Impact of peri-urban forest fires on air quality and aerosol optical and chemical properties: The case of the August 2021 wildfires in Athens, Greece.

Wildfires occurring near urban areas are known to have exceedingly detrimental impacts on the environment, air quality, economy and human health. In this framework, this study examines the effects of peri-urban forest fires on atmospheric chemical composition, and aerosol physical-optical properties in Athens, Greece, during August 2021. Satellite imagery and air mass trajectories showed advection of intense smoke plumes over Athens from three forest fires persisting for 10 days in the greater Athens area and in Central Greece (Euboea). During August 1-20, 2021, daily PM concentrations ranged from 8.9 to 78.7 μg m, and were associated with high OC levels (2.3-27.8 μg m), while BC and BC concentrations on smoke-impacted days were 2.6 μg m and 1.0 μg m, respectively (2-3 times higher than August mean levels). During the peak of biomass burning (BB) smoke transport over Athens, daily-average scattering and absorption coefficients at short wavelengths maximized at 313 Mm and 171 Mm, respectively. There was also a large impact of ambient BrC (brown carbon) absorption (60 Mm), while the OC/EC ratio exhibited characteristically low values (3-4), linked to flaming combustion (modified combustion efficiency of 0.97-0.99). The absorption Ångström exponent (1.38) and single scattering albedo (0.74) indicated highly absorbing BB aerosol, deviating from the normal summer patterns. BB-tracers like nssK displayed strong correlations with OC, EC and BC concentrations, as well as with scattering and absorption coefficients. However, forest fires drastically modified the levels of additional chemical species, with enhancements observed for Ca, NO, Cl, and for organic aerosol (OA) components such as BBOA and less-oxidized oxygenated OA (LO-OOA). Since under climate change conditions, the Mediterranean is anticipated to experience a dramatic rise in the frequency and intensity of wildfires, the results highlight the necessity for prevention and mitigation policies to safeguard urban air quality.