Identification of wildfire aerosols in LIDAR data and model calculations of the vertical propagation of absorbing aerosols

Wildfires are a source of major perturbations in the earth-atmosphere system. Between 1997 and 2008, an area of roughly 4 · 106 km2 has been burned each year. Among other species, wildfire aerosols consist of black carbon that absorbs solar (short wave) radiation. This can lead to a warming of the surrounding air causing buoyancy and vertical transport, an effect known as radiatively induced self-lifting or short self-lifting. To assess the influence of wildfire aerosols on the climate system, it is important to know their altitude distribution. If wildfire aerosols are e.g. lifted into a cloud, they can alter the cloud development through the indirect and the semi-direct aerosol effects. In addition, vertical transport can alter the residence time and transport distance of the absorbing aerosols. Therefore, this study aimed to analyse LIDAR data for wildfire aerosols and to study the effects of radiation absorption on the vertical propagation of the absorbing aerosols. For this, LIDAR data from the ALOMAR research station have been analysed for potential aerosol layers between April and September 2010 and 2011 together with auxiliary data, like SYNOP data and radio-sonde measurements. The source regions of the aerosol layers above ALOMAR have been determined with the help of FLEXPART simulations and were compared to MODIS wildfire data. Two events have been identified with clear wildfire aerosol transport towards ALOMAR. The event showing an aerosol layer from fires west of Lake Superior was chosen for simulations with WRF-Chem. An aerosol layer above ALOMAR was reproduced in the WRF-Chem simulations but uncertainties in emissions and transport caused the simulated aerosol concentrations above ALOMAR to be too low. Therefore, no self-lifting has been found for this plume. However, signs of self-lifting have been found in a plume that originated from stronger wildfires in Kansas during the same time, and in an analysis of domain averaged properties. Two main effects of radiation absorption on the vertical propagation of the aerosols have been found: the additional radiation extinction through the absorption of short wave radiation led to a cooling of the ground at the emission sites, causing a trapping of the aerosols close to the ground above the fires. Later on, short wave heating caused a lifting of strong aerosol plumes. Wet-scavenging was reduced through the absorption of short wave radiation. An analysis of the probability distribution function of the black carbon aerosol concen-trations showed that the overall effect of the radiation absorption was to keep the plumes more intact. The effect without the trapping of the aerosols at the fires and the reduced wet-scavenging led to a dilution of the plume edges, indicating the development of a convectively mixed layer at the plume tops, whereas the plume centres were protected from dilution. Overall, self-lifting was only found important for concentrated plumes.