Analysis of a strong wildfire event over Valencia (Spain) during Summer 2012 – Part 1: Aerosol microphysics and optical properties

摘要

The most intense wildfire experienced in Eastern Spain since 2004 happenedin Valencia during summer 2012. Although the fire was mostly active duringdays 29–30 June, a longer temporal period (from 24 June to 4 July) wasselected for this analysis. Column-integrated, vertical resolved and surfaceaerosol observations were performed continuously at the Burjassot stationthroughout the studied period. The aerosol optical depth at 500 nm showsvalues larger than 2 for the most intense part of the wildfire and anextremely high maximum of 8 was detected on 29 June. The simultaneousincrease of the Ångström exponent was also observed, indicating theimportant contribution of small particles in the smoke plume.An extraordinary increase in the particle concentration near the ground wasobserved and hence the measured scattering coefficient was drasticallyenhanced. The scattering coefficient and the PM level maxima reached theunusually high values of 2100 Mm and 160 μg m,respectively. These records represent an enhancement factor of 26 and 7 withrespect to the climatological averages found in this station during June andJuly. The surface maxima were observed with 1-day lag from the maximum AOD,and this fact is linked with the mixing layer amplitude and thesedimentation of smoke particles.The aerosol microphysical parameters and optical properties were determinedfor the whole period by combination of an inversion procedure and the MieTheory. The smoke particles enhanced drastically the volume concentration ofthe fine mode with a maximum of 0.4 μm μm, which is 10times higher than the climatological summer background in this site. Thesimultaneous presence of dust and smoke particles at different altitudes wasobserved and hence the coarse mode was also significant during the mostintense period of the wildfire episode. Therefore the aerosol singlescattering albedo and the asymmetry parameter obtained during the smokecases display high variability which is partially modulated by the volume ofcoarse particles.The smoke episode highly contributed to increase the load of particlesremaining in the atmosphere after the event, especially in the fine mode,although similar aerosol microphysical and optical properties were observedbefore and after the event. In addition, the particle concentration observedat surface level due to the wildfire episode largely contributes to exceedthe EU annual limits for the particulate matter in the studied region.