Long-term record of air quality data
The compilation of the data from the three sampling stations in the Klang Valley (Petaling Jaya, Shah Alam and Gombak) and their comparison to the background station (Jerantut) is summarised in Table 1. The averaged concentration of PM10 recorded at Petaling Jaya, Shah Alam and Gombak are 58.10 µgm−3 (27.00–172.00 µgm−3), 54.84 µgm−3 (22.00–150.00 µgm−3) and 56.41 µgm−3 (30.00–218.00 µgm−3), respectively. The averaged concentration of PM10 recorded at these three stations was found to differ significantly between the stations (p < 0.01; Table 2) and was far below the the value suggested by the Recommeded Malaysian Air Quality Guideline (RMAQG) for the 24 h averaged concentration (150 µgm−3). However, these values are remarkably higher than the averaged concentration value of 38.64 µgm−3 (17.00–152.00 µgm−3) recorded at Jerantut, Pahang, which was used as the background station (Station B). Nevertheless, the averaged concentration of PM10 at these three sampling stations exceeded the value of 50 µgm−3 suggested by the European Comission for PM10. Urban background, due to the number of motor vehicles, industries and level of street dust, is expected to contribute to the amount of suspended particulate matter in the atmosphere at these three stations compared with the background area.
The long-term records of PM10 data as shown in Fig. 2 indicated that the concentration of PM10 was found to be higher during certain months in that particular year due to the haze episode e.g. in August–September, 1997, where the concentration surpassed 200 µgm−3. In this particular episode, almost all air quality monitoring stations in the Malaysian Peninsular, including the Jerantut stations, were affected by the transboundary sources of biomass burning around Southeast Asia, especially from Sumatra, Indonesia as shown in Fig. 2. The haze event in 1997 was the worst ever experienced by the country. Short-term observations using continuous monitoring systems during the haze episodes clearly showed that suspended particulate matter was the major air pollutant during this period and was transboundary in nature as a result of large-scale forest fires in parts of Sumatra (Awang et al. 2001; Keywood et al. 2003; Mott et al. 2005; Radojevic and Hassan 1999).
The averaged concentration of all gases is still under the threshold limit by the Recommended Malaysian Air Quality Guideline (RMAQG) for ambient air (Table 1). There are significant differences between the gas concentrations recorded at the various stations. This suggests that the local surroundings influence the concentration of gases at each station (Table 2).The mean and range of CO found resulted in it being the most prevalent gas recorded in the atmosphere at all three stations followed by NO2, O3 and SO2. CO was clearly recorded at a higher concentration at Petaling Jaya with a mean value of 1.853 ppm (1.420–2.340 ppm) compared with Shah Alam (1.002 ppm, 0.490–2.200 pmm) and Gombak (0.953 ppm, 0.630–1.180 ppm). The other gas which was also recorded at a very high concentration at Petaling Jaya is NO2, which had an average concentration of 0.033 ppm (0.024–0.051 ppm) compared with Shah Alam (0.019 ppm, 0.002–0.031 ppm) and Gombak (0.017 ppm, 0.009–0.025 ppm). The concentration of these two gases is due to the emissions from motor vehicles as indicated by several other studies e.g. Dor et al. (1995), Mayer (1999), Janssen et al. (2001) and Morawska et al. (2002).
Even though there are clear differences between the SO2 and O3 levels recorded in the Klang Valley and Jerantut (the background station), among the three different stations within the Klang Valley, no significant differences are indicated (Table 1). SO2 is usually the result of industrial activities (Pereira et al. 2007), but at these three stations, the main source of SO2 is expected to be from motor vehicles predominantly diesel-engined trucks and buses. SO2 was found to be at high levels during haze episodes. This would be expected due to the composition of the sulphur compound, particularly from biomass burning as cited by Clairac et al. (1988). The reduction of sulphur in petrol and diesel was found to contribute to the amount of SO2 in the atmosphere, more notiably in urban and suburban areas. O3 is a secondary gas produced by the interaction of hydrocarbon, oxides of nitrogen and sunlight (Atkinson 2000). Based on the results, the level of O3 at these three stations can be seen to be much more closely related to the intensity of sunlight and the contribution of other precursors rather than nitrogen dioxide and hydrocarbon from motor vehicles. This can be argued due to its low concentration at Petaling Jaya (S1), in comparison to Shah Alam (S2) and Gombak (S3). A high concentration of NO is expected to be produced by motor vehicles in the urban areas; this has the capability to interact with O3 to form NO2 in the atmosphere.
The results from Fig. 3 show that there is a clear peak of concentration for PM10, CO, NO2 and SO2 at all three stations when compared with the background station (Jerantut). The daily concentration of all the pollutants was also clearly recorded as being at the highest concentration at Petaling Jaya. Traffic congestion, particularly during the morning peak between 7:00 am–9:00 am, led to the higher amount of CO in the the atmosphere. The peak of CO concentration recorded at Shah Alam and Gombak decreased significantly towards the afternoon while the peak of CO concentration recorded at Petaling Jaya declined only slightly. This phenomenon is considered to correlate directly with the number of motor vehicles at the two stations (Shah Alam and Gombak), which are located in residential areas, and at these particular times, the number of motor vehicles increased significantly as people were going to work and children to school. In the latter part of the afternoon, CO concentration peaked from 5:00 pm in line with the evening rush hour when people started returning home from work. The late evening peak can also be attributed to meteorological conditions, particularly atmospheric stability and wind speed (Afroz et al. 2003; Awang et al. 2000).
The other two parameters which show the same daily pattern as CO are NO2 and SO2. NO2 is usually produced by motor vehicles, and its concentration was found to stable from midday towards the late afternoon. This trend was also recorded at Gombak but at lower values of NO2 concentration. Nevertheless, the trend of NO2 recorded at Shah Alam indicates a significant peak of NO2 concentration at around 10:00 am before it decreased towards midday and then increased again in the early evening. These fluctuations give an indication of the movement of motor vehicles around Shah Alam's residential areas where most of the residents travel by motor vehicles on the outskirts of Shah Alam. The level of traffic during working hours was expected to be low and at the same time to lower the concentration of NO2 in the atmosphere. For SO2, the daily trend at all three locations was nearly the same, and the results were considered to be influenced by motor vehicles emissions, particularly from buses, lorries and trucks near the monitoring stations themselves.
Fig. 2 shows O3 trends and demonstrates that the formation of this gas is influenced by sunlight through photochemical reactions (Atkinson 2000; Sillman 1999). The concentration of it at all three stations as well as the background station was found to increase in the morning and return to its earlier and lower concentration at 9:00 pm in the evening (Fig. 3). The highest concentration of ozone was recorded around 2:00 pm in the afternoon, whilst the highest concentration of ozone during peak hours was recorded at Shah Alam, followed by Gombak and Petaling Jaya. These results did not reflect the results of the other gases which were recorded at the highest level in Petaling Jaya. Due to the lower concentration of NO2 in Shah Alam (0.002–0.031 ppm) compared with Petaling Jaya (0.024–0.51 ppm), it is considered that other factors, such as volatile organice carbon (VOC), probably contributed to ozone formation in this location. This result is supported by Paoleti (2009), who found higher a concentration of ozone in suburban areas compared with urban areas. These were considered to be most likely due to the complex interactions between the production of VOC and the ability of several species, such as: NO, to remove ozone from the atmosphere (Sillman 1999). Moreover, meteorological factors such the local temperature at the sampling stations would also have the ability to increase the concentration of ozone in the atmosphere as indicated by Geddes et al. (2009).
Correlation between air pollutants and meteorological factors
The correlations beween atmospheric pollutants at all stations are shown in Table 3. Petaling Jaya station only recorded a positive significant correlation (p < 0.01) between CO and SO2. Nevertheless, all parameters recorded in Gombak were found to be correlated to each other. The strongest significant correlation recorded at this station was between CO and NO2 (r = 0.625, p < 0.01). This correlation pattern indicates that there were the same sources of atmospheric pollutants recorded in Gombak and Petaling Jaya. Petaling Jaya itself, is located in a congested, urban and industrial area, which would lead to the existence of atmospheric pollutants from various sources in comparison to Gombak, which is more influenced by the movement of motor vehicles. Shah Alam appears to be in the middle of these two stations with the concentration of PM10, CO and NO2 correlated between each other.
The meteorological data recorded at each air monitoring station are summarised in Table 4. The average temperature for all stations was in the range of 25.9 to 27.9°C, while wind speed, UVB and humidity were in the range of 2.8 to 4.5 km/h, 156 to 175 J/m2/h and 74.38 to 81.96%, respectively (Table 4). These meteorological conditions are typical for a tropical environment. Correlation between the most significant air pollutants (PM10) and meteorological factors (Fig. 4) indicates that the PM10 concentration has a significant positive correlation with ambient temperature (r = 0.650, p < 0.01). A high temperature in the tropics usually increases the quantity of biomass burning and the evaporation of materials e.g. soil dust from the earth's surface. High humidity is commonly related to the number of rain occasions, and this reduces the number of particles due to the wash-out processes of the atmospheric aerosols in the atmosphere. Therefore, there is a negative correlation between humidity and PM10 in ambient air. Other factors such as wind speed and UV radiation were found to slightly influence the concentration of particulate matter in the atmosphere in the study areas.
The southwest monsoon occurs between June and September each year and is usually associated with the generation of haze episodes in the Malaysian Peninsular as a result of biomass burning from Sumatra, Indonesia. Back trajectory analysis using the HYSPLIT Model (Fig. 5) shows that the southwest monsoon wind from Sumatra can arrive at all three monitoring stations within 48 h. During the intermonsoon season (October and April) the wind systems are large north-westerlies as is shown by trajectories using the HYSPLIT Model. The north-east monsoon, which occurs annually between November and March, is usually connected to the wet season and therefore does not contribute to the amount of anthropogenic sources, particularly from biomass burning, coming from east part of the Malaysian Peninsular.