Assessment of Air Pollution in Bangkok Metropolitan Region , Thailand

Analysis of gaseous criteria pollutants in Bangkok Metropolitan Region (BMR), Thailand, during 2010-2014 reveals that the hourly concentrations of CO, SO2 and NO2 were mostly below the National Ambient Air Quality Standards (NAAQs) of Thailand. However, the hourly concentrations of 10 O3 exceeded the Thailand NAAQs. The maximum concentrations of O3 ranged from 120-190 ppb. On average, the number of hourly O3 exceedances ranged from 1-60 hours a year depending on monitoring station locations. The exceedances occurred during the summer and winter, dry seasons. Interconversion between O3, NO and NO2 indicates crossover points between species occur when the concentration of NOx ([NOx] = [NO]+[NO2]) is ~60 ppb. However, when [NOx] < 60 ppb, O3 is the dominant species; 15 conversely, NO dominates when [NOx] > 60 ppb. The calculated photochemical reaction rate (the reaction between NO2 with sunlight), during photostationary state ranges from 0.12 to 1.22 min . Linear regression analysis between the concentrations of Ox ([Ox] = [O3]+[NO2]) and NOx provides the role of local and regional contributions to Ox. Both the local and regional Ox contributions enhance the concentration of Ox. Values of the local and regional Ox contributions during non-episode were ~44-54 20 ppb and ~ 0.13[NOx] to 0.33[NOx], respectively. Those values were about double during O3 episodes ([O3] > 100 ppb). Ratio analysis suggests that the major contributors of primary pollutants over BMR are mobile sources (CO/NOx = 19.8). The Air Quality Index (AQI) for BMR was predominantly between good to moderate. Unhealthy O3 categories were observed during episode conditions in the region.


Introduction
Bangkok (BKK), the capital city of Thailand, has the largest population and population density in Thailand.Bangkok Metropolitan Region (BMR) refers to Bangkok and five adjacent provinces, including Nakhon Pathom, Pathum Thani, Nonthaburi, Samut Prakan, and Samut Sakhon.These five provinces are linked to BKK in terms of traffic and industrial development (Zhang and Oanh, 2002).Since 1995, BKK has experienced exceedances in Thailand National Ambient Air Quality Standard (NAAQs) for particulate matter (PM) and ozone (O3) (PCD, 2015).The largest number of O3 exceedances ([O3]hourly > 100 ppb) occurred in the year 2000 with 174 hours of exceedances (Oanh and Zhang, 2004).Furthermore, BMR is considered as a region with the worst air quality in Thailand (Watcharavitoon et al., 2013).The transportation and industrial sectors are considered to be the major sources of air pollutants in BKK (Watcharavitoon et al., 2013).The number of vehicles in Thailand has increased since 1989.During 2014, about 36 million new vehicles were registered and 29 % of these cars were registered in BKK (DLT, 2015).About 56 % and 28 % of the registered vehicles in BKK were gasoline and diesel engines.The remaining 16 % is Compressed Natural Gas (CNG).According to the database of the Department of Industrial Work (DIW), Thailand, the number of registered manufacturing plants in Nakhon Pathom, Pathum Thani, Nonthaburi, Samut Prakan, andSamut Sakhon are 3,282 (DIW, 2016), 3,756 (DIW, 2016a), 1,981 (DIW, 2016b), 7,357 (DIW, 2016c) and 6,035 (DIW, 2016d).A variety of manufacturing facilities are located on the outskirts of BKK, including, metal, auto parts, paper, plastic, food, chemical manufacturing and power plants.
In this study, gaseous criteria pollutants including carbon monoxide (CO), nitrogen oxide (NO), nitrogen dioxide (NO2), sulfur dioxide (SO2) and O3 concentrations and trends in BMR during 2010-2014 are investigated.O3 and its precursors (only NO and NO2) are analyzed since they are the species that were measured at a majority of the monitoring sites.Moreover, BMR experiences primarily O3 exceedances amongst all the other gaseous criteria pollutants.Interconversion between O3 and its precursors and photochemical reaction rate during photostationary state are examined to assess O3 formation over BMR.
Local emission, regional contribution and possible emission sources of pollutants that associate with O3 formation are identified.During the dry season, storms may occur especially during seasonal transitions (TMD, 2015).Due to its location in the coastal area of the Gulf of Thailand, land and sea breezes may play an important role on pollution dispersion over BMR.Phan and Manomaiphiboon (2012) showed that sea breezes from the Gulf of Thailand frequently occur during winter.Strong sea breezes that penetrated inland 22-55 km were found during the early to mid afternoon.
Hourly observations collected by Pollution Control Department (PCD), Thailand, from 15 monitoring sites located in BMR are analyzed in this study.It is assumed that the monitoring sites used were representative of BMR specific patterns and trends.The monitoring sites are categorized into three categories-Bangkok (BKK) sites, Roadside sites, and BKK suburb sites).Seven Bangkok sites including 3T, 5T, 10T, 11T, 12T, 15T and 61T sites, refer to the air quality monitoring sites that are located within BKK's residential, commercial, industrial and mixed areas.These monitoring sites are ~50-100 m away from the road.Two roadside sites including 52T and 54T sites, refer to the monitoring sites that are located in BKK within 2-5 m from the road (Zhang and Oanh, 2002).Six BKK suburb sites including 13T, 14T,  14T and 27T)).Figure 1 shows a map of BMR with the major monsoon winds over this region and the monitoring sites' location.

Data Collection and Data Analysis
The data sets in this study were provided by the PCD.Quality assurance and quality control on the data set were performed by PCD prior to receiving the data.Hourly observations of gaseous species and meteorological parameters including wind speed (WS), wind direction (WD), temperature (T) and relative humidity (RH) were automatically collected with auto calibration at the monitoring stations.Manual quality control was performed when unusual observations were found.The equipment and monitoring stations were calibrated every year.
Gaseous species were measured at 3 m above ground level (AGL).CO was measured using nondispersive infrared detection (Thermo Scientific 48i), NO and NO2 were measured using chemiluminescence detection (Thermo Scientific 42i), SO2 was measured using ultraviolet (UV) fluorescence detection (Thermo Scientific 43i) and O3 is measured by using UV absorption photometry detection (Thermo Scientific 49i).The meteorological parameters including wind speed (WS) and wind direction (WD) were measured at 10 m AGL by cup propeller and potentiometer wind vanes; temperature (T) and relative humidity (RH) were measured at 2 m AGL by thermistor and thin film capacitor, respectively (Watchravitoon et al., 2013).All the meteorological measurements were made by Met One or equivalent instruments.Data analysis, statistical data analysis (t-test) and plots are performed using Excel 2016.Predominant wind directions over BMR are illustrated by wind rose diagrams which are performed using WRPLOT program (free software from Lake Environmental).During the 5 years of the study period, maximum hourly concentrations of CO, NO2 and SO2 were mostly lower than their hourly standard.Elevated CO and NO2 concentrations were frequently found at roadside sites.The average concentrations of CO, were ~1.0±0.7 ppm over roadside sites and ~0.7±0.4 ppm over BKK sites and BKK suburb sites.The hourly maximum concentrations of CO ranged from ~3-8 ppm.
The average concentrations of NO2 were ~32.2±17.7 ppb, 21.1±13.6ppb and 16.3±11.9ppb over roadside sites, BKK sites and BKK suburb sites, respectively.The hourly maximum concentrations of NO2 ranged from 62-180 ppb (an exceedance was found at 52T monitoring station, during 2013).High SO2 concentrations were frequently found over BKK suburb sites.The average concentrations of SO2 were ~3.8±3.9 ppb, 3.0 ± 2.1 ppb and 2.6±2.3 ppb over BKK suburb sites, BKK sites and roadside sites.The hourly maximum concentration of SO2 ranged from 13-163 ppb.
Even though the hourly maximum concentrations of the other gaseous species were generally lower than their standards, the hourly maximum concentrations of O3 were greater than its standard.The average concentrations of O3 were ~22.0±19.8ppb, 17.9±16.9ppb and 13.3±12.7 ppb over BKK suburb sites, BKK sites and roadside sites, respectively.The hourly maximum concentration of O3 ranged from 68-190 ppb.O3 exceedances at BKK suburb sites were more frequently occurred than those at other sites.The

Diurnal Variation of the Gaseous Species
The primary precursors for tropospheric O3, in the urban environment, are oxide of nitrogen (NOx; refers to NO + NO2) and non-methane volatile organic compounds (VOCs), methane or CO (The Royal Society, 2008, Monks et al., 2009;Cooper et al., 2014).NOx was measured continuously at all the monitoring sites.However, VOCs concentrations were measured periodically only at one monitoring station limiting its usefulness as part of this study.
Diurnal variations of O3 and its precursors over BMR during 2010-2014 are shown in Fig. (3(a)-(c)).The diurnal variations of O3 show a typical single-peak pattern (Aneja et al., 2001) with the concentrations increase after sunrise and reach the peak around 15:00 local time (LT).The concentrations begin to decline in the evening and reach the minimum concentrations around 7:00 LT in the next morning.The concentrations at the peaks of the diurnal variations of O3 were ~40 ppb over BKK sites, ~30 ppb over roadside sites and ~45 ppb over BKK suburb sites.The diurnal variations of NO and NO2, show doublepeak patterns with the concentrations increase around 5:00 LT and reach the first-peak around 7:00-9:00 LT before they decline.The concentrations of NO and NO2 start rising and reach the second-peak around 21:00-22:00 LT.The NO concentrations at the morning-peak over BKK sites, roadside sites and BKK suburb sites were ~40 ppb, 110 ppb and 30 ppb.In the afternoon-peak they were ~23 ppb, 73 ppb and 13 ppb.The NO2 concentrations at the morning-peak over BKK and BKK suburb sites were ~23 ppb and 20 ppb and those at the afternoon-peak were ~28 ppb and 22 ppb.The NO2 concentrations over roadside sites ranged from ~22-37 ppb and were near constant during the day.The diurnal variations of CO show double-peak patterns with the first-and the second-peak occur around 8:00 LT and 21:00 LT.The diurnal variations of CO are similar to those of NO.The concentrations increase around 4:00-5:00 LT and reach the first sharp peaks around 8:00 LT before they decline.The CO concentrations start rising and reach the second-peak at night.The CO concentrations at the morning-peak were ~1 ppm, 2 ppm and 1 ppm and those at the night-peak were ~1 ppm, 1.5 ppm and 1 ppm, over BKK, roadside and BKK suburb sites, respectively.
Diurnal patterns of NO, NO2 and CO correspond to road traffic patterns and similar to those in other big cities (Tiwari et al., 2015).The study of Leong et al. (2002) on air pollution measurement in BKK showed that, in BKK, morning rush hour occurred during 7:00-9:00 LT and evening rush hour occurred during 16:00-18:00 LT.During traffic rush hours, traffic volume was high with low vehicle speeds.While the first peak of the diurnal pattern of pollutants occurred during the morning traffic rush hour, the second peak occurred ~3-5 hours after the evening traffic rush hour.This is due to a combination of pollutants emissions and collapse of the planetary boundary layer during this time.The evening planetary boundary layer is characterized by weak turbulence and diffusion, allowing pollutants to accumulate in the layer (Arya, 1999;Jacobson, 2012).
The concentrations of SO2 start increasing around 5:00 LT and reach maximum around 8:00 LT before the decline.The concentrations of SO2 at the morning-peak were ~3 ppb over BKK sites and roadside sites, and ~6 ppb over BKK suburb sites.The concentrations of SO2 increase again in the afternoon and reach a second-peak around 21:00 LT over roadside site.Over BKK sites and BKK suburb sites, the concentrations of SO2 are nearly constant after 19:00 LT.The concentrations of SO2 at the second-peak over roadside sites were ~3 ppb and ~3-4 ppb over BKK sites and BKK suburb sites.The double-peak pattern of SO2 over roadside sites indicates that SO2 is influenced by emission primarily from vehicle exhaust using high sulfur content fuel, especially high sulfur diesel.The study of ambient air SO2 patterns in European cities by Henschel et al. (2013) showed that diurnal patterns of SO2 had a double-peak pattern which the morning peaks more likely related to emission during rush hour, evening peaks were possibly caused by traffic and meteorology-collapse of the planetary boundary layer.It is noteworthy that BKK has a large diesel engine fleet (an estimated 25 % of registered vehicles) (DLT, 2015a).The diesel fuel contains ~0.035 %wt Sulphur (DOEB, 2017).Given the timing of SO2 peak (morning automotive rush hour), it is likely that SO2 is emitted by automotive diesel engine exhaust.

Interconversion between O3, NO and NO2 and Photochemical Reaction
In this study, the photostationary state (PSS) is applied through all chemical reactions for O3 formation during 10:00-16:00 LT.This time window is chosen due to the fully developed atmospheric boundary layer with well-mixed condition (Pochanart et al., 2001) in order to avoid accumulation due to surface inversion.To eliminate effects of the removal process by wet deposition, analysis and calculation are performed only during dry season.The relationship among three chemical species (NO, NO2 and O3) under PSS is presented by Eq. ( 1) (Seinfeld and Pandis,1998) Where [ 3 ]  is the concentration of O3, at PSS, j1 and k3 are reaction rate coefficient of photochemical reaction of NO2 and reaction rate coefficient of chemical reaction between NO and O3, respectively.
According to Eq. ( 1), the concentration of O3 depends on the ratio of NO2 and NO.Therefore, other chemical reactions or processes that affect NO2 and NO species will also affect O3 concentrations in the atmosphere (Jacobson, 2012).
The ratio of NO2 and NO are calculated only during dry season.During dry season, the values of the rations range from 0.54-4.33 in winter and from 0.87-4.33 in summer.T-test values for the ratios exhibit no significant difference with season (P-value > 0.05).While there is no significant difference with season, the t-test values exhibit a significant difference with locations of monitoring sites.The ratios of NO2 and NO show significantly different between roadside sites and non-roadside sites (BKK sites and BKK suburb sites) with P-value < 0.05.
In this study, j1 is calculated based on Eq. ( 1), since we cannot directly measure it.The values of j1 range from 0.12-1.22min -1 in winter and from 0.13-0.90min -1 in summer (Table (1)).T-test values for j1 exhibit no significant difference with season and location (P-value > 0.05).The values of j1 from this study are similar to those values at an urban background site in Delhi, India (values of j1 ranged from 0.4-1.8min - 1 and the average was 0.8 min -1 ) (Tiwari et al., 2015) and those values collected during a November daytime in the UK (values of j1 was ~0.14 min -1 ) (Clapp and Jenkin, 2001).
The values for k3 (ppm -1 min -1 ) is calculated by Eq. ( 2) (Seinfeld and Pandis, 1998;Tiwari et al., 2015).(2) During dry season, the values of k3 range from 28.3-29.8ppm -1 min -1 in winter and from 30.0-30.9 ppm - 1 min -1 in summer.T-test values for k3 exhibit a significant difference with season (P-value < 0.05) and no significant difference with locations of the monitoring sites (P-value > 0.05) ( function of temperature (T), therefore, the maximum values of k3 (29.6 and 30.8 ppm -1 min -1 in winter and summer, respectively) occur during the afternoon (around 15:00 LT) when the temperature is highest.
The maximum values of k3 from this study conforms to the k3 value (29.3 ppm -1 min -1 ) that was found at an urban background site in Delhi, India, which the peak occurred at 15:00 LT (Tiwari et al., 2015).
Due to high value of j1, high O3 concentrations are expected to be found at 11T, 20T and 52T sites.
However, high O3 concentrations were found only at 20T and 52T sites, but low at 11T site.The low level of O3 concentration at 11T site has an association with the titration of O3 by NO, since high NO concentrations were observed at 11T site.In conclusion, the titration of O3 by NO is perhaps one of the more important processes that control O3 concentrations in urban areas.
To gain a better understanding of O3 and its precursors over BMR, the concentrations of NO, NO2 and O3 are plotted against the concentrations of NOx.Polynomial trend lines are added in order to investigate the interconversion among these species.

Local and Regional Contribution to Ox
The Ox concentration is the summation of O3 and NO2 concentration.Under the PSS condition, concentration of NO, NO2 and O3 approach an equilibrium and the concentration of Ox may be considered constant (Keuken et al., 2009).Since the conversion between O3 and NO2 in the urban and suburban atmosphere is rapid, the use of Ox to represent production of oxidants is more appropriate than only using O3 (Lu et al, 2010).The local or NOx-dependent contribution refers to Ox concentration that is influenced by concentration of the local pollutants.The regional contribution or NOx-independent refers to the background concentration of Ox that is not influenced by changes of the local pollutants (Clapp and Jenkin, 2001;Tiwari et al. 2015).implies the local contribution, and the intercept with the y-axis (c) implies the regional (background) contribution (Aneja et al., 2000;Clapp and Jerkin, 2001;Notario et al., 2012).Table (2) shows the comparison between the fitted linear regressions from this study with other studies.The average background Ox concentrations over BMR during non-episodes and episodes are ~48 ppb and 95 ppb, respectively.The local and regional contributions during the episode days, in general, were about double of those during the non-episode days.Therefore, O3 formations during the episode days were influenced by both the local and regional contributions of Ox.It is noteworthy that the pattern of the local and regional contributions at roadside sites during non-episode period is composed of two NOx concentration regimes.
The low NOx regime (NOx < 60ppb) resembles the local and regional contributions during non-episode over BKK suburb sites.The high NOx regime (NOx > 60ppb) may represent typical characteristic of air quality near roads.
The local contributions from the fitted linear regressions are compared with the local contribution that is calculated from delta O3 method.A delta O3 (ΔO3) analysis was performed to reflect on the intensity of O3 production in BMR area (Lindsay and Chameides, 1988).We utilized hourly O3 concentrations during 10:00-16:00 LT reflecting the role of photochemistry in O3 formation.Thus, the difference between O3 concentration measured at Samut Sakhon Provincial Administrative (site 27T) and Bangkok University Rangsit Campus (site 20T) during the predominant wind direction should reflect the difference in the amount of O3 leaving and entering the city whenever the winds are out of the Southwest or Northeast direction (Fig. ( 6)).This analysis provides the net increment of photochemical O3 added to an air mass over the course of the day as it advects over the city.For a more rigorous delta O3 analysis, we need to consider the role of wind speed.Lindsay et al. (1989) analyzed high-O3 events in Atlanta, GA, and showed that rural background O3 during episode days ([O3] > 80 ppb) in Atlanta Metropolitan Area were higher than its average and the concentration of O3 increased from ~15-20 ppb when the air mass travelled across the city, enhancing the total O3 concentration to 80-85 ppb.In our study, the concentrations of O3 over the upwind and downwind monitoring sites are averaged during 10:00-16:00 LT in dry season when concentrations at the upwind monitoring stations.However, a negative ΔO3 may be found.The negative ΔO3 suggests deposition of O3 and/or O3 was consumed as it passes over the city and/or there may have been a wind reversal so that air already polluted by the metropolitan area was brought back in to the city (Lindsay et al., 1989).The ΔO3 in BMR ranged from -53 to 86 ppb (average about 10.4 ppb.), and ranged from -66 to 96 ppb (average ~9.4 ppb.) when the predominant wind direction advecting into the city were from NE and SW, respectively.Thus, we find that there was ~10 ppb enhancement of the O3 concentration during the air pollution high O3 concentration in BMR ([O3] > 80 ppb), which corroborates local O3 production analysis based on linear regression.

Local Sources Analysis
Characteristic of emission sources are often determined by the ratios between CO/NOx and SO2/NOx.In general, the major sources of NOx are point sources and mobile sources.However, NOx from point sources is more likely correlated with SO2.NOx from mobile sources is more likely correlated with CO (Parrish et al., 1991).Therefore, the characteristics of mobile source are high CO/NOx ratios and low SO2/NOx ratios.In contrast to mobile sources, the characteristic of point sources are low CO/NOx ratios and high SO2/NOx ratios (Parrish et al., 1991;Rasheed et al., 2014).
Table (3) shows the comparison between the CO/NOx and SO2/NOx ratios from this study and when compared with other studies.The ratio of CO/NOx is 19.8 and the ratio of SO2/NOx is 0.1 over BMR.This suggests that the major contributors of primary pollutants over the BMR are mobile sources.However, this region may also be influenced by manufacturing facilities' point sources (SO2 contributor) on the outskirts of the BKK.These point sources will impact the concentrations of SO2, NOx and CO.

Effects of Pollutant Transport
In general, O3 has a short (approximately hours) lifetime in polluted urban atmosphere.However, O3 has a longer lifetime of several weeks in the free troposphere.This occurrence may allow O3 to be transported over continental scales (Stevenson et al., 2006;Young et al., 2013;Monks et al., 2015).from this study are supported by an earlier study (Sahu et al, 2013) that showed pollution concentrations over BKK related with local wind direction.

Air Quality Index for O3 Management
Enhanced ambient air pollution has an association with increased risk of adverse cardiovascular morbidity and mortality for humans.For example, increased levels of O3 causes coughing, reduces lung function, enhances pulmonary inflammation and may increase the risk of death due to respiratory diseases (US.EPA, 2017c).While adverse health effects may occur in healthy people, enhanced ambient air pollution is a serious threat to sensitive groups (i.e.children, elders and people with respiratory system diseases).Increased lifetime exposure of tropospheric O3 was a cause of decreased lung function in young adults (Targer et al., 2005).Buadong et al. (2009) studied the association between O3 exposure and hospital visits for cardiovascular diseases (CVD) in the central of BKK, Thailand.The study showed a positive relationship between exposure to O3 on the previous day with increasing number of hospital visits for CVD in elderly patients (≥ 65 years).Fann et al (2011) studied the relationship between O3 exposure with the national public health burden in the U.S. and found O3 associated with premature death in metropolitan areas where these numbers were greater than other habitable environs.The study of the Global Burden of Disease, Injuries, and Risk Factor study 2013 (GBD 2013) for 188 countries by For air pollutant species in the US, the AQI for each species is categorized into 6 categories (good, moderate, unhealthy for sensitive groups, unhealthy, very unhealthy, and hazardous).These categories are nonlinear and relate to human health (US.EPA, 2017EPA, , 2017aEPA, , 2017b)).In Thailand, the NAAQs for the air pollutant species is pegged at an AQI value of 100.In the US AQI rating system the results were the following for Thailand: the majority of air quality over BMR were in the good AQI category (~93-99 %); followed by the moderate air quality category.However, unhealthy for sensitive group (88-632 hours), unhealthy (19-209 hours) and very unhealthy (2-59 hours) O3 air quality categories were found during the study period.In general, BKK suburb sites have higher number of hours that were found in the unhealthy for sensitive group, unhealthy and very unhealthy categories than BKK and roadside sites.The average number of hours that were found in unhealthy for sensitive group, unhealthy and very unhealthy categories over BKK suburb sites were 425.8, 146.7 and 28.7 hours.This study provides measurements and analysis for the gaseous criteria pollutants.However, in order to provide a well-established air quality management policy, the integration of multidisciplinary analysis is needed.This will include scientific, socioeconomic and policy analysis (Aneja et al, 2001).The results from this study reveal evidence of violations for O3 for air quality resulting in adverse health effects, human welfare, economics and environment over BMR.Source analysis suggests to control pollution emission from local sources that emissions from mobile sources should be the first priority.The complexity between O3 and its precursors and the effects of pollution transport shows that decreasing only NOx emissions and/or local emissions may not be an effective policy to reduce O3 since regional air pollution transport contributes to O3 exceedances.To identify the proportional contribution between local and regional sources of O3 concentrations during selected O3 episode days, atmospheric modeling is needed to quantify various processes that contribute to the ambient concentration at specific locations.This scientific analysis provides a frame work for the process of establishing an air quality policy while developing socioeconomic impacts.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1063Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 13 December 2017 c Author(s) 2017.CC BY 4.0 License.BKK is located at latitude and longitude of 1345' N and 10085' E, over the low flat plain of Chao Praya River, elevation height ~2.3 m above mean sea level.Thailand has three official seasons-summer (around February-May), rainy (around May-October) and winter (around October-February) as per the Thai Meteorological Department (TMD) (TMD, 2015).During rainy season, this region is influenced by Southwest monsoon wind that travels from the Indian Ocean to Thailand.This marine air mass contains high moisture, resulting in the wet season in Thailand.During this season, Thailand is characterized by cloudy weather with high precipitation and high humidity.Around October-April, this region is influenced by Northeast monsoon wind that travels from the northeastern and the northern parts of Asia (China and Mongolia).This monsoon wind brings a cold and dry air mass, resulting in the dry season in Thailand.The dry season in Thailand can be classified into two minor local seasons-winter and summer.The local winter in Thailand is characterized by cool and dry weather, while the local summer is characterized by hot (~35 ℃-40 ℃) to extremely hot weather (> 40.0℃) due to the strong solar radiation.
Figure (4(a)-(c)) show relationship and crossover points between the species.The crossover points occur when the concentration of NOx is ~60 ppb.At this point, two regimes are identified-low NOx regime and high NOx regime.Under low NOx regime ([NOx] < 60 ppb), O3 is the dominant species among the others, and NO2 concentrations are higher than NO for NOx species.On the other hand, under high NOx regime ([NOx]> 60 ppb), NO and NO2 increase and, the concentrations of O3 rapidly decrease.Under the high NOx regime, the decrease of O3 trend-lines may describe O3 removal process through the titration of O3 by NO.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1063Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 13 December 2017 c Author(s) 2017.CC BY 4.0 License.The effects of local and regional contributions to Ox concentration are analyzed by plotting Ox concentrations against NOx concentrations and fitting the plot with a linear regression (y = mx + c).The concentration of NOx and Ox are referred by x and y, respectively.The slope of the linear regression (m) Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1063Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 13 December 2017 c Author(s) 2017.CC BY 4.0 License.backward trajectories from the National Oceanic and Atmospheric Administration (NOAA) HYSPLIT model reveal N-NE, S-SW wind directions with high O3 concentrations ([O3] > 80 ppb) at the monitoring sites.The O3 concentrations at the downwind monitoring stations are expected to be greater than the O3 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1063Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 13 December 2017 c Author(s) 2017.CC BY 4.0 License.
Figure 6 shows high O3 concentrations ([O3]hourly > 100 ppb) with the predominant wind directions over BMR during 2010 to 2014.The results show that O3 exceedances are associated with the local wind directions which are related to locations of the monitoring sites.High O3 concentrations are associated with the three predominant wind directions; westerly, northerly and southerly winds.Elevated O3 concentrations associated with northerly winds were at 11T, 13T, 14T, 22T and 27T sites.At sites 3T, 5T, 10T, 12T, 15T, 19T, 54T and 61T, high O3 concentrations are associated with southerly winds.At sites 52T and 20T high O3 concentrations were predominantly observed with westerly wind directions.The results Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1063Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 13 December 2017 c Author(s) 2017.CC BY 4.0 License.Forouzanfar et al. (2015) reported the increased number of deaths during 1990 to 2013 (from 133 to 219 deaths in thousands) due to ambient O3 pollution.World Health Organization (WHO) Regional Office for Europe, Economic Co-operation and Development (OECD) (2015) estimated the annual economic cost of premature deaths and those of morbidity from air pollution between US $1.431 trillion and $1.575 trillion across the countries of the WHO European Region.

Figure 2 :
Figure 2: Maximum (vertical bars) and average (solid line) concentrations of (a) CO, (b) SO2, (c) NO2 and (d) O3 from 15 monitoring stations, during 2010-2014, are compared with the 1-hour NAAQs (dotted line) of Thailand.Three different sites, the BKK sites, roadside sites and the BKK suburb sites are represented by light blue, purple and blue colors.The number of 5 exceedances of hourly O3 concentration are shown by (e) monitoring stations, and (f) months.

Figure 3 :
Figure 3: Diurnal variations of gaseous species including NO, NO2, CO and SO2 at (a) BKK site (b) roadside sites and (c) BKK suburb sites.

Figure 5 :
Figure 5: Effects of local and regional contributions on Ox during non-episode and episode days over BMR (a) BKK sites (b) roadside sites and (c) BKK suburb sites during 2010-2014.

Figure 6 :
Figure 6: Relationship between high O3 concentration ([O3]hourly > 100 ppb) (monitoring site ID is listed at the center of the Wind Rose Plots) and wind directions over BMR during 2010 to 2014.The map illustrates several industrial areas located near the study area and the monitoring stations, blues, purples and dark blues identify BKK sites, roadside sites and BKK suburb 5 sites; greys identify industrial areas

Table (
exceed the NAAQs of Thailand.The O3 exceedances occur during the dry season (summer and winter) and most frequently occur over BKK sites and BKK suburb sites than roadside sites; which O3 titration by NO played an important role to decrease O3 concentrations.Interconversion between O3, NO and NO2 and photochemical reaction shows that O3 has a non-linear relationship with its precursor with high concentrations of O3 which occur when NOx concentration is less than 60 ppb.After this point, O3 concentrations rapidly decrease, while NOx concentrations increase.Under high NOx regime, the concentration of O3 is influenced by NO through the titration process.The result for the study shows that decreasing NOx emission will not directly decrease O3 concentration over BMR.The regression curves reveal a background Ox concentration of ~48 ppb (non-episode) and ~95 ppb (episode) over BMR.During an O3 episode, both local and regional contributions play an important role in the increase of Ox concentrations.The result reveals that, decreasing emission from only local sources may not improve air quality during O3 episodes, since regional air pollution transport contributes to O3 formation.Sources analysis suggests that to control pollution emission from local sources, the emissions from mobile roadside sources should be the first priority.Air Quality Index for O3 reveal evidence of violations for O3 for air quality resulting in adverse health effects, human welfare, economics and environment over BMR.Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1063Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 13 December 2017 c Author(s) 2017.CC BY 4.0 License.