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24 hour averages of ambient PM₁₀ level in Colombo over the years have remained relatively within the 60 to 82 μg/m³ range with a slight decreasing trend from 2003 to 2008 (Fig. 2). These values, consistently exceeded WHO latest guideline value of 50 μg/m³ for PM₁₀. However, there is a slight decreasing trend of PM₁₀ from 2003 to 2008 (Fig. 2) and considerable decreasing trend since 2008 (CEA, 2017). This could be due to the introduction of vehicle emission testing program and promotion of tax concession on newer cleaner vehicles. 24 hour averages values of ambient NO₂ level in Colombo over the years have remained relatively within the 0.030 ppm to 0.050 ppm range with a slight decreasing trend from 2003 to 2008 (Fig. 2). High pollutant concentration was observed during the months of December to April having dry weather conditions. On the other hand, low pollutant concentration was observed during the months of June to September having wet weather conditions. 24 hour average values of ambient CO level in Colombo over the years have remained relatively within the 1 ppm to 2 ppm range with a remarkable decreasing trend from 2005 to 2008 (Fig. 2). USEPA Standard for CO is 9 ppm (10 mg/m³) and therefore CO is not a major air pollutant in Sri Lanka. 24 hour average values of ambient SO₂ level in Colombo over the years have remained relatively within the 0.02 ppm to 0.08 ppm range with an increasing trend from 2003 to 2008 (Fig. 5). USEPA Standard for 24 hour SO₂ is 0.014 ppm. Thus city of Colombo was exposed to high SO₂ pollution during this period.

Fig. 2. 
PM₁₀, NO₂, CO, SO₂ and O₃ 24 hour average values from 2003 to 2008 at Colombo Fort Air Pollution Monitoring Station. *Concentrations of O₃ and SO₂ are shown in secondary axis.

Recognizing the importance of the problem, government of Sri Lanka has proposed the fuel quality and air quality improvement road map to reduce the SO₂ emissions. This will enhance the quality of fossil fuels for managing air quality in Sri Lanka. From this road map it had introduced high quality fuels in the country as follows; 500 ppm sulfur diesel as auto diesel distributed island-wide with effect from 1st of January 2014. Further, 350 ppm sulfur diesel as auto diesel and 150 ppm sulfur diesel as super diesel with effect from 2016. 24 hour average values of ambient O₃ level in Colombo from July 2007 to June 2008 one year period shows that moderate peaks in December and January and prominent high peaks in April and June (Fig. 2). This also could be explained as a results of dry weather condition during these four months. Table 1 shows the results of the 1998 to 2008 for the above five pollutant exceedance of the Sri Lankan national standards and the WHO guideline values.

Surface ozone over the continents has a marked seasonal cycle (Zvyagintsev, 2004). A number of studies have been investigated on seasonal variation as explained below. Historically, high ground-level ozone has been reported in urban areas during hot, stagnant summer weather. According to the latest research findings, maximum can occur in winter/early spring (Almadov et al., 2015; Oltmans et al., 2008; Oltmans et al., 2006; Gros et al., 1998), in spring, or in spring/summer (Ahammed et al., 2006; Felipe-Sotelo et al., 2006). A complex interaction of photochemical and dynamic processes controls the key features of surface ozone variations (Lelieveld and Dentener, 2000) and the shape of the seasonal cycle (Monks, 2000; Oltmans and Levy, 1992). The lifetime of O₃ in the lower troposphere varies from 4-5 days to 1-2 weeks depending on season (Wang et al., 2011). Due to the limited data available on O₃, local variation can’t be explained in detail.

Table 2 shows the comparison of 24-hour average values of pollutant at five different locations in Sri Lanka. It shows that high level of air pollutants have shown in different location base on the pollutant source. Due to the industrial air pollution at Rathnapura highest particulate pollution (PM₁₀-64 μgm^-3, PM_2.5-40 μgm^-3, ppm where in both cases WHO guideline value has exceeded), high oxides of nitrogen (0.125 ppm), and high non-methane hydrocarbon (NMHC-0.997 ppm) was observed. Colombo has high air pollutant levels mainly coming from vehicular emission (Perera et al., 2018). In Colombo high particulate pollution was observed for both PM₁₀ and PM_2.5 (PM₁₀-55 μgm^-3, PM_2.5-28 μgm^-3, ppm where in both cases WHO guideline value has exceeded), high oxides of nitrogen both NO and NO₂ (NO-0.044 ppm and NO₂-0.026 ppm), and high non-methane hydrocarbon (NMHC-0.669 ppm) was observed. Jaffna and Anuradhapura have comparatively low air pollutant concentration. At Anuradhapura having higher Paddy fields high CH₄ (CH₄-1.828 ppm) concentration was observed. With the favorable temperature (28.4°C) in Anuradhapura, production of O₃ (0.017 ppm) was also higher. Low concentration of pollutant at Jaffna could be due to the high sea breeze in the area.

Data signifies that two concepts exist for NO_x-VOC-O₃ sensitivity. In some conditions, the process of O₃ formation is controlled almost entirely by NO_x and is largely independent of VOC (i.e., NO_x-sensitive regime), while for other conditions O₃ production increases with increasing VOC and does not increase (or sometimes even decreases) with increasing NO_x (i.e., VOC-sensitive regime). In the NO_x-sensitive regime (with relatively low NO_x and high VOC), O₃ increases with increasing NO_x and changes little in response to increasing VOC. As an example, Fig. 3 shows that this phenomenon occurs in Colombo. The city is located in the wet zone. The average high temperature is around 31°C from March to April. Instruments are located down wind. At the Colombo site, wind direction is mainly from West and Southwest. Similarly, NO_x-sensitive regime occurs in Kurunegala and Jaffna as well. Further, it was observed the trend patterns of NO, NO₂ and NO_x in these cities in Sri Lanka. These cities are with high traffic congestion and witnessed trends are probably due to traffic related emissions (Perera et al., 2010; Perera and Emmanuel, 2005). The Sri Lanka vehicle emission testing program (SLVET) was established in 2008. Sri Lanka has attempt to reduce air pollution by vehicle emission reduction methods including promotion of cleaner fuels and technologies (such as shifting to electric vehicles, fixing catalytic converters at the exhausts line), tax concession of importing newer and cleaner vehicles, i.e. 25% on electric vehicles and 50% on hybrid vehicles (MOF, 2010; Perera and Jayaweera, 2008), reduction of traffic by transport demand management, etc.

Fig. 3. 
NO_X-sensitive regime in Colombo.

In the NO_x saturated or VOC sensitive regime O₃ decreases with increasing NO_x and increases with increasing VOC, which is predominant in city of Rathnapura (Fig. 4). NO concentration vs wind direction at the Rathnapura location shown that the source or the industry may placed in 60° to 80° direction Rathnapura is located in the wet zone. The town receives rainfall mainly from south-western monsoons from May to September. The average temperature varies from 24 to 35°C, and there are high humidity levels. At the Rathnapura site, wind direction is mainly from North West directions. In the presence of low NO concentration, marked O₃ concentration is seen in Fig. 4. However, with very high NO concentration, O₃ concentration has reduced. This is due to the removal of O₃ through reaction with NO as described in the introduction section. Further NO concentration vs wind direction at Rathnapura shows that pollutants are coming from the direction of 60-80° implying that a point emission source such as industry is located in that particular direction. Further, high peaks of NO concentration were observed when the industrial actions are involved. This type of condition needs to handle carefully as reduction in NO concentration may result in high production of O₃. Therefore, it is essential to reduce both pollutants NO and VOC simultaneously to mitigate formation of O₃. Intermediate of this high O₃ production and how the titration of produced O₃ concentration suddenly reduced could be clearly showed in Fig. 5 at the Anuradhapura site. Anuradhapura is usually hot and humid throughout the year and the average temperature remains 25-30°C. The town receives rainfall mainly from Southwest monsoon season begins in mid-May to October. At the Anuradhapura site, wind direction is mainly from North west and South east. The large thermal power plant such as Norocholai Coal Power plant, Kelanitissa Oil Power plant, Sapugaskanda Oil Power plan are away from the measurement sites. Accordingly, effects of power plant emissions could be excluded in the present study. Further, validation results confirm the two regimes of NO_x-VOC-O₃ sensitivity. Accordingly in Matara, Badulla, Pollonnaruwa, and Gampaha show the NO_x-sensitive regime. In Kandy and Nuwaraelliya shows the VOC-sensitive regime as shown in Fig. 6.

Fig. 4. 
NO_X-saturated or VOC-sensitive regime in Ratnapura.

Fig. 5. 
Intermediate of NO_X-sensitive regime and VOC-sensitive regime in Anuradhapura.

Fig. 6. 
Validation of NO_X-sensitive and VOC-sensitive regime in other cities.