Ground/space, passive/active remote sensing observations coupled with particle dispersion modelling to understand the inter-continental transport of wildfire smoke plumes

Abstract

During the 2017 record-breaking burning season in Canada/United States, intense wild fires raged during the first week of September in the Pacific northwestern region (British Columbia, Alberta, Washington, Oregon, Idaho, Montana and northern California) burning mostly temperate coniferous forests. The heavy loads of smoke particles emitted in the atmosphere reached the Iberian Peninsula (IP) a few days later on 7 and 8 September. Satellite imagery allows to identify two main smoke clouds emitted during two different periods that were injected and transported in the atmosphere at several altitude levels. Columnar properties on 7 and 8 September at two Aerosol Robotic Network (AERONET) mid-altitude, background sites in northern and southern Spain are: aerosol optical depth (AOD) at 440¿nm up to 0.62, Ångström exponent of 1.6–1.7, large dominance of small particles (fine mode fraction >0.88), low absorption AOD at 440¿nm (<0.008) and large single scattering albedo at 440¿nm (>0.98). Profiles from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) show the presence of smoke particles in the stratosphere during the transport, whereas the smoke is only observed in the troposphere at its arrival over the IP. Portuguese and Spanish ground lidar stations from the European Aerosol Research Lidar Network/Aerosols, Clouds, and Trace gases Research InfraStructure Network (EARLINET/ACTRIS) and the Micro-Pulse Lidar NETwork (MPLNET) reveal smoke plumes with different properties: particle depolarization ratio and color ratio, respectively, of 0.05 and 2.5 in the mid troposphere (5–9¿km) and of 0.10 and 3.0 in the upper troposphere (10–13¿km). In the mid troposphere the particle depolarization ratio does not seem time-dependent during the transport whereas the color ratio seems to increase (larger particles sediment first). To analyze the horizontal and vertical transport of the smoke from its origin to the IP, particle dispersion modelling is performed with the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) parameterized with satellite-derived biomass burning emission estimates from the Global Fire Assimilation System (GFAS) of the Copernicus Atmosphere Monitoring Service (CAMS). Three compounds are simulated: carbon monoxide, black carbon and organic carbon. The results show that the first smoke plume which travels slowly reaches rapidly (~1¿day) the upper troposphere and lower stratosphere (UTLS) but also shows evidence of large scale horizontal dispersion, while the second plume, entrained by strong subtropical jets, reaches the upper troposphere much slower (~2.5¿days). Observations and dispersion modelling all together suggest that particle depolarization properties are enhanced during their vertical transport from the mid to the upper troposphere.