Diurnal and Seasonal Variations of Surface Ozone and Its Precursors in the Atmosphere of Yanbu , Saudi Arabia

Surface ozone (O3) and its precursor’s gases (nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrogen oxides (NOx), carbon monoxide (CO), methane (CH4) and non-methane hydrocarbons (NMHCs), and sulfur dioxide (SO2) were measured in Yanbu, Saudi Arabia from January 2004-December 2004. The annual average concentrations of O3, NO, NO2, NOx, SO2, CO, CH4, TNMHCs and THC were 22.51 ppb, 15.58 ppb, 17.25 ppb, 23.84 ppb, 6.66 ppb, 165.13 ppb, 3.44 ppm, 0.56 ppm and 3.88 ppm, respectively. The SO2/NOx and SO2/NO2 concentration ratios during the period of study indicate that the mobile emissions are the predominant sources in yanbu. The diurnal cycles of O3 concentrations revealed highest levels in the daytime and lowest levels in night time. The diurnal variations of NO, NO2, NOx, SO2, CO, NMHCs concentrations were similar and showed peaks concentrations linked to traffic density, boundary layer mixing processes and chemical processes in the atmosphere. The mean concentrations of O3 in different seasons follow the order of summer > spring > autumn > winter. In contrary to O3, NO has higher concentrations in autumn and winter than those in summer and spring seasons. The highest concentrations of NO2, NOx, SO2, CO, and TNMHCs were found in autumn and spring, whereas the lowest concentrations were found in summer and winter. THC and CH4 have no significant change over winter, autumn and summer seasons, while their concentrations decrease sharply in spring. Although the studied area does not suffer from NO2, O3, SO2 and CO pollution and no health risk, comprehensive and long-term air quality management programmes are needed in order to keep air quality in a good condition.


Introduction
Anthropogenic air pollutants caused by technological progress, industrialization, and urban overpopulation had led to the deterioration of environmental air quality [1].Toxic air pollutants emitted in close proximity of population areas have the potential to subject such population to health risks [2].Tropospheric ozone (O 3 ) is an important air pollutant threatening human health and vegetation growth [3] [4].People living in cities with high O 3 levels had an over 30% increased risk of dying from lung disease [5].O 3 is also one of the key species affecting the chemical properties of the atmosphere where it is a precursor for the highly reactive hydroxyl radical [6] [7].Surface O 3 and its precursors play an important role in affecting regional climates and causing adverse effects on human health and vegetation [8].
There are two sources of tropospheric O 3 , viz.photochemical production [9] and downward transport of ozone from the stratosphere [10].Ground level O 3 is a major constituent of smog in urban areas where motor vehicles are the main anthropogenic emission source of its precursors [11].It is produced mainly by photochemistry involving pollutants that are released from various industrial and other anthropogenic activities, e.g., CH 4 , CO and [12].NO x and hydrogen oxide radicals act as catalysts in this process.The relation between O 3 and its main precursors represents one of the major scientific challenges associated with urban air pollution [13].
Ozone concentrations depend on the absolute and relative concentrations of its precursors and the intensity of solar radiation [14].Variations in O 3 concentration are controlled by a number of processes including photochemistry, physical/chemical removal, and transport, which occur on local, regional and global scales [15].The variations in local meteorological conditions, such as wind direction, wind speed, temperature, and relative humidity, can affect the temporal variations in O 3 and its precursors [16].An analysis of the influences of meteorological parameters on O 3 and its precursors at a specific site can contribute to a better understanding of the local and regional causes of O 3 pollution [8].The seasonal and diurnal variations of surface O 3 and its precursors and the related meteorology have been extensively studied around the world [17] [18].These studies showed that O 3 chemistry and the effects of meteorological conditions could vary depending on the characteristics of the climate and air pollutants emissions in the place of interest.The temporal variations of O 3 have been reported at many sites including rural, urban, coast and mountain sites in India [19], Japan [20], Thailand [21] and Kuwait [14].
Large quantities of non-methane hydrocarbons (NMHC) are emitted in the troposphere from vegetation and from a variety of anthropogenic sources.These anthropogenic sources include incomplete combustion sources (vehicle and fossil fueled power plant emissions), fuel storage and transport, solvent usage, emissions from industrial operations, landfills, and hazardous waste facilities [22] [23].NO x are also emitted, or produced in the troposphere.Nitrogen monoxide (NO) is emitted from soils and natural fires and formed in situ in the troposphere from lightning, and is emitted from combustion processes such as vehicle emissions and fossil fueled power plants [22] [23].NO is short lived because it oxidizes to produce nitrogen dioxide gas (NO 2 ) and plays a major role in O 3 production.In urban areas, NMHC and NO x , from anthropogenic sources dominate over NMHC and NO x from biogenic sources, and the reverse is generally the case in rural and remote areas [24].
Biomass burning [25], combustion of fossil fuels, and oxidation of hydrocarbons released from automobiles and industrial solvents are the main sources of atmospheric CO [26].Its oxidation leads to O3 formation or destruction, depending upon the NO concentration [27].The reaction of CO with OH radicals is the primary removal process from the atmosphere.Through this mechanism, CO acts as a major precursor to photochemical ozone [9].
In order to reduce ambient ozone concentrations, anthropogenic VOC emissions, which predominantly consist of non-methane hydrocarbons (NMHC s ) have been the subject of extensive control programs in many countries.Accurate characterization of ozone precursors is extremely important for understanding tropospheric ozone formation and crafting effective control strategies to better address ozone air quality management issues [28].The objective of the present study is to investigate the seasonal and diurnal variations of surface O3 and its precursors (NO, NO 2 , NO x , CO, THC, CH 4 and TNMHCs), and SO 2 in the atmosphere of yanbu Al Sinaiyah, and compare the concentrations of these pollutants with the standard limits.

Study Area
Yanbu Al Sinaiyah is a city in the Hejaz region of western Saudi Arabia.It has a population of more than 66,381 people.It is located about 350 km northwest of Jeddah.It is a terminal for oil pipelines and gas, which, stretching from the production areas in eastern Saudi Arabia to the west and it is the largest port on the Red Sea coast for the export of crude oil [29].There are two types of industries in Yanbu Al Sinaiyah, which are traditional local industries and modern industries.The traditional local industries include building of sailing ships and fishing boats, furniture, traditional agricultural machinery, wicker industry, leather industry, woolens industry and jewelry industry.The modern industries include oil refining, sort of liquid natural gas, petrochemical industry, lubricants and oil additives industry.
Yanbu has a hot desert climate.The climate of Yanbu Al Sinaiyah is characterized by high temperature and dryness.The maximum temperature reaches up more than 40˚C in the summer, while temperature in the winter is between 28˚C and 15˚C.The relative humidity is between 25% (dry) and 83% (humid).The average annual rainfall is lower than 50 mm.Over the course of the year typical wind speeds vary from 0 mph to 23 mph (calm to fresh breeze), rarely exceeding 30 mph (strong breeze).The wind is most often out of the west (31% of the time).The wind is least often out of the north (3% of the time), south east (3% of the time), north east (5% of the time), south (5% of the time), and east (5% of the time).

Air Sampling
Air quality monitoring mobile station (Environment SA, France) was used to conduct the measurements of pollutants including: NO, NO 2 , NO x , O 3 , CO, SO 2 , THC, NMHCs and CH 4 .Calibration was daily done and no data was recorded during that period.Average hourly measurements were taken in the period of January 2004 to December 2004.

Status of O3 and Its Precursors
The statistical summary of daily average concentrations of O 3 , NO, NO 2 , NO x , SO 2 , CO, THC, CH 4 and TNMHC during the period of study (January 2004 to December 2004) are shown in Table 1.Seasons are defined as: spring is March, April and May; summer is June, July and August; autumn is September, October and November; winter is December, January and February.The daily average concentration ranged from 5.4 4 to 50.68 ppb (with a mean value of 22.51ppb) for O 3 , 6.04 to 45.47 ppb (with a mean value of 15.58 ppb) for NO, 5.42 to 36.74 ppb (with a mean value of 17.25 ppb) for NO 2 , 8.83 to 89.70 ppb (with a mean value of 32.84 ppb) for NO x , 1.00 to 53.56 ppb (with a mean value of 6.66 ppb) for SO 2 , 32.92 to 1006.67 ppb (with a mean value of 165.13 ppb) for CO, 0.76 to 6.27 ppm (with a mean value of 3.88 ppm) for THC, 0.64 to 5.65 ppm (with a mean value of 3.44 ppm) for CH 4 , 0.10 to 3.45 ppm (with a mean value of 0.56 ppm) for TNMHC during the period of study.
The results of daily (24-h) minimum, maximum and average concentrations of NO 2 , O 3 , SO 2 and CO were compared with the available air quality standards in different countries.The concentrations of these pollutants did not exceed the available standards.Although, the studied area does not suffer from NO 2 , O 3 , SO 2 and CO pollution and no health risk, comprehensive, long-term air quality management programmers are needed in order to keep air quality in a good condition.

Diurnal Variations in O3 and Its Precursors
The diurnal variations in O 3 , NO, NO 2 , NO x , CO, and SO 2 concentrations during the period of study are represented graphically in Figure 1.From this figure, it can be noticed that the diurnal variation of O 3 is characterized by high concentrations during daytime and low concentrations during late night and morning.The minimum O 3 concentration appears during the morning hours (7.00 -9.00).The maximum O 3 concentration appears at around 11.00 h in summer, at around 12.00 h in spring, at around 14.00 -17.00 h in autumn and at around 17.00 h in winter.The increase in O 3 concentration during daytime is due to the increase in solar radiation intensity which favors the conditions to power the photochemical reactions [30].Significant increasing O 3 values during daytime were observed at an urban site in tropical India [31].The decrease in O 3 concentration during late night and morning is due to the absence of the photochemical reactions and the consumption of O 3 by deposition and or reaction with NO and NO 2 which act as a sink for O 3 [30].
The diurnal variations of O 3 for the four seasons show a similar pattern; however, the amplitudes of the varia-  tions and position of peaks are different.The maximum amplitude is the highest in summer and spring, the second highest in autumn, and the lowest in winter.This may be attributed to the higher photochemical reactions during the summer seasons.The seasonal variation of O 3 at an urban area, Nagoya, Japan, showed that higher concentrations were observed from late spring to summer, while lower concentrations were observed in winter [32].Elevated levels of O 3 , frequently encountered in the large urban centers to exhibit variable concentration profiles [33] with maximum levels occurring during the summer months [34] are becoming a cause of global concern.
The ozone precursors, NO, NO 2 , NO x and CO, show an almost opposite diurnal variation pattern to O 3 (Figure 1), characterized by high concentrations during night and morning and low concentrations during daytime, especially noon and afternoon.Each pollutant has two main peak concentration levels; the first peak appears in the morning rush hours (6:00 -9:00); the second one appears around late night (22:00 -2:00), but for CO the second one appears around (22:00 -24:00).The morning rush hour peak is mainly caused by traffic.These precursors reach their minimum values at around 4:00 -6:00 and at around 15:00 -17:00 when O 3 reaches its high values.The double peak patterns of NO, NO 2 , NO x and CO may be atypical signature of urban influence boundary layer processes, surface wind pattern, emissions, transportation/work cycle and chemical processes.During the morning time, the increase in the emission rate from traffic, accompanied with a poorer dispersive conditions and lower photochemical reaction lead to accumulate the concentrations of air pollutants.Latha and Badarinath [35] reported that high levels of NO x during morning and late evening can be attributed to combinations of anthropogenic emissions, boundary layer processes, chemistry as well as local sources and wind patterns.In the present study, the low concentrations of NO and NO 2 during the mid-day time may be due to the high dispersion and high dilution conditions under the effect of high temperature, which increases the thermal turbulence currents.Moreover, the high temperature and solar radiation intensity during the mid-day time lead to increasing the photochemical reactions and consequently increasing the chemical loss of those pollutants.This leads todecreasing the NO x concentrations in the mid-day time.On the other hand, during the late evening and nighttime, the boundary layer descends and remains low until early morning, thereby resisting the mixing of anthropogenic emissions with the upper layer.Hence, pollutants get trapped in a shallow surface layer resulting in raising levels of NO, NO 2 and NO x in the present of study.This is in agreement with Rao et al. [36] [37] who attributed that the variations in NO x to variations in boundary layer mixing processes, chemistry, anthropogenic emissions, and local surface wind patterns.
The diurnal variation of SO 2 in the present study is characterized by low concentration in early morning, evening and night and high concentration in the other time of the day (Figure 1).The SO 2 trend is characterized by two maxima for all seasons except for summer where it has three maxima.The maximum SO 2 concentration appears at 12.00 in autumn, at 11.00, 14.00 and 15.00 in summer, at 11.00 and 13.00 in spring and at 13.00 and 14.00 in winter.The high concentration of SO 2 in morning, noon and afternoon may be attributed to the primary emissions from factories in this industrial area.
The diurnal variations of THC, CH 4 and TNMHC during the period of study are represented graphically in Figure 2. From this figure, it can be noticed that the diurnal variation of THC, CH 4 and TNMHC is fluctuated along the day.THC concentration is fluctuated in the range 3.32 -5.73 ppm in winter, 1.68 -4.31 ppm in spring, 2.57 -6.65 ppm in summer and 2.89 -4.81 ppm in autumn.CH 4 concentration is fluctuated in the range 2.91 -5.36 ppm in winter, 1.47 -3.80 ppm in spring, 2.35 -5.77 ppm in summer and 2.48 -4.34 ppm in autumn.TNMHC concentration is fluctuated in the range 0.35 -0.50 ppm in winter, 0.57 -0.87 ppm in spring, 0.31 -0.55 ppm in summer and 0.56 -0.80 ppm in autumn.The trend of THC and CH 4 are the same where autumn has trend opposite to that of the other seasons of the year.This may be due to the presence of additional emission sources of THC and CH 4 in the study area.The trend of TNMHC is the same for all seasons of the year.

Seasonal Variations in O3 and Its Precursors
The monthly and seasonal variations of O3, NO, NO 2 , NO x , CO, and SO 2 during the period of study are represented graphically in Figure 3 and Figure 4. From these figures, it can be seen that, O 3 shows a well-defined seasonal variation.The low values appear in late autumn and winter (colder periods), while the high values appear in late spring, summer and early autumn (warmer periods).Studies conducted around the world reported high O 3 levels during the warmer periods [11] [14].This common behavior of O 3 was justified by the dominant photochemical production process of O 3 due to the presence of its precursors and intense solar insolation during the warmer periods.The low levels of O 3 during late autumn and winter (colder periods) is almost the same as that observed in Kuwait [11] and in urban site in China [8].
The O 3 precursors, NO, NO 2 , NO x and CO, show similar seasonal variations (Figure 3 and Figure 4).The higher values appear in late autumn and winter, while the lower values appear in summer.The seasonal variation pattern of these precursors is almost opposite to that of ozone.This pattern is similar to the results of previous studies performed in many areas around the world [11] [32] [38].One of the causes of this pattern might partly be attributed to the variation in temperature.For example, the greater photochemical reaction due to the higher solar radiation, and the stronger vertical atmospheric mixing in summer than in the other seasons, contribute to the low values of these pollutants in summer.Another cause is the increase in air pollutant emission in winter due to heating fuels consumed.The monthly and seasonal variation of SO 2 in the present study (Figure 3 and Figure 4) is characterized by high concentration in autumn and early winter and the minimum concentration in winter and summer.
Figure 5 shows the seasonal variations of THC, CH 4 , and TNMHC during the four seasons of year 2004.The seasonal variation of THC and CH 4 is the same, where the maximum was in summer (July) and the minimum was in spring (April).The highest concentrations of TNMHC were found in autumn and early winter, whereas the lowest levels were detected in summer (July).The photochemical removal (primarily by the hydroxyl (OH) radical) and dilution due to atmospheric mixing are two important factors that affect the seasonal variation of TNMHCs [39].The mixing layer in summer is much higher than that in winter.The dilution for airborne pollutants from the ground source emissions in summer is stronger than that in winter.OH concentration in summer in mid-latitude is about 10 times its value in winter.High OH concentration speeds up the degradation of TNMHCs.
Mobile emission sources are predominantly characterized by high NO x and CO concentrations, and point emission sources by high SO 2 and NO x .So, the mobile sources have relatively low SO 2 /NO x ratios [40].In the current study, the concentration ratios were 0.27, 0.38, 0.37, 0.48 and 0.39 for SO 2 /NO 2 and 0.15, 0.21, 0.19, 0.24 and0.20 for SO 2 /NO x during the winter, spring, summer, autumn and annual, respectively.These results indicate that the mobile emissions are the predominant source within the study area.The SO 2 /NO 2 concentration ratios at the study area during the period of study were similar to those found in Kolkata, India (ranged from 0.38 -0.41), [41].

Comparison of O3 and Its Precursors Levels in Yanbu Al Sinaiyah with Other International Cities
The Saudi Arabia air quality standard for hourly O 3 concentration is 150 ppb.Moreover, photochemical smog usually occurs when O3 concentration exceeds 100 ppb [42].2, the average concentrations of NO, NO 2 , NO x , SO 2 , O 3 , CO, CH 4 and NMHCs in the present study were lower/higher or similar to those detected in other cities of the world.For example, the O 3 mean concentration in Yanbu Al Sinaiyah was similar to that found in Al-Taneem, KSA; Eskisehir, Turkey, and Nanjing, China, while it was higher than those found in Holy Makkah but it was lower than those found in Haram, Giza, Egypt.NO concentration was lower than those found in other locations except those found in Eskisehir, Turkey and Nanjing, China.NO 2 concentration was lower when compared with those reported in other locations.NO x concentration was similar to that observed in Al-Taneem, Holy Makkah, KSA, while it was higher than that observed in Holy Makkah and it was lower than that recorded in Muna, KSA.SO 2 concentration was lower than those found in other locations.The CO concentration was lower when compared with those reported in other locations.CH 4   concentration was much higher than those found in other locations.The TNMHC concentration was higher than those recorded in other locations.Generally, the variation in the levels of atmospheric measured ozone and its precursors among the different locations of the world was presumably due to the difference in the traffic density, industrial activities, and intensity of human activities, land use patterns and the frequency of rainfall prior to sample collection.

Figure 1 .
Figure 1.Diurnal variation in O 3 , NO, NO 2 , NO x , CO and SO 2 concentration in Yanbu during the different seasons.

Figure 2 .
Figure 2. Diurnal variation in THC, CH 4 , and TNMHC concentration in Yanbu during the different seasons.

Figure 3 .
Figure 3. Monthly variation in concentrations of O 3 , NO. NO 2 , NO X , CO and SO 2 in Yanbu during the period of study.
The frequency percentage distribution of the hourly concentrations of O 3 during the period of study is graphically presented in Figure 6.From this figure, it can be seen that the hourly O 3 concentrations during the period of study not exceed the maximum O 3 hourly set by

Figure 4 .
Figure 4. Seasonal variation in concentrations of O 3 , NO. NO 2 , NO X , CO and SO 2 in Yanbu during the period of study.Saudi Arabia air and the photochemical smog formation not occur during the period of study.The annual average concentration of O 3 and its precursors in Yanbu Al Sinaiyah were compared with these found in different locations in Saudi Arabia and over the world[8] [20][30] [41]  [43]-[54].As shown in Table2, the average concentrations of NO, NO 2 , NO x , SO 2 , O 3 , CO, CH 4 and NMHCs in the present study were lower/higher or similar to those detected in other cities of the world.For example, the O 3 mean concentration in Yanbu Al Sinaiyah was similar to that found in Al-Taneem, KSA; Eskisehir, Turkey, and Nanjing, China, while it was higher than those found in Holy Makkah but it was lower than those found in Haram, Giza, Egypt.NO concentration was lower than those found in other locations except those found in Eskisehir, Turkey and Nanjing, China.NO 2 concentration was lower when compared with those reported in other locations.NO x concentration was similar to that observed in Al-Taneem, Holy Makkah, KSA, while it was higher than that observed in Holy Makkah and it was lower than that recorded in Muna, KSA.SO 2 concentration was lower than those found in other locations.The CO concentration was lower when compared with those reported in other locations.CH 4

Figure 5 .
Figure 5. Seasonal variation in concentrations of O 3 , THC, CH 4 and TNMHC in Yanbu during the period of study.

Figure 6 .
Figure 6.Frequency distribution of hourly ozone concentration during the different seasons.

Table 1 .
Statistical summary of daily averaged concentrations of O 3 , NO, NO 2 , NO x , SO 2 , CO, THC, CH 4 and TNMHC in different seasons over the period from January 2004 to November 2004.

Table 2 .
Concentration of Ozone and its precursors at different sites all over the world.