Ana Maria Silva

Properties of dust aerosol particles transported to Portugal from the Sahara desert

Aerosol properties of mineral particles in the far field of an African desert dust outbreak were investigated that brought Saharan dust over the Mediterranean in different layers to Portugal. The measurements were performed inside the project Desert Aerosols over Portugal (DARPO) which was linked to the Saharan Mineral Dust Experiment (SAMUM). The maximum particle mass concentration was about 150μgm−3 and the corresponding scattering coefficient was 130Mm−1 which results in a mass scattering efficiency of 0.87m2 g−1. The aerosol optical depth reached values up to 0.53 and the lidar ratio was between 45 and 50 in the whole dust loaded column. A comparison between particle size distributions and refractive indices derived from different instruments and models showed a general good agreement but some minor differences could also be observed. Measurements as well as calculations with a particle transport model suggest that there is a relatively higher concentration of very large particles in the upper region of the dust layer than on the surface which is likely connected with meteorological conditions at the observational site ( Évora, Portugal).

Aerosol radiative forcing efficiency in the UV region over southeastern Mediterranean: VELETA2002 campaign

[i] Atmospheric aerosol effects on spectral global UV irradiance were evaluated during the VELETA2002 field campaign between 8 and 19 July 2002 in southeast Spain. In the first stage, seven UV spectroradiometer and six CIMEL Sun photometer measurements were carried out simultaneously, allowing them to be calibrated and intercompared. The mean ratio obtained for the global irradiance between the spectroradiometers, with regards to a reference instrument, ranges from 0.98 up to 1.04 with standard deviations that oscillate between ±0.01 and ±0.17. In particular, the two spectroradiometers used to obtain the aerosol forcing efficiencies have a ratio of 1.000 ± 0.001. The aerosol optical depth (AOD) obtained with the CIMEL Sun photometers has a standard deviation of lower than ±0.01 for all the channels. Under clear sky conditions, the diurnal aerosol forcing efficiency (ΔDF e ) and fractional diurnal forcing efficiency (ΔFDF e ) was calculated for two Mediterranean stations: Armilla (691 m.a.s.l.) within the boundary layer and Sabinas (2200 m.a.s.l) on the lower limit of the free troposphere and 25 km away from the first station. The ΔDF e values obtained at Armilla range between -2.72 ± 0.45 W m-2/τ 380 and -2.88 ± 0.45 W m-2/τ 440 and between -3.22 ± 0.61 W m-2/τ 380 and -3.40 ± 0.62 W m-2/τ 440 at Sabinas station; the ΔFDF e values range from -8.0 ± 1.4%/τ 380 to -8.6 ± 1.3%/T 440 and -12.0 ± 2.3%/τ 380 to -12.6 ± 2.3%/τ 440 at the two stations, respectively. Also, an experimental aerosol transmittance factor, C T , used to obtain UV satellite derived products was found as a result of the dependence of the global irradiance with the AOD, under cloudless conditions. The average aerosol attenuation factor, η, obtained from the C T , is 6 ± 2% under weakly absorbing aerosols, with a negligible spectral dependence.

Infrared lidar overlap function: an experimental determination.

The most recent works demonstrate that the lidar overlap function, which describes the overlap between the laser beam and the receiver field of view, can be determined experimentally for the 355 and 532 nm channels using Raman signals. Nevertheless, the Raman channels cannot be used to determine the lidar overlap for the infrared channel (1064 nm) because of their low intensity. In addition, many Raman lidar systems only provide inelastic signals with reasonable signal-to-noise ratio at nighttime. In view of this fact, this work presents a modification of that method, based on the comparison of attenuated backscatter profiles derived from lidar and ceilometer, to retrieve the overlap function for the lidar infrared channel. Similarly to the Raman overlap method, the approach presented here allows to derive the overlap correction without an explicit knowledge of all system parameters. The application of the proposed methodology will improve the potential of Raman lidars to investigate the aerosol microphysical properties in the planetary boundary layer, extending the information of 1064 nm backscatter profiles to the ground and allowing the retrieval of microphysical properties practically close to the surface.

Solar irradiation inside a single span greenhouse

A model is offered through which the solar irradiation can be computed inside a single span hemicylindrical tunnel greenhouse. The model is mathematically exact. From the physical point of view it is founded on the assumption that the cladding surface is fully diffusive and that the radiation diffused through the atmosphere and by the ground is fully isotropic. It is found that the solar irradiation inside the greenhouse depends upon its orientation. A greenhouse with its longitudinal axis aligned along the north-south direction collects more radiation in summer and less in winter as compared with a greenhouse with its axis aligned along the east-west direction. On the other hand the east-west oriented greenhouse is more efficient in collecting solar radiation in winter than in summer, while the north-south oriented greenhouse exhibits a constant efficiency the year round. Finally, it is shown that the collected radiation depends upon the optical properties of the cladding surface being mainly determined by its transmittance.

Porous materials to control climate behaviour of enclosures: an application to the study of screened greenhouses

The aim of this paper was to study the influence of porous materials (screens, shelters, filters, etc.) on the climate behaviour of enclosures. First, a theoretical approach, describing the climate behaviour of a multi-zone enclosure, was proposed, being afterwards validated against experimental data. Having obtained a satisfactory agreement between the theoretical predictions and the experimental data, applications related with the use of porous screens in greenhouses were researched. This research concerned screened greenhouses, but other aspects, such as indoor climate control in buildings with ventilation filters, can greatly benefit from the approach presented in this paper.

Remote sensing of water quality parameters over Alqueva Reservoir in the south of Portugal

In this study, the potential of MEdium Resolution Imaging Spectrometer (MERIS) to describe variations of optically active substances over Alqueva artificial lake is investigated. Limnological laboratory analyses of the water samples collected monthly, from 2003 to 2006, are used in combination with MERIS. The water surface spectral reflectance is derived from Level1b MERIS data, using radiative transfer calculations to account for the atmospheric effects. The lake water spectral surface reflectance is combined with laboratory analyses of cyanobacteria total densities as well as chlorophyll a concentrations and empirical algorithms for both quantities are derived. The results obtained are compared with independent laboratory analyses from 2007, with good correlation coefficients obtained both for cyanobacteria (R = 0.93) and chlorophyll a(R = 0.80). The methodology proposed here has been developed to inexpensively monitor Alqueva Reservoir water quality in terms of cyanobacteria and chlorophyll a on a regular basis, and to provide useful information to the authorities.

Wind-induced airflow through permeable materials, Part I: the motion equation

An approach to describe the air infiltration in enclosed spaces is proposed. The approach is obtained by developing the momentum equation in terms of the method of volume averaging. The physical meaning of each equation term is presented and the range of validity and applicability are analysed.

Thermal radiation inside a single span greenhouse with a thermal screen

A model is offered which yields the net thermal radiation at the ground of a single span greenhouse fitted with thermal screens, in terms of the outside radiative environment, the properties of the cladding and screening surfaces and the temperatures of the ground, screen and covering. Radiation is assumed to be diffusely emitted, transmitted and reflected, both by the cladding and the screening surfaces. The importance of the geometry and temperature of the screening surface, and of the radiometric properties of the screen in determining the radiative environment inside are indicated. On the basis of this model, together with the outside radiation fluxes, temperature of the ground, screen and cover and data on the properties of the cladding and screening surfaces, the net thermal radiation inside the greenhouse can be determined, both in clear and overcast night sky conditions. These results are found to be in fairly good agreement with the measured net thermal radiation at ground level inside the greenhouse.

PollyNET: a network of multiwavelength polarization Raman lidars

PollyNET is a growing global network of automatized multiwavelength polarization Raman lidars of type Polly (Althausen et al., 2009). The goal of this network is to conduct advanced remote measurements of aerosol profiles and clouds by the same type of instrument. Since 2006 this network assists the controlling and adjustment activities of Polly systems. A central facility receives the data from the Polly measurements. The observational data are displayed in terms of quicklooks at http://polly:tropos.de in near real time. In this way, the network serves as a central information platform for inquisitive scientists. PollyNET comprises permanent stations at Leipzig (Germany), Kuopio (Finland), Evora (Portugal), Baengnyeong Island (South Korea), Stockholm (Sweden), and Warsaw (Poland). Non-permanent stations have been used during several field experiments under both urban and very remote conditions - like the Amazon rainforest. These non-permanent stations were lasting from several weeks up to one year and have been located in Brazil, India, China, South Africa, Chile, and also aboard the German research vessels Polarstern and Meteor across the Atlantic. Within PollyNET the interaction and knowledge exchange is encouraged between the Polly operators. This includes maintenance support in system calibration procedures and distribution of latest hardware and software improvements. This presentation introduces the PollyNET. Main features of the Polly systems will be presented as well as recent instrumental developments. Some measurement highlights achieved within PollyNET are depicted.

Monitoring of atmospheric ozone and nitrogen dioxide over the south of Portugal by ground-based and satellite observations.

The SPATRAM (Spectrometer for Atmospheric TRAcers Monitoring) instrument has been developed as a result of the collaboration between CGE-UE, ISAC-CNR and Italian National Agency for New Technologies, Energy and the Environment (ENEA). SPATRAM is a multi-purpose UV-Vis-scanning spectrometer (250 - 950 nm) and it is installed at the Observatory of the CGE, in Evora, since April 2004. A brief description of the instrument is given, highlighting the technological innovations with respect to the previous version of similar equipment. The need for such measurements automatically taken on a routine basis in south-western European regions, specifically in Portugal, has encouraged the development and installation of the equipment and constitutes a major driving force for the present work. The main features and some improvements introduced in the DOAS (Differential Optical Absorption Spectroscopy) algorithms are discussed. The results obtained applying DOAS methodology to the SPATRAM spectrometer measurements of diffused spectral sky radiation are presented in terms of diurnal and seasonal variations of nitrogen dioxide (NO(2)) and ozone (O(3)). NO(2) confirms the typical seasonal cycle reaching the maximum of (6.5 +/- 0.3) x 10(+15) molecules cm(-2) for the sunset values (PM), during the summer season, and the minimum of (1.55 +/- 0.07) x 10(+15) molecules cm(-2) for the sunrise values (AM) in winter. O(3) presents the maximum total column of (433 +/- 5) Dobson Unit (DU) in the spring season and the minimum of (284 +/- 3) DU during the fall period. The huge daily variations of the O(3) total column during the spring season are analyzed and discussed. The ground-based results obtained for NO(2) and O(3) column contents are compared with data from satellite-borne equipment (GOME - Global Ozone Monitoring Experiment; SCIAMACHY - Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY; TOMS - Total Ozone Monitoring Spectrometer) and it is shown that the two data sets are in good agreement. The correlation coefficient for the comparison of the ground-based/satellite data for O(3) is of 0.97.