Long range transport of Canadian wildfire smoke to Europe in 2023: aerosol properties and spectral features of smoke particles

The Canadian wildfires of 2023 had an unprecedented biomass burning season spanning from mid-April to late October. Towards the end of this long biomass burning season, there was a rare observation of smoke properties that occurred in about a week interval, that were different from the whole biomass burning season and that makes it quite notable. The observed aerosol properties were studied using remote sensing and in situ measurements for both short-range and long-range transported plumes across North America and Europe. One of the highlights was the observation of concave spectral curvature in aerosol optical depth (AOD) having maxima at higher wavelengths than the minimum measured wavelength (i.e., 340 nm) which led to negative values of Ångström exponent in spectral ranges below 500 nm. Along with this, large accumulation mode size distributions with volume median diameters reaching about 800 nm were observed. Another rare observation was the non-monotonic spectral curvature in single scattering albedo (SSA) associated with submicron size particles. For most of the stations, SSA increased in the UV-Visible region and/or further remaining either constant or decreasing at longer wavelengths as observed from column integrated (AOD) retrievals from remote sensing and coefficients from in situ measurements. Additionally, two stations in Canada and one in Europe were found to have a well-defined peak in AOD at 500 nm. These Canadian stations also displayed a non-monotonic spectral SSA with maxima at 675 nm, while the high altitude stations of Europe showed monotonically increasing SSA. Finally, a much higher (approximately 5 times) UV absorption than visible absorption indicated the presence of brown carbon and/or tar balls, which have a strong spectral dependence in imaginary refractive index. The SSA concave spectral curvature denotes the mix of black carbon and non-absorbing particulate matter and influence of particle size, while the AOD concave spectral curvature is attributed to particle size. The comparison of the ground based AOD measurements with satellite observation and model reanalysis showed an AOD underestimation ranging from 0.1 to 1.5.