Atmospheric turbidity over the Angad area of Morocco

Abstract

TheLinketurbidityfactor,whichisdefinedas the ratio of the integral optical depth ofthe total extinction divided by the opticaldepth due to Rayleigh scattering only, is aconvenient measure for the total load ofatmospheric trace compounds in thevertical column (Louche et al. 1986). Thisanalysis is performed from records ofglobal and diffuse radiation measured by aset of two pyranometers under clear skyconditions. The diffuse radiationpyranometer is conducted by using aninstrument equipped with a shadow band.The two pyranometers (Kipp & Zonen,Type CM 11; Fig. 1) are installed at theOujda university campus as a part of anautomatical meteorological station locatedsouth-east of Oujda city (34°40'N, 1°50'W580mofaltitude).Wecanconsiderthattheurban aerosol has very low influenceaccording to the wind direction frequencyand the distance to the city center. Oujdaitself is located at the northern foothills oftheAtlas mountains (about 60 km south ofthe Mediterranean sea). It is often affectedby a land-sea circulation, except for caseswhen major pressure systems or storms arepassing. Thus, daily intrusions of maritimeair masses are replacing the dry continentalair masses, which are advected from thesouth during night time in the boundarylayer from the late night to the morning. Inthis region calledAngad region (north-eastof Morocco), the precipitation is rare andthe sky is often clear. Therefore theatmospheric turbidity reflects above all theatmosphericaerosolload.The Linke turbidity factor was calculatedvia the direct solar radiation determinedfrom actual measurements of diffuse andglobal solar radiation after a Linke dataevaluation scheme (Louche et al. 1986 andSchutzetal.1993).Athree years time series of Linke turbidityfactors (Fig. 2) presents values rangingfrom 1.5 to 12 and shows a summermaximum. The frequency distribution(Fig. 3) shows a maximum for values ofabout 2.7. Compared to other locations, theAngad average atmospheric turbidity hasrelative low values which are found morecommonly at mountain stations than atMediterranean or European cities. Thisresult may be explained by the largenumberof230cleardaysperyear,ofwhichonlydaysduringafourmonthcyclicperiodshow high turbidity values. This fourmonth period starting in May ischaracterized by exceptional advection ofSaharan air masses coming from the south-east of Morocco. During this time highturbidity values larger than 6 can beobserved, and also high massconcentrations are found (Meziani et al.,2000).The large variations during summer areobviously caused by single local andSaharan dust impacts. The general patternhas some similarity to spectralmeasurements (El Amraoui et al., 2000),where only wavelengths with strongaerosol extinction have been investigated.The range of values for Linke turbidityfactor is representative for continentalareas with low pollution (Helmes andJaenicke,1988).The Linke turbidity factor and itsdetermination is a rather low-priced andsimple tool compared to spectralmeasurements to monitor long-termvariations of total columnar loading ofatmospherictracecompounds.Fig. 1: Pyranometers at the Oujda university campus meteorological stationFig. 3: Frequency distribution of Linke turbidity factor for Oujda 1996 to 1998Fig. 2: Time series of Linke turbidity factor for Oujda 1996 to 1998