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

Abstract. The most intense wildfire experienced in Eastern Spain since 2004 happened in Valencia during summer 2012. Although the fire was mostly active during days 29–30 June, a longer temporal period (from 24 June to 4 July) was selected for this analysis. Column-integrated, vertical resolved and surface aerosol observations were performed continuously at the Burjassot station throughout the studied period. The aerosol optical depth at 500 nm shows values larger than 2 for the most intense part of the wildfire and an extremely high maximum of 8 was detected on 29 June. The simultaneous increase of the Ångström exponent was also observed, indicating the important contribution of small particles in the smoke plume.

An extraordinary increase in the particle concentration near the ground was observed and hence the measured scattering coefficient was drastically enhanced. The scattering coefficient and the PM_2.5 level maxima reached the unusually high values of 2100 Mm⁻¹ and 160 μg m⁻³, respectively. These records represent an enhancement factor of 26 and 7 with respect to the climatological averages found in this station during June and July. The surface maxima were observed with 1-day lag from the maximum AOD, and this fact is linked with the mixing layer amplitude and the sedimentation of smoke particles.

The aerosol microphysical parameters and optical properties were determined for the whole period by combination of an inversion procedure and the Mie Theory. The smoke particles enhanced drastically the volume concentration of the fine mode with a maximum of 0.4 μm³ μm⁻², which is 10 times higher than the climatological summer background in this site. The simultaneous presence of dust and smoke particles at different altitudes was observed and hence the coarse mode was also significant during the most intense period of the wildfire episode. Therefore the aerosol single scattering albedo and the asymmetry parameter obtained during the smoke cases display high variability which is partially modulated by the volume of coarse particles.

The smoke episode highly contributed to increase the load of particles remaining in the atmosphere after the event, especially in the fine mode, although similar aerosol microphysical and optical properties were observed before and after the event. In addition, the particle concentration observed at surface level due to the wildfire episode largely contributes to exceed the EU annual limits for the particulate matter in the studied region.