Forest fire aerosol over the Mediterranean basin during summer 2003

Abstract

Observations at Lampedusa show that long‐lasting intense episodes of fine (Ångström exponent, α, > 1.5) light absorbing aerosol occurred in the central Mediterranean during summer 2003, along with exceptionally hot and dry conditions throughout continental Europe and the Mediterranean basin. The absorbing particles appear to be produced mostly by large‐scale intense forest fires in southern Europe. In this paper Moderate Resolution Imaging Spectroradiometer (MODIS) observations are used to determine the spatial and temporal extent of the summer 2003 forest fire aerosol episode over the Mediterranean sea. MODIS observations are first compared to measurements obtained with a multifilter rotating shadowband radiometer, MFRSR, at Lampedusa in July and August 2003. Values of the optical depth, τ, at 550 nm show a good agreement (linear correlation coefficient of 0.67, slope of 0.85). Ångström exponents retrieved from MODIS are larger than those from MFRSR at low values of α and are smaller than those from MFRSR at high values of α. In addition to Lampedusa, five other open sea locations representative of different sectors of the Mediterranean basin were chosen to study the evolution of the aerosol properties during July and August 2003. MODIS observations reveal that particles displaying α > 1.3 and relatively large aerosol optical depth are present at four out of the six locations for an extended period (11–16 days) in August. Trajectories ending at the four locations show that in all cases, air masses overpass active fires in southern Europe. MODIS observations between 2000 and 2004 show that the summer 2003 forest fire aerosol episode was the longest lasting and covered the largest area. Normally, summertime episodes lasted about 4–6 days and only covered two locations at a time. The aerosol optical properties observed at Lampedusa are used as input to a radiative transfer model to estimate the absorbing aerosol radiative effects. Aerosol radiative forcing in the 300–800 nm range and atmospheric heating rates are calculated assuming different aerosol vertical distributions. It is estimated that forest fire particles produce an increase in heating rate as large as 2.8 K/day at 20° solar zenith angle at the altitude of the aerosol layer. This large heating may have increased the atmospheric stability that helped to maintain the anomalously hot and dry conditions during summer 2003.