Atmospheric Particulate Matter (PM) is considered one of the most critical air pollutants in terms of its detrimental health impacts, environmental degradations and visibility. Particles size, their chemical composition and atmospheric levels are important factors for determining their adverse health impacts. In this paper various aspects of PM 2.5 are analysed including PM 2.5 /PM 10 ratios and association with meteorological parameters using data collected from January 2014 to September 2015 in Makkah Saudi Arabia. During the study period, mean PM 2.5 /PM 10 ratio was found to be 0.64, whereas median and maximum ratios were 0.69 and 0.99, respectively. Diurnal, weekly and annual cycles of PM 10 , PM 2.5 and their ratios were analysed, which demonstrated considerable variations during various hours of the day, days of the week and months of the year. PM 2.5 /PM 10 ratios were lower in summer (June and July) and higher in winter (November and December), likewise the ratios were lower during afternoon and higher in the morning and evening. As expected, there was a positive correlation between PM 10 and PM 2.5 (r = 0.51) and both PM 10 and PM 2.5 showed negative association with relative humidity and positive with wind speed and temperature. Furthermore, PM 2.5 /PM 10 ratios were lower (< 0.45) at lower relative humidity (< 16%) and higher (> 0.70) at higher relative humidity (35-90%), indicating a shift towards high PM 2.5 concentrations at higher relative humidity. Polar plots showed lowest ratios at high wind speed (> 3 m s -1 ) blowing from west and southwest direction in summer, and highest ratios at low wind speed (< 2 m s -1 ) in winter. Polar plots were successfully applied to show the interaction between various meteorological parameters and PM 2.5 /PM 10 ratios. Further work on source apportionment and receptor modelling of PM is required to help develop air quality index and prepare an effective air quality plan for Makkah.

Particulate matter originates from a variety of sources in Makkah, Saudi Arabia. Since Makkah is situated in an arid region and is a very busy city due to its religious importance in the Muslim world, PM 10 concentrations here exceed the international and national air quality standards set for the protection of human health. The main aim of this paper is to model PM 10 concentrations with the aid of meteorological variables (wind speed, wind direction, temperature, and relative humidity) and traffic related air pollutant concentrations (carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO 2 ), sulphur dioxide (SO 2 ) and lag_PM 10 concentrations), which are measured at the same location near Al-Haram (the Holy Mosque) in Makkah. A Generalized Additive Model was developed for predicting hourly PM 10 concentrations. Predicted and observed PM 10 concentrations are compared, and several metrics, including the coefficients of determination (R 2 = 0.52), Root Mean Square Error (RMSE = 84), Fractional Bias (FB = -0.22) and Factor of 2 (FAC2 = 0.88), are calculated to assess the performance of the model. The results of these, along with a graphical comparison of the predicted and observed concentrations, show that model is able to perform well. While effects of all the covariates were significant (p-value < 0.01), the meteorological variables, such as temperature and wind speed, seem to be the major controlling factors with regard to PM 10 concentrations. Traffic related air pollutants showed a weak association with PM 10 concentrations, suggesting road traffic is not the major source of these. No modeling study has been published with regards to air pollution in Makkah and thus this is the first work of this kind. Further work is required to characterize road traffic flow, speed and composition and quantify the contribution of each source, which is part of the ongoing project for managing the air quality in Makkah.

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