What happened on Monday 16th October 2017? From diverse countries (France, United Kingdom, the Netherlands, Finland), people reported to have observed a “red Sun”. This is a direct result of natural events, although issued from different sources and locations, which combined together over the whole Europe.
During a couple of days, between 16th and 18th October 2017, northern Portugal and Spain were victims of violent wildfires ravaging parts of their territory. These fires released large amounts of fine black particles, also named aerosols, that can absorb the Sun light.
The Copernicus Atmosphere Monitoring Service (CAMS) forecasts of aerosol optical depth (AOD) show high values extending from the Iberian peninsula to the British Isles.
In addition to these fires, a tropical storm, named Ophelia, appeared 1300 km south-west of the Azores islands and some 2000 km off the African coast. Originally classified as a tropical storm, it was upgraded to a hurricane. The storm moved north-easterly, towards Spain and Britain, collecting sand from the Sahara desert. The related dust particles were then mixed with the black carbon from Portugal fires. Their scattering properties with the solar light led to this red Sun observed by many Europeans, instead of its natural yellow colour in clear sky or white/milky in presence of thin cloud.
The dust particles can be observed through the visible colour composite image from the MODIS instrument, on-board Aqua, on 16.10.2017: some yellow colours are mixed with some thin clouds.
It remains, overall, challenging for satellite measurements acquired in the visible spectrum to easily distinguish dust particles from transparent clouds or cirrus.
GOME-2 and OMI satellite sensors also reveal through the AAI index, with high values in red, the presence of black absorbing particles (i.e. smoke from biomass fires in Portugal) in large quantity. These particles were released in the North of Portugal before being transported to the North (UK) and then East (the Netherlands, Finland and Russia).
Although the aerosol particles were the visible part of the pollutant transport, IASI sensor revealed the additional presence of CO – Carbon monoxide, a toxic gas issued from incomplete biomass combustion by the fires. CO is a gas pollutant that cannot be visible in the eyes. It can only be measured in the shortwave or thermal infrared spectrum such as the IASI measurement.
This impressive pollutant transport is quite unique due the combination of heterogeneous sources at different locations. But, similar episodes of particle and gas pollution were observed last Summer in Canada as well, as described in our last post Canada wildfires from space and ground – Seeing beyond the flames: a series of observations.
All these worldwide satellite pictures very well illustrate that, although emissions can be national, mix of pollutants and their transport are not contained within the limits of borders. This shows how much pollution and their scientific and societal challenges are an international concern!
More information?
- CAMS, Saharan dust and smoke over France and UK here
- GOME-2 AAI index by KNMI, EUMETSAT AC SAF & TEMIS here
- OMI AAI maps on the TEMIS website here
- The Ophelia storm as seen by the ESA Sentinel-3 mission here
- OMI sensor here & IASI sensor here
- Canada wildfires from space and ground – Seeing beyond the flames: a series of observations WebPage here
Homepage Julien Chimot: a journey in Earth observation satellite science 
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