Transport of desert dust mixed with North African industrial pollutants in the subtropical Saharan Air Layer

S Rodríguez,X Querol,E Cuevas,M Pandolfi,J De La Rosa,J Abreu-Afonso,S Alonso-Pérez,N Pérez,M Viana,A Alastuey

doi:10.5194/acp-11-6663-2011

Abstract

Abstract. An analysis of chemical composition data of particulate matter samples (TSP, PM10 and PM2.5) collected from 2002 to 2008 in the North Atlantic free troposphere at the Izaña Global Atmospheric Watch (GAW) observatory (Tenerife, Canary Islands) shows that desert dust is very frequently mixed with particulate pollutants in the Saharan Air Layer (SAL). The study of this data set with Median Concentrations At Receptor (MCAR) plots allowed the identification of the potential source regions of the dust and particulate pollutants. Areas located at the south of the southern slope of the Atlas mountains emerge as the most frequent source of the soil desert dust advected to the northern edge of the SAL in summer. Industrial emissions occurring in Northern Algeria, Eastern Algeria, Tunisia and the Atlantic coast of Morocco appear as the most important source of the nitrate, ammonium and a fraction of sulphate (at least 60 % of the sulphate <10 μm transported from some regions) observed in the SAL. These emissions are mostly linked to crude oil refineries, phosphate-based fertilizer industry and power plants. Although desert dust emissions appear as the most frequent source of the phosphorous observed in the SAL, high P concentrations are observed when the SAL is affected by emissions from open mines of phosphate and phosphate based fertilizer industry. The results also show that a significant fraction of the sulphate (up to 90 % of sulphate <10 μm transported from some regions) observed in the SAL may be influenced by soil emissions of evaporite minerals in well defined regions where dry saline lakes (chotts) are present. These interpretations of the MCAR plots are consistent with the results obtained with the Positive Matrix Factorization (PMF2) receptor modelling. The results of this study show that North African industrial pollutants may be mixed with desert dust and exported to the North Atlantic in the Saharan Air Layer.

Highlights

Desert regions of Northern Africa are the largest source of the soil dust suspended in the atmosphere of the Earth
The results of this study show that North African industrial pollutants may be mixed with desert dust and exported to the North Atlantic in the Saharan Air Layer
The simultaneous increases in PM10 and in Al concentrations registered at Izana indicates that increases in the aerosol mass above the clean free troposphere background are prompted by the advections of Saharan dust, when PM10 may reach values as high as 150 μg m−3

Summary

Introduction

It is estimated that annual emissions range within the interval 300–1600 Tg y−1 They account for 60–70 % of global desert dust emissions and they are 2–3 times larger than those of the Asian deserts, the second most important dust source region (Ginoux et al, 2004; Engelstaedter et al, 2006). Dust plays an important role in processes affecting climate, biogeochemistry and air quality. The presence of dust influences the energy distribution in the atmosphere, due to its scattering and absorbing radiative properties (Haywood et al, 2003). Dust particles may act as cloud condensation nuclei and they influence rain and cloud radiative properties (Levin et al, 1996).

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Conclusion