Analysing PM 2 . 5 and its Association with PM 10 and Meteorology in the Arid Climate of Makkah , Saudi Arabia

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 PM2.5 are analysed including PM2.5/PM10 ratios and association with meteorological parameters using data collected from January 2014 to September 2015 in Makkah Saudi Arabia. During the study period, mean PM2.5/PM10 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 PM10, PM2.5 and their ratios were analysed, which demonstrated considerable variations during various hours of the day, days of the week and months of the year. PM2.5/PM10 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 PM10 and PM2.5 (r = 0.51) and both PM10 and PM2.5 showed negative association with relative humidity and positive with wind speed and temperature. Furthermore, PM2.5/PM10 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 PM2.5 concentrations at higher relative humidity. Polar plots showed lowest ratios at high wind speed (> 3 m s) blowing from west and southwest direction in summer, and highest ratios at low wind speed (< 2 m s) in winter. Polar plots were successfully applied to show the interaction between various meteorological parameters and PM2.5/PM10 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.


INTRODUCTION
Air pollution is becoming a growing environmental issue in both developing and developed countries throughout the world (Colls, 2002;Harrison, 2001). Growing urbanisation, transport vehicles on the roads, and biomass and fossil fuels burning to meet the energy needs of growing population have resulted in large amount of emissions of both gaseous pollutants and particulate matter (PM). In this study we focus on PM with aerodynamic diameter of up to 10 µm (PM 10 ) and 2.5µm (PM 2.5 ). PM 10 and PM 2.5 have attracted a vast number of research investigations (e.g., COMEAP, 2009COMEAP, , 2010Harrison et al., 2010;AQEG, 2012;Barmpadimos et al., 2012;Vu et al., 2015) due to their detrimental impact on public health, visibility, vegetation and other environmental impacts. However, the health impacts of PM over weigh the other environmental impacts, therefore the most concerning fact regarding PM is its health impact (Walters and Ayres, 2001;WHO, 2003). Many scientists have investigated the health impacts of PM and have reported that PM can cause various health issues, including respiratory diseases, rhinosinusitis, cardio-vascular diseases, asthma, and chronic obstructive pulmonary diseases (Walters and Ayres, 2001;WHO, 2003). The health effects of PM are mostly related with the fine fraction (PM 2.5 ), which can penetrate deeper into the respiratory system (COMEAP, 2010).
The levels of PM 10 and PM 2.5 are dependent not only on their emission sources but also on meteorological parameters, such as wind, temperature, relative humidity, atmospheric pressure and boundary layer height (Munir et al., 2013a). Furthermore, the levels and composition of PM 10 and PM 2.5 are affected by the local geological characteristics and land usage type (Khodeir et al., 2012). The levels and composition of PM 10 and PM 2.5 vary from one region to another, depending on their background levels and emission sources. It is, therefore, important to conduct monitoring and modelling investigations locally to be able to make informed decision regarding local air quality management. Makkah is situated in an arid hot region and is surrounded by large sandy deserts. The city receives little rain and experiences high temperature throughout the year. The city is expanding rapidly and large amount of construction-and-demolition activities take place, which include expansion of the Holy Mosque, developing a train network inside the Makkah city and from Makkah to other large cities of the countries, mountains digging, and building multi-storeyed building for residents and religious visitors (Munir et al., 2013b). The city also experiences frequent sand-and-dust storms. Furthermore, increasing number of road-traffic vehicles and frequent congestions during the busy hours add to atmospheric pollution in the city. Makkah is one of the densely populated city. Moreover, it receives million of visitors every year to perform Hajj and Umrah. This puts further burden on the local resources and environmental quality, including air quality. Like most of the other Asian and African cities, Makkah also lacks an effective air quality management plan, therefore there is a need for research work into air quality related issues to develop the basic science and characterise temporal and spatial variability of various air pollutants, including PM. Because of the geological characteristics and arid climatic conditions, PM 10 and PM 2.5 are considered the pollutants of concern in this region as they exceed national and international air quality standards frequently (Othman et al., 2010).
Previously several researchers (e.g., Al-Jeelani, 2008, 2009Othman et al., 2010;Seroji, 2011) have investigated the levels of PM, especially PM 10 in Makkah and have reported that PM 10 levels frequently exceeded national and international air quality standards. These studies included both monitoring and modelling investigations and analysed the temporal and spatial variability of PM 10 (Munir et al., 2013b), modelled its association with other pollutants and meteorology (Munir et al., 2013a;Munir, 2015), and analysed its potential health effects (Habeebullah, 2013a, b). However no published literature was found regarding PM 2.5 . This paper analyses PM 2.5 and its association with PM 10 , focusing on PM 2.5 /PM 10 ratios. The paper also investigates how the ratios vary during different hours of the day, days of the week and months of the year, plus how the ratios are affected by various meteorological parameters, such as temperature, relative humidity, and wind characteristics. Analysis of the ratio is important as it helps understand the relationship between PM 2.5 and PM 10 levels and their emissions sources, which may be helpful for developing an effective air quality management plan in Makkah and elsewhere.

METHODOLOGY
This paper characterises PM 10 , PM 2.5 and their ratios in the Holy City of Makkah, Saudi Arabia, which is situated in an arid region, experiencing little rain and hot temperature all year round. Recently an air quality network is developed in the city of Makkah which monitors not only the concentrations of various air pollutants but also meteorological parameters (Fig. 1).
The data used in this study were collected at the Masfalah air quality monitoring station (AQMS111, Fig. 1). The site is classified as an urban traffic site, located near a busy road a couple of kilometre from the Holy Mosque in Makkah (Fig. 1). PM 10 (µg m -3 ), PM 2.5 (µg m -3 ) , temperature (°C), wind speed (m s -1 ), wind direction (degree from the north) and relative humidity (%) hourly data were collected from January 2014 to September 2015. More than 90% data were available for PM 10 , PM 2.5 and meteorological parameters for the study period. A summary of these variables are presented in Fig. 2 as a time series and in Fig. 3 as polar plots, which also provide a view of the variations of the various parameters during different seasons. Aeroqual AQM 60 air quality monitoring station was installed to monitor the concentrations of PM 10 , PM 2.5 and meteorological parameters. The particles monitor uses a well proven near forward light scattering nephelometer and high precision sharp cut cyclone and has a range of 0-2000 µg m -3 with accuracy of ± 2 for both PM 10 and PM 2.5 . The optical sensor nephelometer uses light scattering from particles to provide a continuous realtime measurement of PM 10 and PM 2.5 . In order to make the collected data useful and to provide a sound scientific basis for comparison against air quality standards, public information or policy development, the data need to be accurate and reliable. Strict QA/QC (Quality Assurance and Quality Control) measures were taken to ensure the quality of data. The QA/QC process ensures that the data are representative of atmospheric concentrations in the areas under investigation over the period of measurement. QA included careful selection of monitoring site, proper installment of instruments, selection of instrument and sample system design, and proper training of operators and personnel. QC measures included calibration of instruments, monitoring calibration gases and instrument response, routine site visits and operations, routine data review and validation and data ratification. After the initial calibration, the zero and span responses of the instrument were checked for drift on weekly basis. For both PM 10 and PM 2.5 the range of measurement was 0-2000 µg m -3 , flow rate was 2 L min -1 , and accuracy ± 2 µg m -3 .
Statistical software R programming language (R Development Core Team , 2014) and its package openair (Carslaw and Ropkins, 2012) was used to perform data analysis and develop various graphs, including polar plots, time variation plots, scatter plots, and time plots (Carslaw and Ropkins, 2012).

RESULTS AND DISCUSSION
A summary of the data, which include PM 10 , PM 2.5 , ratios of PM 2.5 /PM 10 , temperature, relative humidity, wind speed and wind direction is presented in Table 1. The Saudi Arabian national annual air quality standards developed by the Presidency of Meteorology and Environment (PME) for PM 10 and PM 2.5 are 80 µg m -3 and 15 µg m -3 , respectively. The European Union (EU) annual air quality standards for PM 10 and PM 2.5 are 40 µg m -3 and 25 µg m -3 , respectively (EU, 2016). The World Health Organisation (WHO) air quality guidelines for PM 10 and PM 2.5 are 20 µg m -3 and 10 µg m -3 , respectively. The annual average of PM 10 and PM 2.5 during the study period at the Masfalah air quality monitoring station (AQMS111, Fig. 1) was 44 and 23, respectively, indicating that long term PM 2.5 levels have exceeded the PME air quality limits, whereas PM 10 levels have exceeded the EU limits. Furthermore, the WHO long terms guidelines are exceeded by both PM 10 and PM 2.5 . In Makkah maximum temperature gets as high as 56.70°C, and the ratios of PM 2.5 /PM 10 ranged from 0.001 to 0.99 with an average value of 0.64 (Table 1).
Time variations of PM 10 , PM 2.5 and their ratios during the study period in Makkah are presented in Fig. 4. As expected PM 10 and PM 2.5 follow mostly the same trends in their diurnal, weekly and annual cycles. However, PM 2.5 levels are significantly lower than PM 10 levels. On diurnal basis, lowest levels of both PM 10 and PM 2.5 were observed in the morning (about 05:00 hour) and at night, whereas highest levels were observed in the late afternoon (about 16:00 hour). In Makkah most of the atmospheric PM comes from resuspended and windblown dust particles, generated by construction-and-demolition activities and raised from the surrounding sandy deserts (Munir et al., 2013b;Habeebullah et al., 2015). The amount of resuspended and windblown dust is greater during daytime due to atmospheric instability and greater vertical and horizontal wind movement. Traffic activities are also greater during daytime, which may play a role in increasing the PM concentrations during daytime. On weekly basis, lowest level of PM 10 was observed on Saturday and that of PM 2.5 on Friday, whereas highest level   Makkah and Leeds UK and reported opposite trends in the diurnal and annual cycles. In Leeds PM 10 concentrations were higher during winter probably due to stagnant atmospheric conditions which hinder the dispersion process, whereas in Makkah higher levels of PM 10 were observed in summer probably due to atmospheric conditions, such as high temperature which encourage re-suspension and windblown dust and sand particles.
Recently Mohammed et al. (2016) observed that PM 10 concentrations were higher in the north western part and lower in the south-eastern part of Makkah city. PM 10 levels ranged from 56 to 391 µg m -3 in Makkah. In another study, Mohammed et al. (2015) reported that PM 10 and PM 2.5 levels were 145.4 and 13.2 µg m -3 , respectively during Hajj days in Makkah. Munir et al. (2013b) investigated PM 10 levels in Makkah and reported that PM 10 ranged from 0 to 999 µg m -3 having mean concentration of 174.6 µg m -3 in 2012. In another investigation Munir et al. (2013a) reported that PM 10 concentration had positive trend in Makkah during 1997-2012 and had a mean value of 112.8 µg m -3 and maximum value of 821 µg m -3 . Several authors have investigated the levels of atmospheric aerosols around the world. Their reported PM 2.5 levels were lower in Turkey (9.7 µg m -3 , Kocak et al., 2007) and Brazil (13.8 µg m -3 , Mariani and Mello, 2007) and higher in China (451 µg m -3 , Wang et al., 2003), India (55.7 µg m -3 , Rengarajan et al., 2011) and Pakistan (83.5 µg m -3 , Mansha et al., 2011) than those observed in this study in Makkah. Furthermore, PM 10 levels were reported to be lower in Turkey (26.7 µg m -3 , Kocak et al., 2007) and Brazil (34.4 µg m -3 , Mariani and Mello, 2007) and higher in China (682.0 µg m -3 , Wang et al., 2003) and India (171.0 µg m -3 , Rengarajan et al., 2011). Nasrallah and Seroji (2008) reported that daily PM 10 concentration ranged from 191 to 262 µg m -3 in Makkah, Saudi Arabia. Khodeir et al. (2012) estimated that the main sources of PM 2.5 and PM 10 in Jeddah were heavy oil combustion, resuspended soil, industrial sources, traffic sources, and marine aerosols.
The time variations of PM 2.5 /PM 10 ratios are presented in Fig. 4 (lower-panel). Here it can be observed that lowest ratios are observed at mid-day and in summer months, when re-suspended and windblown dust and sand particles are proportionately in higher quantity. It is important to emphasise that resuspended and windblown dusts are mostly in the coarse range (PM 10 -PM 2.5 ), therefore has low PM 2.5 /PM 10 ratio. In contrast PM emitted by combustion processes are mostly in the fine particulate (PM 2.5 ) range and have higher PM 2.5 /PM 10 ratio.
A scatter plot of PM 10 versus PM 2.5 in Makkah using hourly data from January 2014 to September 2015 is shown in Fig. 5. The scatter plots are coloured by the ratios of PM 2.5 /PM 10 . Based on the levels of ratios there are 3 distinct regions coloured as red, green and blue in Fig. 5 ( upperpanel). Probably the red colour, which represents high ratio of PM 2.5 /PM 10 (> 0.8) are those particles which are emitted by direct combustion processes or are made in the atmosphere, known as secondary particles, such as sulphate and nitrate ions. The blue colour represents course particles generated by mechanical means, such as grinding, construction-anddemolition and digging, having very low PM 2.5 /PM 10 ratio (< 0.2). Green colour having moderate ratios from 0.2 to 0.8 (roughly) are a mixture of the fine and coarse particles. Fig. 5 (lower-panel) which shows the counts of particles in each portion of the plot, shows that in the red region (counts > 200), there seems to be a very strong correlation between PM 10 and PM 2.5 , however as concentrations increase and the counts decrease, the plot spreads like a fan and makes a V -sort of shape, which weakens the correlations.
In Fig. 6 the correlation plot shows the association of PM 10 and PM 2.5 with each other and with wind speed, relative humidity and temperature. PM 2.5 is positively correlated with PM 10 (r = 0.51), temperature (r = 0.27) and with wind speed (r = 0.12) and negatively correlated with relative humidity (r = -0.17). Similarly, PM 10 is positively correlated with temperature (r = 0.24) and wind speed (r = 0.19) and negatively correlated with relative humidity (r = -0.30). Meteorological parameters, such as wind, temperature and relative humidity play an important role in transport,  dispersion, and removal (dry and wet deposition) of PM 10 and PM 2.5 from the atmosphere. They also affect atmospheric chemistry and hence the formation of secondary PM in the atmosphere (Andersson et al., 2006;Munir et al., 2013a). The effect of meteorological parameters varies in different regions of the world. In Makkah, previously Munir et al. (2013a) reported a strong positive effect of wind speed and temperature on PM 10 concentrations. High wind speed and temperature play a vital role in the dispersion and transportation of air pollutants from one place to another, ranging from local to regional or global scale, thus encouraging vertical and horizontal movement of the particles. In addition, in arid region like Makkah high wind speed and temperature may encourage wind turbulence and re-suspension of dust particles. Furthermore, Makkah receives little rain and is surrounded by large deserts, in such conditions wind blowing at high speed may lift dust and sand particles and increase particle concentrations in Fig. 6. Correlation plots of PM 10 , PM 2.5 , relative humidity, wind speed and temperature using hourly data from January 2014 to September 2015 in Makkah. the atmosphere. Previously Munir et al. (2013a) in a modelling study on PM 10 reported stronger (than reported in this study) positive correlation between PM 10 concentration and wind speed (r = +0.42) and temperature (r = +0.38). Relative humidity on the other hand has negative association with both PM 10 and PM 2.5 , probably due to washout effect and wet deposition by rainfall. However, Charron and Harrison (2003) reported that ultrafine particles had higher concentrations during the rainy periods, which might show that relative humidity has different effect on the concentrations of fine and course particles. Probably this is the reason that relative humidity showed stronger correlation (r = -0.30) with PM 10 than PM 2.5 (r = -0.17). The effect of meteorological parameters on the ratios of PM 2.5 and PM 10 is further elaborated by the help of polar plots (Figs. 8 and 9). Fig. 7 presents the ratios of PM 2.5 and PM 10 associated with wind speed and wind direction in various seasons of the year in Makkah. Generally the values of ratios are lower (< 0.6) in summer and higher (> 0.6) in winter and autumn at all wind speed and wind direction. In spring high ratios are associated with low wind speed (< 2 m s -1 ) from all directions and low ratios are associated with high wind speed blowing mostly form east and west direction. Fig. 7 shows the complex nature of the effect of meteorology on PM, which means the effect of wind direction and wind speed is not the same throughout the year. It rather changes in each season of the year. The Masfalah AQMS, from which the data were obtained, is situated at a roadside in a busy location in the centre of Makkah, therefore at low wind speed locally emitted air pollutants are not dispersed quickly. Traffic related PM has a high PM 2.5 /PM 10 ratio, this is the reason why the polar plots show high ratios at low wind speed in Fig. 7. The difference in the ratios observed in winter and summer is mostly due to temperature and atmospheric stability which are responsible for atmospheric turbulence and hence vertical pollutants dispersion. Winter is characterised by low temperature and stable atmospheric conditions, hindering the dispersion of locally emitted pollutants, whereas summer on the other hand is characterised by high temperature and unstable atmospheric conditions which encourage dispersion of locally emitted traffic related air pollutants and enhance resuspension of sand and dust particles, which consist mostly of coarse particles and hence reduce the ratios.
To further elaborate the effect of meteorology on the ratios of PM 2.5 and PM 10 , in Fig. 8 polar plots show the effect of temperature along with wind speed and direction. This figure also confirms that seasonal variations in the ratios are mostly due to the levels of temperature. There is a general negative association between temperature levels and PM 2.5 /PM 10 ratios i.e., higher ratios are linked with low temperature and vice versa. However, this relationship is affected by wind speed and wind direction. At low temperature (< 34.5°C), the ratios are greater than 0.60, whereas at high temperature (> 43.4°C), the ratios are less than 0.60. Furthermore, low wind speed is shown to encourage high PM 2.5 /PM 10 ratios and vice versa. At temperature from 34.5 to 38.4°C, high wind speed (> 2 m s -1 ) blowing from the west decreases the  ratios. Similarly high wind speed (>2 m s -1 ) at temperature range from 38.4 to 43.4°C causes reduction in the ratios almost from all directions. The effect of wind speed is linked with dispersion and re-suspension of the particles, whereas the effect of wind direction is linked with the emission sources. When the wind direction is linked with low ratios, it shows that in that direction probably there is a source of PM dominated by coarse particles. On the other hand when wind direction is linked with high ratios, it shows the presence of fine PM dominated source, such as road traffic or other combustion sources. Fig. 9 shows the effect of relative humidity on the ratios of PM 2.5 and PM 10 . There is a clear positive association between the levels of relative humidity and the ratios of PM 2.5 and PM 10 . The interesting thing is that unlike temperature, the association of relative humidity with the ratios of PM 2.5 and PM 10 does not seem to be wind speed or direction dependent and the ratios are the same at all wind speed and direction. Although relative humidity has negative correlation with both PM 2.5 and PM 10 concentrations (Fig. 6), it seems to have a stronger negative effect on PM 10 than on PM 2.5 . Due to washout effect and wet deposition, rainfall and high relative humidity may cause reduction in the concentration of PM, however due to differential effect fine and ultrafine particles are observed to have higher concentrations in the rainy season (Charron and Harrison, 2003). This shows the importance of meteorological parameters on PM and how meteorological parameters have different effects on smaller and larger particles and thus affecting the ratios of PM 2.5 and PM 10 . The effect of meteorological parameters also vary from place to place  due to variations is local meteorological conditions and emission sources.

CONCLUSIONS
In this paper PM 2.5 , PM 10 and their ratios are analysed during the period from January 2014 to September 2015 in the Holy City of Makkah Saudi Arabia. The ratios of PM 2.5 and PM 10 are analysed in light of their diurnal, weekly and seasonal cycles. Furthermore, the effects of some meteorological parameters, such as wind speed, wind direction, temperature and relative humidity on PM 2.5 , PM 10 and their ratios are investigated with the help of correlation analysis and polar plots. 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. Overall, temperature and wind speed had negative associations, whereas relative humidity had positive association with the ratios, which means that the former encourages high levels of PM 10 , whereas the latter encourages high levels of PM 2.5 . The effect of temperature and relative humidity on PM is complicated by wind speed and wind direction, which is demonstrated by the polar plots. Makkah is part of an arid region, which receives little rainfall and experiences high temperature, especially in summer when temperature reaches as high as 50°C. Meteorology seems to play an important role in determining the levels of PM 2.5 and PM 10 and their ratios. The ratios are higher in winter and during night-time and lower in summer and during hot hours of the day. Further work is required on source apportionment and receptor modelling of PM 10 and PM 2.5 which will indentify the emission sources of PM and help prepare an effective air quality plan in Makkah.