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EVALUATION OF MERIS AEROSOL PRODUCTS FOR NATIONAL AND
REGIONAL AIR QUALITY IN AUSTRIA
Robert Höller (1),*, Christian Nagl (1), Herbert Haubold (1), Ludovic Bourg (2),
Odile Fanton d’Andon (2), and Philippe Garnesson (2)
(1) Umweltbundesamt, Spittelauer Lände 5, 1090 Wien, Austria
(2) ACRI-ST, 260, route du Pin Montard - B.P. 234,06904 Sophia Antipolis Cedex, France
ABSTRACT/RESUME
The Umweltbundesamt (Austrian Federal Environment Agency, FEA) is currently evaluating the potential of satellite
data for assessing national and regional air quality. Within the framework of the GSE project PROMOTE, Service Level
Agreements (SLA) were signed between the Umweltbundesamt and various data providers. In this paper, the ESA
standard MERIS aerosol product, provided by ACRI-ST in near-real time, is evaluated. Data are also provided
temporally aggregated for air quality reporting. In this paper, we present a first evaluation of ENVISAT/MERIS aerosol
products during the period March to August 2005 and comparisons to data from the in-situ ground based monitoring
network.
1 INTRODUCTION
The situation of the air quality in Austria improved quite strongly in the last decades in Austria. Concentrations of most
classic air pollutants were continuously reduced, and per capita emissions of SO2 and NOx are among the lowest of all
EU member states. Pollution levels are generally lower than the limit values for lead, benzene, and CO. SO2
exceedances are rare and usually caused by transboundary air pollution. Nevertheless, limit values for PM10
concentrations (50 µg m-3 daily average not to be exceeded more than 35 times per year; 40 µg m-3 yearly average) are
frequently exceeded in agglomerations (Figure 1), but exceedances are measured in nearly all regions of Austria,
particularly in the north-eastern part of the country, Alpine valleys and basins. Statistical analyses of back trajectories
showed that a considerable amount of the total PM10 levels in the eastern part of Austria is caused by long-range
transport, but also local sources add substantially to PM10 levels. Due to its geographic situation Austria is, therefore,
strongly affected by local, regional, and long-range air pollution. Therefore, the FEA supports further emission
reduction strategies on a national and international level [1].
The legal framework for monitoring air quality in Austria are the Austrian air quality protection act, ozone act, clean air
*Corresponding author. Tel.: +43-1-31304-3312, Fax.: +43-1-31304-3700; E-mail: [email protected]
act for steam boilers, as well as international obligations, such as the EC Air Quality Framework Directive and DD, and
the UNECE Convention of Long-Range Transboundary Air Pollution (CLRTAP) [2].
Figure 2 shows the air quality monitoring sites in Austria that are operated by the FEA. The total number of sites in
Austria, including the sites operated by the Federal Provinces, already exceeded 90 in 2003. For an analysis of
long-term trends, unfortunately only TSP measurements exist, monitoring of PM10 did not start until 2001. More
recently, sites measuring PM2.5 are added to the monitoring network.
Fig. 1. Number of days with daily average concentrations of PM10 higher than 50 µg m-3 [1].
Fig. 2. Air quality monitoring sites in Austria operated by the Umweltbundesamt [1].
For an analysis of the state of the atmosphere over Austria, besides the legally required monitoring and analysis
methods, the FEA is currently also evaluation the potential of satellite remote sensing and modelling methods. Within
the framework of the GEMS Service Element (GSE) project PROMOTE [3], several data products are evaluated and, if
necessary and possible, optimized to the user’s requirements. The mission of PROMOTE is to deliver the Atmosphere
GMES Service Element by constructing and delivering a sustainable and reliable operational service to support
informed decisions on atmospheric policy issues. The aim is an incremental enhancement of services during the lifetime
of the project.
2 MERIS DATA
MERIS aerosol data are delivered to FEA within a Service Level Agreement signed with ACRI-ST in the framework of
PROMOTE [3]. The service lasts for the period of the project, with a continuous delivery of near real-time (NRT)
aerosol products, that is, the aerosol optical thickness (AOT) and the Ångstöm exponent, as well as RGB images of the
same area. The time delay between data acquisition by the satellite and the delivery of the aerosol product and the
RGBs is presently about one day. Part of the SLA is also access to archive data (2003 and 2004), assistance to exploit
the data products, and the production of monthly, seasonal, and yearly average maps. The resolution of the aerosol
product is 1 km, with a latitude coverage of 44N - 50N, and a longitude coverage of 8E - 20E. A detailed description of
the standard MERIS aerosol algorithm is given in Santer et al. [4] and the MERIS aerosol algorithm ATBD [5, 6].
3 RESULTS
Fig. 3. Example image of the aerosol optical thickness over Austria and surroundings on July 30, 2005.
Figure 3 shows an example of the aerosol optical thickness over Austria and surrounding on July 30, 2005. Typically,
only for part of the area data is available, which is due to the coverage of the MERIS sensor. Also, the data only give a
snapshot of the aerosol at the time of the overpass of the satellite. It can be well recognized in this image that the
aerosol concentration is higher in valleys with lower altitude. A slightly higher AOT can also be recognized in the
south-east of Austria. Cloudy areas are screened out, but it is clear that the cloud-screening algorithm still needs to be
improved due to the high aerosol concentrations in the vicinity of clouds.
Fig. 4. Monthly mean of the aerosol optical thickness during March 2005.
Fig. 5. Monthly mean of the aerosol optical thickness during August 2005.
Figures 4 and 5 show average of the AOT during spring (March 2005) and summer (August 2005) over Austria and
surroundings. In Figure 4 it can be seen that even in a monthly average image, no full coverage of the area can be
achieved during spring. During winter (not shown here), the situation is even worse due to frequent cloud coverage and
due to the fact that the dark-target method of the MERIS algorithm cannot retrieve data over highly reflecting surfaces
such as snow and ice. It has to be mentioned that also for areas were data are available, only a small number of
measurements (up to 8) are made for one pixel. Therefore, the “monthly average” value is calculated from a much
smaller number of measurements than the values that are retrieved from ground in-situ stations, that measure the
aerosol concentration every hour during the day and night time
During summer, nearly full coverage of the area can be achieved, except for some high mountainous regions (Figure 5).
For one pixel, up to 15 measurements per month are achieved with fewer measurements in areas with frequent cloud
coverage. The clearly visible higher AOT in the southeastern part of Austria and over Slovenia might be an artefact due
to cloud contamination, but could also be due to higher emissions in this area. Therefore, data for this period have to be
investigated into more detail.
Several groups already performed comparisons of AOT data from satellite measurements with ground-based PM data
[7, 8, 9]. Here, for a comparison with ground-based data of PM10 measurements three background sites in Austria were
chosen, that is, Pillersdorf, Illmitz, and Enzenkirchen. All of these sites are only slightly influenced by local air
pollution. Figure 6 shows the PM10 concentrations (blue line) measured at Pillersdorf in the northeastern part of
Austria, near to the Czech border (see Figure 2). Also shown are the retrieved AOT values from the MERIS aerosol
product. The two datasets compare reasonable good, but the amount of data is still to small to make a more detailed
assessment. It is planned to use archived data from the years 2003 and 2004 for a long-term comparison. Also, PM2.5
data will be used for sites where these are available.
Pillersdorf
0
10
20
30
40
50
60
70
80
17.02.2005 19.03.2005 18.04.2005 18.05.2005 17.06.2005 17.07.2005 16.08.2005
mas
s co
ncen
tratio
n ( µ
g m
-3)
0
0,5
1
1,5
2
2,5
3A
OT
Fig. 6. Comparison of PM10 concentrations and aerosol optical thickness values (AOT) for the site Pillersdorf between
February and August 2005.
4 CONCLUSIONS
From this preliminary evaluation of the ENVISAT/MERIS aerosol product, several lessons could be learnt.
Disadvantages of MERIS aerosol data for air quality monitoring are missing data for cloudy pixels, and the low
temporal frequency of data compared to ground-based data. Also, the dark-target approach of the MERIS aerosols
algorithm does not provide aerosol information above high reflecting surfaces such as snow and ice, which limits the
usefulness of data in the winter period, during which usually the highest PM10 levels are observed. Moreover,
compared to ground-based measurements, the diurnal variability of the aerosol concentration is not covered. The
advantage of satellite data compared to the operational in-situ monitoring network clearly is the wide spatial coverage
of data. Especially in a country with a complex topography such as Austria, this could provide valuable additional
information for areas where no ground-based monitoring stations exist. Satellite data can also give valuable information
about long-range transport of pollution, and a synoptic view of the area of interest. One possible solution to the
undersampling might be the future use of data from geostationary satellites, possible combined with data from data
from sensors on LEO satellites to achieve higher accuracy.
5 ACKNOWLEDGEMENTS
This work was financially supported by the ESA GSE Project ‘PROMOTE’.
6 REFERENCES
1. Schneider, J., et al. (Umweltbundesamt, Ed.), Schwebestaub in Österreich, pp. 410, 2005 (in German).
2. State of the Environment in Austria (Umweltbundesamt, Ed.), Vol. 7, pp. 472, 2003.
3. PROMOTE webpage: www.gse-promote.org
4. Santer, R., V. Carrere, P. Dubuisson, and J. C. Roger, Atmospheric corrections over land for MERIS, Int. J. of Rem.
Sens., 20, 1819-1840, 1999.
5. Santer, R, Carrere, V., Dessailly, D., Dubuisson, P., and J.-C. Roger, MERIS Algorithm theoretical basis document,
ATBD 2.15, Atmospheric corrections over land.
6. Ramon, D., and R. Santer, MERIS Algorithm theoretical basis document, ATBD 2.19, Atmospheric corrections
over land: correction of directional effects over DDV.
7. Chu, D. A., et al. , Global monitoring of air pollution over land from the Earth Observing System –Terra Moderate
Resolution Imaging Spectroradiometer (MODIS), J. Geophys. Res., 108(D21), 4661, doi: 10.1029/2002JD003179,
2003.
8. Engel-Cox, J. A., R. A. Hoff, and A. D. J. Haymet, Recommendations on the use of satellite remote-sensing data
for urban air quality, J. Air & Waste Manage. Assoc., 54, 1360-1371, 2004.
9. Hutchison, K.D., Smith, S., and S. J. Faruqui, Correlating MODIS aerosol optical thickness data ground-based
PM2.5 observations across Texas for use real-time air quality prediction system, Atmos. Environ., 39, 7190–7203,
2005.