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: robert.hoeller@umweltbundesamt.at

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

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17.02.2005 19.03.2005 18.04.2005 18.05.2005 17.06.2005 17.07.2005 16.08.2005

mas

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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

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