Manfred Wendisch

Saharan Mineral Dust Experiments SAMUM-1 and SAMUM-2: What have we learned?

Two comprehensive field campaigns were conducted in 2006 and 2008 in the framework of the Saharan Mineral Dust Experiment (SAMUM) project. The relationship between chemical composition, shape morphology, size distribution and optical effects of the dust particles was investigated. The impact of Saharan dust on radiative transfer and the feedback of radiative effects upon dust emission and aerosol transport were studied. Field observations (ground-based, airborne and remote sensing) and modelling results were compared within a variety of dust closure experiments with a strong focus on vertical profiling. For the first time, multiwavelength Raman/polarization lidars and an airborne high spectral resolution lidar were involved in major dust field campaigns and provided profiles of the volume extinction coefficient of the particles at ambient conditions (for the full dust size distribution), of particle-shape-sensitive optical properties at several wavelengths, and a clear separation of dust and smoke profiles allowing for an estimation of the single-scattering albedo of the biomass-burning aerosol. SAMUM–1 took place in southern Morocco close to the Saharan desert in the summer of 2006, whereas SAMUM–2 was conducted in Cape Verde in the outflow region of desert dust and biomass-burning smoke from western Africa in the winter of 2008. This paper gives an overview of the SAMUM concept, strategy and goals, provides snapshots (highlights) of SAMUM–2 observations and modelling efforts, summarizes main findings of SAMUM–1 and SAMUM–2 and finally presents a list of remaining problems and unsolved questions.

Assessing 1D Atmospheric Solar Radiative Transfer Models: Interpretation and Handling of Unresolved Clouds

Abstract The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended? One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport ...

Dust mobilization and transport in the northern Sahara during SAMUM 2006 – a meteorological overview

The SAMUM field campaign in southern Morocco in May/June 2006 provides valuable data to study the emission, and the horizontal and vertical transports of mineral dust in the Northern Sahara. Radiosonde and lidar observations show differential advection of air masses with different characteristics during stable nighttime conditions and up to 5-km deep vertical mixing in the strongly convective boundary layer during the day. Lagrangian and synoptic analyses of selected dust periods point to a topographic channel from western Tunisia to central Algeria as a dust source region. Significant emission events are related to cold surges from the Mediterranean in association with eastward passing upper-level waves and lee cyclogeneses south of the Atlas Mountains. Other relevant events are local emissions under a distinct cut-off low over northwestern Africa and gust fronts associated with dry thunderstorms over the Malian and Algerian Sahara. The latter are badly represented in analyses from the European Centre for Medium–Range Weather Forecasts and in a regional dust model, most likely due to problems with moist convective dynamics and a lack of observations in this region. This aspect needs further study. The meteorological source identification is consistent with estimates of optical and mineralogical properties of dust samples.

Cloud droplet nucleation scavenging in relation to the size and hygroscopic behaviour of aerosol particles

The size distributions and hygroscopic growth spectra of aerosol particles were measured during the GCE cloud experiment at Great Dun Fell in the Pennine Hills in northern England. Hygroscopic growth is defined as the particle diameter at 90% RH divided by the particle diameter at 10% RH. The fraction of the aerosol particles scavenged by cloud droplets as a function of particle size was also measured. The general aerosol type was a mixture of marine and aged anthropogenic aerosols. The Aitken and accumulation mode numbers (average ± 1 S.D.) were 1543 ± 1078 and 1023 ± 682 cm−3 respectively. The mean diameters were in the range 30–100 nm and 100–330 nm. The hygroscopic growth spectra were bimodal about half the time. The less-hygroscopic particles had average growth factors of 1.06, 1.06, 1.03, 1.03, and 1.03 for particle diameters of 50, 75, 110, 165, and 265 nm, respectively. For the more-hygroscopic particles of the same sizes, the average hygroscopic growth was 1.34, 1.37, 1.43, 1.47, and 1.53. The effects of ageing on the aerosol particle size distribution and on hygroscopic behaviour are discussed. The scavenged fraction of aerosol particles was a strong function of particle diameter. The diameter with 50% scavenging was in the range 90–220 nm. No tail of smaller particles activated to cloud drops was observed. A small tail of larger particles that remained in the interstitial aerosol can be explained by there being a small fraction of less-hygroscopic particles. A weak correlation between the integral dry particle diameter and the diameter with 50% scavenging was seen.

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.

STAAARTE-MED 1998 summer airborne measurements over the Aegean Sea 2. Aerosol scattering and absorption, and radiative calculations

descent over a ground-based site in northeastern Greece (40� 24 0 N, 23� 57 0 E; 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm � 1 , and the particle absorption coefficient was around 15 Mm � 1 , giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6–9% of the total accumulation mode particle mass (<1 mm diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 m mi s up to � 64 W m � 2 at the surface and up to � 22 W m � 2 at the TOA. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 9335 Information Related to Geographic Region: Europe; KEYWORDS: aerosols, Aegean Sea, optical properties, vertical profiles, direct radiative forcing

Aerosol optical properties and large‐scale transport of air masses: Observations at a coastal and a semiarid site in the eastern Mediterranean during summer 1998

Spectral measurements of the aerosol particle scattering coefficient σs and the aerosol optical depth τa were conducted at Ouranoupolis (Greece, 40°23′N, 23°57′E) and at Sde Boker (Israel, 30°51′N, 34°47′E) between June and September 1998. Measurements were related to 5-day three-dimensional back trajectories at 950, 850, and 550 hPa to assess the influence of long-range transport from particular source regions to the aerosol load at the two sites. Our measurements show that the eastern Mediterranean basin is moderately to highly polluted during summer. Daily average σs values at 550 nm were typically in the range of 30–200 Mm−1 at both sites. The range obtained for the summer regional aerosol optical depth τa was 0.03–0.52 at 500 nm. Enhanced aerosol extinction was related to transport of polluted air masses from western and eastern Europe. High-altitude transport of mineral dust from northern Africa was observed at both sites, particularly in Israel.

An Airborne Spectral Albedometer with Active Horizontal Stabilization

Abstract An airborne albedometer including a low-cost, precise, and fast sensor head horizontal stabilization system was developed to measure spectral down- and upward irradiances between 400- and 1000-nm wavelength. It is installed on a small research aircraft (type Partenavia P68-B), but it can easily be mounted on other aircraft as well. The stabilization unit keeps the two radiation sensor heads (up- and downward looking) of the albedometer in a horizontal position during the flight with an accuracy of better than ±0.2° over a range of pitch and roll angles of ±6°. The system works properly for angular velocities up to 3° s-1 with a response time of the horizontal adjustment of 43 ms. Thus it can be applied even under turbulent atmospheric conditions. The limitations of the stabilization have been determined by laboratory and in-flight performance tests. As a result it is found that the new horizontal stabilization system ensures that misalignment-related uncertainties of the measured irradiances are ...

Night-time formation and occurrence of new particles associated with orographic clouds

Abstract The formation and occurrence of new ultrafine aerosol particles were studied in association with an orographic cloud during a field experiment at Great Dun Fell (GDF), Northern England. Three size spectrometers to measure submicrometer aerosol particles were located upwind, on top, and downwind of GDF Summit to investigate changes in the aerosol size distribution. During two night-time cloud periods, ultrafine particles were observed downwind of the hill while no particles were detected upwind of the hill. During one cloud event, there was some evidence of entrainment. In this case, the occurrence of ultrafine particles may have been due to entrainment from aloft or by homogenous nucleation downwind of the hill. During the other cloud event, the formation of an ultrafine particle mode (nucleation mode) occurred probably after the cloud passage. There was no evidence of entrainment during this time period. Multicomponent homogeneous nucleation models were used to simulate the formation of new particle downwind of an orographic cloud. Possible homogeneous nucleation processes for this could be the formation of sulphuric acid or ammonium chloride due to outgassing of hydrochloric acid. It was not possible, however, to simulate formation rates of new particles as observed downwind the hill using a model for the binary or ternary homogeneous nucleation process of ammonia and hydrochloric acid. During the first event with high sulphur dioxide concentrations, the formation of new particle via binary homogeneous nucleation of sulphuric acid and water could be only predicted using a high nighttime hydroxyl radical concentration. No formation of sulphuric acid particle could be simulated during the second event with low sulphur dioxide concentrations.

Possibility of refractive index determination of atmospheric aerosol particles by ground-based solar extinction and scattering measurements

Abstract A method is developed to infer the optical properties of aerosol particles from ground-based solar extinction and scattering measurements under cloudless conditions. On the basis of the spectral aerosol optical thickness (extinction measurement) radiative transfer calculations are carried out yielding the diffusely scattered radiances at the ground. These calculated radiances are compared with the scattering measurement. The unknown refractive index and consequently all the other optical aerosol properties are varied in the model calculations until best agreement to the measurement is achieved. Thus a complete set of aerosol characteristics is deduced describing the measurements in an optically equivalent sense. The method is examined by means of aerosol models. These tests show that the method works well if many optically effective fine particles are present and in the case of slight absorption. The imaginary part of the refractive index cannot be obtained with the present version of the method. The application to real atmospheric measurements stresses the need to consider the nonsphericity of the aerosol particles, especially if the relative humidity is low and the portion of large particles is high. A comparison with the results of independent nucleopore filter measurements yields good agreement. Furthermore typical values of the real part of the refractive index for different geographical regions are given.