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Tropospheric aerosol scattering and absorption over central Europe: A closure study for the dry particle state
This work analyses optical properties of the dry tropospheric aerosol measured at the regional Global Atmosphere Watch (GAW) observation site Melpitz in East Germany. For a continuous observation period between 2007 and 2010, we provide representative values of the dry-state scattering coefficient, hemispheric backscattering coefficient, absorption coefficient, single scattering albedo, and scattering Ångström exponent. Besides the direct measurement, the aerosol scattering coefficient was alternatively computed from experimental particle number size distributions using a Mie model. Within pre-defined limits, a closure could be achieved with the direct measurement. The achievement of closure implies that such calculations can be used as a high-level quality control measure for data sets involving multiple instrumentation. All dry-state optical properties show pronounced annual and diurnal variations, which are attributed to the corresponding variations in the regional emission fluxes, the intensity of secondary particle formation, and the mixing layer height. Air mass classification shows that atmospheric stability is a major factor influencing the dry aerosol properties at the GAW station. In the cold season, temperature inversions limit the volume available for atmospheric mixing, so that the dry-state aerosol optical properties near the ground proved quite sensitive to the geographical origin of the air mass. In the warm season, when the atmosphere is usually well-mixed during daytime, considerably less variability was observed for the optical properties between different air masses. This work provides, on the basis of quality-checked in situ measurements, a first step towards a climatological assessment of direct aerosol radiative forcing in the region under study.
Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles
The solar optical properties of Saharan mineral dust observed during the Saharan Mineral Dust Experiment (SAMUM) were explored based on measured size-number distributions and chemical composition. The size-resolved complex refractive index of the dust was derived with real parts of 1.51–1.55 and imaginary parts of 0.0008–0.006 at 550 nm wavelength. At this spectral range a single scattering albedo ωo and an asymmetry parameter g of about 0.8 were derived. These values were largely determined by the presence of coarse particles. Backscatter coefficients and lidar ratios calculated with Mie theory (spherical particles) were not found to be in agreement with independently measured lidar data. Obviously the measured Saharan mineral dust particles were of non-spherical shape. With the help of these lidar and sun photometer measurements the particle shape as well as the spherical equivalence were estimated. It turned out that volume equivalent oblate spheroids with an effective axis ratio of 1:1.6 matched these data best. This aspect ratio was also confirmed by independent single particle analyses using a scanning electron microscope. In order to perform the non-spherical computations, a database of single particle optical properties was assembled for oblate and prolate spheroidal particles. These data were also the basis for simulating the non-sphericity effects on the dust optical properties: ωo is influenced by up to a magnitude of only 1% and g is diminished by up to 4% assuming volume equivalent oblate spheroids with an axis ratio of 1:1.6 instead of spheres. Changes in the extinction optical depth are within 3.5%. Non-spherical particles affect the downwelling radiative transfer close to the bottom of the atmosphere, however, they significantly enhance the backscattering towards the top of the atmosphere: Compared to Mie theory the particle non-sphericity leads to forced cooling of the Earth-atmosphere system in the solar spectral range for both dust over ocean and desert.
Studying the vertical aerosol extinction coefficient by comparing in situ airborne data and elastic backscatter lidar
Vertical profiles of aerosol particle optical properties were explored in a case study near the San Pietro Capofiume (SPC) ground station during the PEGASOS Po Valley campaign in the summer of 2012. A Zeppelin NT airship was employed to investigate the effect of the dynamics of the planetary boundary layer at altitudes between ∼ 50 and 800 m above ground. Determined properties included the aerosol particle size distribution, the hygroscopic growth factor, the effective index of refraction and the light absorption coefficient. The first three parameters were used to retrieve the light scattering coefficient. Simultaneously, direct measurements of both the scattering and absorption coefficient were carried out at the SPC ground station. Additionally, a single wavelength polarization diversity elastic lidar system provided estimates of aerosol extinction coefficients using the Klett method to accomplish the inversion of the signal, for a vertically resolved comparison between in situ and remote-sensing results. Note, however, that the comparison was for the most part done in the altitude range where the overlap function is incomplete and accordingly uncertainties are larger. First, the airborne results at low altitudes were validated with the ground measurements. Agreement within approximately ±25 and ±20 % was found for the dry scattering and absorption coefficient, respectively. The single scattering albedo, ranged between 0.83 and 0.95, indicating the importance of the absorbing particles in the Po Valley region. A clear layering of the atmosphere was observed during the beginning of the flight (until ∼ 10:00 LT – local time) before the mixing layer (ML) was fully developed. Highest extinction coefficients were found at low altitudes, in the new ML, while values in the residual layer, which could be probed at the beginning of the flight at elevated altitudes, were lower. At the end of the flight (after ∼ 12:00 LT) the ML was fully developed, resulting in constant extinction coefficients at all altitudes measured on the Zeppelin NT. Lidar estimates captured these dynamic features well and good agreement was found for the extinction coefficients compared to the in situ results, using fixed lidar ratios (LR) between 30 and 70 sr for the altitudes probed with the Zeppelin. These LR are consistent with values for continental aerosol particles that can be expected in this region.
Vertical profiling of Saharan dust with Raman lidars and airborne HSRL in southern Morocco during SAMUM
Three ground-based Raman lidars and an airborne high-spectral-resolution lidar (HSRL) were operated duringSAMUM 2006 in southern Morocco to measure height profiles of the volume extinction coefficient, the extinction-to-backscatter ratio and the depolarization ratio of dust particles in the Saharan dust layer at several wavelengths. Aerosol Robotic Network (AERONET) Sun photometer observations and radiosoundings of meteorological parameters complemented the ground-based activities at the SAMUM station of Ouarzazate. Four case studies are presented. Two case studies deal with the comparison of observations of the three ground-based lidars during a heavy dust outbreak and of the ground-based lidars with the airborne lidar. Two further cases show profile observations during satellite overpasses on 19 May and 4 June 2006. The height resolved statistical analysis reveals that the dust layer top typically reaches 4–6 km height above sea level (a.s.l.), sometimes even 7 km a.s.l.. Usually, a vertically inhomogeneous dust plume with internal dust layers was observed in the morning before the evolution of the boundary layer started. The Saharan dust layer was well mixed in the early evening. The 500 nm dust optical depth ranged from 0.2–0.8 at the field site south of the High Atlas mountains, Ångström exponents derived from photometer and lidar data were between 0–0.4. The volume extinction coefficients (355, 532 nm) varied from 30–300Mm−1 with a mean value of 100Mm−1 in the lowest 4 km a.s.l.. On average, extinction-to-backscatter ratios of 53–55 sr (±7–13 sr) were obtained at 355, 532 and 1064 nm.
Vertically resolved dust optical properties during SAMUM: Tinfou compared to Ouarzazate
Vertical profiles of dust key optical properties are presented from measurements during the Saharan Mineral Dust Experiment (SAMUM) by Raman and depolarization lidar at two ground-based sites and by airborne high spectral resolution lidar. One of the sites, Tinfou, is located close to the border of the Sahara in Southern Morocco and was the main in situ site during SAMUM. The other site was Ouarzazate airport, the main lidar site. From the lidar measurements the spatial distribution of the dust between Tinfou and Ouarzazate was derived for 1 d. The retrieved profiles of backscatter and extinction coefficients and particle depolarization ratios show comparable dust optical properties, a similar vertical structure of the dust layer, and a height of about 4 km asl at both sites. The airborne cross-section of the extinction coefficient at the two sites confirms the low variability in dust properties. Although the general picture of the dust layer was similar, the lidar measurements reveal a higher dust load closer to the dust source. Nevertheless, the observed intensive optical properties were the same. These results indicate that the lidar measurements at two sites close to the dust source are both representative for the SAMUM dust conditions.