The monthly variation of PM₁₀ concentrations as obtained from the daily mean recordings across the whole study period (February 2012 to January 2013) is shown in Figure 1. Results indicate that the highest PM concentrations occurs mainly in February, March and April, which is the period with the most frequent dust-storm

events [15] . The highest PM₁₀ concentration (2234 µg・m⁻³) is recorded during March 18^th, closely associated with a severe dust event. One can, therefore, conclude that the intense dust storms taking place on specific days during spring and early summer are predominantly responsible for the large month-to month variations at all PM concentrations. The annual pattern of PM₁₀ shows a significant increase in spring and summer where monthly mean concentrations of up to 696 μg・m⁻³ were recorded in March. PM₁₀ concentrations during winter months are significantly lower, with 65 μg・m⁻³ measured in November and 79 μg・m⁻³ measured in December. January exhibits PM₁₀ concentrations of little above 100 μg・m⁻³, which persists until February when PM₁₀ levels increase as a result of dust events.

The PM₁₀ data when grouped by month (Figure 2), indicated a consistent and systematic pattern decreasing in the order: spring (February-April, 53%) followed by summer (May-July, 30%) and fall and winter (September- January, 18%). The spring Peak of PM₁₀ is coinciding with dust events that commonly occur during spring in Riyadh. In contrast, the lowest PM concentration in winter can be attributed to the absence of dust events during winter. The variation of monthly mean PM_2.5 (Figure 3, Figure 4) is quite similar to PM₁₀; (March maximum). The similarity in the annual variation between PM₁₀ and PM_2.5 suggests some similarities in source regions for these aerosol sizes.

Some studies conducted in other urban environments, in Turkey [16] and in Athens, Greece [17] found, contrary to our results, that in winter both PM concentrations were higher, which was attributed to larger use of fossil fuels in winter. A monthly mean PM₁₀ concentrations in Athens ranging from 60.3 μg・m⁻³ (January) to 88.9 μg・m⁻³ (December), with an annual mean value of 75.5μg・m⁻³ was reported [17] . In Barcelona, Spain, the ambient PM₁₀ and PM_2.5 were in the range of 39 to 42 μg・m⁻³ and 25 to 29 μg・m⁻³, respectively over the period 2003-2006 with 97 daily values exceeding 50 μg・m⁻³ [18] while the mean annual PM10 concentration ranges from 20 to 37 μg・m⁻³ in Rio de Janeiro, Brazil [19] . Comparing the present results with those of the above- mentioned studies, it is concluded that the city of Riyadh is exposed to much higher PM concentration levels. This is not only the case for summer, when the area is affected by natural phenomena, but also for winter. This highlights the fact that PM concentrations over Riyadh can be regarded as a genuine environmental problem that poses a real risk to quality of life and threatens human health.

The monthly mean values of PM₁₀, PM_2.5, and PM₁ in 2012-2013 are shown in Figure 5-7, and the month wise percentage share of PM₁₀ in 2012 is shown in a pie chart (Figure 8). The concentrations of PM₁₀ are highest in summer (June-July 23% and 24% respectively). The concentration decreases in September and October (14%) and reaches its minimum in November (10%). The monthly variability of PM_2.5 (Figure 9) followed the same trend except the relatively higher percentage which was registered in December (20%). On the other hand it is noticeable that the monthly variability of PM₁ (Figure 10) reflected higher percentage in winter months, (December, November and October, 21%, 17% and 17% respectively). The difference in the monthly variation

between PM₁₀ on one hand and PM _2.5 and PM_1.0 on the other hand, may suggests differences in source regions for these aerosol sizes. The main anthropogenic source of PM_2.5-1 in Riyadh urban environment can be confined to vehicular traffic, fossil-fuel combustion, been observed within urban environments in other cities [20] - [22] . In addition, the boundary layer mixing height is lower in winter and traps the pollutants near the ground as a result of temperature inversions. This may explain the relatively higher concentration of small-sized particles (PM_1.0) in winter. In contrast, during summer months thermal heating at the surface and the increase of the mixing layer height favors buoyancy and the dilution of anthropogenic aerosols (PM_1.0). Apart from desert dust, a natural contribution to the total PM (mainly to PM_2.5 and PM₁₀) is also expected to originate from eolian and traffic- driven re-suspension of dust, since the scarce rainfall favors the accumulation of road dust in summer.

The variation of monthly mean PM₁₀ in 2013 showed more or less similar results to 2012 during month June, July, September and October. Higher concentrations were observed during spring and summer (12%, 17% and 13% in Feb., March and April respectively and 17, 15% during June and July,). Starting from September and October PM₁₀ concentrations started to drop to 7% and 8% respectively (Figure 11). Data of November and December is expected to drop further as was demonstrated in 2012 at both KF and AP stations. As described above, a characteristic seasonal variation can be observed during 2013 with relatively elevated PM concentrations observed in the warmer seasons peaking in March, June and July (Figure 11). This is consistent with the earlier results at the airport station. Higher summertime PM concentrations can be attributed to elevated wind speed that enhances wind induced re-suspended PM concentrations. Wind speeds are higher in the summer. A somewhat similar pattern―if reduced in magnitude was observed during 2012

In contrast, the variation of monthly mean PM_2.5 and PM₁ concentrations vary less seasonally compared to PM₁₀ mass concentrations (Figure 12, Figure 13) This has been observed previously during 2012. This could be due to the fact that a major source for PM_2.5 and PM₁ which is motor-vehicle emissions is not directly affected by seasons. In contrast, secondary atmospheric formation of fine particles through photochemical processes is higher during the summer (relatively higher concentrations are observed from May?October). Stronger source strength in the summer however is counterbalanced by the lower mixing height in winter months thus reducing seasonal variability.

In general, the results of PM₁₀, PM_2.5, and, PM₁ concentrations, obtained during the current work were little higher than those found in other studies. Results obtained by studies of PM_2.5 concentrations conducted in China [23] reported a mean PM_2.5 concentration of 67.6 µg/m³. Also [24] reported 89 µg/m³ during the spring. These values are little less than the values reported in this study. On the other hand, some other studies reported more or less similar results. For an example, PM₁₀ concentrations higher than 3000 and produced by dust events were

observed in a study conducted in Iraq, Kuwait and Saudi Arabia [25] . In contrast, Meng and Lu [26] observed mean PM_2.5 concentrations of up to 216.7 µg/m³, which was higher than the results presented in this study (almost double). The explanation for this substantial difference is that the PM_2.5 is measured during winter months where fossil fuel combustions which considered as the primary source of fine particulate are highest.

The frequency of occurrence for PM₁₀ and PM_2.5 concentrations for the studied period in A.P station showed that 4.5% of the PM10 values were more than 1000 μg・m⁻³ occurring all in spring. Approximately 9.5%, 9% and 16% occurred at 500 - 1000, 340 - 500 and 200 - 340 μg・m⁻³ interval, respectively. A peak in the distribution of PM₁₀ concentrations occurred at 100 - 200 μg・m⁻³, (32%). The percentage of occurrence of values 50 - 100 μg・m⁻³ and less than 50 μg・m⁻³ were 19% and 9% respectively. PM₁₀ values <50 μg・m⁻³ is observed in winter months (Table 1, Table 2).

Regarding the frequency distributions of PM_2.5 in all seasons the largest frequency is observed for values between 35 and 100 μg・m⁻³, (52%), while in spring the largest frequency shifts towards higher values (100 - 600 μg・m⁻³, 18%. Similarly to PM10, spring presents a broader distribution for PM_2.5, with values >100 μg・m⁻³

range. Apart from these similarities in the PM₁₀ and PM_2.5 frequency distributions, some differences in winter and summer reveal a possible different source of aerosols in these seasons (natural or anthropogenic). For example, the intense dust storms during summer do not have such a pronounced signal in PM_2.5 concentrations as in PM₁₀ ones, while the larger contribution of anthropogenic aerosols in winter rather increases the PM_2.5 levels. However, it should be noted that dust events may also affect significantly the PM_2.5 levels, as observed during a severe dusty day in March.

The frequency of occurrence for PM₁₀ and PM_2.5 concentrations for each season is depicted in table, respectively. In summer 2012 (June-September), 55% of the PM₁₀ values were between 180 and 340 μg・m⁻³; 25% of the PM₁₀ values were more than 500 μg・m⁻³. In October 2012 to January 2013, the frequency distribution shifts towards lower values, with values above 500 μg・m⁻³ only 6% and the peak frequency occurred for values between 200 - 340 μg・m⁻³ (40%). On the other hand, a considerable fraction (24%) of PM₁₀ values <500 μg・m⁻³ is observed in spring (Feb.-March). The frequency of occurrence for PM10 during June to October is some who resembles the values of the summer months. Peak concentrations occurred for values between 100 - 200 μg・m⁻³ (43%), 15% of the values were above 500 μg・m⁻³. Regarding the frequency distributions of PM_2.5 in all seasons except June- October, the largest frequency is observed for values between 50 and 100 μg・m⁻³, while in June-October the largest frequency shifts towards lower values (<50 μg・m⁻³), which is opposite to that observed for PM₁₀. However, similarly to PM₁₀, summer presents a broader distribution for PM_2.5, with values > a 100 μg・m⁻³ range. Apart from these similarities in the PM₁₀ and PM_2.5 frequency distributions, some differences in winter and summer reveal a possible different source of aerosols in these seasons (natural or anthropogenic). For example, the intense dust storms during summer do not have such a pronounced signal in PM_2.5 concentrations as in PM₁₀ ones, while the larger contribution of anthropogenic aerosols in winter rather increases the PM_2.5 levels. Also note that the mean PM_2.5 in winter (32 μg・m⁻³) is similar to that of spring and larger than that of autumn, despite the fact that winter PM₁₀ is the lowest. However, it should be noted that dust events may also affect significantly the PM_2.5 levels, as observed during a severe dusty day in June (PM_2.5 daily value of 182 μg・m⁻³) (Table 1, Table 2).

During this study period, we observed that PM₁₀ and PM_2.5 concentrations mainly depend on dust storms occasions and increased significantly during dust storms; however, there is no universally accepted definition of dust storms in terms of hourly dust concentrations. Previous researchers have classified dust storms by their intensities on the basis of hourly mean PM₁₀ concentrations together with wind speed and visibility. The classification system proposed by Hoffmann et al., [27] is based on the hourly mean PM₁₀ concentration in combination with visibility and wind speed as follows: “dusty air” (haze) means hourly mean PM₁₀ concentrations higher than 50 μg・m⁻³; “light dust storm”, >200 μg・m⁻³; “dust storm”, >500 μg・m⁻³; “strong dust storm”, >2000 μg・m⁻³; and “serious strong dust storm”, >5000 μg・m⁻³.Wang et al. [28] used hourly mean PM₁₀ concentrations of <200 μg・m⁻³ for “suspended dust,” 200 - 5500 μg・m⁻³ for “blowing dust,” 5500 - 15,000 μg^?3 for “sand and dust storm”, and >15,000 μg⁻³ for “severe sand and dust storm”.

In Taiwan an Asian dust storm event is defined each day that hourly mean PM₁₀ concentrations observed at the Yangmingshan station exceed the air quality standard (125 μg・m⁻³) for a period of at least 3 h [29] . Dust storms are defined in Korea when the measured PM₁₀ concentration exceeds 190 μg・m⁻³ for at least 2 h [30] . The KMA has developed an Asian Dust Warning System with alarm, advisory, and warning levels depending on the dust concentration. Alarms, advisories, and warnings are issued when the hourly mean dust concentrations (PM₁₀) are expected to exceed 300 μg・m⁻³, 400 μg・m⁻³, or 800 μg・m⁻³, respectively, for over 2 h [30] . Park et al. (2010) [31] identified dust events as the peak PM10 concentration exceeding one standard deviation of the annual mean concentration. We arbitrary choose this criterion. Our criterion for dust storm events was an hourly mean PM₁₀ concentration exceeding one standard deviation of the mean concentration at the airport station during February 2012 to January 2013 months (mean = 282.11.34 µg/m³ and standard deviation = 372). Based on this criterion, we identified 16, 21, 10, 6, 5, 6 and 1 dust events at February, March, April, May, June, July and December 2013. Based on the same criterion, we identified 8, 17, 7, 7, 3, and 6 dust events at June, July, September, October, November and December 2012 and 14, 11, 20, 16, 7, 9, and 4 at January, February, March, June, July September and October 2013 respectively.