Vehicular emissions on main roads in Makkah, Saudi Arabia—a dispersion modelling study

Abstract

Particulate matter (PM) is the atmospheric pollutant of main concern in Makkah; therefore, there is a need for its effective monitoring, modelling and management. In this study, Atmospheric Dispersion Modelling System (ADMS)-Urban model is employed, which is a well-known atmospheric dispersion modelling system. Traffic data were collected for several years (2007–2012) on six main roads in Makkah during the months of Ramadhan and Hajj. Data analysis showed that on average, there were 83% light-duty vehicles and 17% heavy-duty vehicles on Makkah roads; however, this percentage slightly varied both spatially and temporally. The number of vehicles demonstrated increasing trend from 2007 to 2012 on the six roads. Traffic characteristics, such as vehicle speed, vehicle type and number, were used to calculate the emissions of PM₁₀ and PM_2.5. Along with pollutant emissions, ADMS-Urban requires meteorological parameters such as wind speed and direction, temperature, relative humidity, cloud cover and boundary layer height. Concentrations were predicted in three different forms: (a) for six receptors, (b) as diurnal cycles and (c) as contour maps for the whole Makkah City. Modelled concentrations were compared with the observed concentrations at Masfalah and Presidency of Meteorology and Environment (PME) monitoring stations. ADMS-Urban underestimated both PM₁₀ and PM_2.5 concentrations; however, the difference was much greater at the PME (about 73%) than at the Masfalah station (about 24%). Reasons for the discrepancies are discussed, and various statistical metrics are calculated to assess the model performance. More emission data are required to improve the performance of the model and minimise the gap between observed and predicted concentrations.