Light scattering by dust and anthropogenic aerosol at a remote site in the Negev desert, Israel

Abstract

[1] We investigated aerosol optical properties, mass concentration, and chemical composition over a 2 year period at a remote site in the Negev desert, Israel (Sde Boker, 30°51 0 N, 34°47 0 E, 470 m above sea level). Light-scattering measurements were made at three wavelengths (450, 550, and 700 nm), using an integrating nephelometer, and included the separate determination of the backscatter fraction. Aerosol coarse and fine fractions were collected with stacked filter units; mass concentrations were determined by weighing, and the chemical composition by proton-induced X-ray emission and instrumental neutron activation analysis. The total scattering coefficient at 550 nm showed a median of 66.7 Mm À1 (mean value 75.2 Mm À1 , standard deviation 41.7 Mm À1 ) typical of moderately polluted continental air masses. Values of 1000 Mm À1 and higher were encountered during severe dust storm events. During the study period, 31 such dust events were detected. In addition to high scattering levels, they were characterized by a sharp drop in the Å ngström coefficient (i.e., the spectral dispersion of the light scattering) to values near zero. Mass-scattering efficiencies were obtained by a multivariate regression of the scattering coefficients on dust, sulfate, and residual components. An analysis of the contributions of these components to the total scattering observed showed that anthropogenic aerosol accounted for about 70% of scattering. The rest was dominated by the effect of the large dust events mentioned above and of small dust episodes typically occurring during midafternoon.

Climatological aspects of aerosol optical properties in Northern Greece

“…6b one can clearly see the influence of the Sahara dust events on the size of aerosols, as revealed by the distinct peak around 0.4-0.6 found for the S sector. This maximum is less well defined forå σ , probably because most of the dust transport occurs at higher altitudes and does not influence surface aerosol properties as much as the column-integrated values (Formenti et al, 2001b;Andreae et al, 2002). The frequency distribution forå τ in the W sector is shifted to higher values serving again as a transition to the NW and N sectors, for which nearly identical and relatively broad distributions with maxima near 1.7 are found.…”

“…Such disturbances do not produce large changes to daily averages, but they are useful for certain case studies. The operating principles of the TSI nephelometer as well as the methodology used to extract σ sp and σ bsp are extensively described by Andreae et al (2002). Corrections for the angular truncation and the nonlambertian errors were performed using the correction equations proposed by Anderson and Ogren (1998).…”

“…In the last decade, a substantial number of experimental studies on aerosols have focused on the eastern Mediterranean area (Luria et al, 1996;Pinker et al, 1997;Mihalopoulos et al, 1997;Paronis et al, 1998;Papayiannis et al, 1998;Ichoku et al, 1999;Andreae et al, 2002;Formenti et al, 2001aFormenti et al, , b, 2002aLelieveld et al, 2002;Kouvarakis et al, 2002). This is because of the maximum in the net direct radiative forcing by sulfate aerosols predicted to occur in this region by various models (Charlson et al, 1991;Kiehl and Rodhe, 1995;Boucher and Anderson, 1995).…”

Smoking Rain Clouds over the Amazon

“…The net effect of aerosols on the atmospheric radiation budget and climate constitutes the greatest uncertainty in attempts to model and predict climate (1). Aerosols can counteract regional greenhouse warming by reflecting solar radiation to space or by enhancing cloud reflectance (2) or lifetime (3,4).…”

“…Several hundred thousand deforestation and agricultural fires burn in Amazonia during the dry season each year, covering vast areas with dense smoke (1,2). The smoke's radiative impact suppresses surface heating and evaporation and stabilizes the lower troposphere.…”