Extreme wildfires over northern Greece during summer 2023-Part A: Effects on aerosol optical properties and solar UV radiation

In Mediterranean countries, such as Greece, the frequency of forest fires has increased in recent years, primarily due to the widespread impacts of climate change. At the end of August 2023, Greece experienced a recordbreaking heatwave that triggered severe wildfire incidents, significantly impacting the atmospheric conditions of several cities, among them Thessaloniki. A significant number of wildfires occurred in northern Greece (Evros), burning thousands of hectares of the protected Dadia forest (Natura 2000) and releasing a significant load of smoke into the atmosphere. According to the European Strategy Forum on Research Infrastructures (ESFRI) a total area of 90,000 ha was burnt collectively by the Evros extreme wildfires during August 2023. The emissions led to severely adverse air pollution conditions, causing reduced visibility across an extended eastern Mediterranean region for several days. In this work, we analyze the influence of the transported biomass burning particles on the aerosol properties in the free troposphere, as well as on the surface UV radiation levels over Thessaloniki during the last week of August 2023. The transported smoke plume was detected over the Laboratory of Atmospheric Physics (LAP) in Thessaloniki, approximately 240 km away from the burning area, from August 22nd to the 25th. During this period, the presence of smoke led to exceptionally increased levels of aerosol optical depth (AOD), reaching up to 3.4 at 340 nm (the highest ever recorded in Thessaloniki), as well as high & Aring;ngstro<spacing diaeresis>m exponent (AE) values, peaking at 2.4 for the 440-870 nm range, followed by high Fine Mode Fractions (FMFs), indicating the prevalence of fine-mode smoke aerosol particle. Moreover, during the event, the presence of the biomass burning aerosols led to a strong attenuation of the solar UV irradiance by up to 90 %, reaching unprecedented levels, similar of a solar eclipse. The primary goal of this study is to highlight the extensive impacts that wildfires have, which are anticipated to increase in frequency in the near future, due to the predicted rise in the rate of occurrence of summer heatwaves especially for Mediterranean areas and specifically for Greece. Furthermore, the great value of the synergistic monitoring of the event with ground-based remote sensing instrumentation, along with satellite aerosol observations and modeling tools, is made prominent.