Analysis of nighttime aerosols and relation to covariates over a highly polluted sub- Saharan site using wavelet coherence approach

Abstract

Abstract High emissions of aerosols and trace gases during nighttime can cause serious air quality, climate, and health issues, particularly in extremely polluted cities. In this paper, an effort has been made to examine the variations in aerosols and trace gases over a sub-Saharan city of Ilorin (Nigeria) during nighttime. We have used Aerosol Robotik Network data of aerosol optical depth at 500 nm (AOD), Angstrom Exponent (440/870) (AE) and precipitable water (WVC). We also analyzed nighttime data of CO, CH4 and O3 from Atmospheric Infrared Sounder (AIRS), and aerosol subtypes from CALIPSO. AOD, AE, and WVC average values are found to be 0.64 ± 0.33, 0.74 ± 0.24, and 3.40 ± 0.97, respectively. As a result of northeasterly winds carrying Saharan dust during the dry season, the greatest value of AOD was discovered in February. Desert dust aerosols (37.63%) were the most prevalent type, followed by mixed aerosols (44.15%). Winds at a height of 1500 m AGL were likely transporting Saharan dust to Ilorin. CALIPSO images revealed that Ilorin's atmosphere contained dust, polluted continental, clean maritime, and polluted dust on high AOD days. The NOAA's vertical sounding profiles showed that the presence of high AOD values were caused by the inversion layer trapping aerosol pollution. Average nighttime concentrations of CO, O3 and CH4 were measured to be 12718 ppbv, 29.72.1 ppbv, and 1822.612.7 ppbv, respectively. The wavelet coherence spectra exhibited significant quasi-biannual, and quasi-annual oscillations at statistically significant level.