Aerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean basin or land stations in the region from multi-year ground-based AERONET observations with a focus on the Absorbing Aerosol Optical Depth (AAOD), Single Scattering Albedo (SSA) and their spectral dependence. The AAOD and Absorption Angström Exponent (AAE) dataset is composed of daily averaged AERONET level 2 data from a total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and/or soil dust. This dataset covers the 17-yr period 1996–2012 with most data being from 2003–2011 (~89% of level-2 AAOD data). Since AERONET level-2 absorption products require a high aerosol load (AOD at 440 nm > 0.4), which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angström exponent < 1.0 in order to study absorption by carbonaceous aerosols. The SSA dataset includes AERONET level-2 products. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 ± 0.01 (resp. 0.040 ± 0.01) and 0.050 ± 0.01 (0.055 ± 0.01) for urban (dusty) sites. Analysis shows that the Mediterranean urban-industrial aerosols appear "moderately" absorbing with values of SSA close to ~0.94–0.95 ± 0.04 (at 440 nm) in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing (SSA ~0.89–0.90 ± 0.04). The aerosol Absorption Angström Exponent (AAE, estimated using 440 and 870 nm) is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust (iron) and/or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate a possible East-West gradient, with higher values over the eastern basin (AAE_East = 1.39/AAE_West = 1.33). The North-South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols.

The objective of this study is to investigate the contribution of biomass burning emissions to O₃ production during small-scale dry-grass fires over Western Russia (24 April–10 May 2006) as well as to quantify the effect of biogenic emissions in this environment. By using the Factor Separation methodology, we evaluate the pure contribution of each one of these two sources and we appoint the significance of their synergistic effect on O₃ production. The total (actual) contribution of each source is also estimated. Sensitivity simulations assess the effect of various fire emission parameters, such as chemical composition, emissions magnitude and injection height. The model results are compared with O₃ and isoprene observations from 117 and 9 stations of the EMEP network, respectively. Model computations show that the fire episode determines the sensitivity of O₃ chemistry in the area. The reference run which represents grass fires with high NO_x/CO emission ratio (0.06) is characterized by VOC-sensitive O₃ production. In that case, the pure impact of fire emissions on surface O₃ is up to 40–45 ppb, while their synergistic effect with the biogenic emissions is proven significant (up to 8 ppb). Under a lower NO_x/CO molar ratio (0.025, representative of agricultural residues), the area is characterized by NO_x-sensitive chemistry and the maximum surface O₃ predictions are almost doubled due to higher O₃ production at the fire spots and lower fires' NO emissions.