High values of particulate matter (PM10) were recorded in the air in many places across South-East Europe on 27 March 2020. The source of dust was traced back to Aralkum Desert.
By Natasa Strelec Mahovic and Jose Prieto (EUMETSAT), Amela Jericevic and Goran Gasparac (Croatia Control) and Ivan Smiljanic (CGI)
A continuous increase in the observed hourly values of particulate matter (PM10) was recorded in the continental part of Croatia, many parts of Slovenia, Hungary and Serbia, and even as far as northern Italy, in the period from 26 to 27 March.
In Zagreb, Croatia, observed PM values were ~25 µg/m3 at three different monitoring urban stations at 26 March at 12:00 UTC (blue lines in Figure 1), which then jumped to ~150 µg/m3 on 27 March at 07:00 UTC.
The highest values up to ~400 µg/m3 were observed in Zagreb on 27 March at 14:00 UTC, when Croatian capital became the most polluted city around the world (according to IQAir).
High PM concentrations were first observed at the monitoring stations Osijek (urban, red line in Figure 1) and Kopački rit (rural background, orange line in Figure 1) located in eastern Croatia. This indicates that pollution was transported in the easterly flow. Reports from Serbia and Hungary suggested the same.
In Figure 3 the transport of air pollution from the east to the west is confirmed with the trajectories calculated using the Hysplit model. Three days worth of backward trajectory arriving at sites in Zagreb, Croatia at 15:00 UTC on 27 March are shown.
Many cities in Central and South Eastern Europe experienced poor to extremely poor air quality on the afternoon of 27 March, see Figure 2 and dusty Zagreb photo (credit: Kristina Klemencic).
Satellite data analysis
Model trajectories of the dust source pointed to the area close to the Caspian Sea, and some media stated the Karakum Desert was the source. However, analysis of satellite data shows that the source of pollution was, in fact, the Aralkum Desert on the border between Kazakhstan and Uzbekistan.
The Meteosat-8 Dust RGB animation (Figure 4) shows that there were several episodes of dust being lifted-off the Aralkum Desert.
Figure 4: Meteosat-8 Dust RGB animation, 23 March 04:00 UTC–27 March 18:00 UTC
After 25 March, due to the synoptic situation, the dust reached the deformation zone and stretched in a prevailing east-west direction (Figure 5). From there it took a path towards SE Europe.
As well as the sand from the Aralkum Desert, there was also a substantial intrusion of Saharan dust into the cyclone over Balkans, as seen in Figure 5.
Before 27 March, due to cloudy conditions above most of the affected area of SE Europe, the dust could not be seen in the satellite images. On 27 March a dust cloud was vaguely visible in Suomi NPP VIIRS Natural Color imagery (Figure 6).
Changing dust colour in Dust RGB
An interesting feature could be observed in the dust cloud coming from the Aralkum Desert — during morning hours the typical pink colour of dust in the Dust RGB changed to a more white-green-light pink, as can be seen in the animation in Figure 4 and also in a comparison between the Dust RGB images at 02:00 UTC and 09:00 UTC on the 25 March (Figure 7).
This colour change is the result of green beam of the Dust RGB gaining values with the Sun rising, meaning that the IR10.8 µm channel becomes warmer than the IR8.7 µm channel.
There are two possible reasons for that. The change in the dust colour can be related to absorption/emission properties of the sand. Since the Aralkum Desert is made of dried sea, the sand there probably contains more salt particles, which are much smaller than sand grains (salt: 0.035–0.5 µm, very fine sand: 62 µm). For very small particles (<10 micron) emissivity in the IR8.7 µm channel is larger than emissivity in IR10.8 µm.
However, this does not explain why the 8.7–10.8 µm difference becomes larger during the day, compared to the night. The more probable reason is that the dust cloud was very thin (transparent) or rather low, having little influence on the day-time difference in either the IR10.8 µm or 8.7–10.8 µm difference. As a result of the strong day-night thermal variations, the difference between the 8.7 and 10.8 µm channels gets bigger for higher brightness temperatures.
Figure 8 shows that during the night the IR10.8 µm temperature in the 'dusty' area was between 268 and 271 K, whereas in the morning the values rose to between 290 and 295 K. In the same area the 8.7–10.8 µm difference was between 0 and 3 K in the night (03:00 UTC) and between 4 and 9 K in the morning (09:00 UTC).
The same analysis was done for an adjacent dust-free area and it showed that the IR10.8 µm temperatures were nearly the same as in the dusty area, whereas the 8.7–10.8 µm difference was about 1 K less during the night and about 1 K higher during the day in the dusty area, compared to the dust-free area. This leaves the larger day-time difference in favour of 10.8 µm under the responsibility of the ground, with rather small impact from the dust cloud.
Previous case studies
Dust cloud with unusual RGB colour (2 April 2009)