High Nighttime Ground-Level Ozone Concentrations in Kemaman : NO and NO 2 Concentrations Attributions

High nighttime ozone (O3) concentration levels were observed in Kemaman, Terengganu, and results were compared with those in other places in Malaysia. In this study, the contribution of precursors [nitric oxide (NO) and nitrogen dioxide (NO2)] and meteorological factors wind speed, and wind direction) toward long-term high nighttime O3 over the period of 1999 to 2010 was evaluated. During this period, the recorded highest nighttime O3 ground level was 89 ppb with more than 25% surpassing 20 ppb. Analysis shows that minimal decreasing trends were measured in Kemaman. Lower nitrogen oxide (NOx) concentrations restricted the sinking agents; thus, reducing the depletion rates allowed O3 to remain in the atmosphere. Minimal associations were observed between the O3 concentration level and the speed and direction of wind. Accordingly, the largest contributor toward high nighttime O3 ground level concentration in Kemaman was most probably NOx concentration.


INTRODUCTION
Ozone (O 3 ) is an important constituent of air that plays significant roles as an oxidant and greenhouse gas.Most of O 3 concentrations occupy in ozone layer which, is located in the middle of the stratosphere and acts as a shield that protecting Earth from harmful radiation.However, a small concentration of O 3 also exists in the troposphere.At the ground level, O 3 is considered as a secondary air pollutant, which is typically associated with degrading air quality, and induces harmful effects on human health, crop production, material quality, and the ecosystem.O 3 is emitted from various anthropogenic activities.In particular, the formation and accumulation of O 3 are induced by the emissions of nitrogen oxide (NO x ) and volatile organic compounds (VOCs).At the ground level, NO x plays an important role in the O 3 chemistry (Seinfeld and Pandis, 2006).In the presence of sunlight, nitrogen dioxide (NO 2 ) undergoes photochemical reactions to produce free oxygen atom (O), which mainly reacts with oxygen molecule (O 2 ) to form O 3 (Duenas et al., 2004;Abdul-wahab et al., 2005;Ghazali et al., 2010).The variation in O 3 concentrations is influenced by meteorological factors.Such variation acts as a catalyst of photochemical reactions as well as a mixing and dispersion agent.
O 3 has received substantial attention worldwide because of its negative effects.Ghosh et al. (2013) reported that background O 3 has increased over the last decade and is expected to continuously increase in the subsequent years.Many countries, including Malaysia, monitor the current O 3 condition and have set guidelines against this air pollutant.Considerable research employed the variability of O 3 concentrations on daily, seasonal, weekend, and weekday bases.The variability of O 3 concentrations in urban (Alvarez et al., 2000;Ghazali et al., 2010;Hassan et al., 2013), suburban (Reddy et al., 2011;Swamy et al., 2012), and rural (Duenas et al., 2004) conditions were also described by several studies.Toh et al. (2013) and Ghozdic et al. (2011) explored the role of meteorology in the variation of O 3 concentrations and concluded that O 3 is significantly responsive toward temperature, UV radiation, relative humidity, as well as wind speed and direction.Intense solar radiation, high temperature, minimal rainfall, low wind speed, and low relative humidity can raise O 3 concentration (Toh et al., 2013).Substantial research also realized the possibilities of adopting multivariate analysis to produce prediction and forecasting models of O 3 concentrations (Pires et al., 2008;Kovac-Andric et al., 2009;Jimenez-Hornero et al., 2010;Ghozdic et al., 2011).Ishi et al. (2007) and Mohammed et al. (2013) examined the phytotoxic effects of O 3 on crop production.At present, the number of studies that have investigated the variations of nighttime O 3 remain insufficient.
The increase of nighttime O 3 concentrations has gained scholarly attention.Such a phenomenon in different regions of the world has been observed and reported by several researchers.Sousa et al. (2011) studied the frequency, concentration, and time of the occurrence of high nighttime O 3 maxima in urban and suburban sites in Portugal from 2005 to 2007.Consequently, Sousa et al. (2011) discovered that the among meteorological parameters analyzed maxima scenarios were only correlated with wind direction with higher occurrences transpired in urban areas more than in suburban sites.According to Kulkarni et al. (2013), increasing nocturnal O 3 is associated with prevailing meteorological conditions and transport process because the photochemical production of O 3 ceases with the absence of sunlight.Meanwhile, Ghosh et al. (2013) explored the nighttime chemistry between O 3 and its precursors (NO x ) in terms of NO 2 -NO 3 -N 2 O 5 cycle over urban Kolkata, India; the researchers found that the NO 2 -NO 3 -N 2 O 5 cycle plays a significant role in the nighttime O 3 chemistry as the cycle heavily affects the depletion process.
This study primarily aims to determine the high nighttime O 3 concentrations in Kemaman, Terengganu.The magnitude, frequency, and time of high nighttime O 3 concentrations are analyzed with respect to the precursors of O 3 such as NO 2 , NO, and CO concentration and its prevailing meteorological parameters, including of wind speed, and wind direction.The possible nighttime reactions of NO x and O 3 are also critically explored and presented in this paper.

METHODS
O 3 , NO 2 , nitric oxide (NO) concentrations in Kemaman, Terengganu were progressively measured together wind speed and direction.Daylight hour was calculated based on the total number of hours between sunrise and sunset (Clapp and Jenkin, 2001), and the remaining hours were considered as nighttime.Based on the local time, the nighttime hours were between 7 PM and 7 AM (Mohammed et al., 2013).The monitoring was conducted every hour for a period of twelve year, specifically from January 1, 1999 to December 31, 2010.

Sampling Site
Kemaman [N04°16.260,E103°25.826],one of the districts of Terengganu State, is located at the east coast of Peninsular Malaysia (Fig. 1).With a total area of 2535 km 2 or approximately 20% of the total area of Terengganu (Toriman et al., 2009), Kemaman is known for the discovery of a petroleum site near its town Kerteh in the early 1980s.In general, this district is relatively underdeveloped, except for a few places along the coastline, in which steel and petrochemical plants are located (Sulong et al., 2002).Only two major industrial sites (i.e., Kerteh Petrochemical and Gebeng Industrial Area) are located in and near Kemaman (Ismail et al., 2011).In terms of weather conditions, this district experiences a wet equatorial climate characterized by high temperatures all year round, with seasonal heavy rains during the northeast monsoon from November to January (Sulong et al., 2002).The annual rainfall in Kemaman is 2390 mm on average, with a standard temperature of 27°C and relative humidity at approximately 81% year-round (Toriman et al., 2009;MMD, 2013).In 2010, the total population of Kemaman was 166,750.

Instrumentations
Continuous hourly ground level O 3 concentrations and other air pollution levels were established across Malaysia by the Department of Environment (DoE) to measure and detect significant changes in air quality.Mohammed et al. (2012) claimed that hourly O 3 concentrations can be measured with the UV absorption Ozone Analyzer Model 400A, a microprocessor controlled device.This O 3 analyzer applies a system based on the Beer-Lambert law to measure low ranges of O 3 concentrations in ambient air and gaseous media (Ghazali et al., 2010).In this study, ambient O 3 concentrations were detected from the internal electronic resonance of O 3 molecules with the absorption of 254 nm UV light emitted from an internal mercury lamp (Teledyne, 1999).Meanwhile, the samples of hourly NO 2 were collected with the NO/NO 2 /NO x Analyzer model 200A (Ghazali et al., 2010).This analyzer applies the chemiluminescent detection principle to detect NO 2 concentrations in ambient air.Teledyne (1999) concluded that microprocessor technologies were incorporated into the analyzer to enhance its concentration monitoring ability.The analyzer has been proven to provide the sensibility, stability, and ease of use for ambient or dilution continuous monitoring requirement (DoE, 2008).Mohammed et al. (2012) reported that the secondary data obtained from the DoE, Malaysia were regularly subjected to standard quality control and quality assurance procedures.Wind speed (km/h) and wind directions (°), during nighttime were also included in the analysis.Wind speed and wind direction, were measured Met One 010C sensor, Met One 020C, respectively.

Cluster Analysis
Cluster analysis (CA) is a set of multivariate techniques commonly used to primarily assemble objects with similar characteristics.CA is a method to categorize objects into a number of clusters or groups, so that the objects in a cluster are similar to one another and the objects in different clusters are different from one another (Lau et al., 2008;Pires et al., 2008).CA maximizes the similarity of cases within each cluster while minimizing the dissimilarity between groups that are initially unknown (Lu et al., 2011).In this analysis, each object is considered as a separate cluster before it is connected by Ward method agglomerate techniques and squared Euclidean distance.These approaches have been used by several researchers in air and water pollution studies (Shresta and Kazama, 2007;Lau et al., 2009).Shresta and Kazama (2007) explained that Ward's method uses an analysis of variance approach to evaluate the distance between clusters in an attempt to minimize the sum of squares of any two clusters that are generated.The classifications of objects in CA are illustrated by a dendrogram (tree diagram), which shows the measured similarity or distance between any two variables.The variability of nighttime O 3 concentrations monitored from 1999 to 2010 was determined by conducting CA and with the linkage of reported distance in terms of Dlink/Dmax.

High Nighttime Ozone
The monitoring results reveal that O 3 exhibited strong day-to-day variations (Ghazali et al., 2010).In the presence of sunlight, O 3 was formed by photochemical reactions, which involved its precursors and UV radiations.Maximum O 3 concentrations were observed during noontime that coincided with maximal UV radiation.By contrast, O 3 during nighttime was generally low and in a stable condition because of the absence of photochemical reactions.Further reduction in nighttime O 3 concentrations were also attributed to deposition processes and chemical reactions (Sousa et al., 2011).Fig. 2 demonstrates the box plot of hourly average nighttime O 3 concentrations in Kemaman, Kuching, Taiping, Kajang, Kota Kinabalu, Gombak, Johor Bahru, Seberang Jaya, and Terantut.The findings suggest that nighttime O 3 concentrations were higher in Kemaman than in other stations.Mean nighttime O 3 in Kemaman surpassed was 14 ppb, whereas constant mean concentrations ranging from 5 ppb to 6 ppb were measured in other stations.The mean nighttime O 3 concentrations in Kemaman were twice as high as the concentrations in other stations, which verify the high nighttime O 3 occurrences in the district.Table 1 exhibits the mean concentrations of nighttime O 3 in Kemaman and several places.Nighttime O 3 concentrations in Kemaman were relatively higher than the reported nighttime concentrations in Kolkata and Portugal.
The variability of nighttime O 3 in Kemaman was further investigated by conducting CA using the hourly average nighttime O 3 concentrations over the period of 1999 to 2010.Nine monitoring stations (i.e., Kemaman, Kuching, Taiping, Kajang, Kota Kinabalu, Gombak, Johor Bahru, Seberang Jaya, and Jerantut) were applied to Hierarchical Agglomerative Cluster Analysis (HACA).A dendrogram in Fig. 3 illustrates the classification of the monitoring stations based on nighttime O 3 .The results of the investigation confirm that Kemaman was solely grouped in Cluster 2 (C2) and all other stations were placed in the same cluster (C1).The differences between nighttime O 3 concentrations in Kemaman and other locations are believed to contribute to the results.Nighttime O 3 in Kemaman was high, whereas daytime O 3 was relatively low.The mean and median of daytime O 3 in the district were 26.9 and 26 ppb, respectively.Throughout the 12-year study period, zero exceedances were observed in Kemaman.
This specific finding clarifies that none of the daytime O 3 concentrations surpassed the Malaysian Ambient Air Quality Guidelines (MAAQG) for 1-hour averaging of O 3 concentration at 100 ppb.The maximum daytime O 3 in Kemaman was only 89 ppb, which was almost half of the maximum concentration recorded in Gombak (175 ppb).High daytime O 3 concentrations were observed in most of the urban stations, such as Kajang and Gombak (Azmi et al., 2010;Ghazali et al., 2010;Latif et al., 2012

Nighttime Ozone Concentration Variations
The fluctuations of nighttime O 3 concentrations over the 12-year study period are depicted in Fig. S1 (see the  S1, numerous high concentrations were measured, as indicated by concentration spikes that could exceed 50 ppb.These high concentrations were generally measured early at night (7 PM to 10 PM), confirming the results reported by Sousa et al. (2011).Meanwhile, the monthly nighttime O 3 is illustrated in Fig. S2 (see the Supplementary Material, SM) with a box and whisker plot.The highest monthly average of nighttime O 3 was measured in January (19.2 ppb), but gradually decreased to the lowest concentration in April (8.9 ppb).The highest hourly average nighttime O 3 concentration was eventually observed in July with 40.2 ppb.The variation in monthly air pollutants is closely related to monsoonal changes (Md Yusuf et al., 2010;Toh et al., 2013).
Malaysia annually experiences four distinct monsoonal changes characterized by the northeast monsoon (November-March), southwest monsoon (June-September), first intermonsoon (April-May), and second intermonsoon (October-November).Nighttime O 3 concentration during intermonsoon is relatively low.According to Majid et al. (2012), late afternoon thunderstorms are frequently observed during the intermonsoon season.These thunderstorms are a localized phenomena, but they are periodically accompanied by heavy rainfall and gusty surface winds that are stronger and more turbulent than monsoon winds.Low O 3 concentrations were also observed at the beginning of the northeast monsoon (November).Suhaila and Jemain (2009) elucidated that the effect of the northeast monsoon is more prominent in east coast Peninsular Malaysia than in other locations.East coast Peninsular Malaysia is considered as the wettest location during the monsoon season.An increase in rainfall intensity promotes wet deposition process in the atmosphere, minimizing the pollutant concentration in ambient air, and strong and turbulent wind speed promotes the transport of air pollutants to other locations (Colls, 2002).The distribution of nighttime O 3 from 1999 to 2010 is displayed by a histogram in Fig. S3 (see the Supplementary Material, SM).The results of the investigation imply that most nighttime O 3 concentrations ranged from 5 ppb to 10 ppb, followed by 10 ppb to 15 ppb, and 15 ppb to 20 ppb.Few cases of nighttime O 3 were above 50 ppb.

Role of NO and NO 2 Concentrations
In Fig. 5, the diurnal variations of the hourly average of O 3 and its precursors (NO 2 and NO) illustrate their individual characteristics.O 3 production was photochemically driven; thus, the diurnal characteristic of O 3 concentrations displayed an increasing trend after sunrise, reaching the maximum around noon; minimum concentrations were recorded in the afternoon.In the troposphere, NO x plays the most important role in forming and destroying O 3 concentration.The photochemical reactions of O 3 production are as follows: The lowest O 3 productions were observed in the morning around 8 AM.Similar characteristics of O 3 diurnal trends recorded by the current study have been reported by previous research (i.e., Duenas et al., 2004;Kim and Guldmann, 2011; Reddy et al., 2011;Song et al., 2011).O 3 concentrations at night are relatively low and more constant (Ghosh et al., 2013).These low concentrations are primarily attributed to the absence of photochemical reactions, which convert O 3 precursors into O 3 (Seinfeld and Pandis, 2006).O 3 concentration during nighttime can also be reduced by chemical loss via NO titration (R3) and deposition process.
The diurnal plot indicates that nighttime O 3 concentrations decreased significantly from 7 PM to 7 AM.The test conducted in Kemaman revealed that NO and NO 2 concentrations were remarkably low with mean values of 1.1 and 4.5 ppb, respectively (Fig. 6).The plots also exhibited slightly unclear diurnal trends of NO 2 concentration.The maximum NO 2 and NO concentrations were recorded at 8 PM and 8 AM.In terms of magnitude and trends of NO 2 and NO concentrations, similarities were noted between Kemaman and Jerantut, which serve as background stations in Malaysia.Ghazali et al. (2010) posited that the typical NO 2 diurnal trends in Malaysia show two significant peaks at early morning (9 AM-10 AM) and in the evening (8 PM-10 PM), in which the second peak is lower because of emission intensity and prevailing meteorological parameters.Meanwhile, Banan et al. (2013) reported that the typical patterns of NO concentration diurnal trends are similar to that of NO 2 concentration, in which relatively high concentrations are observed at night and peak concentrations in the morning are attributed to vehicle emissions.
The absence of sunlight ceases O 3 production and enhances the efficiency of the removal mechanism of the pollutant concentration in the atmosphere.Ground level O 3 concentrations are removed from ambient air either via dry deposition or chemical reactions with NO x and VOCs (Seinfeld and Pandis, 2006;Ghosh et al., 2013).Principal reactions via R3 contribute to the destruction of daytime and nighttime O 3 concentrations.During daytime, the newly produced O 3 reacts with NO concentration to reproduce NO 2 concentration.Jenkin and Clemitshaw (2000) attested that R3 occurs at approximately 1 min at a typical boundary layer concentration of 30 ppbv.O 3 only accumulates when the rates of NO 2 photolysis (R1) are higher than those of R3.Nighttime O 3 chemistry is predominantly controlled by R3.However, the mean of nighttime NO concentrations in Kemaman is 1.1 ppb, which is extremely low and contributes to the reduction of NO titration rates.Reduction of NO titrations directly affects nighttime O 3 removal, thereby allowing O 3 to remain in the atmosphere.Consequently, the concentration of NO 2 eventually decreases with the decrease of NO titration, during which NO 2 is produced.Similar mechanism is believed to occur during high nighttime O 3 concentrations that were observed from June 2007 to February 2008 (Fig. S1).During that period, mean for nighttime ozone is 27.46 ppb, while mean NO 2 and NO concentration is 2.71 ppb and 0.80 ppb, respectively.Colls (2002) stated that, other than NO titration, NO 2 is another significant sink agent of nighttime O 3 via the following reactions: At night, the reactions between O 3 and NO 2 yield nitrate (NO 3 ) radicals through R4.The produced NO 3 radicals directly react with NO 2 , producing dinitrogen pentoxide (N 2 O 5 ).Ghosh et al. (2013) argued that R5 is thermally unstable and may disassociate N 2 O 5 back into NO 2 and NO 3 radicals.However, N 2 O 5 may also undergo reaction with H 2 O to form nitric acid (HNO 3 ) (R6).The increase of water vapor at nighttime indicated by high relative humidity (Fig. 5) enhances R6.HNO 3 is highly soluble in water; thus, it 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008  can be easily removed from the atmosphere via precipitation; in this case, HNO 3 becomes acid rain (Colls, 2002).The mean of nighttime NO 2 concentration in Kemaman was only approximately 4.6 ppb because of low NO titration rates and low emissions.Limited nighttime NO 2 concentrations directly reduce the reaction rates of R4 and R5.This study observed that interrupted removal reactions allowed O 3 to remain in the atmosphere.Fig. 5 exhibits that nighttime O 3 concentrations were still decreasing mainly because of dry deposition and some additional chemical reactions with hydrocarbons trough reaction with NO 3 (Jenkin and Clemitshaw, 2000).However, in the current studies effect of hydrocarbons is unconsidered due to limitation of monitoring data and understanding.Differed from other years, relatively higher nighttime NO 2 concentrations is measured on 2005 (Fig. 6).Mean and maximum NO 2 concentrations on 2005 is 22.58 ppb and 96 ppb, respectively.Result suggested that increasing in nighttime NO 2 concentrations will eventually enhancing R4, R5 and R6 reactions thus promote destructions of nighttime O 3 .The mean concentrations of nighttime O 3 on 2005 is 12.51 ppb which is relatively smaller than average value for 12 years (14 ppb).
In summary, NO x plays an important role in nighttime O 3 chemistry.In particular, low nighttime NO x concentration in Kemaman evidently decreased O 3 removal rates, thereby allowing O 3 to remain at high concentration at night.This finding concurs with the observations reported by Ghosh et al. (2013) regarding the importance of O 3 -NO-NO 2 chemistry in nighttime O 3 .Jenkin and Clemitshaw (2002) mentioned that nighttime O 3 chemistry is predominately controlled by NO 3 and N 2 O 5 concentrations; such control ceases at sunrise.
The observed high nighttime O 3 concentration in Kemaman was chiefly attributed by low nighttime NO x concentration, which acts as the main component in the pollutants sinking process.During the 12-year study period, both NO 2 and NO were relatively at very low concentrations, except in 2005.MAAQG sets the limit against NO 2 at 170 ppb, and the United States Environmental Protection Agency sets 100 ppb for 1-hour averaging.However, the mean concentrations of NO 2 in Kemaman ranged from 3 ppb to 22 ppb, and the maximum concentrations ranged from 22 ppb to 96 ppb (Fig. 6).Both values are far below the set limit.Meanwhile, NO concentration was considered at a minimal although no standard or guideline limit has been set against this particular pollutant.The mean concentrations of NO were relatively lower than those of NO 2 , whereas in most years, the maximum concentrations of NO were higher than those of NO 2 .The daily average of NO 2 in Kemaman is higher than that in Jerantut and is relatively similar to Sibu, Kota Kinabalu, and Tawau (Table 2).The NO 2 concentrations in urban cities such as Shah Alam, Kuching, Klang, Ipoh, and Johor Bahru were distinctively higher than those in Kemaman.Meanwhile, the daily average of NO concentration in Kemaman was the lowest among the selected locations listed in Table 2. Sousa et al. (2011) claimed among several meteorological parameters which always been associated with ozone variations, wind speed and directions is the only significant factors that mays contributed to variations in nighttime ozone.Unlike daytime, cease in solar radiations during nighttime attributed to insignificant effect of UVB radiations, temperature and relative humidity towards ozone concentrations.The scenario is occur due relationship between ozone, temperature and relative humidity are depended on sunlight.However, wind speed and direction are significant agent that controlling ozone transport and dilution (Ghazali et al., 2010;Kim and Guldman, 2011;Toh et al., 2013) in both daytime and nighttime.Ghozdic et al. (2011) reported that lower wind speed is always associated with higher concentration of ozone and as well as other primary air pollutions.In abs Fig. S4 (see the Supplementary Material, SM) shows the monthly wind rose plot of nighttime wind speed and direction in Kemaman from January to December.In January, southerly and southwesterly winds accounted for more than 20% of wind directions, in which the majority of the winds blew between 3 and 6 km/h.Similar trends were also exhibited by the wind rose plot in February and March, in which most winds blew from the southerly and southwesterly directions.A slight change in wind direction was observed in April.More than 30% of the dominant wind direction came from southerly and southeasterly direction.During the second month of the first intermonsoon, most of the winds blew from the southerly and southeasterly direction (12%) but relatively higher wind speeds between 9 and 12 km/h were measured from northeast.Higher wind speeds blowing from north and northwest directions and up to 18 km/h were observed during the southwest monsoon, which Kemaman annually endures from June to September.However, during this monsoon, south and southeast dominate the wind blowing direction.Meanwhile, during the second intermonsoon (October-November), south and southeasterly is the dominant wind blowing direction.In December, southerly becomes the dominant wind blowing direction at Kemaman.Fig. S5(a) and Fig. S5(b) illustrated composite plots of monthly average O 3 , NO 2 and NO concentrations from 1999 to 2010. Figure suggested that highest monthly nighttime ozone is observed on January coincided with major air coming from southerly directions.Meanwhile, high nighttime ozone was also observed on August with average concentration of 18.35 ppb.On August wind speed relatively lower with most of wind is in the range of 3 to 6 km/h and may contributed to lower nighttime ozone transports.Low nighttime ozone also observed on April (10.32 ppb) coincided with relatively low wind speed which majorly blowing from south directions.However, on April, relatively higher NO concentrations (1.31 ppb) were measured which is believed significantly control nighttime ozone destructions via NO titrations reactions (R3).Similar conditions is most probably occurred on October.Wind factor is considered relevant in the transport of air pollutants (Kim and Guldmann, 2011;Toh et al., 2013).Lower wind speed may contribute to high nighttime O 3 concentration because of low transport emissions of air pollutants, which allows O 3 to remain at one particular locations.The transport of air pollutants from other locations may also be one of the contributors of O 3 concentrations.The result of this study suggests that, high nighttime O 3 concentrations were observed in January that coincided with prevailing winds from the southerly direction.Kuantan City is located in the south of Kemaman and may influence the air pollution in the district via pollution transport.Kuantan is one of the biggest cities in east coast Peninsular Malaysia and characterized by high population and traffic density, numbers of industrials and residential establishments.From wind rose analysis (Fig. S4), the main direction of wind during northeast

CONCLUSION
O 3 production at ground level is highly influenced by its precursors which is oxides of nitrogen.In this study, the occurrences of high nighttime O 3 in Kemaman, Terengganu were explored in terms of the contribution of NO x concentration and effect of wind speed and direction.The mean concentration of nighttime O 3 in Kemaman was 14 ppb, which was relatively higher than that in the major cities of Malaysia such as Kajang, Johor Bahru, Gombak, and Seberang Jaya.High nighttime O 3 concentrations were observed in from June 2007 to Febuary 2008 mainly attributed by lower NO titrations that capable in removing ozone from atmosphere.Whereas the lowest nighttime concentration was measured on April and October anually, which coincided with the intermonsoon period.This study discovered that low concentration of NO with mean concentration of 1.1 ppb significantly reduced NO titration reactions during nighttime in Kemaman.Lower rate of NO titration minimized the O 3 sinking process, which allowed high O 3 concentrations to remain in the atmosphere.Nighttime NO 2 might also contribute to high nighttime O 3 .Significantly low NO 2 concentrations measured in Kemaman might cause the reduction in O 3 to sink via acid nitric production, inducing O 3 to remain in the atmosphere.Meanwhile, a small possibility of pollutant transport governed by wind speed and direction affected the nighttime condition in Kemaman.However, the role of NO x concentration as nighttime O 3 sinking agent was the primary element that contributed to the high nighttime O 3 concentrations in Kemaman.

Fig. 6 .
Fig. 6.The mean and maximum concentrations of NO 2 and NO during 1999-2010.

Fig. S1
Fig. S1 Time series plot of yearly nighttime ozone variations at Kemaman during 1999-2010

Table 1 .
; Banan et Comparison of mean nighttime ozone between this study and reported studies.
*median concentrations were used.

Table 2 .
Comparison of mean NO 2 and NO between Kemaman and several locations in Malaysia.