The Mediterranean basin is characterized by the coexistence of various aerosol types such us anthropogenic, desert dust, biomass burning (BB) and background marine particles, with relatively high aerosol load affecting the regional radiative budget. This diversity results in complex aerosol properties with high temporal and spatial variability, which is enhanced by the sporadic character of dust transport and wildfire events. The impact of aerosol load on the regional or local radiative budget is intensified during the summer cloud-free period due to the intense solar radiation. During the warm period the Mediterranean experiences the impact of BB emissions when forest and shrubland fires occur due to favorable meteorological conditions. BB aerosols consisting of two major components, black carbon which is the most absorbing aerosol species and organic aerosols that scatters solar radiation, can have a strong impact on the solar radiation budget, altering atmospheric temperature lapse rates and dynamics over a region. In this work an assessment of temporal variability and spatial distribution of biomass burning aerosol optical properties over the broader Mediterranean basin for the period 2002-2016 is conducted. To that end satellite databases are used. At first, wildfire events are identified in terms of ignition date and geographical location based on MODIS (MODerate resolution Imaging Spectroradiometer) collection 006 data. For our analysis wildfires with a burned area of at least 500ha are considered. Then, for each event aerosol optical properties namely spectral AOD, Ångström Exponent (AE), Fine Fraction (FF) and Aerosol Index (AI), that describe the atmospheric load and size of BB aerosols, are assessed. The relevant data are derived from Collection 006 MODIS - Terra database, except for AI which is taken from TOMS (Total Ozone Mapping Spectrometer) and OMI-Aura (Ozone Monitoring Instrument) databases for the periods 2002-2004 and 2005-2016, respectively. The spatial distribution and the inter-annual variability of BB aerosol optical properties are analyzed. The AI values are greater than 2 with maxima attaining 4.3. Ångström Exponent presents a mean value around 1.5 with maxima exceeding 2.5, whereas mean value of FF is about 0.84. AOD (AOD550) can reach values up to 3.3. For selected large wildfire events (mega-fires) the spatial expansion of smoke plume is estimated through forward - trajectories using the HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory Model) model and then BB aerosol properties are calculated over areas affected by the smoke.

Wildfires have been recognized as an intrinsic factor of the Earth system affecting vegetation functioning, structure and distribution with consequent impacts on terrestrial ecosystems, biogeochemical cycle, atmospheric composition, surface albedo and climate. At the same time, large uncontrolled fires are an environmental hazard due to their adverse effects on natural systems, the economy and human health. Wildfires regime is controlled by both, natural and human factors, such as vegetation type and cover, climate, weather, land management practices including human ignitions. Weather in particular, plays a key role in fire ignition and fire behavior as well, in terms of spreading, severity and suppression. Frequently, large fire events are the synergistic result of fuel load and anomalous atmospheric conditions. This work examines the atmospheric circulation characteristics associated with wildfires events occurring in the Mediterranean basin, a region which counts the most fires, and hence burned areas, in Europe. Wildfire data, namely ignition date and total burned area, are taken from MODIS (MODerate resolution Imaging Spectroradiometer) database over the period 2002-2016. For our analysis 2533 wildfires with a burned area of at least 500ha, which occurred in 871 days during the warm period of the year (May to September) are considered. In order to identify the atmospheric circulation characteristics associated with those wildfires, gridded (2.5°x 2.5°) NCEP/NCAR reanalysis data of 500 and 1000hPa geopotential height and 1000hPa temperature over the broader European area and 850hPa relative humidity across the Mediterranean basin are analyzed. Following the synergistic application of Factor Analysis for data dimensionality reduction, and Cluster analysis for the objective classification of daily synoptic conditions, seven (7) homogenous and distinct to each other mean atmospheric circulation patterns emerged. Specific characteristics of the revealed patterns favor the ignition and spread of wildfires in certain regions of the Mediterranean Basin. Wildfires in the Balkan Peninsula are favored by the intensification of the northerly flow over the region, due to the strengthening of the pressure gradient between the enhanced and spatially extended Azores Subtropical Anticyclone and the Asian thermal low. Iberian Peninsula wildfires are associated with the dominance of dry and windy conditions, controlled by the relative position and strength of the anticyclonic circulation over the Atlantic Ocean, whereas a thermal low developed over southern Spain and/or northwestern Africa plays also a key role.