Analysis of Diurnal and Seasonal Behavior of Surface Ozone and Its Precursors ( NOx ) at a Semi-Arid Rural Site in Southern India

Surface measurements of O3, NO, NO2 and NOx have been made over a semi-arid rural site, Anantapur (14.62°N; 77.65°E; 331 m asl) in southern India, during January-December 2010. The highest monthly mean O3 concentration was observed in April (56.1 ± 9.9 ppbv) and the lowest in August (28.5 ± 7.4), with an annual mean of 40.7 ± 8.7 ppbv for the observation period. Seasonal variations in O3 concentrations were the highest during the summer (70.2 ± 6.9 ppbv), and lowest during the monsoon season (20.0 ± 4.7 ppbv), with an annual mean of 40.7 ± 8.7 ppbv. In contrast, higher NOx values appeared in the winter (12.8 ± 0.8 ppbv) followed by the summer season (10.9 ± 0.7 ppbv), while lower values appeared in the monsoon season (3.7 ± 0.5 ppbv). The results for O3, NO and NO2 indicate that the level of oxidant concentration ([OX] = NO2 + O3) at a given location is the sum of NOx-independent “regional contribution” (background level of O3) and linearly NOx-dependent “local contribution”. The O3 concentration shows a significant positive correlation with temperature, and a negative correlation with both wind speed and relative humidity. In contrast, NOx have a significant positive correlation with humidity and wind speed, and negative correlation with temperature. The slope between [BC] and [O3] suggests that every 1 μg/m increase in black carbon aerosol mass concentration causes a reduction of 4.7 μg/m in the surface ozone concentration. A comparative study using satellite data shows that annual mean values of tropospheric ozone contributes 12% of total ozone, while near surface ozone contributes 82% of tropospheric ozone. The monthly mean variation of tropospheric ozone is similar to that tropospheric NO2, with a correlation coefficient of +0.80.


INTRODUCTION
Industrialization, urbanization, rapid traffic growth and increasing levels of anthropogenic emissions have resulted in a substantial deterioration of air quality over Asia (Levy et al., 1999;Streets and Waldhoff 1999;Yienger et al., 2000).A recent study by Akimoto (2003) indicates that the NO x emission rate from Asia now exceeds the amounts emitted in North America and Europe and the trend is expected to continue over the next two decades.In contrast to the thinning of the ozone (O 3 ) layer in the stratosphere, the O 3 burden in the troposphere is generally increasing because of increasing emissions of precursors such as nitrogen oxides NO x (NO + NO 2 ) and volatile organic compounds (VOC).NO x and O 3 levels in urban air and amounts of nitrate in the aerosol particles over South Asia indicate that NO x levels are not negligible (Lal et al., 2000;United Nations Environment Programme (UNEP), 2002).
Tropospheric ozone is one of the important greenhouse gases and contributes to global warming and climate change (IPCC, 2007;Kulkarni et al., 2011).Tropospheric ozone is an important air pollutant threatening human health and vegetation growth (Lippmann, 2009).Moreover, it is also one of the key species affecting the chemical properties of the atmosphere (Sitch et al., 2007).Variations in ozone concentration are controlled by a number of processes including photochemistry, physical/chemical removal, and transport, which occur on local, regional and global scales.At the surface, ozone is a secondary air pollutant because its formation occurs in the presence of sunlight and its precursors, e.g., nitrogen oxide (NO x ) and volatile organic compounds (VOCs) (Tsuang et al., 2009).Recent studies suggest that long term exposure to O 3 has been shown to increase risk of death from respiratory illness, adverse effects on the human health and well-being of children exposed to air pollution (Bates, 1995).Besides, O 3 is a greenhouse gas, affecting climate (Staehelin et al., 2001).
In addition to emissions of precursors to O 3 into the troposphere, NO x are also emitted, or produced in, the troposphere (Jallad and Jallad, 2010).Nitrogen monoxide (NO) is emitted from soils and natural fires and formed insitu in the troposphere from lightning, and is emitted from combustion processes such as vehicle emissions and fossil fueled power plants (Guenther et al., 1995;Placet et al., 2000;Sawyer et al., 2000).NO is short lived because it oxidizes to produce NO 2 which plays a major role in O 3 production (Jallad and Jallad, 2010).It is clear that more efforts are needed to understand the spatial and temporal evolution of surface ozone and regional NO x over India with respect to the increasing demand of energy consumptions, rapid urbanization and efforts to stabilize the increasing level of greenhouse gas concentrations as well as improve the air quality of this region.
The study presented in this paper aimed at evaluating the O 3 pollution levels in association with its precursors, in the University area of Anantapur (14.62°N,77.65°E,331 m asl), a tropical semi-arid station in south India, as recorded during the period of 1 year, from January to December, 2010.The objectives of this work were: to monitor ambient tropospheric levels of O 3 and its precursors and to compare the annual average values reported from different sites in India (Lal et al., 2000;Naja and Lal, 2002;Nair et al., 2002;Naja et al., 2003;Ghude et al., 2008;Reddy et al., 2010;David and Nair, 2011), to assess their health effects, and study their diurnal and seasonal trends.First-cut results on the temporal behavior of near-surface ozone at this location have already been published by Ahammed et al. (2006) and Reddy et al. (2008a).The major focus of the present study is on the interdependence of ozone and its major precursor, NO x .Furthermore, the variations in [O 3 ] and [NO x ] with meteorological parameters and comparison between [O 3 ], [NO x ] and [BC] were demonstrated and analyzed.Lastly, comparison of monthly variations of near surface ozone to the monthly variations in tropospheric ozone, NO 2 and total ozone are also examined by using satellite based data.

STUDY AREA AND METEOROLOGY
The measurements have been carried out at the Department of Physics, Sri Krishnadevaraya University (SKU) campus, Anantapur, India.The geographical location of the experimental site is shown in Fig. 1.Anantapur located in southern India represents a very dry semi-arid, rain shadow and continental region of Rayalaseema, Andhra Pradesh, India.Within a 50 km radius, this region is surrounded by a number of cement plants, lime kilns, slab polishing and brick making units.These industries, the national highways (NH 7 and NH 205) and the town area are situated in the north to southwest side of the sampling site.The study area is also at a short distance from two major capital cities, about 200 km away from Bangalore and approximately 400 km away from Hyderabad and is close to the highway that surrounds the city.The production of black carbon is mainly due to the biomass burning together with heavy traffic (mainly diesel vehicles).The old part of the city has rather narrow streets responsible of heavy traffic in some areas of the city, especially at rush hours.
The continental conditions prevailing at this site are responsible for large seasonal temperature differences, providing hot summers (March-May) and cool winters (December-February).Most of the rainfall occurs during the monsoon (southwest monsoon; June-September) and post monsoon (northeast monsoon; October and November) seasons.During the year 2010, the annual rainfall in the study area is 805 mm (see Table 1) by which it secures least rainfall when compared to adjoining places in Rayalaseema region and other parts of Andhra Pradesh.The rainfall during the southwest monsoon period is 619 mm, which forms more than 77% of the total annual rainfall, whereas 186 mm only for the northeast monsoon period, which forms 23% of annual rainfall for the study period.Daily mean wind speed (WS), wind direction (WD), air temperature (AT), relative humidity (RH) and total rainfall (RF) have been obtained from automatic weather station (AWS) which is installed in the observation site near to the department.The statistical data for the monthly mean prevailing meteorological conditions during the period of study over the experimental site is shown in Table 1.The monthly mean wind speed was maximum during the month of June, which is around 3.4 m/s and minimum in February ~1 m/s followed by January which is about 1.1 m/s.Most of the winds are prevailing from southwesterly direction during the total observation period.The monthly average maximum (minimum) AT of about 34.2°C (26°C) was noticed in the month of April (August) and maximum RH (minimum) of around 71.0% (27.0%) is observed in August (April).

INSTRUMENTATION
Surface ozone is measured using an analyzer (O 3 41 M; Environment S.A., France) based on absorption of Ultraviolet (UV) radiation at 253.7 nm by ozone molecules.Contribution by other species in the absorption and scattering of the radiation in the cell is eliminated by comparing the measurement with ozone free air in reference mode.Systematic and regular measurements of surface ozone have been made at Anantapur with the ozone analyzer.The instrument is kept at a height of ~12 m from the ground level and a five meter long Teflon tube is arranged as the intake tube for taking the outside air sample, with a particulate filter to prevent particles from entering the instrument.An inverted Teflon funnel is fitted at the entrance of the tube to avoid the dust and rainwater  from entering the tube and the systems.A mercury lamp is the source of this radiation that is absorbed by the ozone molecules present in the ambient air filled in the absorption cell of length of about 60 cm.The absorbed signal as well as the reference signal is measured by a detector.Ozone concentration is estimated using the Beer-Lambert law.This is kept at constant temperature and, based on the lamp intensity (by changing the current to the UV lamp), the concentration of O 3 varies.The O 3 determination is based on a commercial instrument using UV mercury absorption of 253.7 nm radiation.The calibration factor is not required in this process.The detector is employed before and after the absorption takes place in the fixed length flow path, so the variations in the intensity of the light are balanced.In order to check the zero reading of the analyzer, zero air has to be admitted which is free of ozone.If the analyzer reads higher value for zero air, then the ozone scrubber in the analyzer will be changed.Also the analyzer scrubber continually checks the zero every 10 sec and goes to the sample line.These are reformed for 15 min each on a daily basis for an initial period of 1 year and then once in every 3-4 weeks afterwards.The minimum detection limit of the analyzer is about 1 ppbv and its response is about 10 sec (Reddy et al., 2008b).Lal et al. (1998) and Nair et al. (2002) have also employed the same O 3 analyzer described above in their study.The absolute accuracy of these types of system is reported to be 5% (Kleinman et al., 1994).NO, NO 2 and NO x were continuously measured using an ambient analyzer (Model APNA-370, HORIBA, and Germany).The APNA-370 uses a combination of the dual cross flow modulation type chemiluminescence principle and the referential calculation method.This gives it the advantages of the single-detector method plus the ability to do continuous measurements of NO x , NO, and NO 2 .The design gives great stability and extremely high sensitivity (0.1 ppm).Standard equipment includes a drier unit with an automatic recycle function to provide dry ambient air as the ozone source.This makes long-term continuous measurements possible.The detector uses a silicon photodiode sensor to reduce size and prolong working life.All the necessary features are built right into a single racksized unit, including a reference-gas generator, an ozonesource drier unit, an ozone decomposer, and a sampling pump.No supplemental gas is required.In order to acquire stable, accurate data, we have been perform calibration when starting measurements and at regular intervals by using certified standard gas cylinders, as well as with the certified permeation tubes of fixed permeation rate (concentration), which can be placed in a heated permeation bench in the analyzer and maintained at constant temperature.
In order to understanding the comparison of near surface ozone to tropospheric and total ozone, the satellite data have been used for this study.The data collected by the Ozone Monitoring Instrument (OMI) and the Microwave Limb Sounder (MLS).The OMI and MLS instruments were launched in July 2004 on the board the Aura spacecraft into a polar Sun-synchronous orbit (Bovensmann et al., 1999).OMI is a nadir scanning instrument that detects back scattered solar radiance over 270-500 nm to measure column O 3 with near global coverage at a resolution of 13 km × 24 km at nadir (Levelt et al., 2006) and a swath width of 2600 km.Total column ozone is derived from OMI using the Total Ozone Mapping Spectrometer (TOMS) algorithm (version 8.5).MLS provides measurements of stratospheric column ozone by the standard method of pressure integration of ozone volume mixing ratio (version 2.2).Tropospheric column ozone is determined using the residual method, which involves subtracting measurements of MLS stratospheric column ozone from OMI total column ozone after adjusting for the intercalibration differences of the two instruments using the convective cloud differential method (Ziemke et al., 2006).Total ozone with a resolution of 0.25° × 0.25° can be obtained online at http://toms.gsfc.nasa.gov/pub/omi/data, and tropospheric ozone with a resolution of 1° latitude × 1.25° longitude is archived at ftp://jwocky.gsfc.nasa.gov/pub/ccd/data_monthly_new.The precision uncertainty for derived gridded ozone is 5 Dobson units (DU) (7 ppbv), with a mean offset of 2 DU (OMI and MLS) (Ziemke et al., 2009).
The SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) instrument on board the ENVISAT satellite provides the capability for global retrieval of atmospheric NO 2 columns through observation of global backscatter (Bovensmann et al., 1999).The satellite was launched in March 2002 into a Sunsynchronous orbit, crossing the equator at about 10:00 LT in the descending node.The SCIAMACHY instrument observes the atmosphere in the nadir view with a typical surface spatial resolution of 30 km along track by 60 km across track.Global coverage is achieved every 6 days.

Diurnal and Seasonal Variations of O 3 , NO, NO 2 , and NO x
The average concentrations of surface level NO, NO 2 , NO x , and O 3 in the study area were observed to be 3.0 ± 0.5, 2.1 ± 0.6, 5.1 ± 0.7 and 40.7 ± 3.1 ppbv respectively, during the study period.These concentrations are different as reported from other locations (eg., Beig et al., 2007;David and Nair, 2011;Singla et al., 2011) in India (Table 2).These variations are mainly due to the differences in the concentrations of precursor gases, chemical processes, anthropogenic activities prevailing in the concerned areas and meteorological parameters (Reddy et al., 2010).The NO 2 concentrations were less than NO concentrations, indicating a lower oxidizing level of the environment in this region.The concentrations of NO and O 3 were higher during day time while opposite trends exist for NO 2 and NO x .This is mainly attributed to the photochemical reactions of NO x and O 3 and the balance between the emissions (natural and anthropogenic) and mixing processes of both horizontal and vertical convections.(Lal et al., 2000;Mazzeo et al., 2005;Tu et al., 2007).
The diurnal cycle of NO, NO 2 and NO x are shaped like double waves with similar diurnal patterns observed in NO x and NO 2 .The morning peak is higher in magnitude than the late evening peak.The morning peak of NO 2 appears 1 hour after the NO peak and the O 3 peak appears 6 hours after NO peak and 5 hours after NO 2 peak.This difference reflects the time taken for conversion of NO to NO 2 involving NMHC.During early morning hours (03:00-07:00 h), the NO x concentrations at the observation site increased rapidly, which is due to the photochemical processes and emissions-dilution balance of NO x and O 3 , reflecting increased emissions of motor vehicles during the morning rush hours and also from industrial activities.The newly emitted NO could react with O 3 without solar radiation and producing more NO 2 and reducing O 3 concentrations.During the noon hours, the solar radiation increased greatly and the photochemical processes that produce O 3 dominated, especially after the sunrise.Oxygen atoms produced in the photolysis of NO 2 could react with O 2 and to produce O 3 through the chemical reactions which are given below.
where M (usually N 2 or O 2 ) represents a molecule that absorbs the excess vibrational energy that there by stabilizes the O 3 molecule formed, h represents the energy of photon (with > 0.424 µm).O 3 showed peak values when NO x had the lowest concentrations at around 14:00 h.During this time, NO x accumulations were not significant because of high NO x photochemical consumption and increased air mass dilution as the height of the boundary layer increases.The boundary layer height should reach a maximum in the afternoon and additional venting of the boundary layer by convection.After this, the photochemical production of O 3 decreased while NO 2 level increased.The reduction in O 3 level is mainly due to the decrease of solar radiation which then would lower the level of the photochemical production.Finally during the night hours, the NO x and O 3 were maintained balance quickly; as there was no solar radiation and both the source emission and dilution effects decreased significantly.Furthermore, the increasing traffic in the evening hours usually starts from 18:00-22:00 h.A combination of more stable atmospheric conditions and more vehicle emissions causes to second peak (late night peak) of the precursors from the late evening to early night.
The seasonal variations in pollutant concentrations can be associated with changes in synoptic wind patterns, prevailing air mass type and seasonal differences in solar radiation which is shown in Fig. 3. O 3 showed a well defined seasonal variation pattern on a diurnal scale with high levels (70.2 ± 6.9 ppbv) during the summer and low (20.0 ± 4.7 ppbv) during the monsoon with an annual mean of 40.7 ± 8.6 ppbv.This pattern is mainly attributed to the more intense solar radiation and higher temperatures often associated with the presence of weak, slow moving and persistent high pressure systems, which favor the photochemical production of O 3 (Logan, 1989).All these conditions are satisfied over the observation site during the summer, resulting in observed extensive ozone utmost.During the monsoon period, insufficient sunshine for photochemical ozone production and rain washout of pollutants cause stumpy ozone levels.And also the observation site, where the measurements were carried out, experiences 619 mm of rainfall out of the total annual rainfall of about 805 mm during the monsoon season.It is also essential to note that stumpy O 3 levels is perceived in all seasons during the morning hours for the reason that the lower boundary layer height largely reduces the mixing process between the ozone-poor surface layer and ozone-rich upper layer.The increase of ozone in the troposphere is the result of increase in human produced ozone precursor's emissions.
NO x levels attain high levels during the winter (12.8 ± 0.8 ppbv) followed by the summer (10.9 ± 0.7 ppbv) seasons as they get transported through northeasterly wind flow from the polluted regions.During the winter season, the pollutants emitted from various anthropogenic and natural sources, are trapped in the boundary layer due to frequent temperature inversions, while in the summer months, this polluted air mixes well with the free tropospheric air causing dilution of the pollutants and the greater photochemical reactions due to the higher solar radiation.A stumpy level of NO x during the monsoon and the postmonsoon seasons is due to intense cloud formation and frequent rainfall activity over the measurement station, resulting in the washout of pollutants precursors.
The variations of hourly average daytime and night time concentrations of NO 2 and O 3 with the NO x levels are shown in Fig. 4. Daytime and night time values were obtained using data averages, recorded between 08:00-20:00 h and 20:00-08:00 h.It is depicted that the mean concentrations of O 3 decreases with increase in [NO x ], while NO and NO 2 levels increase with [NO x ].The largest concentration of NO, NO 2 and O 3 during daytime were 6.1 ± 0.3, 4.3 ± 0.4 and 57.0 ± 4.2 ppbv and during nighttime 7.0 ± 0.2, 8.7 ± 0.5 and 37.0 ± 2.1 ppbv, respectively.These concentrations indicate that the highest mean concentrations of O 3 during daytime were greater than nighttime.While NO and NO 2 noticed higher mean concentrations at nighttime than in daytime period.

Local and Regional Contributions to Oxidant Concentration
The mean variations in daytime and nighttime of oxidant concentrations (

Weekday and Weekend Analysis
In order to investigate the weekday-weekend analysis over the measurements site, the authors divided the total sampling period into two parts: weekdays (WD, Monday-Friday) and weekends (WE, Saturday and Sunday).NO x showed a difference in both weekdays and weekends.Compared with weekends (WE), the concentrations of NO, NO 2 and NO x were higher on weekdays (WD), while an opposite trend was observed in the case of [OX] where concentrations were lower during weekdays (see Fig. 8).This is in consistence with the results obtained from the earlier studies carried out in other locations as reported by Han et al. (2011), Qin et al. (2004a) and Song et al. (2011).This variation is mainly due to characteristics features of the experimental site, e.g., variations in direct emissions of NO 2 owing to variations in local driving conditions or vehicular fleet composition; local sources of biogenic hydrocarbons which can amplify photochemically induced NO to NO 2 conversion under VOC limited conditions and overnight  accumulation of free radical precursors.The output from diesel vehicles daily running on the nearby national highways, not only contains more NO x generally, but also likely to have a higher proportion of the NO x as NO 2 (Clapp and Jenkin, 2001).In addition, the average diurnal variation on weekdays was greater for NO than for NO 2 .This is because NO 2 has a longer lifetime than the more reactive NO (Debaje and Kakade, 2006).Fig. 8 also shows that daily variation of the mean values of the NO x and [OX] levels during weekdays and weekends.The average maximum value of [OX] at weekends was higher than weekdays.This type of temporal variability presented here can also be found in other cities (Mayer, 1999).

Influence of Meteorological Factors on [O 3 ] and [NO x ]
Meteorological conditions play an important role in ozone formation, transfer and dispersion.Variations of local meteorological conditions, such as solar radiation, wind speed, direction, rainfall and relative humidity can greatly affect the temporal variation of ozone.The hourly average concentrations of O 3 and NO x with air temperature and relative humidity (RH) are shown in Fig. 9.The relationship between [O 3 ] and temperature is well known and the highest average ozone concentration recorded at a temperature of about 34°C during the study period.As the temperature decreases, the average concentration of O 3 decreases, which shows ozone, is strongly correlated with temperature.In contrast, the lowest average concentrations of NO x occur at the highest temperature which is 34°C.With the decreasing temperature, the average [NO x ] increases.However, there are some differences among the precursors.NO shows the highest values at temperature lower than or equal to 23°C during the study period.Therefore the precursors have significant negative correlation with temperature and positive correlation with RH.The diurnal changes in meteorological parameters reflect the diurnal changes in solar radiation and boundary layer stability, which have significant influence on photochemical reactions and air pollutant dispersion.

Comparison of [O 3 ], [BC] and [NO x ]
Depending on the reaction mechanisms and reaction probabilities, soot (black carbon, BC) may significantly influence the tropospheric trace gas chemistry by its surface reactions.If an ozone molecule collides with an active site on the surface of carbon sample, one of its oxygen atoms get adsorbed while the resultant oxygen molecule is liberated.The adsorbed oxygen atom can then combine with another adsorbed oxygen atom to form molecular oxygen.This catalytic reaction causes dramatic ozone depletion in the atmosphere (Fendel et al., 1995).Fig. 10 clearly shows that the minimum O 3 values correspond to maximum values of BC aerosols and NO x mixing ratios and vice-versa.This generally happens at 07:00 h local time, a period characterized by windless condition and less stratified boundary layer.For these transition hours, the measurements show a build-up of BC while ozone is titrated away by the reaction with NO.
Further the authors have made an attempt to study the correlation between tropospheric ozone and BC aerosols and the analysis suggests an inverse relation between BC and tropospheric ozone (see Fig. 11).It shows that a negative correlation between [BC] and surface ozone with a correlation coefficient (R) of 0.64.The slope between the black carbon aerosols and tropospheric ozone has been found to be -4.7 suggesting that an increase of every 1 μg/m 3 black carbon aerosol mass concentrations causes a reduction of 4.7 μg/m 3 in surface ozone concentration.This reduction value of surface ozone at Anantapur is somewhat high compared to Hyderabad (3.5 μg/m 3 ) (Latha and Badarinath, 2004).This reduction is due to the aggregate structure of soot particles, which offer a large specific area for heterogeneous interactions with relative trace gases like ozone (Fendal et al., 1995).Emissions from two-stroke engines (Crutzen and Andrea, 1990) contain VOC and NO x ratios higher than emissions from four-stroke engines or diesel vehicles (Pitts and Pitts, 2000a).

Effect of Water Vapour on Surface Ozone
Water vapor content in the atmosphere also plays a crucial role in the production and destruction of ozone.Since RH depends on temperature and is anti-correlated (see Fig. 9).Instead of RH, we use absolute water vapor content for the present study.From simultaneous measurements of temperature and RH, water vapor content (ρ v ) (g/m 3 ) can be estimated using the following empirical relation (Kneizys et al., 1980;David and Nair, 2011): where ρ s is the saturation density of water vapor at ambient temperature, T o = 273.15K, t is temperature in °C, R v is the gas constant for water vapor (4.615 × 10 −3 mbar g/m 3 /K) and P is the total pressure in mbar.Monthly variation of ρ v along with surface ozone was shown in Fig. 12.It is clearly seen that increase of ρ v is associated with decrease in ozone in the atmosphere and attain high values during the monsoon season.This can cause depletion of ozone through reactions involving OH radicals, for which water vapor is the source (David and Nair, 2011).

Comparison of Monthly Mean Variation of near Surface, Tropospheric and Total Ozone
A comparative study was also made by using a satellite data discussed in section 3 over the observation site on the monthly mean variations of near-surface ozone, tropospheric ozone, tropospheric NO 2 and total ozone.The data corresponding to the grid containing the observation site were used.Total columnar ozone and tropospheric NO 2 data are available on daily basis and tropospheric ozone data are available for monthly basis from MLS and OMI.The monthly mean near surface ozone and tropospheric ozone are shown in Figs.13(a) and 13(b).These two are having similar variations with low values from June through September and increasing trend is observed from December onward, reaching peak observed in summer months (April).
The monthly mean variation of tropospheric NO 2 exhibits similar to tropospheric ozone (See Fig. 13(c)).Moreover a positive correlation between tropospheric ozone and tropospheric NO 2 is clear visible in the scatter plot in Fig. 14 with a correlation coefficient of 0.80.Fig. 13(d) shows a monthly mean variation of total ozone, which is opposite pattern of the tropospheric and surface ozone variations.The monthly mean total ozone increased from March and reached a peak in May, remained high until October and decreased in November Onwards to till February.This variation is mainly due to the strong photochemical activity combined with vertical transport processes characteristics of tropics and is well understood (Staehelin et al., 2001).It has been established that the tropics is a region of intense photochemistry and strong vertical motion of air masses associated with convective activity (Holton et al., 1995).The vertical motions enable the mixing and transport of trace gases and aerosols from the lower troposphere into the upper troposphere and lower stratosphere (UTLS) region.
Recent studies on the climatology of vertical winds at altitudes of 4-22 km at the tropical site of Gadanki (13.5°N, 79.2°E) using the mesosphere-stratosphere-troposphere radar have shown that the months from April to September are characterized by strong vertical winds in the UTLS region (Thampi et al., 2009).The reduction in tropospheric ozone, surface ozone, and tropospheric NO 2 during this period (Figs. 13(a) to 13(c)) may be partly associated with this strong vertical motion.

CONCLUSIONS
Simultaneous and continuous measurements of surface ozone (O 3 ) and its precursors NO, NO 2 and NO x have been carried out at a semi arid rural site, Anantapur in southern  In contrast, NO x have significant positive correlation with relative humidity and wind speed and negative correlation with temperature.5.The slope between the black carbon aerosols and tropospheric ozone has been found to be -4.7 suggesting that every 1 μg/m 3 increases in black carbon aerosol mass concentration causes a reduction of 4.7 μg/m 3 surface ozone concentration.6.A comparative study by using satellite data shows that annual mean values of tropospheric ozone contributes 12% of total ozone and near surface ozone contributes to 82% of tropospheric ozone.

Fig. 1 .
Fig. 1.Location map of (top panel) the Sri Krishnadevaraya University (SKU) campus area in Anantapur (bottom panel) satellite aerial view of monitoring site building in the SKU campus indicated with an arrow head.

Fig. 2
shows the observed hourly mean diurnal pattern for pollutant concentrations of O 3 , NO, NO 2 and NO x .The diurnal variations of O 3 at this sampling site were characterized by high concentration (25-95 ppbv) during the daytime and low concentration (5-27 ppbv) during the late evening and early morning hours.The minimum O 3

Fig. 2 .
Fig. 2. Hourly variations in NO, NO 2 , NO x , and surface O 3 at Anantapur during the study period.

Fig. 3 .
Fig. 3. Seasonal variations of O 3 and NO x on a diurnal scale over Anantapur.
Fig. 4. Variation of daytime mean values of NO, NO 2 and O 3 with the level of NO x .

Fig. 5 .
Fig.5.It is possible to consider that the [OX] at a given location has a NO x -independent, and a NO x -dependent contributions.The former is effectively a regional contribution which equates to the regional background of [O 3 ] level, whereas the latter is effectively a local contribution which correlates with the level of primary pollution.Due to the influence of the photochemical reactions on the formation of [OX], differences should be arising between values observed during daytime and nighttime.During nighttime, NO x -dependent (local) contribution is more compared to daytime is mainly attributed to high emission of NO 2 from heavy duty diesel trucks over the observation site.The data also indicate that the regional contribution to the oxidant depends on the wind direction.The slope of the relationship between [OX] and [NO x ] representative of the local contribution to the oxidant concentration and the value of [OX] when [NO x ] = 0 representative of the regional contribution to the oxidant concentration.Based on the photostationary state, it is possible to infer an expected variation in daytime mean [NO 2 ]/[OX] values with [NO x ].This variation is shown in Fig. 6 along with the fitted empirical expression.The data shows that a progressively greater proportion of [OX] is in the form of NO 2 as the level of NO x increases.Low level ratios of [NO 2 ]/[OX] due to the high concentration of O 3 during daytime.On the other hand, during nighttime it is expected the complete conversion of NO and O 3 to NO 2 .If photochemical processes have an influence on [OX] levels in polluted areas, then a difference between the behavior of [OX] during daytime and nighttime would be expected.Fig. 7 shows the diurnal variations of mean values of [OX] concentration.This variation is similar to the variation of [O 3 ].It shows a mid-day peak and lower nighttime concentrations.The [OX] slowly rises after sunrise, reaches a maximum during day, and then decrease until the next morning.This is due to photochemical O 3 formation.Hence we suggest that the [OX] level at this site was less influenced by NO x emissions.

Fig. 5 .
Fig. 5. Variation of daytime (left panel) and nighttime (right panel) mean values of [OX] with level of [NO x ].

Fig. 9 .Fig. 10 .
Fig. 9. Variation in [O 3 ] and [NO x ] with air temperature (Temp) and relative humidity (RH) for the present study period prevailing over the measurement site.

Fig. 14 .
Fig. 14.Scattered plot of monthly mean values of tropospheric ozone with tropospheric NO 2 .

Table 1 .
Statistical data of monthly mean meteorological parameters with ± 1 σ variation noticed at Anantapur during January-December 2010.

Table 2 .
Comparison of monthly mean minimum and maximum levels of surface ozone (O 3 ) and oxides of nitrogen (NO x ) measured at different locations in India.