P. Formenti

Radiative properties and direct radiative effect of Saharan dust measured by the C‐130 aircraft during SHADE: 1. Solar spectrum

[1] The physical and optical properties of Saharan dust aerosol measured by the Met Office C-130 during the Saharan Dust Experiment (SHADE) are presented. Additional radiation measurements enable the determination of the aerosol optical depth, taerl, and the direct radiative effect (DRE) of the mineral dust. The results suggest that the absorption by Saharan dust is significantly overestimated in the solar spectrum if standard refractive indices are used. Our measurements suggest an imaginary part of the refractive index of 0.0015i is appropriate at a wavelength l of 0.55 mm. Different methods for determining taerl=0.55 are presented, and the accuracy of each retrieval method is assessed. The value taerl=0.55 is estimated as 1.48 ± 0.05 during the period of heaviest dust loading, which is derived from an instantaneous DRE of approximately � 129 ± 5 Wm � 2 or an enhancement of the local planetary albedo over ocean of a factor of 2.7 ± 0.1. A comparison of the DRE derived from the C-130 instrumentation and from the Clouds and the Earth’s Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite is presented; the results generally showing agreement to within a factor of 1.2. The results suggest that Saharan dust aerosol exerts the largest local and global DRE of all aerosol species and should be considered explicitly in global radiation budget studies. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1640 Global Change: Remote sensing; 3359 Meteorology and Atmospheric Dynamics: Radiative processes;

The mean physical and optical properties of regional haze dominated by biomass burning aerosol measured from the C‐130 aircraft during SAFARI 2000

Original article can be found at: http://www.agu.org/journals/jd/ Copyright American Geophysical Union DOI: 10.1029/2002JD002226 [Full text of this article is not available in the UHRA]

Measurement and modeling of the Saharan dust radiative impact: Overview of the Saharan Dust Experiment (SHADE)

[1] Aerosols are known to be important in determining Earth’s radiative balance. Dust aerosols are of particular interest since, in addition to their scattering and absorbing properties that affect the solar radiation, they also perturb the terrestrial radiation. Recent studies have shown that a significant proportion of mineral dust in the atmosphere may be of anthropogenic origin, and therefore they may have an important role in climate change by exerting a significant radiative forcing. However, the optical and radiative properties of dust are not yet very well-determined, and even the sign of the resulting forcing is still questionable. The Saharan Dust Experiment (SHADE) was designed to better determine the parameters that are relevant for computing the direct radiative effect. Two aircraft combining in situ and remote sensing instruments were coordinated with satellite overpasses and ground-based observations during the experiment, which was based in the Cape Verde area during the period 19–29 September 2000. These in situ and remotely sensed data provide new valuable information on the microphysical, optical properties, and radiative effects of a large mineral dust outbreak. In addition, a global chemical transport model was used for assessing the radiative impact of these events, which are shown to be important on regional and global scales. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: Saharan dust, physical and optical properties, dust direct radiative forcing

Saharan dust in Brazil and Suriname during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA)- Cooperative LBA Regional Experiment (CLAIRE) in March 1998

Advection of Saharan dust was observed via chemical and optical measurements during March 1998 in Brazil and Suriname during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA)-Cooperative LBA Airborne Regional Experiment (CLAIRE)-98 experiment. In Brazil the dust outbreak produced an increase of a factor of 3 in the daily mean mass concentration (up to 26±7 μg m−3) of particles smaller than 10 μm equivalent aerodynamic diameter (EAD), and in the daily mean aerosol particle scattering coefficient σs (up to 26±8 Mm−1 STP, ambient humidity). Background levels of aerosol scattering (ambient) were σs ∼ 10 Mm−1. The effect of dust advection was evident for all major crustal elements (Al, Si, Ca, Ti, Mn, and Fe), as well as the sea-salt elements (Na, Cl, and S), as the dust layer was transported at low altitude (below 800 hPa). Coarse P and organic carbon (OC) concentrations were not influenced by the occurrence of dust, and were mainly emitted by the rain forest. The dry scattering mass efficiency of dust (particles smaller than 10 μm EAD) was estimated to be between 0.65 (±0.06) and 0.89 (±0.08) m2 g−1. Airborne profiles of aerosol scattering showed two distinct types of vertical structure in the dust layer over Suriname, either vertically uniform (15, 26 March), or plume-like (25 March). Dust layers extended generally up to 700 hPa, while scattering layers occasionally encountered at higher altitudes resulted from smoke emitted by biomass burning in Venezuela and Colombia. Observations in South America were supported by measurements in Israel and Tenerife (Canary Islands), where the dust outbreaks were also detected.

Chemical composition of mineral dust aerosol during the Saharan Dust Experiment (SHADE) airborne campaign in the Cape Verde region, September 2000

containing up to 54 m gm 3 Al (submicron plus supermicron fraction) were measured during particularly intense dust events. Mixing of dust with anthropogenic aerosols, mainly NH4HSO4, was observed in the fine fraction. It was associated with air masses that had originated over Europe and then traveled over North Africa. The ratio of nitrate to non-sea-salt sulfate was around 0.3. This argues against any significant influence of biomass burning emissions, which have much higher nitrate. However, we also encountered aged fossil fuel pollution plumes, likely from North America. The geochemical signature of mineral dust was consistent with previous results in the area. Si, Fe, and Ti were not enriched with respect to the soil composition, while other elements, such as Ca and S, were. Ca is prevalently present as calcite in African soils, but it is also found as calcium sulfate in the atmosphere. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0335 Atmospheric Composition and Structure: Ion chemistry of the atmosphere (2419, 2427); 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 9305 Information Related to Geographic Region: Africa;

Seasonal variations of the physical and optical characteristics of Saharan dust: Results from the Dust Outflow and Deposition to the Ocean (DODO) experiment

[1] NorthAfricandustisimportantforclimatethroughitsdirectradiativeeffectonsolarand terrestrial radiation and its role in the biogeochemical system. The Dust Outflow and Deposition to the Ocean project (DODO) aimed to characterize the physical and optical properties of airborne North African dust in two seasons and to use these observations to constrainmodelsimulations,withtheultimateaimofbeingabletoquantifythedepositionof iron to the North Atlantic Ocean. The in situ properties of dust from airborne campaigns measured during February and August 2006, based at Dakar, Senegal, are presented here. Average values of the single scattering albedo (0.99, 0.98), mass specific extinction (0.85 m 2 g � 1 ,1 .14 m 2 g � 1 ), asymmetry parameter (0.68, 0.68), and refractive index (1.53–0.0005i, 1.53–0.0014i) for the accumulation mode were found to differ by varying degrees between the dry and wet season, respectively. It is hypothesized that these differences are due to different source regions and transport processes which also differ between the DODO campaigns. Elemental ratios of Ca/Al were found to differ between the dry and wet season (1.1 and 0.5, respectively). Differences in vertical profiles are found between seasons and between land and ocean locations and reflect the different dynamics of the seasons. Using measurements of the coarse mode size distribution and illustrative Mie calculations, the optical properties are found to be very sensitive to the presence and amount of coarse mode of mineral dust, and the importance of accurate measurements of the coarse mode of dust is highlighted.

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.

AMMA dust experiment: An overview of measurements performed during the dry season special observation period (SOP0) at the Banizoumbou (Niger) supersite

In the frame of the African Monsoon Multidisciplinary Analysis-Special Observation Period 0 (AMMA SOP0) field campaign, a sophisticated instrumental setup specially designed for documenting the characteristics of aerosols in the surface boundary layer has been operated for 1 month (13 January to 13 February 2006) at the Banizoumbou supersite in Niger. The analysis of the long-term routine sunphotometer measurements shows that the intensive measurements were performed during a transition phase between a period when biomass burning aerosols are present in the atmospheric column and another when mineral dust is clearly predominant. On the basis of ground-level observations, three types of events can be defined. (1) Type B events are those corresponding to advection of Biomass burning plumes from southern Sahel. They represent only 3% of the measurement period. (2) Type D events are those during which mineral dust concentrations are larger than 200 mu g m(-3). They represent 35% of the measuring time, and coincide with advection of mineral dust from distant sources to the measurement site. The composition of the dust differs significantly from one episode to the other, depending on the trajectory of air mass that transported it. (3) Type L events are those that can be explained by occurrence of local erosion and that are of short duration. During them, the aerosol size distribution is dominated by a coarser mode (9 mu m) than during the events of the D type dominated by a finer mode (4.5 mu m). Elemental analysis also shows that mineral dust is relatively rich in Fe and poor in Ca, which is typical of the Sahelian aerosols.

Optical properties of Saharan dust during ACE 2

The Aerosol Robotic Network (AERONET) of automatic Sun/sky radiometers collected data on Tenerife, Canary Islands, in June-July 1997 during the second Aerosol Characterization Experiment (ACE 2). Initially, two instruments were deployed at Izana observatory (2360 m above sea level) and one at a mountain station Teide (3570 m above sea level). Repeatability of the calibration constants (Langley method) for all instruments was less than 0.5%. Aerosol optical depths measured by colocated sunphotometers and column size distributions, retrieved from spectral sky radiance data, were in good agreement. Later, one of the instruments was relocated at sea level. On July 8, 17, and 25, Saharan dust outbreaks were observed. Diurnal variations of spectral aerosol optical depth are presented. Relative diurnal stability of Saharan dust optical properties has been observed. Volume size distributions at various heights (sea level and 2360 m above sea level) show that the main portion of coarse particles is situated above 2360 m level. Measurements on July 25 showed how incoming dust has changed the magnitude and spectral dependence of aerosol optical depth and volume spectra of columnar aerosol. Mean optical depth and Angstrom parameter values for Saharan dust outbreaks during the ACE 2 experiment agree well with the Atlantic Ocean and Bermuda data obtained during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) in July 1996, as well as with previously reported Atlantic Ocean results. Also, there is a good agreement between ACE 2 data for Saharan air masses and data obtained on certain sites of the AERONET network.