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Universidade de Aveiro (2012)

Variação temporal das propriedades óticas do aerossol em Cabo Verde

Almeida, Daniel Santiago Brandão de

Titre : Variação temporal das propriedades óticas do aerossol em Cabo Verde.

Auteur : Almeida, Daniel Santiago Brandão de

Université de soutenance : Universidade de Aveiro

Grade : Mestrado em Engenharia do Ambiente 2012

The archipelago of Cape Verde is located in the Oriental side of North Atlantic Ocean and west from the African Coast. This region may be hit every year by north or northeastern winds carrying mineral dust from Saharan desert region, especially during the periods from November to March. In this work a characterization of the concentrations of particle matter and its particle size distribution was done for the year 2011. An analysis of the equivalent black carbon concentration (CNeq) was also done and using an empiric model we tried to separate this variable into two others, the pure black carbon (CN) and the mineral dust. For the characterization of the particulate matter a direct measurement method was used (GRIMM) and for measurement of CNeq was used the Aethalometer AE31. To understand the origin of particulate matter we used an atmospheric model the HYSPLIT. Thus it was possible to assess the air quality in the region and the importance that desert particle sources have in the Cape Verde atmosphere. Measurements made with the GRIMM showed high concentrations of PM for the months of January and February, even reaching a daily average of 300 and 150 mg/m3 of PM10 and PM2,5 respectively. These months have passed up several times the daily limit value, according to the European Directive 2008/50/EC guidelines. The remaining months also showed some exceedances however concentrations were lower. The particle size mass distribution, proved to be different for the two types of selected events, under the influence of wind desert showed a distribution formed by three modes in which the dominant mode was the fraction having diameters between 0.9 and 8 mm, while, for days with influence of sea breezes the dominant fraction was the one with diameters between 8 and 36 mm. The analysis of backward trajectories showed that the highest concentrations of PM were associated with air masses from the northeast, originating in the Sahara Desert, or that passed over this region. The concentrations of CNeq measured with the Aethalometer were quite different for each wavelength in January, February and December. For lower wavelengths CNeq concentrations were higher because of the presence of iron oxides. The model proved to be more suitable for the separation of CNeq into CN and mineral dusts in situations under the influence of desert winds. The air quality in Cape Verde is greatly affected by high concentrations of PM, especially in periods of massive transport of dust from the Sahara.

Mots clés : Engenharia do ambiente ; Qualidade do ar - Cabo Verde ; Física da atmosfera ; Poeiras minerais ; Aerossois atmosféricos


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