Analysis of Ground Level O3 and Nox Measured at Kannur, India

Air pollution will be an important and challenging problem in the coming decades that will affect sustainable development of nations [1-3]. The rapid growths in industrial activities due to globalization result in vast amounts of potentially harmful gases and particles being emitted into the atmosphere on a global scale [4]. Prominent air pollutants are sulphur compounds (SO2 and H2SO4) nitrogen compounds (NOx and NH3), carbon monoxide (CO) and organic compounds (hydrocarbons, VOCs, halogenated compounds). These trace gas species in the atmosphere produce other secondary pollutants via thermal, chemical and photochemical pathways [5,6]. Surface O3 is identified as one of the air pollutants that effect human health and the ecosystem [7-9]. Surface level O3 concentration has steadily increased since the 19th century [10,11]. The sources of surface O3 are in-situ photochemical production, transport from other in-situ production sites and downward transport from the stratosphere [12-15].


Introduction
Air pollution will be an important and challenging problem in the coming decades that will affect sustainable development of nations [1][2][3]. The rapid growths in industrial activities due to globalization result in vast amounts of potentially harmful gases and particles being emitted into the atmosphere on a global scale [4]. Prominent air pollutants are sulphur compounds (SO 2 and H 2 SO 4 ) nitrogen compounds (NO x and NH 3 ), carbon monoxide (CO) and organic compounds (hydrocarbons, VOCs, halogenated compounds). These trace gas species in the atmosphere produce other secondary pollutants via thermal, chemical and photochemical pathways [5,6]. Surface O 3 is identified as one of the air pollutants that effect human health and the ecosystem [7][8][9]. Surface level O 3 concentration has steadily increased since the 19th century [10,11]. The sources of surface O 3 are in-situ photochemical production, transport from other in-situ production sites and downward transport from the stratosphere [12][13][14][15].
The photochemical production of O 3 is enhanced by the chemistry involving CO, CH 4 , Non-Methane Hydro Carbons (NMHC) and other Volatile Organic Compounds (VOC) and occurs in the presence of sufficient amount of NO x and depends on the concentrations of hydroxyl and other radicals which act as catalysts [16][17][18][19][20][21]. NO x and VOCs are emitted from anthropogenic sources such as fossil fuel combustion, power plants, industrial activities and transportation, as well as from natural sources such as lightning, and vegetation (biogenic VOCs such as isoprene). Recently, elevated concentrations of O 3 have been observed even over large rural areas in developed countries due to anthropogenic activities and biogenic emissions where industrial activities are limited [22][23][24][25][26]. Two mechanisms have been proposed to account for the high rural O 3 [27][28][29]. One is the transport of O 3 from urban areas, and the other is the transport of its precursors like NO x and non-methane hydrocarbons [30], followed by photochemical O 3 production. Ozone is a strong oxidizer in the atmosphere and its chemistry is complex. However, the relationship between O 3 and its precursors represents one of the major scientific challenges associated with urban air pollution [31,32]. Further, O 3 being a strong greenhouse gas, increasing levels of O 3 can warm the troposphere, and induce significant changes in the climate over an extended period of time [33,34]. Thus, a long-term investigation of surface O 3 and its precursors is essential to understand the ambient air quality over a location. Meteorological parameters including wind speed, wind direction, temperature, solar radiation and relative humidity have substantial influences on O 3 concentrations [35,36]. Generally, O 3 is observed to be higher in both winter and summer at many measurement sites over the globe [37].
Surface O 3 in the atmosphere has been monitored sporadically at various sites around the globe for the past several decades [38,39] Many studies have been carried out on the photochemical characteristics of O 3 and its precursor gases in the United States [40,41] and most networks in Europe were implemented at the beginning of 1980s. Atmospheric concentrations of O 3 and its precursors like NOx and VOCs are increasing rapidly in Southeast Asia as a result of the fast growth of the transportation and industrialization [42] found that the transport sector contributed 37% of the NO x emissions, while 27% came from power generation and 18% from industry. Measured and modeled O 3 , NOx and VOCs in Shanghai, China by [43] shows that O 3 concentrations are higher in rural area than in center of the city, suggesting that there are O 3 depression processes in center of city and air pollutant emissions are not favorable for the O 3 chemical formation. The studies on the significant role of O 3 influenced on the ambient air quality were reported by [44,45]. Further [46] found that Surface O 3 at a coastal site in Hong Kong is strongly influenced by the outflow of Asian continental air during the winter and the inflow of maritime air from the subtropics in the summer. Modeling studies have been emerged as one of the promising methods to evaluate the production and transport of O 3 confined in a region. Such studies were carried out by many groups [47][48][49][50] which were quite promising.
In the Indian continent, available observations [51][52][53][54] and modeling studies [32,55] show higher O 3 levels during spring winter and summer extending until May. Such variations over India are due to higher levels of precursors and the availability of abundant solar radiation flux during these seasons. In India, various groups have been involved in carrying out long-term measurements of surface O 3 and its precursors; their observations are quite relevant in understanding the regional air quality [28,32,45,52,[56][57][58][59][60][61][62][63][64][65][66]. In order to explore O 3 chemistry and its transport at Kannur, a tropical coastal site in the state of Kerala in India, a continuous observation has been on-going with the support of Atmospheric Chemistry Transport and Modelling (AT-CTM) project under Geosphere Biosphere Programme (GBP) of the Indian Space Research Organization (ISRO). This study describes the key observations of both seasonal and diurnal variations of surface O 3 and its precursor NO x and their variations with meteorological parameters. The location is rural with limited industrial activities. The observation of ambient air quality over this location for a period of one year from November 2009 to October 2010 revealed that the seasonal variation of O 3 was influenced by meteorology and air mass movement. This manuscript begins with a brief introduction.

Monitoring site
The location of the sampling site at Kannur University campus (KUC) (11.9 º N, 75.4 º E 5m) is shown in figure 1. Observation site is in the northern part of Kannur district in Kerala state which is surrounded on one side by a river basin and the other side by the Arabian Sea. This site is in the valley of the mountains of Western Ghats. It is a rural location with no major industrial activities except a few small-scale industries such as plywood and mattress manufacturing units. The distance to the seashore to the west is 4 km and that to the Western Ghats on the east is 50 km. The land area of Kannur is about 3000 km 2 with an average population density of 1000 per km 2 . KUC is situated in an open land which receives plenty of sunshine (peak average of 1 ± 0.3 KWhr/m 2 ) throughout the day, and the land is surrounded with green vegetation throughout the year.

Meteorological background
A prominent meteorological characteristic of this location is the intense monsoon rainfall occurring in two spells every year. The southwest monsoon is active during the months of June-August, and this intense rainfall has been classified as the Monsoon Season. This is followed by the return monsoon or northeast monsoon in November. Hence September, October and November are earmarked as the postmonsoon (autumn) season with scattered showers accompanied by heavy thunder and lightning. The intensity of summer is masked by the southwest monsoon season over the entire region. About 80% of the total rainfall occurs from June to August which constitutes the main monsoon season. The months, December, January and February with scant rainfall and relatively low humidity constitute the winter season. From March to May the season is summer with scorching heat and high convective flow. Thus, we have Easterly wind during winter months from December to February, the Westerly and South-Westerly winds during summer, and the monsoon and post-monsoon season. The period from January to March records the maximum sunshine hours of more than 9.1 hours/day and between June and August records the minimum sunshine due to cloudy skies. Figure 2 shows the monthly variations of meteorological (wind speed, temperature, relative humidity and rainfall) in Kannur during the period of observation. The temperature was high between March and May and was low during June through August. The average monthly high temperature ranged from 29.6 to 37.1ºC and low temperature ranged from 22.9 to 25.8ºC.
The maximum humidity was measured during monsoon (June, July, August) and minimum was observed during winter months (December, January, February). The maximum monthly average relative humidity ranged from 55.5% to 88.4% and minimum of 45% to 80% at this location. The maximum rainfall was recorded during monsoon, while minimum was observed during the winter season. Figure 3 shows the monthly mean wind speed and wind direction at 1000 hPa during the study period at the observation site. The wind speed is steady during monsoon season and minimum during winter season. It was pretty high during the period from June to September and low from December to April. The maximum average wind speed ranged from 2.4 to 5.9 km per hour and the minimum from 1.3 to 4 km/hr during the period of observation.
The meteorological parameters like temperature, relative humidity, wind speed and wind direction were collected from the local automatic weather station, which is one of the stations managed by the Meteorological and Oceanographic Satellite Data Archival Centre (MOSDAC) and established by the Indian Space Research Organization (ISRO).

Measurement techniques
The concentrations of surface O 3 and NO x were measured simultaneously with the aid of respective gas analyzers from Environment S.A., France. The ambient air collecting sampling inlet hood was placed at a height of 6 m from the ground and data were continuously recorded from November 2009. The concentrations of O 3 were obtained using the ozone analyzer (Model O 3 42M) and the details of the analyzer and its principle were described elsewhere [67][68][69] Calibration of the analyzer was performed at regular intervals using ozone free air and pure ozone produced by the appropriate gas generators in the analyzer. It has a lower detectable limit of 0.4 ppbv with a minimum response time of 20 seconds. This analyzer is provided with an in-built correction for temperature and pressure variations as well as intensity fluctuations of the light source. Similarly total NO x was monitored with the aid of a standard chemiluminescent analyzer (Model AC32M). The method was found to have higher sensitivity and specificity [70][71][72]. The lower detectable limit was 0.4 ppbv with a response time of 30s. The calibration of the analyzer was carried out using reference gas cylinders.

Diurnal variation of ozone
Eight hour averaged (09:00-17:00 h) surface O 3 concentration measured from November 2009 to October 2010 on a day-to-day basis is shown in figure 4. Despite considerable short-term variations, a seasonal change was seen for the entire period. Among the maximum mixing ratio of O 3 , eight-hour average highest and lowest values were observed in December (46.9 ppbv) and in July (16.4ppbv). From the figure 4, it is clear that O 3 levels were observed to be the highest in winter and lowest during the monsoon season.
The enhanced O 3 mixing ratio observed during winter season was mainly due to the easterly air flow that favours advection of precursors The daytime increase in O 3 mixing ratio is mainly from the photooxidation of industrial and anthropogenic hydrocarbons, carbon monoxide, and methane in the presence of sufficient amount of NO x [58,73,74] With the onset of sunshine, O 3 concentration gradually increases and it reaches a maximum value at 14:00 h noontime, due to its photochemical formation through the photolysis of NO 2 via the following set of reactions.
It was observed that the mixing ratio of O 3 started to decline after 16:30 h in the evening on all days. The low concentration of O 3 observed during nighttime is due to the absence of photolysis of NO 2 and loss of O 3 by NO through the following titration reaction and surface deposition.
Loss of O 3 also occurs due to both dry and wet deposition that results in a minimum concentration at sun rise. [28] Have observed a similar diurnal variation at Gadanki, a rural site in south-east India (13.5 º N, 79.20E, 375m asl). To substantiate the annual surface O 3 variation, seasonal average daytime, nighttime O 3 mixing ratios and the rate of change during morning and evening are presented in (Table.1).
From the table 1, it is evident that the observed higher O 3 mixing ratios were in December (46.9 ppbv) while lower mixing ratios were in July (16.4 ppbv). This may be due to the variations in precursor gases and the influence of changing meteorological parameters. The daytime high average O 3 mixing ratio observed in December (31.9 ± 4.7) ppbv was due to the lower boundary layer that occurs in winter and the low relative humidity. The daytime low average O 3 mixing ratio was in July (11.0 ± 3.4) ppbv, which is mainly due to the intense southwest monsoon which removes pollution over this location. The highest rate of O 3 increase (9.2ppbv/h) was observed in December between 08:00-11:00 h Indian Standard Time (IST) and lowest (2.4ppbv/h) was found in August. The O 3 removal rate was the highest in December (-14.0ppbv/h) between 17:00-19:00 h IST and the lowest was in June (-2.4ppbv/h).  [25]. Similar patterns of seasonal variations have also been observed at Gadanki [28] Anantapur [75] and Trivandrum [66] in south India. The minimum and maximum mixing ratios of O 3 found at this site during monsoon were (2.3 ppbv, 18.5 ppbv) and post monsoon were (3.3 ppbv, 28.0 ppbv) respectively. From this it is evident that O 3 exhibits a distinct seasonal variation over this site, which may not only be controlled by solar radiation, but also by atmospheric dynamics as well. There is also a possibility of enhanced transport of O 3 from the stratosphere during the winter season [64,74]. Further the active photochemistry of NO x under clear sky conditions in winter season would enhance the O 3 abundance.

Seasonal variation of O 3
This in turn offers a suitable environment for the production of added O 3 during winter season at this site. Moreover, the meteorological parameters have a strong influence on the seasonal changes in the concentration of O 3 . An enhancement in the boundary layer height and increased cloud cover over this location during summer induce a relatively small mixing ratio observed in summer than in winter. A relatively low amount of surface O 3 was observed during monsoon seasons than summer and winter seasons. During the monsoon period, the absence of sunshine and high relative humidity reduce the O 3 concentration at this location. Seasonal average variations of O 3 during day time and night time are shown in (Table 2).
From the table 2, it is clear that daytime average O 3 concentration became a maximum (29.2 ±4.4) ppbv during winter while nighttime O 3 became higher during summer (5.3 ±1.8) ppbv due to the land air mass influence in winter and oceanic influence in summer as observed from the backward trajectory depicted in figure 14 of Air trajectories during the period of observation. The minimum and maximum observed mixing ratios of O 3 during winter were 2.9 ppbv, 44.0 ppbv and that during summer was 33.4ppbv, 4.1 ppbv respectively. From this it is evident that O 3 variation is not only controlled by solar radiation, but also by atmospheric dynamics. The favorable conditions for photochemical production of O 3 are satisfied at KUC during winter months. Moreover, the meteorological parameters have a strong influence on the seasonal changes in the concentration of O 3 . A relatively small mixing ratio observed in summer than in winter is due to high air temperature induced convective activities and increased cloud cover. The minimum and maximum observed mixing ratios of O 3 during monsoon were 2.3 ppbv, 18.5 ppbv and that for post monsoon were 3.3 ppbv, 28.0 ppbv respectively.  and NO x observed at KUC. The morning peak of NO 2 appears 2h after the NO peak. Subsequently, after the morning peak, NO diminishes until it reaches its lowest value at 15:00 h and has a gradual increase till midnight. During nighttime, surface emission of NO were limited inside the nocturnal planetary boundary layer, and NO reached its second highest peak at midnight. Likewise, a decline in NO 2 mixing ratio observed during daytime was due to the enhanced photolysis by which O 3 is formed at this site. It was observed that NO x mixing ratio started declining from (3.1 ± 0.28) ppbv at 09:00 h IST and it became a minimum of (1.53 ± 0.26) ppbv at 16:00h and began to regain in magnitude during nighttime. This reduction in NO x concentration boosts the production of O 3 through photolysis induced by sunlight in daytime. At night, the relatively low air temperature near the surface could prevent the vertical dispersion of NO x , contributing to its accumulation and resulting in high nighttime mixing ratio. Higher levels of NO x during early morning and night hours are due to the   combination of anthropogenic emissions, boundary layer processes, chemistry as well as local surface wind pattern [28,75]. During nighttime, the boundary layer descends and remains low till early morning, hence pollutants get trapped in the shallow surface layer and show higher concentration. In order to attain high levels of O 3 during noon, large amounts of precursors are needed, and this may leads to low levels of NO x during daytime [57].

Diurnal and seasonal variations of NO, NO 2 and NO x
At this site, the annual average mixing ratio of NO x is found to be about 2.5ppbv, which is relatively small compared to that of rural sites, where mixing ratios can be much higher [28]. The diurnal variation of NO x for different months is shown in figure 8. The maximum and minimum concentrations of NO x observed at KUC were (3.9 ± 0.5) ppbv and (1.0 ± 0.3) ppbv in December and January respectively. The maximum rate of change of NO x occurs in December in which O 3 concentration is high and the minimum rate is in July in which O 3 concentration is lowest.
The maximum magnitudes of average NO x concentration (2.9 ± 0.8) ppbv were observed in summer and minimum during monsoon (2.3 ± 0.4) ppbv. During winter the maximum average concentration was (2.5 ± 0.6) ppbv and post-monsoon it was (2.7 ± 0.7) ppbv. Thus the concentration of NO x did not show significant variation during the period of observation at this site but only had a small seasonal variation. NO x mixing ratios are generally stronger during early morning and nighttime in all the seasons. The minimum and maximum observed mixing ratios of NO x were 1.3 ppbv, 3.5 ppbv (winter), 1.6 ppbv, 3.3 ppbv (summer), 1.6 ppbv, 3.2 ppbv (monsoon) and 1.5 ppbv, 3.4 ppbv (post monsoon).

Analysis of O 3 , NO, NO 2 and NO x
The basic process leading to the photochemical formation of O 3 in the lower troposphere and the polluted urban atmosphere involves the photolysis of NO 2 to yield NO and a ground-state oxygen atom. O 3 is formed and destroyed in a series of reactions involving NO and NO 2 as described by (R1), (R2) and (R3) in Diurnal variation of ozone where M denotes a third body like a nitrogen or oxygen molecule that absorbs the excess vibrational energy and eventually stabilizes the O 3 molecule thus formed. In the absence of other chemical species, these three reactions govern O 3 concentration. Figure 9 (a) represents the variation of O 3 concentration with NO mixing ratio within a sample interval of 15 minutes. It is evident that O 3 concentration diminishes as NO mixing ratio increased, which may be due to the removal of O 3 by titration with NO. The most appropriate fit to this distribution is exponential and the empirical relation is Thus, a fast decay in O 3 concentration is observed at high NO mixing ratios from late evening to early morning. Further, the daytime variation between O 3 and NO 2 observed on the maximum number of clear sky days available during winter mornings is shown in figure 9(b). A linear fit with excellent correlation with a significance level (p < 0.01)   (2) Figure 10 (a) represents the variation of NO 2 and NO x using the mean observed data. A linear fit with a good correlation is obtained and the observed empirical relationship is and it shows the active competition between NO 2 and NO x on the chemistry of O 3 at this location.
The correlation between NO and NO x shown in figure 10 ( (4) and this enhanced influence of NO mixing ratio is the primary reason for the nighttime reduction in O 3 concentration at this site. However, the wet and dry depositions of O 3 to land and marine surfaces are significant due to the proximity of Arabian Sea and with fairly good vegetation in the area. Figure 11 shows a negative linear correlation between daytime O 3 and NO x observed during the whole period of observation suggesting the possible VOC-sensitive characteristics of the study location as described by [76,77] and the linear relation is as The coefficient of this correlation is -0.74, which is quite significant.
The organic emissions from nearby wetlands and biogenic VOCs from vegetation make this location VOC sensitive assuming that the higher O 3 levels appear when the VOC concentrations increase. Fishing boats and ship emissions are other strong contributors of organic species over the Arabian Sea [78]. The significant contribution to NO 2 , SO 2 , Black Carbon (BC) and Organic Compounds (OC) is large over coastal zones. Secondary species formed from fishing boat emissions have large chemical lifetimes and are transported in the atmosphere over several hundreds of kilometers. Thus, they can contribute to air quality problems on land [79]. In general, the emission perturbation is most effective in increasing ozone formation over regions with low background pollution [19]. The region in the sea which is adjacent to our sampling location is an estuary with a good catchment area.

Respirable Dust High Volume Air Sampler (ENVIROTECH, India
Model APM 460 NL) at this site and the chemical analysis employing gas chromatography revealed the presence of several organic species during the day time and is listed ( Table 3).
Thus the presence of various organic species detected over this location substantiates the existence of a large amount of VOCs in this region arising from both anthropogenic and biological origin. Since quantifications of VOC were not available on site, it was not possible to investigate the complex chemistry between VOC-O 3 -NO x in detail. However, the analysis of NO x and O 3 was conducted with the understanding that O 3 production was influenced by NO x concentration. Nevertheless, a detailed analysis of VOC-O 3 -NO x chemistry is to be explored through future investigations.

Diurnal variation of OX and the correlation between [NO 2 ]/ [OX] with [NO x ]
Surface O 3 and NO 2 are inextricably linked due to their strong chemical combination. Therefore, the response to reductions in the nitrogen oxides emissions is remarkably not linear [80] and any resultant reduction in the level of nitrogen dioxide is invariably accompanied by an increase in the concentration of ozone. In addition, its interplay with VOCs represents a complex mechanism whose detailed characterization requires a significant number of measurements of source emissions and meteorological variables. At night, O 3 reacts with NO to form NO 2 , which further reacts with O 3 to yield NO 3 and N 2 O 5 and in daytime in the presence of sunlight NO 3 and N 2 O 5 reconvert to NO 2 and O 3 , these compounds may be grouped as a single chemical family, odd oxygen or OX [81][82][83] Thus, OX (nocturnal) = O 3 +NO 2 + 2NO 3 + HNO 4 +3N 2 O 5 (R4) For daytime, OX is only the sum of NO 2 and O 3 since the daytime abundances of NO 3 , HNO 4 and N 2 O 5 are quite meager. The interconversion of O 3 , NO and NO 2 under atmospheric conditions can be summarized by the primary reactions (R1), (R2) and (R3) mentioned above. These equations constitute a cycle. The overall effect of reactions (R1) and (R2) is the reverse of Reaction (R3). These reactions represent a closed system in which the NO x and the OX (O 3 +NO 2 ) components are related. In addition, the increasing ozone background concentration influences on the local levels of O 3 and NO 2 and on the efficiency of local emission controls. As a result, different authors [6,[84][85][86][87][88][89][90] studied the relationships between ambient levels of NO and NO 2 , O 3 in order to improve the understanding of their chemical coupling. Figure 12 (a) shows the diurnal variation of OX with one sigma standard deviation. Similar to the variation of O 3 , OX concentration shows a mid day maximum and minimum during night. The OX concentration gradually increases after sunrise and reaches a maximum at 1500 hrs in the afternoon and decreases slowly and reaches a minimum during night time. The photochemical activities are quite intense during midday hours, the O 3 production is higher while the concentrations of NO 2 and NO starts declining by which OX shows a maximum during noon time. A key factor to be considered on the photochemical activity and the behaviour of atmospheric pollutants including OX is the mixing processes caused by the heating of the ground layer and the variation in height of the planetary boundary layer. Vertical convection is a principal mechanism for transporting air from close to the surface to the free troposphere [76,91].
The combination of fresh traffic emissions that occurring from the nearby highway during the early morning, mixed with aged pollutants due to convective activity and the variation of the height in the boundary layer throughout the day, and the atmospheric cycle between O 3 , NO and NO 2 with intense photochemical production of O 3 at these hours leads to a daily maximum of OX around noon time. Further, the level of OX, an oxidizing level indicator of the NO x , O 3 regime, could be increased in two ways: (1) the net increase of NO 2 through NO x emissions and then oxidization of NO to NO 2 by radical species such as RO 2 (where R is a hydrogen atom or carbon containing fragment), OH etc. [5] and (2) Where J1 is the rate of NO 2 photolysis and k 3 is the rate coefficient for the reaction of NO with O 3 . Coefficient J 1 is a function of solar radiation intensity and k 3 is function of the temperature (T) [76] propose the following expression for k 3 : The variation of the mean values of J 1 /k 3 estimated applying Equation In this scatter plot, points with higher values represent the mixing ratios of NO 2 and O 3 during early morning and late evening of a day. The low values of O 3 and higher values of NO 2 are the main reasons for this. While in the noon time, O 3 is found to be increasing and NO x starts declining. Hence, the points with lower values are found to be in the noon time.

Influence of humidity and temperature on ozone
Comparison between O 3 and meteorological parameters like temperature and humidity during winter, summer, monsoon and post monsoon season has been made at KUC. Figure13 elucidates the relationship between O 3 , relative humidity and temperature for the period of observations. The correlation coefficient between O 3 concentration change with relative humidity, wind speed and temperature were -0.84, -0.85 and 0.87 respectively during the period of observation. In winter, summer, monsoon and post-monsoon, the correlation coefficients between O 3 and humidity were 0.82, -0.85, -0.83 and -0.86 respectively Likewise, the correlation between O 3 and temperature in these seasons were 0.88, 0.86, 0.84 and 0.90 (p< 0.01) respectively [92].
It is evident that O 3 variation is directly correlated to temperature and is inversely related to humidity. Monsoon season is associated with the maximum number of cloudy days and minimum sunshine hours; and it also provides minimum temperature and maximum humidity. Thus, in the presence of low temperature and high humidity, O 3 production is kept minimum. In the post monsoon season, the morning sky is clear followed by few showers in the afternoon. The existence of clear sky after the monsoon offers an environment to build O 3 and aerosols at this location. The positive correlation between O 3 and temperature is due to the fact that the radiation controls the temperature and hence the photolysis efficiency will be higher. When the humidity becomes higher, the major photochemical paths for removal of O 3 will be enhanced. Moreover, higher humidity levels are associated with large cloud cover and atmospheric instability, the photochemical process is slow down and the surface O 3 is depleted by deposition on water droplets in this location, hence the O 3 concentration has a strong dependence on humidity.    Forum, Japan. From this, it is clear that the air mass movement during winter and post monsoon months was from east of this site and confined from west during summer and monsoon seasons. During winter season, long-range transport of air mass contributes to the observed high O 3 levels in addition to its photochemical formation. It is further observed that air masses appear to originate from the eastern part of Kannur that is influenced by the transport of pollutants from nearby industrialized land during winter. While, during post monsoon, short-range air masses at low altitude (500m) appear to originate from the east of this site inducing the transport of precursors, which seems to have little contribution. Hence during post monsoon, in addition to photochemical production, inventories within a short range may effectively contribute O 3 formation at this location. During summer and monsoon seasons, the movement of air mass trajectories originated over the Arabian Sea and traversed through a smaller area of landmass before reaching Kannur. Since, these air masses have a strong marine influence, the observed enhancement of surface O 3 is only at the expense of photochemistry rather than the transport of pollutants. During monsoon season, winds were strong in the westerly direction, back trajectories were of oceanic origin, and marine air mass prevailed over the Indian subcontinent.

Comparison with other measurements in india
Seasonal variation of O 3 observed at KUC show a typical pattern of rural site, which is highly influenced by the seasonal changes, like some other locations in Indian sub continent. Figure15 shows a comparison of monthly average surface O 3 at KUC and other observational sites in India.
From the figure 15, it is evident that Nainital, a high altitude site located in the central Himalaya region and in Delhi, the capital city of India recorded the highest O 3 mixing ratio during late spring. At Anantapur, Pune, Joharapur and Tranquebar O 3 concentrations show distinct maxima in summer season whereas at Dayalbag, Agra and Gadanki it became high during summer/ winter season. At Ahmadabad and Mt. Abu, O 3 concentration reaches its peak during autumn/winter and at Trivandrum [66], a tropical coastal site shows a maximum O 3 concentration during winter/ post monsoon seasons. Among the minimum O 3 mixing ratio recorded at these locations, Agra, Gadanki, Dayalbag and Tranquebar register lower concentrations. It is further observed that the O 3 minima were found during monsoon and post monsoon seasons at all locations except Gadanki, while the maxima changes with season. Variations of surface O 3 concentration recorded at different locations in the Indian subcontinent were mainly due to latitude/longitude variation, population, meteorological parameters, availability of intense solar radiation, differences in the concentrations of precursor gases and anthropogenic activities.

Conclusion
This paper describes the diurnal and seasonal variations of surface O 3 and its precursor NO x observed for one year from November 2009 through October 2010 at Kannur University, a rural site lying along the west coast of India. The higher surface O 3 concentration observed in the midday and lower concentrations during nighttime was in tune with the solar UV flux. A significant seasonal variation for O 3 and NO x mixing ratios at this site was observed. The average O 3 mixing ratios were maximum during winter and minimum during the monsoon period. O 3 mixing ratio was higher in the winter months at KUC. This may be due to the dominant transport of biogenic VOCs in winter, from the Western Ghats lying east of this site since the air mass movement is from that direction. While in summer, in spite of higher solar flux, O 3 concentration was low at this site. This may be due to the cloud cover and the higher abundance of humidity due to proximity with the Arabian Sea. During the southwest monsoon period, O 3 concentration was found to be a minimum and it gradually builds up in the post-monsoon season. The seasonal changes in O 3 levels are found to be different due to the atmospheric pollution levels. We found good correlations between O 3 , NO and NO 2 which revealed the chemistry of O 3 production from NO 2 and titration with NO at this location. Further a linear relationship between NO and NO x and NO 2 and NO x obtained could be useful in unfolding the chemistry of O 3 at this location. The net effect of NO x on O 3 concentrations was negative with a decaying exponential correlation indicating a possible VOC sensitive location, which reveals the prominent role of abundant biogenic VOCs on the production of O 3 even at lower levels of NO 2 present over KUC.
A good correlation between O 3 , temperature and humidity were obtained which shows that the seasonal variation of O 3 has strong dependence on meteorological parameters. This observation further reveals that the marine influence is well defined in this location and other prominent precursors is essential in the O 3 chemistry over this location. These observations are useful in understanding the ambient air quality of a location with high population density and the enhanced anthropogenic activities that lead to deterioration in the air quality. The elevated levels of O 3 in an industrially less developed area can also affect human health. This will be further ascertained through an analysis of indoor air quality in the area. The results of this study are preliminary and need confirmation with more observation to explore the O 3 chemistry and transport with the aid of other gas analyzers like CO, CH 4 and VOC.