Aerosol Optical Depths during Two Harmattan Seasons in Ile-Ife, Nigeria

Abstract

Aim: To quantify the atmospheric aerosol loading in order to predict the severity and accompanying consequences of aerosols at a tropical location in Ile-Ife, southwest Nigeria.
 Place and Duration of Study: Department of Physics and Engineering Physics, Obafemi Awolowo University, Ile-Ife, Nigeria, between November 2017 and March 2019.
 Methodology: Daily measurements of Aerosol Optical Depth (AOD) at about the local noon (12:30 pm–1:30 pm) for two consecutive Harmattan seasons (November 2017–March 2018; and November 2018 – March 2019) were carried out at three different wavelengths, 465 nm, 540 nm and 619 nm using a manually operated hand-held sun photometer (model Calitoo).
 Results: The mean values of AOD were 0.98, 0.87 and 0.83 in the 465 nm, 540 nm and 619 nm wavelengths respectively for November 2017 – March 2018; and 0.94, 0.83 and 0.78 in the 465 nm, 540 nm and 619 nm wavelengths respectively for November 2018 – March 2019. The values assume high levels of haziness at the study location. Intense Harmattan dust storm was experienced on some typical days with AOD values > 2. The resulting elevated level of atmospheric haziness led to visibility deterioration and visibility values greatly reduced to 1 km on such days. December, January and February months were the peak of the Harmattan. The distribution of the particle size indicated that the dominated aerosol is the coarse mode Harmattan dust during the period of study.
 Conclusion: The study location experiences a polluted atmosphere during the Harmattan season.

Estimation of Atmospheric Turbidity Parameters in Ile-Ife, Nigeria

“…The high values of β obtained in the dry season suggest high aerosol loading in the atmosphere as a result of episodes of Harmattan dust storms which are frequent occurrences at location at this period [26]. Conversely, the minimum values of β during the wet season can be linked to washout of aerosols in the atmosphere due to increase in precipitation.…”

“…A high visibility signifies good air quality, i.e. low atmospheric turbidity, while visibility deterioration is a consequence of high atmospheric turbidity [26].…”

“…The study area is located within the tropical zone of West Africa at the department of Physics and Engineering Physics, Obafemi Awolowo University (OAU), Ile-Ife, Nigeria (7.31 During this time, the north-easterly winds bring cold, dry and stable continental air masses from the Sahara desert where they originate. These winds are locally called the 'Harmattan' [26]. The harmattan wind transports fine dust to the study area having had a long trajectory over the northern parts of Nigeria lasting between the end of November and up to the middle of March.…”