Extreme wildfires over Northern Greece during Summer 2023 – Part B. Adverse effects on regional air quality

The frequency, severity, and duration of wildfires are on the rise worldwide, underscoring the importance of comprehending the health impacts of wildfire exposure. The main aim of this work is to examine the effects that the Northern Greece forest fires during August 2023 had on regional air quality levels. On August 21st, a massive forest fire started within the Dadia Forest National Park, a protected area, nearby the city of Alexandroupoli. During the nearly ten-day duration of the fire, approximately 80,000 ha burned, while the strong prevailing easterly winds transported smoke particles and biomass burning gases over Northern Greece and the Mediterranean Sea, reaching the coasts of Italy and Tunisia. Using both spaceborne observations of nitrogen dioxide (NO₂), formaldehyde (HCHO) and carbon monoxide (CO), as well as ground-based instrumentation in the city of Thessaloniki, the extend of the degradation of air quality has been quantified. The spatially resolved Sentinel-5P TROPOspheric Monitoring Instrument (S5P/TROPOMI) observations showed how the fire plumes spread throughout Northern Greece, degrading massively the air quality near the fire event as well as over Thessaloniki, some 300 km to the west. NO₂ levels were enhanced up to 82 % over the fire location whereas in the metropolis of Thessaloniki by 36 %, testifying to the extent of the fire, the dispersion of the plume and the emission strength. During the main fire days HCHO levels were shown to be enhanced by 54 % in the greater area of Alexandroupoli and by 114 % in the vicinity of Thessaloniki, in line with other works. The longer-lived CO was found similarly enhanced in both locations, ∼26 %, with a longer lasting effect to regional air quality. Ground-based remote sensing resulted in similar findings with MAX-DOAS spectrophotometer observations of nitrogen dioxide capturing the build-up of the transported NO₂ load over Thessaloniki with morning levels reaching 30 Pmolec/cm², testifying to the magnitude of the transported NO₂ loads well beyond the stable variability of the typical August mean NO₂ levels of 10 Pmolec/cm². The MAX-DOAS also measured increased formaldehyde levels throughout the days of the fire event, between 25 and 35 Pmolec/cm², unaffected by the photochemical processes destroying NO₂, and well beyond the typical summertime levels of ∼12 Pmolec/cm². FTIR spectroscopy observations, also routinely performed over Thessaloniki, revealed that the long-lived carbon monoxide levels exceeded the expected background loads by more than 100.00 ± 10.61 ppb on the second major fire day. The predicted frequency increase of such major wildfire events places an imperative need for changes in the typical urban air quality monitoring practices currently in place, to account for unexpected degradation of the breathable air, as well as changes on how mitigation measures are presently applied by regional and central government.