Ambient Levels of TSP , PM 10 , PM 2 . 5 and Particle Number Concentration in Al Samha , UAE

The Arabian Peninsula experiences elevated levels of airborne particulate originated from both natural and anthropogenic sources. This study is mainly aimed to determine the ambient levels of TSP, PM10 and PM2.5) at one of the monitoring locations “Al Samha” that is located in the northeast quadrant of UAE. Mass concentrations, particle count, as well as meteorological parameters were simultaneously measured using a spectrometer, PM10 beta attenuation monitor and weather sensors for the period from April 10 to December 31, 2011. The hourly mean concentrations of TSP, PM10, PM2.5-10 and PM2.5 were 245, 110, 64 and 46 μg/m, respectively. About 34%, 15% and 56% of the monitored days had daily concentrations above the allowable limits for TSP, PM10 and PM2.5, respectively. Diurnal peak occurred at 14:00 for TSP, at 10:00 for PM10, and at 04:00 for PM2.5 reaching values of up to 410, 122, and 54 μg/m, respectively. The highest concentrations were observed on Saturdays for TSP and PM10, but on Sundays for PM2.5. July had the greatest monthly level of PM compared to other months of this study. The average ratios of PM10/TSP, PM2.5/TSP and PM2.5/PM10 were 0.61, 0.31 and 0.47, respectively. Weak relationships were found between the particle number and mass concentrations, while very strong to moderate correlations were observed among all PM size fractions as well as between TSP and wind speed. The measurement results of the light scattering spectrometer were strongly correlated with the beta attenuation monitor, but the mean value of the spectrometer was higher by 18%.


Introduction
Air pollution kills about 7 million people, 12.5% of the global deaths, every year across the world [1], and it is expected to become the top environmental cause of global mortality by 2050 [2] .Predominantly, airborne particulates contribute greatly to poor air quality and are considered to be one of the biggest threats to human health in urban environments [3] [4] [5] [6] [7].
Airborne particulate can be classified in various ways based on their properties such as; size, shape, formation mechanism, and composition.However, the most common classification is according to their characteristic size [8].Total Suspended Particles (TSP) refers to all particles up to 50 micrometers (μm) in diameter that can remain suspended in the atmosphere for significant periods of time [9].More precisely, Particulate Matter (PM) is usually labeled by a number indicating its aerodynamic diameter.For instance, PM 10 (respirable) and PM 2.5 (fine) refer to particles with a nominal mean aerodynamic diameter of less than or equal to 10 µm and 2.5 μm, respectively [10] .The notation PM 2.5-10 is used to represent the coarse particles with an aerodynamic diameter between 2.5 μm and 10 μm [11].
The sources of PM are divided into three major categories; natural, anthropogenic and secondary.Windblown dust, sea sprays, volcanoes, fires and pollen are examples of natural sources.On the other hand, anthropogenic sources are further classified into stationary and mobile subcategories; stationary sources are fixed-site producers such as power plants, factories, mines, farms, and waste-disposal sites.Whereas, mobile sources are mainly the transportation means such as cars, trucks, planes and ships that emit pollutants while moving [12].Finally, secondary fine particles are formed in the atmosphere through chemical reactions among the gaseous pollutants involving; sulfur dioxide (SO 2 ), nitrogen oxides (NO x ), volatile organic compounds (VOCs) and ammonia (NH 3 ) [13].
Elevated levels of ambient PM might lead to considerable adverse effects on public health and the environment.On one hand, it contributes to visibility degradation, acid deposition, and influences the climate either directly by scattering and absorbing sunlight radiation or indirectly through providing condensation nuclei for cloud droplets [14].On the other hand, both short and long-term exposures to PM cause respiratory and cardiovascular diseases and are also linked to overall increased mortality [15] .However, the size of the particle plays an important role in its potential hazard.As such, smaller particles have a larger surface area available for physical and chemical interactions, travel farther distances, remain suspended for longer times, and penetrate deeper into the human respiratory system [16].
Therefore, strategic plans have been developed and implemented by many countries across the world to control PM levels and eventually minimize its adverse impacts [17].In order to achieve the desired objectives, these control plans should be established based on reliable monitoring information, which highlights the importance of assessment and evaluation programs [18].
The Arabian Peninsula, including the United Arab Emirates (UAE), experiences elevated levels of PM originating from both natural and anthropogenic sources [19] [20] [21].Thus, comprehensive studies are very essential to understand the temporal and the spatial behavior of the suspended particulates, and to accordingly apply effective measures to achieve and maintain acceptable levels.
In this study, continuous measurements were carried out at Al Samha area for TSP, PM 10 & PM 2.5 mass concentrations, particle count as well as meteorological parameters during the period from April 10 to December 31, 2011.The obtained results were comprehensively analyzed to examine different measurement techniques, verify the compliance with relevant standards, determine temporal variation patterns, and investigate inter-correlations between the measured parameters.The findings of this study might be of great relevance to scientists and decision-makers, providing them with a fundamental basis to establish further research studies and develop effective policies for pollution reduction.

Site Description
The measurements were conducted in Al Samha area (Figure 1), which is located approximately 40 km northeast of Abu Dhabi City at about the midway to Dubai.The area is surrounded by various contributors of particulate matters, from sources such as desert sand, power plants, an aluminium smelter, and construction activities, in addition to road and sea traffic.Furthermore, sandstorms are a common occurrence across the entire region, especially during the summer months.
The UAE generally has a subtropical and arid climate, being hot, humid and very dry during summer (April to September), and becoming cooler with occasional rainfall during the winter season (October to March) [22].

Instrumentation
In this study, TSP, PM 10 , PM 2.5 mass concentrations and Particle Number (PN) were simultaneously measured using a Grimm aerosol spectrometer (Grimm Aerosol Technik GmbH, Germany, model EMD 365), which is also equipped with weather sensors (LufftGmbH, Germany, model WS600) to jointly monitor meteorological parameters such as wind speed, wind direction, relative humidity and temperature.The mass concentrations of the coarse particles (PM 2.5-10 ) were calculated as the difference between PM 10 and PM 2.5 concentrations.Concurrently, PM 10 mass concentrations were measured continuously using a beta attenuation monitor (Environment S.A., France, model MP101M).For comparison and verification purposes, TSP and PM 2.5 daily levels were also gravimetrically determined by collecting of some random samples.
The spectrometer (EDM 365) is designed on the principle of orthogonal light scattering, where air containing multiple particle sizes passes through a flat laser beam.The scattered signal is collected at approximately 90˚ to the beam by a mirror and is detected by a high speed photodiode.Each signal is then counted and classified into different size channels by an integrated pulse height analyzer.Eventually, these counts are converted to a mass distribution using the density factor established for urban environments.The EDM 365 utilizes a diffusion dryer to avoid condensation during measurement, which is activated when the relative humidity exceeds 70%.In the beta attenuation monitor, the sampling stream is slightly heated to avoid water condensation, and the air sample is sucked at a constant flow rate (16.7 L/min) from PM 10 size-selective inlet and pulled through a filter to deposit particles.At the end of a predefined hourly sampling cycle, the loaded filter is positioned between a carbon 14 beta source and a Geiger-Mueller detector to determine attenuation of the beta ray signal which is directly proportional to the mass of dust accumulated on the filter.
Finally, a particulate sampler (Environment S.A., France, model MP162) was used to collect daily random samples of TSP and PM 2.5 , where an air sample is drawn for 24 hours at a constant flow rate of 16.7 L/min through a size-selective inlet (TSP or PM 2.5 ) and then collected on a 47 mm filter membrane.The filters were conditioned and weighted prior and after sampling to determine net weight gain due to the collection of sample and eventually estimate the concentration.

Regulations and Guidelines
Air quality standards for suspended PM have been established by different entities in order to protect public health and the environment (Table 1).These standards identify the maximum acceptable concentrations in ambient air, which should not be exceeded during a specified time interval.In this study, the UAE standards were used to assess the daily concentrations of TSP and PM 10 , while the PM 2.5 daily limit of 35 µg/m 3 was also consulted since it is widely applied in many countries such as the Kingdom of Saudi Arabia (KSA), United States (USA) and others.

Statistical Analysis
All statistical analyses were performed using Microsoft Excel and SPSS Statistics.Pearson's correlation analysis was used to determine the linear correlations between the measured parameters, where the existence and strength of the relationship is assessed based on the correlation coefficient (r) as follows: negligible if r < 0.19, weak if r is between 0.2 and 0.39, moderate if r is between 0.4 and 0.59, strong if between 0.6 and 0.79, and very strong if r > 0.8 [23].

Mass Concentrations and Particle Number
Descriptive statistics of the hourly concentrations obtained throughout the study period are summarized in As shown in Figure 2, elevated daily concentrations were observed during the study period for TSP, PM 10 and PM 2.5 reaching values of up to 1160 µg/m 3 , 657 µg/m 3 and 252 µg/m 3 , respectively.Furthermore, about 34%, 15% and 56% of the monitored days had 24-hour average concentrations above the maximum allowable limits of TSP, PM 10 and PM 2.5 , respectively.These elevated levels might be attributed to various factors including; increased human activities (e.g.industries and traffic), frequent natural events (e.g.dust storms) and the significant influence of climate conditions (e.g.enhanced formation conditions of secondary particles with high temperatures and intense sunlight in addition to re-suspension of surface dusts in dry conditions).

Temporal Variation of PM
As it is obvious in Figure 3, the diurnal variation of PM with different size fractions did not follow a similar pattern because of their divergence characteristics as well as the variance of their behaviors in the atmosphere.The lowest levels of TSP and PM 10 were observed during the early morning hours between 01:00 -02:00 am, where the human activities are minimal and the climate is relatively cool, damp, with an eastern low-speed wind.After sunrise, the concentrations remarkably started to rise in conjunction with increased temperature and wind speed and reduced humidity, reaching primary and secondary peaks at 10:00 am and at 13:00 pm for PM 10 and one hour later for TSP, and then began to decline.The rise in TSP and PM 10 levels might be justified by a longer lifetime of particles at low humidity conditions, re-suspension of surface dusts by higher wind speed, and formation of secondary aerosols at high temperatures.The observed time lag between PM 10 and TSP can be explained by the longer time required to transport larger and heavier particles by the wind, in addition to the contribution of the small particles that are agglomerated and coalesced to form greater ones over time.On the other hand, the least level of PM 2.5 occurred at 12:00 noon associated with high temperature, low humidity, and moderate-speed western wind, and then PM 2.5 level increased gradually to reach its peak at 04:00 am.The humid conditions are associated with high levels of PM 2.5 which might be attributed to the role of moisture in forming secondary fine aerosol such as ammonium nitrate through the gas-to-particle conversion.Changes in prevailing wind directions have no noticeable effect on the average diurnal concentrations.As illustrated in Table 3, the highest mean concentrations were observed on Saturdays for TSP and PM 10 and on Sundays for PM 2.5 .On the other hand, the lowest levels were recorded on Thursdays for TSP and on Wednesdays for PM 10 and PM 2.5 .The elevated PM levels during Saturday and Sunday might be attributed to the increased human and industrial activities during the free-time    4. The highest mass mean concentrations were observed for all size fractions in July primarily due to the frequent occurrence of dust storms during this period of time.The pattern of TSP during April to July is consonant with the wind speed pattern, which indicates that there is a notable influence of wind speed on large particulate levels.As expected, the lowest PM levels were recorded during the cool winter season as a result of the humid and occasionally rainy conditions.As presented in Figure 5, non-identical pattern of higher diurnal concentrations of particulate matters was observed during the summer months (April-August) as compared to the winter months (September, November and December).

PM Mass Ratios
Based on the mean ratio values shown in Table 5, TSP contains nearly 39% of particles with an aerodynamic diameter greater than 10 µm (PM >10 ), and the rest (61%) is PM 10 , which consists of 47% PM 2.5 and 53% PM 2.5-10 .These results are inconsistent with the results of Engelbrecht et al. [24] for daily samples collected in the UAE, where the reported ratios of PM 10 /TSP and PM 2.5 /PM 10 were 0.71 and 0.37, respectively.The deviations between the obtained results and the above mentioned reported results by Engelbrecht et al. are mainly due to the influence of temporal and spatial variation in PM ambient levels.However, our results are closer to the typical PM 2.5 /PM 10 ratio of 0.5 that have been documented for urban areas in developing countries [25], and reported for urban sites in Iran [26] .

Figure 1 .
Figure 1.Map showing the location of the study area.

Figure 3 .
Figure 3.Diurnal variation patterns of the meteorological parameters and airborne particulates during the study period.

in Figure 4 ,Figure 4 .
Figure 4. Diurnal variation patterns during the days when the maximum concentrations were observed as compared with other days for (a) TSP, (b) PM 10 and (c) PM 2.5 .

Figure 5 .
Figure 5.Diurnal variation patterns during summer and winter months for (a) TSP, (b) PM 10 and (c) PM 2.5 .
*Based on a percentile value.

Table 2 .
Based on mean value, TSP was approximately 2.2, 3.8 and 5.3 times greater than PM 10 , PM 2.5-10 and PM 2.5 , respectively; while PM 10 was higher than PM 2.5-10 by a factor of 1.7 and PM 2.5 by a factor of 2.4.Hourly concentrations of the particulate number varied widely from 34,035 cm −3 to 2,085,556 cm −3 with a median of 247,431 cm −3 .

Table 2 .
Statistical analysis results for measurements of hourly concentration conducted during the study period.

Table 3 .
Levels of airborne particulates (µg/m 3 ) during the weekdays and weekends of the study period.

Table 4 .
Monthly levels of airborne particulates (µg/m 3 ) during the study period.
*Data is not available from September 26, 20:00 to October 24, 14:00 due to power supply failure.

Table 5 .
Mass ratios between airborne particulate of different size fractions at the study area.