IOSR Journal Of Environmental Science, Toxicology And Food Technology (IOSR-JESTFT) e-ISSN: 2319-2402,p- ISSN: 2319-2399. Volume 4, Issue 4 (May. - Jun. 2013), PP 71-74 www.Iosrjournals.Org www.iosrjournals.org 71 | Page Evaluation of Residence Time of Dust Aerosols during the Harmattan Season in Sokoto Area of Northwestern Nigeria, Using Visibility Data J. O. Akande 1 , M. Momoh 2 I.G. Saidu 3 , M.I. Ilyasu 3 , D.O. Akpootu 2 , M.B Abubakar 3 and M. B. Abdullahi 2 1 Department of Physics, Federal College of Education (Tech), Gusau Zamfara State, Nigeria 2 Department of Physics, Usmanu Danfodio University, Sokot Sokoto State, Nigeria 3 Physics Unit, Sokoto State Polytechnic Sokoto Abstract:S okoto (13.01 0 N, 5.15 0 E) lies within the hot /dry semi desert climate type i.e. Sahel Savannah climate zone and is identified as one of the regions in West Africa that is severely affected by the dust particles originating from Sahara desert. In this paper, we computed the residence time of dust aerosol in Sokoto region using visibility data for 20 years (1991-2010) obtained from Nigeria Meteorological Agency (NIMET). The method used yielded results which show variations in the values of the monthly and annual mean residence time of values 2.4-12.9 and 5.21 days respectively for the Harmattan months (November to March). These values are in general agreed with measurable values and similar to other naturally occurring aerosols in the atmosphere. Reasons are adduced for the variations. Keywords: Dust aerosols, Harmattan, Residence time, Visibility I. Introduction The presence of dust aerosol in the atmosphere during the harmattan season in the northern hemisphere is a familiar feature of the climate of most parts of West Africa. The dust aerosol are mobilized by wind erosion processes from the Sahara and Sahel regions of West Africa and transported down by the prevailing North Easterly Trade Wind. According to the facts established by many researchers, the harmattern haze, a natural atmospheric aerosol regime which affects most parts of West Africa including Nigeria has its origin in the Sahara desert in North East Africa. For instance Kalu[1], Prospero,[2],Babatunde et al [3] identify the alluvial plain of Bilma (18 o N, 12 0 E) in Southern Niger, and Largeau (18 0 N, 19 0 E), in Chad as the main sources of the dust which affects the greater parts of West Africa. Kalu[1] reported that during desert storms, large quantity of sand and dust are lifted into the atmosphere, transported by the North East trade winds and dispersed throughout West Africa from the month of November of one year to March of the following. According to Papastefanou and Bondnetti[4] and Gong et[5] the residence time of aerosol in the atmosphere is a function of various removal processes, the most important being: (a) dry deposition by impaction, diffusion and sedimentation and (b) wet deposition by raindrops as a result of processes occurring both within and below the rain cloud. They further asserted that there could be variations in the removal rates at different continental locations of the globe, over the oceans, and at high altitudes of the atmosphere due to changes in meteorological conditions. Martell [6] and Kuroda[7] reported that several methods have been used for estimating the mean residence times of atmospheric aerosols which include measurements of the radioactivities and ratios of: (a) fission products from nuclear weapons tests, (b) cosmic ray-produced radionuclides and (c) 222Rn and 22°Rn decay products. However, there is disagreement between the derived values of the residence times due to the various processes, including the fact that they refer to different portions of the atmosphere (e.g. cosmic ray-produced isotopes refer to the upper troposphere and radon daughters to lower troposphere, etc.) and also due to the existence of different sources for some isotopes. In the atmosphere, the aerosol is acted upon by various processes like coagulation, sedimentation, wash out and rain out. These processes affect the lifetime of the aerosols in the atmosphere. According to Rajan [8], at steady state, the residence time of an aerosol system could be defined as the ratio of their concentration present at that time to the production/loss rate. The potential impacts of aerosol loading and the associated desert encroachment have necessitated continuous monitoring of aerosol concentration. The environmental implication of monitoring aerosol concentration particularly in industrialized cities and urban environment cannot be over emphasized as Retalis, et al [9] stated that concentrated aerosols from substances from substances such as silica, asbestos and diesel