Abstract
Abstract. A novel procedure has been developed to retrieve, simultaneously, the optical, microphysical and chemical properties of tropospheric aerosols with a multi-wavelength Raman lidar system in the troposphere over an urban site (Athens, Greece: 37.9° N, 23.6° E, 200 m a.s.l.) using data obtained during the European Space Agency (ESA) THERMOPOLIS project, which took place between 15–31 July 2009 over the Greater Athens Area (GAA). We selected to apply our procedure for a case study of intense aerosol layers that occurred on 20–21 July 2009. The National Technical University of Athens (NTUA) EOLE 6-wavelength Raman lidar system has been used to provide the vertical profiles of the optical properties of aerosols (extinction and backscatter coefficients, lidar ratio) and the water vapor mixing ratio. An inversion algorithm was used to derive the mean aerosol microphysical properties (mean effective radius (reff), single-scattering albedo ω) and mean complex refractive index (m)) at selected heights in the 2–3 km height region. We found that reff was 0.14–0.4 (±0.14) μm, ω was 0.63–0.88 (±0.08) (at 532 nm) and m ranged from 1.44 (±0.10) + 0.01 (±0.01)i to 1.55 (±0.12) + 0.06 (±0.02)i, in good agreement (only for the reff values) with in situ aircraft measurements. The water vapor and temperature profiles were incorporated into the ISORROPIA II model to propose a possible in situ aerosol composition consistent with the retrieved m and ω values. The retrieved aerosol chemical composition in the 2–3 km height region gave a variable range of sulfate (0–60%) and organic carbon (OC) content (0–50%), although the OC content increased (up to 50%) and the sulfate content dropped (up to 30%) around 3 km height; the retrieved low ω value (0.63), indicates the presence of absorbing biomass burning smoke mixed with urban haze. Finally, the retrieved aerosol microphysical properties were compared with column-integrated sun photometer CIMEL data.
Highlights
Atmospheric aerosols have large influence on Earth’s radiation budget
The corresponding potential temperature and relative humidity (RH) values derived from local radiosounding at 00:00 UTC (Fig. 6-left) and water vapor mixing ratio values obtained by the EOLE Raman lidar (01:00–02:42 UTC) (Fig. 6-right) on 21 July 2009, were incorporated into the ISORROPIA II model (Fountoukis and Nenes, 2007) to propose the dry chemical composition of aerosols that is consistent with the retrieved m and ω values from the Raman lidar data (Table 1)
During THERMOPOLIS concurrent ground-based and aircraft measurements of the aerosol optical and microphysical properties were performed over Greater Athens Area (GAA) during July 2009
Summary
Atmospheric aerosols have large influence on Earth’s radiation budget. Recent estimations on the possible impact of aerosols (both direct and indirect effects) on the radiative forcing (cooling effect) in a global average show that they may be of the same order of magnitude as the CO2 effect (warming effect) (Kirkevag et al, 2008; Quaas et al, 2009; Ramanathan and Feng, 2009; Lohmann et al, 2010; Kelektsoglou et al, 2011, 2012; Kelektsoglou and Rapsomanikis, 2011), in addition to having direct impact on precipitation (Levin and Cotton, 2009; Lee, 2011; Wulfmeyer et al, 2011). The total direct aerosol radiative forcing combined across all aerosol types is −0.5 Wm−2, with a 5 to 95 % confidence range of −0.1 to −0.9 Wm−2 (IPCC, 2007) and with a medium-low level of scientific understanding This is especially true for the Eastern Mediterranean region (Stock et al, 2011), where insufficient knowledge of the microphysical properties of aerosol particles as well as their spatial distribution remains a key issue to evaluate their effect on global climate. The procedure and methodology described and followed in the present study is illustrated in Plate 1
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