Investigating the evolution of gaseous air pollutants with prevailing meteorology across selected sites within a pollution hotspot in Ile-Ife, Southwestern Nigeria

Abstract

This study investigates the evolution of gaseous pollutants with prevailing meteorology using a network of Alpha sense low-cost portable air monitoring devices deployed to six selected sampling sites namely the Iron steel smelting factory (ISSF), Modomo, Eleweran, Fire service station, staff quarters, and OAU Teaching and research farm (OAUTRF). All six sampling sites were chosen along the pollution hotspot in Ile-Ife. The low-cost sensor has Optical particle counters (OPC-N2) which measured particulate matter and the gaseous pollutants were monitored with electrochemical sensors. At the sites, each sensor unit measured gaseous pollutants: carbon dioxide (CO2), ozone (O3), carbon monoxide (CO), and oxides of nitrogen (NO and NO2). Inbuilt into the monitoring device are temperature and relative humidity (RH) probes as well as a 2-D sonic anemometer to measure wind speed and directions. The directional dependence of the local source of the gaseous pollutant affecting each site was determined using the conditional probability function (CPF) model. The results showed that the highest mean values of 470.6 ppm, 7.2 ppb, and 20.6 ppb for CO2, NO, and NO2, respectively were recorded at the Modomo site while ISSF and Fire service sites were found to have the highest mean values of 67.9 ppb and 517.6 ppb, respectively for O3 and CO during the observational period. The mean values for temperature were noticed to increase in trend from OAUTRF (24.9 °C) to ISSF (26.0 °C). Also, CO2 gave a good relationship with meteorological parameters with R2 value ranging from 0.26 to 0.89 in all the sampling sites except ISSF. On contrary, weak relationships were found between the meteorological variables and other gaseous pollutants other than CO2 for both day and nighttime periods. The CPF model indicated that a larger percentage of the elevated O3, NO and NO2 were associated with pollutant transport at wind speed >2 m/s due to low event number. This study concludes that there is evidence of a significant effect of meteorological parameters on the gaseous pollutants as observed, while, the CPF plot shows a good agreement with the location of known pollution sources.