Aerosol optical properties at Lampedusa (Central Mediterranean) – 1. Influence of transport and identification of different aerosol types

Abstract

Abstract. Aerosol optical depth and Ångström exponent were obtained from multi filter rotating shadowband radiometer (MFRSR) observations carried out at the island of Lampedusa, in the Central Mediterranean, in the period July 2001–September 2003. The average aerosol optical depth at 495.7 nm, τ, is 0.24±0.14; the average Ångström exponent, α, is 0.86±0.63. The observed values of τ range from 0.03 to 1.13, and the values of α vary from −0.32 to 2.05, indicating a large variability in aerosol content and size. In cloud-free conditions, 36% of the airmasses come from Africa, 25% from Central-Eastern Europe, and 19% from Western France, Spain and the North Atlantic. In summer, 42% of the airmasses are of African origin. In almost all cases African aerosols display high values of τ and low values of α, typical of Saharan dust (average values of τ and α are 0.36 and 0.42, respectively). Particles originating from Central-Eastern Europe show relatively large average values of τ and α (0.23 and 1.5, respectively), while particles from Western France, Spain and the North Atlantic show the lowest average values of τ (0.15), and relatively small values of α (0.92). Intermediate values of α are often connected with relatively fast changes of the airmass originating sector, suggesting the contemporary presence of different types of particles in the air column. The largest values of α (about 2) were observed in August 2003, when large scale forest fires in Southern Europe produced consistent amounts of fine combustion particles that were transported to the Central Mediterranean by a persistent high pressure system over Central Europe. Smoke particles in some cases mix with desert dust, producing intermediate values of α. The seasonal distribution of the meteorological patterns over the Mediterranean, the efficiency of the aerosol production mechanisms, and the variability of the particles' residence time produce a distinct seasonal cycle of aerosol optical depths and Ångström exponent values. Particles originating from all sectors show a summer maximum in aerosol optical depth. The summer increase in optical depth for European aerosols is linked with an increment in the values of α that indicates an enhancement in the number of fine particles. The summer maximum of τ for African particles is associated with a weak reduction in the Ångström exponent, suggesting an increase in the total number of particles and a relatively more intense transport of large particles. The observations were classified according to the aerosol optical properties, and two main classes have been identified: desert dust and biomass burning/urban-industrial aerosols. Values of τ and α averaged over the whole observing period are 0.37 and 0.15 for desert dust, and 0.27 and 1.77 for urban-industrial/biomass burning aerosols. Lampedusa reveals a stronger influence of desert dust compared to other Mediterranean sites (mostly located on the coasts of Europe).

Identification of aerosol types over an urban site based on air-mass trajectory classification

“…Nevertheless, a little change in the boundaries of the sectors does not significantly affect the results on the climatological basis (Pace et al 2006;Kaskaoutics et al, 2009). Also, the use of different back-trajectories with varying duration (either 3 or 7 days) does not significantly modify the average aerosol optical properties in each sector.…”

“…AOD 500 nm ) and particle size (i.e. α) used by several investigators (Eck et al, 1999;Pace et al, 2006;Kaskaoutis et al, 2007Kaskaoutis et al, , 2009Kaskaoutis et al, , 2011Kalapureddy et al, 2009;Pathak et al, 2012;Sharma et. al.…”

“…The understanding of LRT of aerosols and their vertical extent is important in studies related to the estimation of radiative forcing (Badarinath et al, 2008). Moreover, along with aerosol physico-optical properties, the atmospheric chemical composition is also strongly influenced by LRT (Lelieveld et al, 2002;Pace et al, 2006). Therefore, there is an increase in the awareness of the potential of air-mass trajectories in advecting aerosols from distinct source regions and causing changes in the optical depth or composition or physical characteristics at far off locations (Tyson et al, 1996;Krishnamurti et al, 1998;Moorthy et al, 2001, Moorthy et al, 2003Kaskaoutis et al, 2014).…”

“…Pace et al, (2006) In the present work for defining the background aerosol type viz., the continental average (CA) type at the present study location which represents an urban continental environment influenced by human activities, a model given by Hess et al (1998) has been used. According to this aerosol model, magnitude of AOD 550 nm is 0.151 for background aerosols and AE in the spectral range 350 -500 nm is 1.11 while in the range 500 -800 nm, it is 1.42 at relative humidity (RH) of 80%.…”

“…Hence, a detailed knowledge of the optical properties of the key aerosol types is highly essential. Globally, numerous studies have been carried out to identify different aerosol types and to characterize their properties at several observing sites (Ramanathan et al, 2001;Dubovik et al, 2002;Moorthy et al, 2005Moorthy et al, , 2009Jayaraman et al, 2006;Pace et al, 2006;Kaskaoutis et al, 2007a, b;El-Metwally et al, 2008;Ogunjobi et al, 2008;Kalapureddy et al, 2009;Babu et al, 2011;Sinha et al, 2011;Chakravarty et al, 2011;Sinha et al, 2012;Pathak et al, 2012;Kulkarni et al, 2012;Padmakumari et al, 2013;Kanike et al, 2014). Based on these measurements, aerosols can be grouped into four main types viz., forest and grassland fire produced biomass burning aerosols, urban/industrial aerosols from fossil fuel combustions in populated urban/industrial regions, windblown desert dust injected into the atmosphere and marine aerosols (Kaskaoutis et al, 2007a).…”