Abstract
Abstract. A strong Saharan dust event that occurred over the city of Athens, Greece (37.9° N, 23.6° E) between 27 March and 3 April 2009 was followed by a synergy of three instruments: a 6-wavelength Raman lidar, a CIMEL sun-sky radiometer and the MODIS sensor. The BSC-DREAM model was used to forecast the dust event and to simulate the vertical profiles of the aerosol concentration. Due to mixture of dust particles with low clouds during most of the reported period, the dust event could be followed by the lidar only during the cloud-free day of 2 April 2009. The lidar data obtained were used to retrieve the vertical profile of the optical (extinction and backscatter coefficients) properties of aerosols in the troposphere. The aerosol optical depth (AOD) values derived from the CIMEL ranged from 0.33–0.91 (355 nm) to 0.18–0.60 (532 nm), while the lidar ratio (LR) values retrieved from the Raman lidar ranged within 75–100 sr (355 nm) and 45–75 sr (532 nm). Inside a selected dust layer region, between 1.8 and 3.5 km height, mean LR values were 83 ± 7 and 54 ± 7 sr, at 355 and 532 nm, respectively, while the Ångström-backscatter-related (ABR355/532) and Ångström-extinction-related (AER355/532) were found larger than 1 (1.17 ± 0.08 and 1.11 ± 0.02, respectively), indicating mixing of dust with other particles. Additionally, a retrieval technique representing dust as a mixture of spheres and spheroids was used to derive the mean aerosol microphysical properties (mean and effective radius, number, surface and volume density, and mean refractive index) inside the selected atmospheric layers. Thus, the mean value of the retrieved refractive index was found to be 1.49( ± 0.10) + 0.007( ± 0.007)i, and that of the effective radiuses was 0.30 ± 0.18 μm. The final data set of the aerosol optical and microphysical properties along with the water vapor profiles obtained by Raman lidar were incorporated into the ISORROPIA II model to provide a possible aerosol composition consistent with the retrieved refractive index values. Thus, the inferred chemical properties showed 12–40% of dust content, sulfate composition of 16–60%, and organic carbon content of 15–64%, indicating a possible mixing of dust with haze and smoke. PM10 concentrations levels, PM10 composition results and SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray) analysis results on sizes and mineralogy of particles from samples during the Saharan dust transport event were used to evaluate the retrieval.
Highlights
Atmospheric aerosols have a large impact on the planetary radiation budget, and are thought to exert a net cooling effect on climate (Andreae, 1995; Ramanathan et al, 2001; Heinold et al, 2007; Levin and Cotton, 2009; Ramanathan and Feng, 2009; Lohmann et al, 2010)
Our study aims to fulfill the existing gap on the vertical profiling of the aerosol dust properties, focusing on the retrieval of the vertical profiling of the optical, microphysical and composition of aged dust aerosol particles associated with a strong saharan dust event, as they interact with anthropogenic particles in the lower free troposphere over an urban site (Athens, Greece)
The National Technical University of Athens (NTUA) lidar system is located on Campus in the city of Athens (37.97◦ N, 23.79◦ E, 200 m a.s.l.), and has been continuously operating since the initiation of the EARLINET project (Bosenberg et al, 2003) in February 2000
Summary
Atmospheric aerosols have a large impact on the planetary radiation budget, and are thought to exert a net cooling effect on climate (Andreae, 1995; Ramanathan et al, 2001; Heinold et al, 2007; Levin and Cotton, 2009; Ramanathan and Feng, 2009; Lohmann et al, 2010). Specific dust experiments (e.g. SAMUM 1 and 2) (Ansmann et al, 2011 and references their in) focused on the estimation of the radiative effects of dust, their results had a rather regional (Saharan region) and a limited temporal coverage; systematic vertical profiles of aerosol optical-microphysical and chemical data around the globe are still missing, and open questions about the aerosol role on climate yet exist. The gap concerning the vertical profiling of the aerosol properties can be filled by synergy of systematic lidar (ground-based, airborne and space-borne measurements to derive the optical-microphysical properties) and of in situ measurements (to derive the optical-microphysical-chemical properties) (e.g. Kandler et al, 2009; Weinzierl et al, 2009; Lieke et al, 2011), which are able to provide much of the information required to constrain models and reduce uncertainties associated with radiative forcing estimate.
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