NOx-VOC-O3 Sensitivity in Urban Environments of Sri Lanka

Physical phenomenon of the relation among the ground level O3, NOx and VOC governed by complex nonlinear photochemistry in urban environments is explained in detail using the ambient pollutant concentration data of eleven cities in Sri Lanka. The time-series analysis was conducted using the 24-hour average ambient concentrations of PM10, NO2, CO, O3 and SO2 air pollutants obtained from fixed air pollution monitoring station located in Colombo since 2008. Further analysis was carried out from the mobile air pollution monitoring station for eleven cities. The hourly averaged ambient real time air quality data i.e. VOC, NO2, NO, O3 pollutants and the corresponding meteorological parameters were analyzed and presented in weekly results for the base year 2013, 2014 and 2015. It was identified that there exist two regimes of NOx-VOC-O3 sensitivity among these cities. Colombo, Kurunegala, Jaffna, Matara, Badulla, Pollonnaruwa, and Gampaha are the NOx-sensitive regime. While Rathnapura, Anuradhapura, Kandy and Nuwaraelliya are the VOC-sensitive regime. In the NOx-sensitive regime (with relatively low NOx and high VOC), O3 increases with the increasing NOx and slightly changes in response to the increasing VOC levels. In the NOx-saturated or VOC-sensitive regime, O3 decreases with increasing NOx level and increases with increasing VOC levels. In the immediate vicinity of very large emissions of NO, O3 concentrations are depressed through the process of NOx titration. Mathematical relationships were developed to calculate the steady state ozone concentration (O3ss) that gives the values for both NOx-sensitive regime and the VOC-sensitive regime. Establishment of these relationships are essential for Sri Lanka to develop the appropriate interventions for controlling O3 pollution in each city.


INTRODUCTION
Ozone (O 3 ) is produced in the troposphere as a result of a complex set of reactions that involve volatile organic compounds (VOCs) and oxides of nitrogen (NO X ).The interaction among O 3 , NO X and VOC are driven by the complex nonlinear photochemistry (Atkinson and Arey, 2003;Atkinson, 2000).Many Studies have been investigated the relation among NO X , O 3 and VOC reactivity and NO X -VOC-O 3 sensitivity (Simon et al., 2015;Kim et al., 2012;Kim et al., 2011;Shao et al., 2009;Geng et al., 2008;Murphy et al., 2007;Stein et al., 2005;Jiménez and Baldasano, 2004;Sillman, 2003;Zaveri et al., 2003;Sillman, 2002;Sillman, 1999;Jiang et al., 1997).In urban areas this can be affect the design of control strategies to reduce ambient ozone levels.If we were able to introduce a NO X -VOC-O 3 sensitivity to certain measurable species which have different values to calculate this phenomenon, it will greatly help to reduce the O 3 concentration in urban environments.However, there is relatively little research on this topic in Sri Lanka; hence, the comprehensive analysis of NO X -VOC-O 3 sensitivity is an urgent need for effective ozone control in the country.Accurate understanding of the relation among O 3 , NO X and VOC is especially important because it suggests the possible role for developing national air quality management plan.Lack of clear understanding on physical phenomenon among O 3 , NO X and VOC interactions has hindered in developing air quality modeling tools with sufficient accuracy.Consequently, development of interventions for the control of O 3 has become a challenging task for the policy makers and regulatory agencies in Sri Lanka.The objectives of this research are to identify (1) the influential parameters and (2) the governing mechanisms of NO X -VOC-O 3 sensitivity in Sri Lankan cities by establishing pollutants interactions which subsequently assist in developing appropriate interventions for controlling O 3 in a particular city in Sri Lanka.

RESEARCH METHODOLOGY
In this research, the time-series analysis was conducted using the 24-hour average ambient concentrations of PM 10 , NO 2 , CO, O 3 and SO 2 air pollutants obtained from fixed air pollution monitoring station located at Fort, Colombo from 1998 to 2008 according to the data availability.Additional analysis was carried out using the data in 2013, 2014 and 2015 which were collected from the mobile air pollution monitoring station for eleven cities in Sri Lanka.Equipment's of the monitoring unit belongs to CEA, Sri Lanka, was used for the ambient air quality monitoring measurements.PM 10 was measured using Environnement S.A.-PM101M Particulate monitor by Beta attenuation method.CO was measured using Environnement S.A.-CO12M Gas filter correlation carbon monoxide analyzer, EN series using the non-dispersive infrared spectroscopy.NO, NO X , NO 2 were measured using Environnement S.A.-NO, NO X , NO 2 -AC32M by gas phase chemi-luminescence method.SO 2 was measured using Environnement S.A.-AF22M, EN Series, UV Fluorescent sulfur dioxide analyzer by Pulse Fluorescent method.O 3 was measured using Environnement S.A.-O 3 -42M, UV Photometric ozone analyzer, EN series using ultra violet photometric method.Non- methane and methane hydrocarbons were measured using Environnement S.A.-HC51M, FID hydrocarbon analyzer by flame ionization method.Environnement S.A,-MGC 101 computerized multi gas calibrator is also available and equipment's are kept in 20°C temperature.The data in five out of eleven cities were selected for identification of major trend patterns as initial analysis, while the data of remaining cities were used for validation of the two regimes (Fig. 1).The locations of the mobile air pollution monitoring stations considered for the initial analysis are resided in the cities of Jaffna, Anuradhapura, Colombo, Rathnapura and Kurunegala (Table 1) covering five different provinces in the country.The 24-hour average data of various pollutants and meteorological parameters at these five different locations were compared with each other to assess air pollution status in different cities.Further similarities and differences of the trend patterns were analyzed to develop the inter-relationships of VOC, NO 2 , NO and O 3 pollutants, and the NO X -VOC-O 3 sensitivity.The ambient real time air quality data of all air pollutants and meteorological parameters were presented in weekly basis for

1 Ambient Air Pollution Status in Sri Lanka
24 hour averages of ambient PM 10 level in Colombo over the years have remained relatively within the 60 to 82 μg/m 3 range with a slight decreasing trend from 2003 to 2008 (Fig. 2).These values, consistently exceeded WHO latest guideline value of 50 μg/m 3 for PM 10 .However, there is a slight decreasing trend of PM 10 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 NO 2 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/m 3 ) and therefore CO is not a major air pollutant in Sri Lanka.24 hour average values of ambient SO 2 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 SO 2 is 0.014 ppm.Thus city of Colombo was exposed to high SO 2 pollution during this period.
Recognizing the importance of the problem, government of Sri Lanka has proposed the fuel quality and air quality improvement road map to reduce the SO 2 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 O 3 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.

2 The Seasonal Variation of Ozone
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 rese arch 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 O 3 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 O 3 , local variation can't be explained in detail.

3 Air Quality Monitoring Data in Five Sites
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 (PM 10 -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 nonmethane hydrocarbon (NMHC-0.997ppm) 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 PM 10 and PM 2.5 (PM 10 -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 NO 2 (NO- With the favorable temperature (28.4°C) in Anuradhapura, production of O 3 (0.017 ppm) was also higher.Low concentration of pollutant at Jaffna could be due to the high sea breeze in the area.

4 Physical Phenomenon
Data signifies that two concepts exist for NO X -VOC-O 3 sensitivity.In some conditions, the process of O 3 formation is controlled almost entirely by NO X and is largely independent of VOC (i.e., NO X -sensitive regime), while for other conditions O 3 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), O 3 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, NO 2 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.In the NO X saturated or VOC sensitive regime O 3 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 O 3 concentration is seen in Fig. 4.However, with very high NO concentration, O 3 concentration has reduced.This is due to the removal of O 3 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 condi-tion needs to handle carefully as reduction in NO concentration may result in high production of O 3 .Therefore, it is essential to reduce both pollutants NO and VOC simultaneously to mitigate formation of O 3 .Intermediate of this high O 3 production and how the titration of produced O 3 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-O 3 sensitivity.Accordingly in Matara, Badulla, Pollonnaruwa, and Gampaha show the NO X -sensitive regime.In Kandy and Nuwaraelliya shows the VOCsensitive regime as shown in Fig. 6.

5. 1 NO X -sensitive Regime
By considering the chemical reactions of NO X -O 3 cycle, mathematical relationship could be developed to calculate O 3ss in the NO X -sensitive regime as (Atkinson and Arey, 2003;Atkinson, 2000): Since the inter conversion between these species is so fast, a steady state is reached within a few minutes.This photo stationary state relation determines the O 3 concentration.The NO X sensitive steady state O 3 concentration is proportional to the [NO 2 ]/[NO] ratio and it is defined as: where k -depends on Temperature; Ω 0.4 ppm -1 s -1 j -depends on solar radiation; at night = 0; at full sunlight = 0.4 min -1 (Atkinson and Arey, 2003)

5. 2 VOC-sensitive Regime
By considering the chemical reactions of NO X -VOC, split mathematical relationship could be developed to calculate O 3ss in the VOC-sensitive regime.O 3 formation occurs through the following sequence of reactions.The sequence is almost always initiated by the reaction of various VOC or CO with the OH radical (R4 and R5).This is followed by the conversion of NO to NO 2 (through reaction with HO 2 or RO 2 radicals), which also regenerates OH (see R6 and R7).NO 2 is photolyed to generate atomic oxygen, which combines with O 2 to create O 3 , as given in R8 and R9 (Sillman, 1999).
Here, RO 2 represents any of a number of chains of organics with an O 2 attached (replacing H in the original chain).This reacts with NO (R6) and H (which combines with O 2 to form HO 2 ).The rate of ozone formation is controlled primarily by the rate of the initial reaction of VOC with OH.
At the nighttime and in the immediate vicinity of very large emissions of NO, O 3 concentrations are depressed through the process of NO X titration (Sillman, 1999;Gillani and Pleim, 1996).This consists of removal of O 3 thro ugh reaction with NO as given in R10.
where P OH , P HO 2 , and P RO 2 are the production rates of OH, HO 2 , and RO 2 , respectively.S i , S j , and S j denote (radical or nonradical) species that act as reaction partners in sink reactions of OH, HO 2 , and RO 2 , respectively.R11 and R12 reactions have k 1 and k 2 Reaction Rate Constants which are available in literature (Spirig et al., 2002;Staffelbach et al., 1997).k 1 = 8.5 × 10 -12 ; k 2 = 7.7 × 10 -12 ; Rate constants at 298 K in cm 3 molecule -1 s -1 for bimolecular reactions and in s -1 for photolysis reactions (Spirig et al., 2002).
Since subsequent NO 2 photolysis and the reaction of O (3P) atoms with oxygen are reasonably fast.With the peroxy radical concentrations obtained from the steady state approximation (SSA), (O 3ss ) in the VOC-sensitive regime is thus calculated as;

CONCLUSION
There exist two regimes of NO X -VOC-O 3 sensitivity in Sri Lankan cities. Accordingly in Colombo, Kurunegala, Jaffna, Matara, Badulla, Pollonnaruwa, and Gampaha show the NO X -sensitive regime.In Rathnapura, Anuradhapura, Kandy and Nuwaraelliya shows the VOCsensitive regime.The developed mathematical relationship could calculate the steady state ozone (O 3ss ).Further by identification of the regime type it will provide whether it is essential to reduce both pollutants NO and VOC simultaneously or vise vasa and the quantification of pollutants.Establishment of these relationships will assist in developing appropriate interventions to control O 3 in a particular city.

Fig. 1 .
Fig. 1.The eleven air quality monitoring locations of this research in Sri Lanka.
. Validation (http://statisticalout sourcingservices.com/Outlier2.pdf) was carried out for the NO X -VOC-O 3 sensitivity regimes developed using ambient real time air quality monitoring data collected in weekly based on the same methodology in another series of cities (e.g., Matara, Badulla, Pollonnaruwa, Gampaha, Kandy, Nuwaraelliya) for the above mentioned pollutants and the trends were compared to confirm the proposed NO X -VOC-O 3 sensitivity for Sri Lankan cities. Physical phenomenon was developed that explains the chemical reactions of the two regimes.Mathematical relationship was developed to calculate the steady state ozone (O 3ss ) that gives the values for the NO X -sensitive regime and the VOC-sensitive regime.

Fig. 2 .
Fig. 2. PM 10 , NO 2 , CO, SO 2 and O 3 24 hour average values from 2003 to 2008 at Colombo Fort Air Pollution Monitoring Station.*Concentrations of O 3 and SO 2 are shown in secondary axis.

Fig. 5 .
Fig. 5. Intermediate of NO X -sensitive regime and VOC-sensitive regime in Anuradhapura.

k 1 NOFig. 6 .
Fig. 6.Validation of NO X -sensitive and VOC-sensitive regime in other cities.

Table 1 .
Results of the 1998 to 2008 for the pollutant exceedance of the national standards and the WHO guideline values in Sri Lanka.

Table 2 .
Comparison of 24-hour average values of pollutants at five locations.
0.044 ppm and NO 2 -0.026 ppm), and high non-methane hydrocarbon (NMHC-0.669ppm) was observed.Jaffna and Anuradhapura have comparatively low air pollutant concentration.At Anuradhapura having higher Paddy fields high CH 4 (CH 4 -1.828ppm) concentration was observed.