Quantifying the Effect of Chemical Transformation on Pollutant Ground Level Concentrations in Ghana Using the CALPUFF Modelling System

This paper discusses the effect of chemical transformation of dispersed pollutants from the Tema Oil Refinery in Ghana within a60 km2 study area. The MESOPUFF II module within in the California Puff (CALPUFF) modeling system was used coupled with Weather Research and Forecasting (WRF) model data driving the dispersion model. The chemical transformation of SOX and NOX to sulphates and nitrates respectively was modelled using refinery emission rates of 2009, 2010, 2011 and 2013 together with terrain and land use data of the study area and refinery emission stack characteristics. Daily average concentrations of SOX and NOX were predicted at fourteen (14) receptor locations within the study area. Receptors located north and north east of the refinery saw higher levels of SOX and NOX due to the south western prevalent wind direction in the study area. Receptors farther from the refinery also recorded relatively lower concentrations. GLCs of SOX and NOX predicted at 14 receptors within the study area were compared with GLCs of the same pollutants predicted without the consideration of chemical transformation. It was observed that SOX and NOX daily average concentrations for modelling with no chemical transformation were higher than with the chemical transformation module activated, the difference accounting for the sulphates and nitrates. The percentage conversion of the pollutants varied from year to year primarily due to meteorological conditions. It was also observed that there was a reduction in ground level concentrations of SOX and NOX at receptors in the study area with percentage conversion of SOx to sulphates ranging from 2 to 19, and 5 to 22 for NOx to nitrates.