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.

Abstract. We performed a diagnostic study of geomagnetic storm-induced disturbances that are coupled to the mid-latitude D region by quantifying the propagation characteristics of very low-frequency (VLF) radio signals from transmitters located in Cumbria, UK (call sign GQD), and Rhauderfehn, Germany (DHO), and received in southern France (A118). We characterised the diurnal VLF amplitudes from two propagation paths into five metrics, namely the mean amplitude before sunrise (MBSR), the midday amplitude peak (MDP), the mean amplitude after sunset (MASS), the sunrise terminator (SRT) and the sunset terminator (SST). We analysed and monitored trends in the variation of signal metrics for up to 20 storms to relate the deviations in the signal amplitudes that were attributable to the storms. Five storms and their effects on the signals were examined in further detail. Our results indicate that relative to pre-storm levels the storm day MDP exhibited characteristic decreases in about 80 % (67 %) of the events for the DHO-A118 (GQD-A118) propagation path. The MBSR showed decreases of about 60 % (77 %), whereas the MASS decreased by 67 % (58 %). Conversely, the SRT and SST showed amplitude decreases of 33 % (25 %) and 47 % (42 %), respectively. Of the two propagation paths, the amplitude decreases for the DHO-A118 propagation path signal were greater, as previously noted by Nwankwo et al. (2016). To better understand the state of the ionosphere over the signal propagation paths and how it might have affected the VLF amplitudes, we further analysed the virtual heights (h'E, h'F1 and h'F2) and critical frequencies (foE, foF1 and foF2) from ionosondes located near the transmitters. The results of this analysis showed significant increases and fluctuations in both the F-region critical frequencies and virtual heights during the geomagnetic storms. The largest increases in the virtual heights occurred near the DHO transmitter in Rhauderfehn (Germany), suggesting a strong storm response over the region which might account for the larger MDP decrease along the DHO-A118 propagation path.

This study estimated the levels of atmospheric turbidity in Ile-Ife, a tropical location in the Southwest of Nigeria, from November, 2017 to March, 2019. This was with the aim to quantify the degree of atmospheric cleanliness of the study location. The methods of estimation used are: the Angstrom turbidity parameters (α and β), Linke turbidity factor (TL) and horizontal visibility (VH). The values of α and β range between 0.6 and 1.4; 0.10 and 0.91 respectively. The values obtained for TL varied between 1 and 7 while visibility values ranged between 2 and 14 km. Maximum values of β and TL (corresponding to low values of VH) were obtained in the dry season (particularly in the months of January and February) while the lowest values of the same methods of estimation (corresponding to high values of VH) were recorded in the wet season (specifically in August and September). The elevated turbidity observed in the dry season was linked to episodes of Harmattan dust storms usually experienced at the study location. The study concluded that a polluted atmosphere dominates the study location especially in the dry season as indicated by the different atmospheric turbidity parameters.

In this study, an Eddy Covariance (EC) system was deployed to measure the turbulent heat fluxes (sensible and latent heat fluxes) at a farmland located at Obafemi Awolowo University, Ile-Ife, Nigeria. The period of measurement was between 2016 and 2019. The surface of the measurement area was covered by grass (Axonopus fissifolius) and changes from leafy-green during the wet season to dry twigs during the dry season. The sensible heat flux, Hs obtained ranged between -0.8 and 181.9 Wm-2 while the latent heat flux, Hl varied between 0.9 and 218.0 Wm-2. The maximum daytime value of Hs peaked in January while the maximum daytime value of Hl peaked in October. The sensible heat flux was 59 % higher than the latent heat flux during the Harmattan months (January, February and December), while the latent heat flux was 63 % higher than the sensible heat flux during the wet months (April - November). The study concluded that 91 % of the energy available at the surface during the period of study, was used for evapotranspiration while only 9 % was used for sensible heating as shown by the distribution of the Bowen ratio.

<p>Diurnal and seasonal variations of sensible heat (<em>H</em>) and latent heat (<em>LE</em>) fluxes observed at an agricultural site on the campus of Obafemi Awolowo University, Ile-Ife, southwest Nigeria have been reported in this paper. The deductions are made based on half-hourly flux data acquired from an open-path eddy covariance (OPEC) system measured continuously over a two-year observation period (2017-2018) at the study site. The study area is within tropical wet and dry climate of West Africa, thereby experiencing an alternating wet (that is, April – October) and dry (that is, December – February) seasons (monsoonal). Our results showed that peak hourly values of <em>H</em> and <em>LE</em> occurred at about 13:00 LT and 14:00 LT respectively, a lag of approximately one hour between them at the location. The diurnal range for <em>H</em> and <em>LE</em> during wet season was 75.3 W m<sup>-2</sup> and 177.0 W m<sup>-2</sup> respectively, while for dry season it was 182.0 W m<sup>-2</sup> and 89.9 W m<sup>-2</sup> respectively. The daily mean value of <em>H</em> for wet season was 19.7 ± 27.2 W m<sup>-2</sup> and it was 52.1 ± 63.5 W m<sup>-2 </sup>for <em>LE</em>. For dry season, daily mean values for <em>H</em> and <em>LE</em> were 44.0 ± 66.4 W m<sup>-2</sup> and 26.6 ± 33.7 W m<sup>-2</sup> respectively. A transition of seasons from wet (Bowen ratio, <em>Bo</em> < 1) to dry (<em>Bo</em> > 1) was observed in November and reversal in March.</p><p> </p><p><strong>Keywords</strong>: Diurnal and Seasonal Variations, Sensible and Latent Heat Fluxes, Tropical Wet and Dry Climate</p>