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Fig. 2 shows the histogram of PM₁₀ concentration measured at five distinct study areas in Sabah (a) Kota Kinablu, (b) Tawau, (c) Labuan, (d) Keningau, (e) Sandakan from Jan to Dec 2012. Within the measurement period, the average hourly PM₁₀ measured at Tawau was 35.7±17.8 μg/m³, Kota Kinabalu (35.0±18.4 μg/ m³), Labuan (34.7±18.1 μg/m³), Sandakan (32.8±11.5 μg/m³), and Keningau (31.9±18.6 μg/m³). Dispersity of the data was the least at Sandakan with the nominal min and max at 8.0 μg/m³ and 104 μg/m³, respectively. The variability of PM concentrations observed at different sites are quite different where out of the five monitoring stations, Kota Kinabalu measured the highest range (KK>KEN>LBN>TWU>SDK), the highest maxima (KK>KEN>LBN>TWU>SDK), and the second highest average (TWU>KK>LBN>SDK> KEN). This could be explained by their geographical environmental conditions and population density. Undoubtedly, Kota Kinabalu has the highest PM₁₀ concentration because it is the state capital of Sabah with population of 462,963 for total area 352 meter-sq and population density 1,315/km² (Department of Statistics Malaysia, 2010). Of all monitoring stations studied in this work, Kota Kinabalu is the most urbanized area with high concentration of cities, industrial and economic activities. Population density is a fundamental amplifier of air pollution problems, and urban population growth has been an important factor in worsening air quality (Yara et al., 2017). The EPA data also indicates a strong association both between higher population densities and higher traffic densities, higher population densities and higher road vehicle nitrogen oxides (NO_x) emission intensities. The second highest population density is remarked in Labuan for 948.4/km², putting it the mid of all monitoring stations. Besides, Labuan is also a federal territory of Malaysia that best known as being an offshore support hub for deepwater oil and gas activities in the region. On the other hand, Sandakan and Tawau have relatively lower population density at 179.8/km² and 67.06/km², respectively. The lowest population density is remarked in Keningau (50.12/km²) which explains its lowest hourly-averaged PM₁₀ concentration among all other stations. However, it has the second highest range and maxima due to its geographical location near to Kota Kinabalu. The local source from Kota Kinabalu and the advection due to wind could be one of the reasons responsible for transboundary aerosol between these two regions.

Fig. 2. 
Histogram of PM₁₀ concentration for (a) CA0030, (b) CA0039, (c) CA0042, (d) CA0049, and (e) CA0050 from Jan 2012 to Dec 2012.

The highest maximum ~317 μg/m³ was measured at Kota Kinabalu on Jan 17^th during the morning hours 0700h-0800h, while the PM₁₀ concentration of other study areas remained low <80 μg/m³ on the same day and time. The exact reason for the unusual high PM₁₀ occurred only on Jan, 17^th from 0700h-0800h was unknown but we ruled out the regional or transboundary effects as the wind speed and wind direction was quite low at 1.7 km/h (153°N) and showed no unusual pattern throughout the past three days (see Fig. 3). Not only that, CO and NO₂ also showed no irregular pattern and fluctuated within the normal range between 0.10-1.60 ppm and 0.005 to 0.020 ppm, respectively. Besides, the month of January is also not the open biomass burning period in Kalimantan and Sumatera, which is usually the main cause for low visibility and haze episode in Sabah. Therefore, the transboundary effect in this case is unlikely to cause this phenomenon. In fact, it could be due to other causes but not limited to local emissions related to traffic and open burning. Unfortunately, the local VOC data measured at the same site during the episode day was unavailable and this unusual incident was also not widely archived due to the TWA-24 h of the day was still within the EPA standard of less than 150 μg/m³.

Fig. 3. 
Hourly variation in WS, PM₁₀, CO, NO₂ concentration measured in Kota Kinabalu from Jan, 16^th to 18^th, 2012.

Fig. 4 shows the seasonal variation of PM₁₀ concentration measured at the five distinct study areas in Sabah from Jan 2012 to Dec 2012. The temporal distribution indicated that the PM₁₀ levels remain highly concentrated during the south-west monsoon (SWM) and post south-west monsoon (POST-SWM) which is the hot and dry season in Sabah. From the statistics summary, the highest PM₁₀ concentration measured was always during the POST-SWM for all five study areas. Besides, no significant difference was found in the highest average PM₁₀ among the sites, ranging from 37 to 40 μg/m³. After the SWM, the PM₁₀ levels started to decline during the north-east monsoon (NEM) and post north-east monsoon (POST-NEM) for all areas. The lowest PM₁₀ concentration was measured at Keningau (27±13 μg/ m³) during the POST-NEM. This finding is similar to another local study in Klang Valley reported that PM₁₀ level was persistently high during the south-west monsoon and low during the north-east monsoon (Rahman et al., 2015; Sansuddin et al., 2011). During the northeast monsoon, the precipitation of rain will carry the pollutants to the earth; hence, reducing the level of pollutants in the atmosphere. However, during the southwest monsoon, the warmer air near the surface area rises to higher latitude, which results in turbulence and causes the pollutants to become unstable; thus resulting in a high level of pollutants in the atmosphere (Barmpadimos et al., 2011). During the wet seasons, precipitation reduces considerably PM₁₀ concentration by wet deposition. In addition, if it is frontal precipitation, some polluted boundary layer air is replaced by clean air from aloft. This feature may not be possible to depict by a linear relationship, but the pattern is significant and obvious for case-by-case comparisons. For example, as shown in Fig. 4d, the PM₁₀ concentration remained positively correlated with the dry seasons during SWM which peaked in the POST-SWM at an average value 37 μg/m³, and negatively correlated with the wet season during NEM which based in the POST-NEM at an average value 27 μg/m³. Nevertheless, the seasonal variation of PM₁₀ concentration in Sabah has no drastic change but limit to an extent of less than <10 μg/m³ for all study areas. This attribute could be due to the small seasonal variation of planetary boundary layer (PBL) in tropics. Previous study revealed that the influence of planetary boundary layer height stands the highest absolute correlation on the PM₁₀ level in Poznan, Poland (Czernecki et al., 2017). Notably, increasing values of net irradiance are associated with strong decrease of PM₁₀ concentration due to large sensible heat flux which in turn drives the formation of deeper atmospheric boundary layers (Barmpadimos et al., 2011). In contrast, the relationship between the PM₁₀ concentration and net irradiance is reversed for decreasing solar irradiance. All five study areas are in tropic where receive fairly constant amount of solar irradiance throughout the year. Hence, it is possible that through this mechanism, relatively small difference of <10 μg/m³ between seasonal PM₁₀ concentration was remarked for all study areas in Sabah.

Fig. 4. 
Seasonal variation of PM₁₀ concentration measured at (a) CA0030, (b) CA0039, (c) CA0042, (d) CA0049, and (e) CA0050 from Jan 2012 to Dec 2012.

Fig. 5 presents the diurnal variation of (a) PM₁₀, (b) CO, (c) Ozone, (d) NO₂, and (e) SO₂ concentration measured for the total period of 2012 over the study area, Kota Kinabalu in the form of box plot. Each box is determined by the 25^th and 75^th percentiles, and the whiskers are marked at 5^th and 95^th percentiles. The median value at 50^th percentile are marked with solid lines inside the boxes, while the maximum and minimum values are marked with solid lines outside the boxes. The box plot gives a good insight into the dynamics of the PM₁₀ concentration for each hour. The averaged diurnal variation of PM₁₀ showed pronounced variations with two significant peaks during the morning hours at 7 h and the evening hours at 19 h (see Fig. 5a). Fig. 5b shows that the hourly averaged CO concentration rose abruptly at 7 h and gradually decreased to the normal background concentration <0.50 ppm at 10 h and then started to hike up again at 18 h and persisted until 23 h. This bimodal distribution was also found in the hourly averaged NO₂ concentration which peaks at 7 h and 19 h (see Fig. 5d). Nevertheless, the hourly maxima for both parameters CO and NO₂ was still far beyond the hourly Recommended Malaysia Air Quality Guidelines (RMAQG) of less than 30 ppm and 0.17 ppm, respectively. In contrast, the diurnal evolution of ozone and SO₂ has no direct effect on the PM₁₀ trend. The ozone profile was in accordance with the photochemical effects where it peaks at the noon hours for high intensity of UV solar light. Fig. 5c clearly exemplifies this pattern on its unimodal distribution which has a bell-curve shape peaks at 13 h. Among all parameters, SO₂ has the least variability throughout the day where the fluctuation shows no significant anomalies and far behind the hourly RMAQG of <0.13 ppm.

Fig. 5. 
Hourly averaged boxplot for (a) PM₁₀, (b) CO, (c) O₃, (d) NO₂, and (e) SO₂ concentration measured in Kota Kinabalu for the total period in 2012.

The two rushing hours at 7 h and 19 h are associated with the heavy-traffic hours where the number of vehicles on road is the direct reflection of the related emission of CO, NO₂, and PM₁₀. The morning peak hour is when people are going to work and sending their children to school, thus causing traffic congestion and increasing the pollutant levels. Similar findings were also reported in a study at Klang Valley where the daily concentration of PM₁₀, CO, NO₂ and SO₂ was clearly recorded as being the highest during traffic congestion, particularly during the morning peak between 7:00 am-9:00 am, led to the higher amount of CO in the atmosphere (Azmi et al., 2010). In the latter part of the afternoon, PM₁₀ concentration peaked from 7:00 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 (Rahman et al., 2015). The PM levels were lower between 0800h and 1400h because of low traffic volumes and hence low emission rates. Another possible reason could be the increase in the mixing height that favors dispersion conditions (Bathmanabhan et al., 2010). However, during the night time between 2000h and 2300h, the PM concentrations were slightly higher than the daytime (see Fig. 5). The probable reason for this may be the builtup of particles under inversion conditions. Gomiscek et al. (2004) have also reported a similar trend for three urban sites in Austria.

More specifically, we present two scenarios: (1) the morning effect and (2) the evening effect in the urban atmosphere at Kota Kinabalu on Fig. 6. The left panel Fig. 6a shows the spikes on PM levels due to the evening effect and the right panel Fig. 6b shows the spikes of the morning effect. On the evening effect, the rise of PM level started at 1800h and fall abruptly after 2000h. On the morning effect, the PM level started to rise at 0700h reaching maxima 190 μg/m³ and then fall abruptly to reaching 190 μg/m³ at 0800h. These two effects are highly related to the traffic exhaust gaseous emission such as CO and NO₂. Nitrogen dioxides have a substantial impact on PM₁₀ through their atmospheric oxidation to aerosol nitrate and the CO formed from oxidation of VOCs (Wang et al., 2015). The concentrations and compositions of nitrate aerosols are determined primarily by their precursor emissions, which are mainly of anthropogenic origin. Among the major pollutants, CO, nitrogen oxides (NO_x=NO+NO₂), PM₁₀, and some types of VOCs (e.g., BTEX: benzene, toluene, ethylbenzene, and ortho-, meta-, and para-xylenes) are primarily traffic-induced, while O₃ and NO₂ are secondary trace gases formed from precursors in photochemical reactions (Yoo et al., 2015). The increase of CO was substantially driven by the road traffic emissions of the day. Similar findings are also reported in Masiol et al. (2014) where species mainly linked to road vehicle exhaust emissions such as CO, and NO₂ have a bimodal structure due to the peaks in traffic at 7-9 am and 6-8 pm.

Fig. 6. 
The hourly mean PM₁₀ concentration at Kota Kinabalu from 1-Jan to 2-Jan, 2012 (left) and 15-Jan to 16-Jan, 2012 (right). Left panel shows spikes on the evening effect and right panel shows the morning effect.

Fig. 7a and 7c shows the diurnal changes of PM₁₀ with comparison to CO and NO₂, respectively. The morning peak on PM₁₀ level at 0700h was observed concurrently with the rise on CO and NO₂, and the abrupt fall on PM₁₀ between 0900h and 1500h was also concurrent with the decrease on both parameters. Similar pattern was observed for the evening peak on PM₁₀ level at 1800h where the peak was corresponding to the rise on CO and NO₂ and the abrupt fall of PM₁₀ after 2000h was consistent to the decrease of CO and NO₂ (see Fig. 8a and 8c). This pattern was further confirmed by the correlation analysis on the scatter plot PM₁₀ against CO and NO₂ on Fig. 7b and 7d, respectively. Positive correlation R₂ of 0.46 and 0.23 was remarked on PM₁₀ against CO and NO₂, respectively. Same goes to the correlation analysis for the evening effect where positive correlation R₂ of 0.45 and 0.42 was remarked for CO and NO₂ (see Fig. 8b and 8d). The higher correlative strength of CO further implies that PM₁₀ level in Kota Kinabalu is more dominantly dependent on the diurnal evolution of CO concentration. This finding was in good agreement with other studies reporting that the daily patterns of each pollutant for NO, NO₂, CO and PM₁₀ peak during the morning rush hours, and fall abruptly in the noon owing to the decrease in the intensity of emission and the rapid growth of the convective boundary layers (Adame et al., 2014).

Fig. 7. 
Hourly mean concentration of (a) CO, (c) NO₂, (e) SO₂, (g) O₃ plotted on secondary axes with comparison to PM₁₀ plotted on primary axes (Left panel) on Jan, 15-16 for morning effect. Scatter analysis of (b) CO, (d) NO₂, (f) SO₂, (h) O₃ plotted against PM₁₀ (Right panel).

Fig. 8. 
Hourly mean concentration of (a) CO, (c) NO₂, (e) SO₂, (g) O₃ plotted on secondary axes with comparison to PM₁₀ plotted on primary axes (Left panel) on Jan, 1-2 for evening effect. Scatter analysis of (b) CO, (d) NO₂, (f) SO₂, (h) O₃ plotted against PM₁₀ (Right panel).

The profile of SO₂ showed no pronounced changes for both the morning and evening effect. The trend is rather irregular and has no significant pattern against the changes of PM₁₀ level (see Figs. 7e and 8e). Both figures show no concurrent rise of PM₁₀ and SO₂, neither the significant drop of both parameters occurred at the same time. Therefore, the effects of SO₂ on the evolution of PM₁₀ were somewhat less reactive compared to other gaseous e.g. NO₂ and CO. A very weak correlative strength of 0.09 and 0.01 was found in the scatter plot between PM₁₀ and SO₂ for morning effect and evening effect respectively (see Figs. 7f and 8f), indicating the changes of both parameters are not entirely related and significant. Another possible reason owing to the low variability of SO₂ is due to SO₂ itself has a very short lifetime and can easily transform into sulphate SO4 through oxidation and interaction with particles and water vapour (Kai et al., 2007).

Ozone is a reactive oxidant gas that plays an essential role in the photochemical air pollution and atmospheric oxidation processes. Although in the upper atmosphere it acts as a barrier for UV-rays, in the troposphere it is a secondary air pollutant induced through a series of complex photochemical reactions involving solar radiation and ozone-precursors gaseous such as NO₂ (Masiol et al., 2014). When the concentration of NO₂, CO, and PM₁₀ started to increase in the morning hours, surface ozone level decreased due to titration reaction with NO and the rupture of the nocturnal inversion layer (Talib et al., 2014). Figs. 7g and 8g supported this findings by showing the daily pattern of ozone decreased for increasing PM₁₀ and vice versa. Ozone concentrations continued to increase during the noon hours due to photochemical reaction and or horizontal and vertical transport processed and reached to a daily maxima 0.035 ppm at 1330h. The decreased in ozone levels was followed by a reduction in solar radiation when reaching to evening hours at 1700h. This was also associated with the increase of NO₂ and PM₁₀ during the evening hours when emission of traffic started to mount up. Over the Borneo, the air mass evolution during the sunset is faster compared to the Peninsular. In other words, solar intensity was dimmed earlier at 1800h in Borneo regions. This is the reason why the ozone level reached to the daily minima 0.005 ppm at earlier time 1900h where the solar activity induced photochemical reaction was no longer active. Hence, a negative correlative R₂ -1.35 (see Fig. 7h) and -2.77 (see Fig. 8h) was observed between the ozone and PM₁₀ concentration, indicating an increase on PM₁₀ concentration was most likely associated with the decreased on ozone levels and vice versa. In general, CO, NO, and PM₁₀ shows two typical patterns for urban atmosphere: two daily peaks correspond to morning mode and evening mode at 0700h and 1800h, respectively. The modes are divided by a minimum extend from 1100h to 1700h, which is assumed to the results of three factors: (i) lower emission of traffic exhaust, (ii) large availability of ozone inducing the photolysis of NO_x and oxidation of CO, and (iii) enhanced dipersion potential. The last effect is more pronounced in the tropics when ozone levels are always higher during the noon hours and the local atmospheric circulation during daytime is dominated by the sea breezes, which have a key role in blowing air masses from the sea (Masiol et al., 2014).

Fig. 9 shows the daily average PM₁₀ concentration measured in the urban atmosphere of Kota Kinabalu in Jan 2012. In this figure, two effects are highlighted: the weekend effect and the weekday effect. The weekend effect and the weekday effect are separated by the symbol “M” which denotes Monday. It is emphasized the abrupt fall in PM₁₀ concentration is often observed during the weekend and gradual rise is often observed during the weekday. Within the measurement period, the PM level never exceeded 100 μg/m³ over the weekend. The PM concentration was significant low <50 μg/m³ on Sunday and slightly higher on Saturday. Meanwhile, high PM₁₀ concentration >150 μg/m³ was observed on two Mondays of the month on Jan, 2^nd and 16^th. A harmonic pattern was observed on the PM₁₀ levels where the peaks are often during the first or second day of the week and the lows are during the last two days of the week.

Fig. 9. 
Daily distribution of hourly PM₁₀ concentration in Jan 2012 over the urban atmosphere of Kota Kinabalu, Sabah. Alphabet symbol M denotes Monday.

Fig. 10 shows the average diurnal changes of PM₁₀, CO, ozone, NO₂, and SO₂. The weekend-weekday differences of PM₁₀ varied greatly at the peak hour 0700h. The highest values ~160 μg/m³ was remarked on weekday and ~40 μg/m³ on weekend (see Fig. 10a). Similar pattern is observed for CO where the highest difference is observed in the morning peak hours from 0700h to 0800h (see Fig. 10b). The increase in PM₁₀ concentration during morning hours as a result of direct road traffic emission is observed but this increase is more marked in the cases of CO which reflects direct exhaust emissions. Daily cycles of traffic-related exhausts NO₂ is also marked by road traffic evolution on Fig. 10d where the weekday concentration is consistently higher than that of weekend. This effect has been discussed elsewhere that the pollutant number concentration increases with road traffic intensity in the morning is due to the ultrafine particle vehicle emission and formation of new particles during dilution and cooling of the vehicle exhaust (Pérez et al., 2010). Besides, the second peak observed during the noon hours was attributed to new formation of aerosols by photochemically induced nucleation by solar radiation intensity, and by dilution of aerosols due to the growth of the mixing layer (Rodríguez and Cuevas, 2007). During the night hours, the PM₁₀ concentration started to decline (see Fig. 10a) owing to the decrease of the traffic congestion and also probably due to the thinning of the boundary layer depth that favors condensation and coagulation processes that sink the aerosol particles (Minoura and Akekawa, 2005). As a whole, during the weekday the levels of vehicle exhaust emission such as CO and NO₂ follow the traffic intensity with maxima during the morning hours, decreasing during the day because of dilution processes and increasing again the evening. During the night hours, the concentrations remained high owing to the reduction of the boundary layer height and probably lower wind speeds that prevent the dispersion of pollutants (Pérez et al., 2010).

Fig. 10. 
Average diurnal changes of (a) PM₁₀, (b) CO, (c) Ozone, (d) NO₂, (e) SO₂ by the day of the week. Weekday is represented by blue color line and weekend is represented by orange color line.

The average diurnal ozone volume fraction is higher during the weekend compared to weekdays (Fig. 10c). This finding is similar to that report from Gvozdic et al. (2011). This behaviour is contrary to other air pollutants such as PM₁₀, NO₂, CO, and SO₂ where the weekend concentration of traffic pollutions were lower than weekday concentration (see Fig. 11). Ozone level increased in the afternoon during both weekdays and weekends, following the solar radiation intensity and its formation is dependent on the photochemistry process. However, during the weekends, the ozone concentration was higher. This is due to the effect of the lower general pollution levels observed during the weekends, thus reducing the interaction of ozone with other species such as nitrate or carbonaceous compounds (Pérez et al., 2010). Similar results were also reported in Qin et al. (2004) that low emission of NO during the weekend could be the reason for high ozone concentration at night.

Fig. 11. 
Average diurnal variations of ozone, NO₂, SO₂, PM₁₀, and CO during (a) weekday and (b) weekend.

Finally, we present two types of average diurnal variations during the weekday (see Fig. 11a). Firstly, the diurnal variation with a single maxima for ozone and secondly, diurnal variations with double maxima for CO, NO₂, and PM₁₀. The single maxima for ozone occurred during the noon hours from 1300h to 1400h while the double maxima for CO, NO₂, and PM₁₀ occurred during the morning hours and afternoon hours at 0700h and 1900h, respectively. The peak of CO, NO₂, and PM₁₀ concentration are in the front and at the back of the peak for ozone is a representative pattern of urban atmosphere in tropical climatic region (Gvozdic et al., 2011). It is corresponding to the increase of traffic density in the morning hours from 0600h to 0800h, and afternoon hours from 1800 to 2000h. However, this pattern is somewhat less reflective during the weekend (see Fig. 11b). The bimodal distribution of CO during the weekend is still legibly observable but less significant when compared to weekday. The peak of CO during the weekend was observed at 0700h and 1900h, which is similar to that of weekday. However, the peak of PM₁₀ had no extreme variations during the weekend. Same pattern was observed for NO₂ levels. The hourly average PM₁₀ concentration was constantly capped below 50 μg/m³ (see Fig. 11b) which is an indicator of good air quality. Meanwhile, the evolution pattern for SO₂ during the weekdays and weekend have no much difference. This indicates that the traffic-related emissions is less influential on the changes of SO₂. Therefore, the sources of SO₂ are somewhat not directly related to traffic exhausts but possibly due to the industrial emissions. This can be explained by the characteristics of NO_x concentration rationed to CO and SO₂. High CO/NO_x and low SO₂/NO_x ratios values indicate mobile sources while high SO₂/NO_x and low CO/NO_x ratio values typically indicate point sources, e.g. from industrial activities (Talib et al., 2014). Fig. 12 shows that the air quality level in the study area was dominantly influenced by the CO/NO₂ ratio value with values ranging from 80 to 140 for weekday and 40 to 80 for weekend. For SO₂/NO₂ ratio, the values range from 0.10 to 0.25 for weekdays and 0.15 to 0.35 for weekend. These ratio values indicate that air pollution in the study area of Kota Kinabalu, is more seriously influenced by mobile sources rather especially during the weekday period.

Fig. 12. 
Hourly distribution of SO₂/NO₂ ratio and CO/NO₂ ratio during weekdays and weekend over the urban atmosphere of Kota Kinabalu. Clustered column represents SO₂/NO₂ ratio and line with marker represents CO/NO₂ ratio.