The characterization of long-range transported North American biomass burning plumes: what can a multi-wavelength Mie-Raman-polarization-fluorescence lidar provide?

Abstract. This article presents a study of long-range transported biomass burning aerosols (BBA) originated from the North American wildfires in September 2020. The BBA plumes presented in this study were in the troposphere and underwent 1–2 weeks aging before arriving at the observations site. A novel dataset 2α+3β+3δ+φ (α: extinction coefficient, β: backscatter coefficient, δ: particle linear depolarization ratio, PLDR, φ: fluorescence capacity) derived from lidar observations is provided for the characterization of long-range transported BBA. The fluorescence capacity describes the ability of aerosols in producing fluorescence when exposed to UV excitation. In the observations of BBA episode, plumes from different wildfire activities have been characterized. In the BBA plumes, we detected low PLDRs, i.e. lower than 0.03 at all wavelengths, as well as enhanced PLDRs (PLDR355,532,1064 ≈ 0.15–0.18, 0.12–0.14, 0.01–0.02) with a similar spectral dependence that had been observed in the aged BBA plumes in the upper troposphere and lower stratosphere (Canadian smoke in 2017 and Australian smoke in 2019–2020). Obvious variations in Angström exponent (−0.3–1.5), lidar ratios (20–50 sr at 355 nm, 42–90 sr at 532 nm) and fluorescence capacity (1.0 × 10−4– 4.0 × 10−4) are also observed during the BBA episode. These variations are coupled with the variation of altitudes, water vapor content and wildfire events. It reflects that the properties of aged BBA particles are highly varied and depend on complex mechanisms, such as burning process and the aging process. The results also pointed out the inhomogeneity of the aging process in the BBA plumes, which means that particles in the core of the plume aged differently with those at the plume edge due to the impact of water vapor, temperatures, particle concentration and so on. These chemical and physical processes involved in BBA aging and how they could impact the particle properties are not yet well understood. In addition, our observations identified the ice crystals mixing with BBA particles, which indicates that BBA could act as ice nucleating particles (INP) at tropospheric conditions. The lidar fluorescence proves to be an efficient tool in studying the interaction of clouds and BBAs due to its high sensitivity. Recent studies claimed that aged BBA particles are more effective INPs than they were thought. As BBAs are becoming an important atmospheric aerosols with growing global wildfires, our observations could improve our characterization about aged BBA particles and the understanding of their importance in ice cloud formation.

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