High sulphur dioxide concentrations over Bulgaria

By Jochen Kerkmann (EUMETSAT), Valery Spiridonov, Christo Georgiev and Zlatomir Dimitrov (NIMH Bulgaria)

The area near Stara Zagora in the southern part of Bulgaria frequently suffers from high concentrations of industrial SO₂. The source for the SO₂ is several coal power plants around Gulubovo and Radnevo that release SO₂ through four or five chimneys. The total pollution from these plants is about 2200 tons SO₂ per day. An extreme case which was detected by the operational Meteosat satellite occurred on 15 June 2005.

15 June 2005 was completely cloud-free, thus, allowing a clear view on the low-level SO₂ plume from satellites like MSG (SEVIRI), ENVISAT (SCIAMACHY) and AURA (OMI). However, for 15 June 2005, due to the low spatial coverage, no measurements are available from SCIAMACHY (the nearest in time measurements are from 13 June!).

The low-level SO₂ plume can be best detected by Meteosat during day-time when the contrast between the hot land surface and the cooler SO₂ plume is largest. This is shown in the image and the animation of the brightness temperature difference (BTD) between the IR10.8 and IR8.7 channels (Figure X).

During day-time, the brightness temperature of the IR8.7 channel, in the case of SO₂ clouds, tends to be significantly lower than the IR10.8 (due to a SO₂ absorption band around 8.6 microns). The exact BTD depends of course on the SO₂ concentration and the lapse rate: the larger the SO₂ concentration and the larger the temperature difference between the surface and the SO₂ cloud, the larger the BTD between these two channels.

In the case shown below, at 12:00 UTC, the BTD assumes values of +5.3K, which falls outside the normal range for this BTD for clear pixels (for Central Europe, about 0 to +4K, depending on the humidity content). The largest BTD values are derived at pixels close to the point of a measurement of SO₂ concentration of about 760 micrograms per cubic meter. The reported measurement was taken manually, at ground level, in the coal power plant Gulubovo, which mainly produces coal briquettes and operates a small power plant.

The contribution of this power plant to the indicated overall amount of pollution in the region is minor. However, its chimney is only 150m  tall, so the pollution is often above the limit near the ground. There are no measurements of high-level concentrations reported in any of the other sites on 15 June 2005. In this case, the presence of SO₂ near Gulubovo is easily detected by the satellite during the day.

During night-time, the BTDs are significantly smaller (due to lower surface temperatures) and can even be negative in the case of a strong low-level temperature inversion.

It should be noted that the detectability of SO₂ plumes is strongly reduced over arid or semi-arid areas (low emissivity of IR8.7 channel) and at high satellite viewing angles (limb cooling effect). This can be seen in the images below over the Sahara desert (emissivity effect) and over northern Russia (limb cooling effect).

Interestingly, high values of the BTD between IR10.8 and IR8.7 channels (indicating the presence of an absorbing gas, whether SO₂ or something else) are also observed in the German/Dutch border region where there is also heavy industry.