MODELING AEROSOL EFFECTS ON THE ATMOSPHERE DURING THE
APRIL 2020 WILDFIRE EPISODE

Wildfires are among the biggest sources of aerosol emissions, in particular black and organic carbon. These aerosols modify meteorological processes through direct and indirect aerosol effects. Despite our knowledge of the main aerosol-meteorology interactions, their role in the atmosphere often remains uncertain depending on weather conditions. This study presents the analysis of the main aerosol effects observed during the severe wildfire event that occurred in the Chornobyl Exclusion Zone (CEZ) in the north of Ukraine in April 2020.

The study is based on seamless modeling using the Environment-High Resolution Limited Area Model (Enviro-HIRLAM). Simulations were performed at 15 km horizontal resolution with downscaling to 5 and 2 km resolution. The model covered 40 vertical levels with a 3-hour data output. Based on the emissions derived from the IS4FIRES and IASA ECLIPSE, it was simulated the three-dimential distribution, transportation, and deposition of black carbon (BC) and organic carbon (OC), including other main aerosol types (dust, sea salt, and sulfates) for considering real aerosol content. Aerosol components were divided into groups by their size (Aitken, accumulation, and coarse modes) and solubility (soluble or insoluble). Enviro-HIRLAM simulations included runs with aerosol effects (direct (DAE), indirect (IDAE), and both (DAE+IDAE) aerosol effects included) and a reference (REF) run without aerosol effects.

Elevated BC and OC content significantly modified meteorological conditions, mainly because of DAE. During the period of clear-sky conditions, aerosols caused local 2-m air temperature decrease up to -3°C. At the same time, cloudy conditions caused the opposite effect, and the 2-m air temperature increased up to 5°C, especially on the edge of the stationary front, which was observed on April 14, 2020. Emitted aerosols affected the moisture regime and resulted in drier conditions with a lower amount of precipitation. These effects were prevailing despite elevated BC and OC content, which influenced cloud formation in different ways depending on weather conditions. The changes were also observed for wind. However, the suggestion is that the observed wind changes because of DAE and IDAE might happen due to the spatial shifts in wind patterns as a result of the overall impact, not because of a direct influence on air pressure at the local scale.

The presented results were obtained within individual grant INFRAIA-2016-1-730897 “High Performance Computing Europa-3 (HPC-Europa3) Transnational Access programme” while conducting the project “Integrated modeling for assessment of potential pollution regional atmospheric transport as result of accidental wildfires” (2020–2022).

Keywords: Enviro-HIRLAM, seamless modeling, direct and indirect aerosol effects