagnant air over Europe.
See case 5 for an example.
no: in case of pollution trapped in stable stratified air, and shielded by stratus from
satellite view.
It depends: on the extend of the pollution relative to the GOME resolution.
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Are short term pollution episodes resolved ?
GOME overpass is every 1-2 days, and in the morning only. Thus, there is a good
chance that short term episodes (< 1 day) are missed by GOME.
How do GOME NO2 values depend on the meteorological situation ?
Often, low GOME values just mean clouds (see certain regions in case 2). This applies
especially for single day overpasses, but must be considered for means also. A
period long enough to cancel out geographical effects caused by clouds is a mean
over several summer months.
Could one use annual means constructed from cloud free days of a specific location?
Yes, this would be a most useful approach to assess the mean pollution and mean
area of influence from pollution hot spots. When using means, the signal/noise ratio
becomes better, and because of overlapping satellite tracks, geographical resolution
increases.
Could one use clear sky means of several years of GOME for trend analysis ?
In principle, it is possible. One could calculate an observed trend, preferably from
clear sky data, for a specific location. There are pitfalls in relating this observed trend
to emissions, as there is also year-to-year variability in mixing. This efficiency of
mixing is hard to access. Further, the background NO2 concentration is within the
limits of the GOME resolution. Therefore, such investigations should focus on
pollution hot spots.
What happens in case of fog or clouds?
Two effects may occur (compare, e.g., first and second day of case study case 5):
(1) Normally, a low signal occurs as the pollution below the clouds is shielded from
the view of satellites.
(2) The signal of pollution residing above the clouds is enhanced because of the
albedo effect. If this effect is strong, one would expect a correlation between fog
cover and high GOME signal. As this had not been observed in our studies, we
consider the effect not as a major one. No indication was found that high NO 2 values
may be faked by fog, stratus or snow, despite low pollution. It is not excluded a
pollution might become pronounced when a high albedo is present and slightly less
pronounced otherwise.
What causes high GOME values on cloud covered days?
The pollution was lifted from the emissions near the ground to heights above the
clouds, or clouds (fog) formed below the polluted air. Large thunderstorms are also
expected to contribute to NO2 plumes.
Compare with case 3 and case 4.
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Does it often happen that pollution is residing above clouds ?
Yes, in Europe we have fronts passing every few days. Connected with fronts, there
appears a mixing of pollution into upper heights (case 3). In winter, low level stratus
often resides below the polluted air and enhances the signal (case 4).
TRANSPORT PHENOMENA OBSERVED FROM SATELLITE
How is the pollution lifted from its sources at the ground to heights above the clouds ?
Frontal lifting, especially at occlusions, could transport the pollution above the clouds.
Further, an inflow of cold air near the ground may lift the polluted layer, and
sometimes also form fog between the two layers. See case 1.
What happens in the vicinity of fronts?
Warm fronts and cold fronts both have the potential to mix the atmosphere vertically
(from ground to the upper troposphere at about 10km) and horizontally (advection
over hundreds to thousands of kilometres). The large structure of fronts make them
prone to observation from satellite. It is known that the mixing sometimes is
connected to conveyor belts. These can be observed in Meteosat images. Their
impact on the pollution fields can be observed in the GOME tropospheric NO 2. See
case 1.
A further, rather common phenomenon connected to fronts are extended cloud
bands, rain, and subsequent wet deposition of air pollutants. Another influence on
the NO2 concentration can occur by extended thunderstorm activity, large flash-rates
and thus noticeable NO2 production.
What happens at occlusions?
The lowermost, and possibly polluted air masses are lifted above the stratus. Thus,
pollution is residing above the clouds and can be sampled by the satellite. See case 4
for an example.
What causes extremely high NO2 concentrations over large areas of Europe?
Pollutants accumulate in air masses, which remain stagnant over emission regions for
a few days and have limited exchange with fresh air. See case 5 as example.
When does extremely high pollution over the Po-valley appear ?
Sometimes, the pollution accumulates in the air masses travelling slowly to the south.
Thus, air already loaded with pollutants arrive over the Po-valley, and get a further
loading. This causes some of the episodes with highest concentrations in Southern
Europe, see case 2.
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Does the Po-valley plume spread across Europe?
Generally, westerly winds are prevailing, and do spread the Po-valley plume either
south or east. More often, the central Europe pollution adds up in stagnant air and is
then transported over the Po-valley (case 6).
Could GOME detect the spread of the Po-valley plume over Switzerland?
The resolution of GOME is rather poor for this task. When only valleys of Southern
Switzerland are filled by Po-valley pollution, an overall pixel mean including clean
alpine areas may suppress the signal. See case 7 for a discussion on this subject.
In which heights does the European transport take place?
Pollution transport can occur in all heights of the troposphere. A longer lifetime of
NO2 in upper heights and stronger winds there cause a more effective transport over
longer distances. On the other hand, considerable dilution takes place during
transport (case 2).
May we get pollutant transport from North America ?
Yes, but we expect the pollution to be rather diluted.
See case 1 for an example.
COMBINATION OF SATELLITE AND GROUND DATA
Which kind of correlation can we expect with ground-based measurements ?
The GOME pixel size is constant, but the representativity of the ground station is
highly variable and valid for a much smaller scale only. When only background
conditions are selected, normally the NO2 concentration is small and at the resolution
limit of the satellite, respectively at the resolution limit of the method used to extract
tropospheric NO2.
A ground-based station measures only local concentration, not a 3-D column as the
satellite. Thus, in an inhomogeneous atmosphere, we cannot expect the same NO2
measurements. The DOAS system, which also measures a column, albeit in a line
rather than in a 3 D pixel, is expected to have more correlation with GOME.
A serious problem is the representativity of the ground station. The most extreme
NO2 concentrations measured there are normally caused by nearby (nearly unmixed)
emissions, which occur at smaller scale only. On the other hand, GOME is more useful
for sampling spatially extended patches of raised concentration. Because of the
resolution, a correlation between GOME and a ground-based column estimate can be
expected at a monthly to seasonal scale only.
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The seasonal cycle is expected correlated/anticorrelated to stations which are
above/below the mean boundary layer. We observed the total tropospheric column
measured by ground profile and by satellite to show about the similar seasonal
behaviour.
The issue of comparability is elaborated in the DUP-POLPO User Handbook.
How does the difference GOME-ground station change with the meteorological situation?
Clouds are the major influence. When considering cloud-free situations only (which
is recommended when comparing ground-based and GOME), there are still
important influences of the meteorology:
We expect the boundary layer height to determine the measured concentrations at
the ground (even if the emissions were constant). A mountain station above the
boundary layer will not sample the pollution below. The open-path DOAS
measurement of Mt Cimone can detect pollution events only if the pollution is
located higher than the 2156 m.
Firstly, one must answer the question whether the mountain station is above or
below the boundary layer. Secondly, the mixing layer thickness determines the
concentration.
See the DUP-POLPO User Handbook for a more detailed discussion.
ADDED VALUE OF EMPLOYING SATELLITE DATA
For which situations and which time scales can we interpret the GOME data and our
combined data?
GOME data look reliable at a summer mean of three months - then a lot of
pollution hot spots can be identified.
Situations of high GOME, high DOAS and high ground-based station values are
situations easiest to be explained (case 2). Low DOAS and high GOME values are
not expected, and as this occurs seldom, this seems to be because of outliers of
DOAS.
In case 4, a chemical reason for a slighly less DOAS column is discussed.
Low ground-based data and high GOME values occur with inhomogeneous
distribution of pollution in the air column sampled by satellite.
The best single day cases to detect pollution hot spots and pollution spreading
occurred at sunny periods and stagnant air. Favourable for detection of pollution
was cold inflow at the ground and possibly lifting of the polluted air, the fog
enhancing the signal. Pollution transport could be observed often with occlusions,
where the polluted air was lifted above the clouds.
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What can we learn from single day overpasses ?
Single day cases can be carefully examined with respect to meteorology and
transport which took place at this day and the 2-3 days before (which is
approximately the relevant scale). These case studies may shed light on important
processes and typical situations. For instance, we learned that for Switzerland north
of the Alps the influence of the Belgium/Netherland/Germany hot spot is more
pronounced than from the spatially nearer Po-valley.
What can we learn from ensemble means ?
Ensembles are chosen according to rather similar meteorological situations. An
ensemble mean will have decreased time resolution, but spatial resolution can be
gained because of overlapping overpasses. Further, the signal/noise ratio will
improve. Effects from small clouds will be smoothed.
It should be considered there is a difference between summer and winter NO 2 fields
over Europe. This is partly caused by the difference in emission rates. Seasonal
changing emission maxima are expected, e.g., in winter with most emissions
probably occuring in Eastern Europe, whereas in summer, probably Western Europe
emits more. On the other hand, the meteorological activity and the mixing
processes are differently pronounced in summer and winter. Thirdly, the chemical
lifetime of NO2 is longer in winter because of less photochemical activity.
For ensemble means concerning certain wind patterns, it will be possible to discern
the shape of hot spot plumes. However, a possible pitfall is that typical cloud
pattern occur with certain wind fields. These have not to be misinterpreted for low
NO2 from GOME. Our efforts to describe the Po-valley plume with ensemble means
were hindered by non-sufficient cloud information.
The ensemble means, its use and its pitfalls are discussed in detail in chapter 6.2 of
the DUP-POLPO User Handbook.
What is the additional value from employing satellite data ?
1. They provide the extent of pollution patches (spatial scale of the pollution).
2. The connection to various European hot spots can be found employing
meteorological data. Frontal transport phenomena are sometimes displayed.
3. An advantage over ground–based station is that GOME is not disturbed by local
effects
4. Observation of cross-border pollution transport is possible under favourable
conditions.
5. Pollution transport occurring in heights above the ground station could be
detected.
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3 Case list
3.1 Case 1: conveyor belt and three hot spots
of 10 Jan 2000
User questions:
How could NO2 be transported from ground into the middle troposphere?
What happens ahead of fronts?
Answer:
The GOME data in combination with Meteosat data show that it is likely frontal
structures lifted NO2 above lower level stratus. This causes extremely high GOME
NO2 extending several hundered to thousands of km over Europe.
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Discussion of the case:
On the 10th January 2000, three distinct hot spot regions appear over central Europe: (A) Povalley, (B) France and (C) Central Germany, all of them spreading to the east (Fig. 1.1).
Fig. 1.1: GOME tropospheric vertical columns. On 9th January 2000, stratus shielded central
Europe from satellite. The pollution is hidden except for a spot Belgium-Netherlands, which
appear as highly polluted (left). On the 10th January 2000, three distinct hot spot regions
appear over central Europe (right).
Surprisingly, the areas of high NO2 are covered with clouds over large parts, as seen in the
Meteosat visible wavelength image (Fig. 1.2). The infrared image show the cloud had been
higher on 9th , and seem rather low (as they are darker) on 10 th. The weather charts confirm
fog was observed over large parts of central Europe. Thus, the pollution resides above the
stratus.
The question emerges: How could the pollution be transported above the clouds? The
Meteosat infrared image (Fig. 1.3) yields an answer. A comma-shaped high cloud band
extends from North of Iceland over Norway and Scotland. The system extends with a S-shape
and lower clouds to the west coast of Spain. This is a typical infrared image of a mature
cyclon, residing over Ireland. Together with an extended high pressure area over Spain to
Germany, these features determine the transport of air masses over Europe. A rather similar
situation, sampled with aircraft measurements, is described in [Bethan et al., 1998]. Europe
is covered over large parts with low-level stratus or fog.
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Fig. 1.2: Meteosat visible wavelengths show stratus over large parts of Europe.
Fig. 1.3: Meteosat infrared image. Left: Central Europe is covered by bright clouds. A
comma-shaped cyclon develops at the east-coast of North America. Right: a comma-shaped
high (bright) cloud band extends from North of Iceland over Norway and Scotland. The
system extends with a S-shape and lower (darker) clouds to the west coast of Spain.
The trajectory analysis confirms the hypothesis of a conveyor belt system.
From the three most prominent high concentration fields an ensemble of backward
trajectories was calculated. Following conclusions could be drawn:
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(A) Over the Po-valley, trajectories indicate at different levels of height that air from both the
west, and also from Germany, advected east of the Alps, might have contributed to the
observed pollution (Fig. 1.4). Generally, the advected air does not change height over the
last 48 hours. A transport of central Europe boundary layer air to the Po-valley is possible
(but not proved) for two reasons: (1) if pollution was lifted to about 800 hPa before or
(2) because of the inherent height uncertainty of trajectories. However, the contribution
of the background is probably less pronounced, as the air resides rather long (two days)
over the Po-valley. Small movements to the east and the west happened, which easily
explains the smearing of the pollution in these directions as observed in Fig.1.1.
Fig. 1.4: Trajectories calculated with ECMWF fields to estimate air paths reaching the Povalley. Left: horizontal, right: vertical view. Three trajectories were calculated, with slightly
differing starting points. The air paths are consistent on a larger scale: all are residing several
hours above Europe before reaching the Po-valley. The air masses paths in the Po-valley are
not consistent on a shorter scale. Transport west around the Alps and transport east around
the Alps is both likely, maybe the air masses have mixed origins.
(B) The hot spot over France is with high probability not influenced by the Po-valley, but
could be influenced by emissions in Germany, Belgium, and Netherlands (Fig. 1.5).
Over central Europe, trajectories have been uplifted from the lower to the middle
troposphere, e.g., from 1000 hPa to 500 hPa, in 48 hours (Fig. 1.5, right panel). The
trajectories shown in Fig. 1.5 suggest a frontal activity, and the action of a conveyor belt is
likely.
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Fig. 1.5: The trajectories calculated for various starting points and various heights over the
hot spot of France. The air was residing over Germany, the Netherlands and Belgium before
reaching France (left). Note that considerable changes in height took place (right). Air masses
from the lower troposphere had been lifted to the middle and upper troposphere, a transport
of polluted air to the middle atmosphere is likely.
Fig. 1.6: the trajectories indicate air masses which had been residing for one to two days at
the US east coast (left) before having been lifted with the frontal system into the higher
troposphere (right), where fast winds delivered the air to Northern Europe.
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Because of the conveyor belt system, which is able to transport pollution over hundreds to
thousands of kilometres, we checked the trajectories on a longer scale. Such, it was found:
(C) Northern Europe is in the influence zone of Northern America (Fig. 1.6).
See, e.g., [Stohl and Trickl, 1999] for an elaborated analysis of this subject.
Thunderstorms and massive NOx production by lightening can be excluded because of a low
lightening rate at the relevant days (http://www.wetterzentrale.de/).
The flash observations from the OTD satellite cover the planet, but have sparse data. The air
advected from North America seem not to have experienced mayor thunderstorms (Fig. 1.7).
Fig. 1.7: Lightening detection from OTD satellite: plot provided by the Lightning Team of the
Global Hydrology Resource Center of the NASA Marshall Space Flight Centre. Blue areas are
viewed by the Optical Transient Detector (OTD), above: ascending passes, below: descending
passes. Red points indicate lightening flashes. The areas are sampled two times a day. As red
dots are missing in the North Atlantic, no sign for major thunderstorm activity could be found,
although this is no proof none took place. data source:
http://thunder.msfc.nasa.gov/data/otdbrowse.html.
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3.2 Case 2: Pollution transport across Europe
of 19 Jan 2000
User questions:
How do extremely polluted situations develop
over large areas in southern Europe?
Where does this pollution has its origins?
Answer:
The GOME data reveal the heavily polluted area is spreading over northern Italy,
Southern France, Sardinia, Eastern Spain and the Mediterranean. The spread of the
Po-valley plume is to the south-east, indicating weak northerly winds. The wind fields
of the numerical weather prediction models confirm nearly stagnant air over central
Europe, slowly moving to the south.
The conclusion is that the pollution from Belgium, Netherlands, Germany, and the
Swiss plateau added up in the air mass which slowly travelled south to the Po-valley.
The regional pollution of the Po-valley, normally mixed with fresh air supplied by
stronger westerlies, is in this case adding up to an already high background, leading
to the extreme pollution event.
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Discussion of the case study
On 19.01.2000, GOME reveals high pollution over large parts of southern and central Europe
(see Fig. 2.1) . The high pollution was recorded also from the ground station at Jungfraujoch
(Fig. 2.2) , and the DOAS system at Mt. Cimone (Fig. 2.3).
NO2
NO2
[ppb]
Fig. 2.1: Tropospheric NO2 column estimated from GOME (data courtesy A. Richter and J.
Burrows, University Bremen).
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Fig. 2.2: NO2 measured at the ground station Jungfraujoch (data courtesy NABEL). The
pollution event of 19th January is the highest in this month. The elevated concentrations
prevail about one day.
January 2000
Fig. 2.3: DOAS profile at Mt. Cimone. The measurement is performed during sunset. The
tropospheric column value (13.3 1015 molec/cm2) is one of the highest values recorded during
the 2 years analysed.
The meteorological situation is characterised by a strong high pressure system with its centre
over Ireland, which causes the northerly winds over Europe.
The low values over France result from cloud cover, Germany and the Mediterranean are
cloud free as the Meteosat image shows (Fig. 2.4). The snow covered alpine arch is clearly
visible in the Meteosat image.
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The albedo of the snow cover could be expected to influence the GOME result. However, the
absence of any arch-like pattern in the GOME tropospheric NO 2 indicates the influence is a
minor one.
The transport of air across central Europe is confirmed with the BOLAM wind fields (Fig. 2.5
and 2.6). They show nearly stagnant air over central Europe, slowly moving to the south.
Interestingly, the pollution hot spot from Southern France does clearly show up to be not
connected with the one of the Po-valley. Separated by a vortex, pollution is accumulating
from sources within the vortex (although some background might have been provided from
the Po-valley). For better recognition of the separation between the two hot spots, see Fig.
2.7 with a colour scale highlighting the highest values only. A fine structure of the Po-valley
can be recognised, and further the confined high NO2 of the Southern France Plume.
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Fig. 2.4: Meteosat image. The Southern areas are lit stronger by sunlight. However, the
clouds over France can be detected.
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Fig. 2.5: BOLAM wind fields from 18 to 20 January show a constant northerly flow in the
upper layers (500 hPa).
Fig. 2.6: BOLAM surface wind fields from 18 to 20 show similar features: a northerly flow
around and across the Alps. The plume of the Po-valley must be expected to stretch to the
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south-west. Over Southern France, a vortex develops on 19th and intensifies during the day,
isolating a rotating, but stagnant air mass from the surrounding air flow.
Fig. 2.7: GOME data indicate the Southern France pollution hot spot is independent from the
Po-valley.
Conclusions from the case study:
Interpreting the GOME tropospheric NO2 data is straightforward in case of high pollution and
clear sky. Low NO2 values may reflect low NO2, but also clouds shielding NO2 from view.
Therefore it is suggested to interpret GOME data together with cloud information. We rely on
Meteosat images, preferably at the visible wavelength, as with IR images low clouds have
little contrast. From our example of a clear winter day in 2000, we conclude snow cover of
the Alps is a minor problem.
The origin of the pollution can be localised only by employing wind fields and a priori
knowledge about the distribution of European pollution hot spots (see summer mean
2000/2001).
The lifetime of NOx is long enough, thus high concentration patches can be detached from its
pollution source (in our case Northern Europe). The air mass, already enriched in NO 2, is
further charged with NO2 at the Po-valley, causing a very high pollution event there.
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3.3 Case 3: High NO2 and cloudy Northern Europe
17 Feb 2000
User question:
Can we always understand GOME NO2 above clouds?
Answer:
A high pressure system was residing over Europe for the first half of February, with
stagnant air and thus higher concentrations of pollution built up. On 17th Feb,
intermediate clouds covered central and northern Europe, and the Po-valley and
parts of Eastern Europe had clear sky. Over the Po-valley, 12 * 1015 molec/cm2 are
measured. The maximum GOME values (Fig. 3.1) reach exceptional 20 * 1015
molec/cm2 in Eastern Germany, albeit visible clouds are present. Thus, the pollution
is residing above, and was probably transported there by a frontal system.
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Discussion of the case:
The GOME image (Fig. 3.1) shows rather high NO 2 concentrations, which are extending from
north-east to south-west, with a maximum over Germany. A cloud swirl is present there, as
seen in the Meteosat image (Fig. 3.2). This implies the pollution is situated above the clouds
and therefore is probably transported by a frontal system. The wind fields describe a trough
(Fig. 3.3), providing fresh polar air over Great Britain and France to the south, which can
produce such kind of clouds [Bader et al., 1995].
Fig. 3.1: Rather high NO2 concentrations are extending from northeast to southwest,
with a maximum over Germany.
Conveyor belts are expected to rise polluted air before the trough, i.e., above Eastern Europe
and thus not in central Europe. Another feature, an occlusion, is extending over the Baltic Sea
to Northern Germany, but does not resemble the pattern found in the NO2, neither is
pronounced in the PV fields.
In this case it is not possible to conclude on how the pollution was lifted above cloud level
over Germany, and where it does originally stem from. The Po-valley pollution is expected to
move according to the winds near to the ground (Fig. 3.4), not contributing to the high NO 2
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observed over Germany. Wind fields in upper heights (850 hPa), often suitable for accessing
European scale transport (Fig. 3.5), confirm this. The general transport direction is east and
south, thus away from the pollution hot spot over Germany.
The situation in the Po-valley is probably uncorrelated with what happens in central Europe. A
relative stability appears from the BOLAM maps at the ground while westerly winds seem to
be present at higher level (see Fig. 3.5).
Fig. 3.2: In this visible Meteosat image, a cloud swirl can be recognised over France and
Germany. The shape of the clouds indicate polar air is mixed south.
Conclusions from the case study:
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Clouds may enhance the NO2 signal because of albedo effects. However, no coincidence of
cloud pattern and GOME values was found. Therefore we conclude the albedo effect is not a
large one.
Some NO2 from lower layers has been lifted above in the middle-upper troposphere by a
dynamical event. Here the NOx lifetime is expected to be longer and some adding-up may
have occurred. The Po-valley seems to have a different situation: not involved in the swirl
and cloud free, pollution is generated by local sources. No link is present between the two hot
spots from GOME picture 3.1. The Po-valley pollution increases during the day and is
transported by westerly winds towards the Adriatic sea.
Fig. 3.3: A trough is transporting polar air to Europe.
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Fig. 3.4: BOLAM temperatures and wind fields at the ground from 17 of February 2000.
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Fig. 3.5: BOLAM wind fields at 850 hPa from 17 of February 2000.
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3.4 Case 4: Occlusion lifting pollution
17 February 2001
User questions:
In which heights can the European background pollution be transported?
What causes high GOME values on cloud covered days?
What happens at occlusions?
Answer:
Polluted air can be lifted by fronts, or, especially occlusions. Low level stratus can
build up between warm and cold air masses, hindering optical remote sensing
measurements from ground. Ground-based methods cannot assess pollution which is
lifted above station height, transported and mixed. GOME data reveal that heavily
polluted air masses are lifted and moved over Europe above low-level stratus.
In this case, the clouds do not shield the pollution from GOME, but in contrary,
enhance the signal because of a high cloud albedo. Further, high NO2 is typical for
stagnant air over Central Europe, adding both effects yields extremely high GOME
values.
The conclusion is that the NOx pollution transport over Europe occasionally may
happen in the middle troposphere, where NOx has a longer lifetime than at the
ground [Brasseur, 1999 #132]. The frequency and importance of this process may
be accessed using GOME.
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Discussion of an example case study
On 17 Feb 2001, very high NO2 was recorded by GOME over central and eastern Europe. This
was confirmed by a pronounced signal with DOAS at Mt. Cimone (see Fig. 4.2), which was
cloud-free. The tropospheric column values was 1.9x10 15 molec/cm2 that is a tropospheric
loading an order of magnitude grater than climatological values for Mt. Cimone. The error in
tropospheric profile is estimated to be 20% at maximum and the difference with GOME
values for the same day (double with respect to DOAS result) could be due to different
factors. First the non-coincidence of the measurements time (GOME midday overpass, DOAS
sunset measurements) that in winter season is about 6 hours, could cause either the
instruments are not measuring the same air masses (dynamical factor) or the environmental
condition has favoured a decrease of the NO 2 lifetime (chemical factor). The second factor is
the strong gradient in NO2 tropospheric column that is present at the Po-valley south
boundary (as evident in Fig. 4.1). Being Mt. Cimone located along this boundary the definition
of the pixel above which perform the GOME average is a critical procedure for this particular
case. Unfortunately, no NO2 measurements are available at Jungfraujoch.
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Fig. 4.1: High GOME values over central Europe. The GOME values in the arctic cannot be
relied on, because of the stratospheric correction may fail when the vortex is not symmetric
around the pole (Richter and Burrows, 2001). Low values of GOME NO2 may be caused by
clouds shielding NO2. Comparison with Fig. 4.4 reveals that the high NO 2 is mostly situated
above the low-level clouds.
Fig. 4.2: DOAS tropospheric NO2 estimate from Mt Cimone (under clear sky) is very high.
The measurement is performed during local sunset. The GOME tropospheric column in the
Mt. Cimone area is obtained by averaging tropospheric column values above a symmetric
square pixel around the measurement site.
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Fig. 4.3: Live cams recorded the stratus over the Swiss plateau, which remained persistent
the whole day. Left: Northern view from Jungfraujoch (morning) , right: Southern view
(afternoon).
The meteorological situation is dominated by a high pressure system over England and rather
stagnant air over Europe since several days. Large areas are covered by stratus.
The stratus (fog) top reaches the height of 2100-2900 m in Switzerland, as is known from
various live-cams (Fig. 4.3) . A METEOSAT-7 image (Fig. 4.4) shows Southern Europe was
rather cloud free, and indicates stratus over Switzerland and parts of France and Germany.
The meteorological situation is prone to stagnation of air and accumulation of pollution.
The modelled wind fields (BOLAM) confirm rather stagnant air over the Po-valley. The
weather chart (Fig. 4.6) shows an occlusion exactly at the location where the very highest
NO2-values are situated. The occlusion feature is supported by the BOLAM modelled PV (Fig.
4.7), where a dry intrusion marks the occlusion over France and Germany.
Occlusions can shift pollution above the stratus into the middle troposphere (at least above
3500 m) (Fig. 4.5). This kind of pollution transport can only be detected by satellite and
confirmed by meteorological information.
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Fig. 4.4: Meteosat thermal infrared image. Higher clouds are bright, lower stratus are dark
grey.
Fig. 4.5: Schematic picture of an occlusion, as explained (e.g. in Liljequist,1994). An
occlusion can lift polluted air into the middle troposphere where it is subsequently spread or
transported.
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Fig. 4.6: Weather map of the case study. Note the occlusion extending from France over
Southern Germany to the east.
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Fig. 4.7: BOLAM wind fields and PV marking a dry intrusion connected with the occlusion.
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Conclusions from the case study:
In our case of 17 February 2001, pollution over central Europe accumulated over several calm
days before and was lifted above stratus. Thus, a strong NO 2 signal, further enhanced by the
cloud albedo, is obtained by GOME, showing high pollution spreading several hundreds of
kilometres.
The stagnant air above Po-valley let us conclude that the differences between GOME and
DOAS measurements above Mt. Cimone should be attributed more to chemical than
dynamical factors. A reasonable hypothesis is that NO 2 loading is located in the middle-lower
troposphere where the NOx lifetime is short (1 day with respect to 4-7 days in the upper
troposphere) and 6 hours difference in measurement time could be appreciable. These
considerations are consistent with inhibition of polluted air vertical mixing caused by stagnant
air.
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3.5 Case 5: Stagnant air over central Europe
1+2 March 2001
User question:
What causes extremely high NO2 concentrations over Europe?
Answer:
Pollutants accumulate in air masses, which remain stagnant over emission regions
for a few days and have limited exchange with fresh air.
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Discussion of an example case study
Extremely high pollution was observed from GOME from England to Germany on 1st of March
2000 (see Figure 5.1, left). On the next day, these high concentration fields have advanced
slightly to the east (see Fig. 5.1, right). The southern part of Europe was shaded by clouds
from the view of satellite (Fig. 5.2 and 5.3), resulting in low GOME values in southern Europe
(not to be misinterpreted as low pollution!).
Fig. 5.1: NO2 hot spots on two subsequent days. Note the colour scale is factor 3 different
from other case studies.
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Fig. 5.2: Central Europe was rather cloud free on 1st of March 2000 (left), note the dendritic
structure of the snow covered Alps. On 2nd of March (right), cloud cover was sparse over
central Europe, but stratus covered Southern France and whole of Italy.
Fig. 5.3: Meteosat infrared images show a cloud band over France to the Adriatic sea,
marking the warm front.
The regions of extremely high pollution (Fig. 5.1) coincide with the regions of stagnant air.
The rotating air masses in the centre of a cyclon can be relatively isolated from the ambient
fresh air, as it is the case in our example. Actually, two nested rotations are observed. The air
masses are rotating over Belgium/Netherlands, and on a larger scale rotating over Germany
to England (Fig. 5.4). These regions have strong emissions, thus pollutants accumulate.
From the wind fields it becomes evident that the high pollution found by GOME over the
North Sea is influenced from the emissions from Belgium, the Netherlands and Germany.
Further, the Po-valley air is clearly stagnant or advected east, and not contributing to the
high concentrations further north. The Alps act as a kind of barrier for the winds near the
surface, even the propagation of the front is affected by the Alps (Fig. 5.4).
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Fig. 5.4: BOLAM wind fields at 850 hPa and specific humidity (coloured) which indicate moist
air (blue to red) is moving from Spain north-eastward with a warm front. Left: two nested
areas of air masses are marked with ellipses. Because of the cyclonic rotation, the air is
rather secluded, and thus pollution is accumulating. Right: The next day, advection to the
east and a break up of the outer circle is indicated.
3.6 Case 6: Pollution source attribution
18 Sept 2001
User questions:
Can we do qualitatively comparisons between different hot spots,
observed at different days?
Can we identify the sources of large pollution hot spots ?
Answer:
When GOME NO2 is compared qualitatively, clear sky conditions have to be selected.
For clear sky situations, comparisons are possible. Because of the seasonal cycle of
NO2, the compared overpasses should be in the same season. The compared
overpasses should belong to the same weather period for comparable mixing and
transport processes.
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Under favourable conditions, the sources of pollution may be identified. Here it is
done by considering the history of the air masses with trajectory and wind field
analysis.
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Detailed discussion of the case:
During 17-19 September 2001, high NO2 concentration over Central Europe could be
observed from GOME (Fig. 6.1). On 17th, large parts of France and Germany are recorded
with higher NO2 (Fig. 6.1a). A loose cloud cover (which is no stratus) is forming (Fig. 6.2a).
This probably attenuates the signal for the satellite. However, the extend of the vast pollution
can be recognised. Either the satellite measures the pollution between the clouds, or pollution
is residing above. The convection which may produce this type of clouds cannot lift the
pollution above. Thus, the signal stems from between the clouds or possibly some frontal
feature did lift the pollution.
On 18th, high NO2 over central Europe is confirmed. Only two areas seem spared, Eastern
Germany and Southern France. The infrared image of Meteosat (Fig. 6.2d) reveals it is the
higher (and maybe denser) clouds, which produce the low signal in this areas. Rather high
NO2 values are recorded over the rest of central Europe, albeit some loose clouds appear in
the visible Meteosat image (Fig. 6.2c). The Po-valley and most of Switzerland have clear sky,
the snow-covered Alps can be seen (Fig. 6.2c). The NO 2 pollution in Switzerland is rather
high, and adjacent hot spots suggest that a substantial part have been transported there.
On 19th, the coverage of GOME matches the cloud free area over central/eastern Europe and
Italy. Some clouds over the Alps and the northern Po-valley may shield parts of the NO 2
there. Southern and Eastern Germany and most of Poland have clear sky. This allows some
qualitative comparison to the hot spots, which had been pronounced on the 18th of
September. The Western Europe hot spots had more NO 2 albeit some clouds were present.
To conclude that Eastern Europe did release less NO 2 between 17-19 September 2001 would
require modelling of the background pollution reaching different areas. A rather stagnant
deep pressure system (Fig. 6.3), which circulated the air anticlockwise over Europe, supports
the thesis that a lot of pollution was distributed over Europe.
The origin of pollution was investigated for Switzerland and the Po-valley. Generally, the
airflow there was from North to South. In Fig. 6.4 to 6.7, an exemplary set of trajectories is
shown. As the wind fields were changing only slowly, the trajectories give a consistent picture
of the air paths to the Jungfraujoch. Boundary layer air (thus enriched with pollutants) from
Belgium, the Netherlands and France had been lifted slowly to the Jungfraujoch (Fig. 6.4).
This is reflected in the ground-based data (Fig. 6.8), where from 16th to 18th September a rise
in the background is found. From 18th on, sometimes inflow of fresher air seems to take
place, which is confirmed by occasionally higher trajectories (Fig. 6.5), which may bring more
fresh air.
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a)
b)
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c)
Fig. 6.1: GOME measurements at a) 17.9.2001, b) 18.9.2001, c) 19.9.2001.
High tropospheric NO2 values are observed over central Europe.
a)
b)
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d)
e)
f)
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Fig. 6.2: Meteosat visible images (left). Clear sky areas are largest on 19 September (e).
Meteosat infrared images (right) indicate generally low clouds, whereas in (f) of 19
September, higher (brighter) clouds over France are present.
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Fig. 6.3: Weather chart of Europe. Note the deep pressure system at 500 hPa, which
describe the main direction of advection.
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Fig. 6.4: MeteoSwiss trajectories indicate that boundary layer air is transported to the
Jungfraujoch from northern direction.
Fig. 6.5: From 18 September 2001 on, occasionally some air from the middle troposphere
(and not from the ground) is arriving at Jungfraujoch.
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Fig. 6.6: Po-valley boundary layer air arrives at midnight of 18/19 September.
Fig. 6.7: Advection changes to a more cyclonic one during 19. September 2001.
ppb
NO2 at Jungfraujoch September 2001
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Fig. 6.8: Ground-based NO2 measurement series at Jungfraujoch.
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The period 16-19 September can be followed in the ground-based monitored concentrations
(Fig. 6.8). High peaks occur on noon and midnight of the 18th September. These are caused
by different air masses. The trajectories (Fig. 6.6) confirm a short episode of advection from
the Po-valley on 00 UTC of 19th September. Air (originally from Belgium, Netherlands and
France) crossed Switzerland, and has been advected to the Po-valley. Then it turned back to
the Alps. With high probability, some Po-valley boundary layer air was taken up, as is
indicated by the trajectories (Fig. 6.6, right plots). This event coincides with the highest peak
of NO2 sampled in September at Jungfraujoch.
Later in the day, the advection shifts more to the west (Fig. 6.7), and does not pick up Povalley air any more. Subsequently, concentrations at Jungfraujoch are decreasing again.
To summarise, three air masses (diluted to different extends), and sometimes a mix of it,
arrived at Jungfraujoch:
(1) polluted boundary layer air from Belgium, Netherlands and France
(2) higher, cleaner tropospheric air
(3) highly polluted air originating from the Po-valley plume.
At Mt. Cimone, advection of Po-valley air is expected. In addition, probably in the heights
above, the NO2 column is enriched with NO2 by the pollution advected from Central Europe.
This model is confirmed by the FLEXTRA trajectory (Fig. 6.9) which is in reasonable
coincidence with the Jungfraujoch trajectories.
Fig. 6.9: Backward trajectories arriving at Mt. Cimone after residing above Benelux,
Germany, and France for left: 06 am and right: 9 pm of 19th September 02, calculated with
the FLEXTRA model.
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On the 17th no pollution is detected from DOAS.and we have no GOME information on the Povalley for 17th. The wind fields (Fig. 6.11) show a quite stable situation in the Po-valley but
with an anticlockwise circulation that laps the southern part. In the morning of 18 th,DOAS
measured a relatively high NO2 tropospheric column with 1.5 1015 molec/cm2, which is
however 3 times lower than the GOME estimate. According to the statistic of GOME - DOAS
intercomparison (see POLPO User Handbook [Weiss et al., 2002]) this could be explained by
poor vertical transport from the surroundings towards Mt. Cimone. The observed NO 2 might
originate from (1) advection of pollution from Central Europe and (2) from a local contribution
from Po-Valley, where the NO2 caused by (2) contributes to the GOME estimate but not to the
DOAS observation. NO2 loading caused by (2) disappeares in the evening probably due to
the change in local circulation with strong winds coming from south-west direction.
On the 19th we measured a NO2 tropospheric column of 0.7 1015 molec/cm2 and BOLAM
analysis shows no changes in the wind direction with air coming from south-west. Excluding
thus a contribution of Po-valley pollution the transport of NO 2 rich air masses from the south
is a reasonable hypothesis taking also into consideration that GOME picture (Fig. 6.1) shows a
clear polluted pixels in the Rome area for the same day.
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Fig. 6.10: NO2 vertical profile retrieved from the DOAS system at Mt. Cimone from 17 th to
19th September. The concentration axis for AM and PM values is on the bottom and on the
top of the plot, respectively.
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Fig. 6.11: BOLAM wind fields at 850 hpa.
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3.7 Case 7: Po-valley plume to Switzerland
27 Sept 2000
User questions:
How well could an episode be observed by GOME,
where the Po-valley plume is spreading into Switzerland ?
Answer:
The spatial resolution and time resolution of GOME reaches its limits for this task.
Depending on the actual satellite track, the satellite measurement will average over
300 km of partly industrial, partly remote alpine area, blurring the signal. Further,
with coarse resolution it becomes likely that clouds are somewhere present in the
satellite’s field of view, suppressing the signal from NO2 below.
The time resolution (overpasses every 1-2 days) is critical because atmospheric
transport processes happen and change on time scales quicker than that. Further,
the NO2 has a chemical lifetime which is ignored here because GOME overpasses at
constant local time. We assume the NO2/NOx ratio to be constant for a certain time
of day and the NOx concentration preserved over a few days, thus ignore any
chemical concentration changes of NO2. As this simplifications are justified for
interpreting a snapshot, for transport processes it might be less appropriate.
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Discussion of an example case study
On 27th September 2000, the air masses reaching Jungfraujoch are expected to have resided
in the boundary layer of the Po-valley for two days (see trajectories in Fig. 7.1). A significant
accumulation of pollution could be expected, as the wind fields confirm (compare Fig. 7.2).
These had been rather stagnant and not changing over several days. Surprisingly, the
ground-based station at Jungfraujoch did not record risen concentrations (Fig. 7.3).
Fig. 7.1: MeteoSwiss trajectories indicate air from the eastern Po-valley reaches the
Jungfraujoch. The air paths are near to the ground (right). Thus highly polluted boundary
layer air is anticipated.
The GOME image (Fig. 7.4) was investigated too find hints on this puzzle. Switzerland and
the Po-valley is measured completely. High NO2 values stand out at the eastern Po-valley.
The NO2 concentration recorded over the western Po-valley are much lower. To suppose the
western Po-valley has cleaner air is a too rash conclusion.
Two possible pitfalls:
(1) The pixel over the Western Po-valley is covering industrial regions and alpine areas.
(2) Although the Po-valley is cloud free, the pixels are partly covered by clouds (as seen in
the Meteosat images (Fig. 7.5).
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It is also possible that indeed little NO2 was present in the western Po-valley, for some
unknown reasons (chemical or dynamical).
Summary: Albeit favourable conditions for measurements are prevailing, we cannot conclude
in this case whether western Po-valley air is polluted and whether it is reaching Switzerland .
Fig. 7.2: BOLAM surface wind fields are rather stagnant, no direction can be attributed in the
eastern Po-valley. From the stagnant wind fields, high pollution is anticipated.
ppb
NO2 at Jungfraujoch September 2000
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Fig. 7.3: Around 27th of September 2000, no pollution event is in the ground-based data of
Jungfraujoch, although anticipated from the meteorology.
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Fig. 7.4: GOME image displays high NO2 at the western Po-valley, and generally enhanced
pollution over central Europe.
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Fig. 7.5: Meteosat images in the visible (left) and infrared (right) wavelength display little
clouds over Europe. The Po-valley and Switzerland are rather cloud-free. Some higher
(brighter) clouds are present over Southern France.
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4 References
Bader, M.J., G.S. Forbes, J.R. Grant, R.B.E. Lilley, and A.J. Waters, Images in weather
forecasting - A practical guide for interpreting satellite and radar imagery, 2III, 499 S. pp.,
Cambridge University Press, Cambridge, 1995.
Bethan, S., G. Vaughan, C. Gerbig, A. Volz-Thomas, H. Richer, and D.A. Tiddeman, Chemical
air mass differences near fronts, Journal of Geophysical Research-Atmospheres, 103
(D11), 13413-13434, 1998.
Brasseur, G.P., Atmospheric chemistry and global change, 654 pp., Oxford University Press,
New York, 1999.
Stohl, A., and T. Trickl, A textbook example of long-range transport: Simultaneous
observation of ozone maxima of stratospheric and North American origin in the free
troposphere over Europe, Journal of Geophysical Research-Atmospheres, 104 (D23),
30445-30462, 1999.
Weiss, A., A. Petritoli, D. Schaub and P. Bonasoni, DUP-POLPO User Handbook, pp. 52, ESA,
Frascati, 2002.
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