ANALYSIS OF THE CORRELATIONS BETWEEN NO , NO 2 AND O 3 CONCENTRATIONS IN CAMPO GRANDE – MS , BRAZIL

Ozone (O3) is a secondary gaseous pollutant in the urban environment, and its variation correlates well with nitrogen oxides (NOx = NO + NO2). Continuous monitoring has been done in the Campo Grande city urban area, using ozone 49C and NO-NO2-NOx 42CTL gas analyzers. The results show that the maximum concentrations of O3 and oxidant (Ox = O3 + NO2) in Campo Grande often appear in the early afternoon around 15:00 hours. The daily variation of NO concentrations shows a very clear cycle with two peaks, one appearing around 07:00 a.m. and the other at 11:00 p.m. At the lowest level, NO2 is the main component of NOx, while NO dominates the higher mixing ratio. It is also shown that the level of Ox is composed of two factors: the regional and the local contributions. The former is affected by the regional O3 level, while the latter is effectively correlated with the primary pollution level. The daily variation also appears in the concentrations of oxidant components.


INTRODUCTION
One of the main problems caused by air pollution in urban areas is the presence of photochemical oxidizers.Among these, ozone (O3) and nitrogen dioxide (NO2) are particularly important since they are susceptible of provoking adverse effects on human health (OMS, 2000).The formation of ozone at ground level depends on the intensity of the solar radiation, the absolute concentration of NOx and the VOCs (Volatile Organic Compounds), and the ratio between NOx and VOCs.
Various observations have shown that, in shiny days, the ozone concentration increases with the growing intensities of solar radiation and temperature.3][4] It is necessary, hence, to search for a complete comprehension of the relationships involving concentrations of O3, NO and NO2 under different atmospheric conditions.Different authors, [5][6][7] studied the relationship between the environmental levels of O3, NO and NO2 to improve the comprehension of their chemical coupling.
There is not an official network for air quality monitoring installed in the city of Campo Grande.4] It has been developed, recently, the first version of a CO and NOx inventory of emissions. 8 the present study, the values of environmental concentration of O3, NO, NO2 and NOx, continually measured in the city of Campo Grande, are used to investigate, for the first time for this town, the concentration of oxidizers (Ox, O3 and NO2) along with NOx.This analysis shall contribute to a better understanding of atmospheric sources of Ox for this specific urban area.The relationships that were found to describe the concentrations of O3, NO and NO2 are in agreement with the current knowledge of their chemical coupling.

Studied and observational data
Campo Grande is the capital city of South Mato Grosso (MS) state, located in the southern of Brazil Midwest region, sited in the center of the state.Geographically the city is near to the Brazilian border with Paraguay and Bolivia.It is located at 20°26'34'' South latitude and 54°38'47'' West longitude.It occupies a total area of 8,096.051km² or 3,126 mi², representing 2.26% of the total state area, within 860,000 inhabitants (2016) and a corresponding HDI of 0.78.The urban area is approximately 154.45 km² or 60 mi², where tropical climate and dry seasons predominate, with two defined seasons: warm and humid in summer, and less rainy and mild temperatures in winter.During the months of the winter, the temperature can drop considerably, arriving in certain occasions to the thermal sensation of 0ºC or 32ºF with occasional and light freezing.The year average precipitation is 1,534 millimeters, with small up or down variations.The main pollution problems in the city are attributed to the traffic of vehicles, to the raise of building

Ensemble of observational data
The air quality and meteorological variables are monitored by an automatic station operated by the Institute of Physics of the Federal University of South Mato Grosso (UFMS).This station is located on the University campus, 8 km or 5 miles to the west of downtown.The main sources of pollution in that area are the building activities; therefore, there are no significant precursor sources of ozone identified close to the region.The ozone levels of Campo Grande area are stored in a regular database since 2004.The equipment of measurement was installed at the top of a tower from where air samples are extracted throughout vertical pipes that are placed approximately 2 meters above the ground level.The concentrations values for O3, NOx, NO and NO2, the ultraviolet (UV) radiation and other meteorological features, such as the air temperature, relative humidity, wind speed and direction are values measured systematically.
The concentrations of pollutants NO, NO2, NOx (NO+NO2) and O3, were measured continuously during a one-year period (2015).The equipments used for measurements included a nitrogen oxide analyzer (AC31Musing chemiluminescence method), an ozone analyzer (O341M-LCD/UV Photometry).All equipments were made by Environnement S.A.

Hourly variation of O3, NO, NO2, and NOx concentrations
The average per day variation observed for the NO, NO2, NOx and O3 concentrations are exhibited in figure 1.The daily cycle of ozone concentration reaches a peak during the middle day and presents smaller concentrations during the night.The ozone concentration slowly increases after the first rays of sun shining, getting to its maximum value during the daylight period, after which it starts to decrease slowly until the next morning.
The variation pattern of the average concentration of gases was well defined during the analyzed period, and the pronounced peak of the concentration of NO in the first hour of the morning was due to photolysis of the nocturnal NO2 accumulated after sunset (NO + O3 → NO2 + O2), both for NO and O3 values are minimal at this time (Figure 01).After sunset, a concentration of NO increases slowly.This occurred because of the variation of the emission of NO due to the change of temperature and humidity 16 .Other mechanisms of heterogeneous reactions during day and night may be important, although few known, such as the influence of humidity, temperature and precipitation 17 .The NO2 (Figure 1) clearly shows that not only the lowest concentration values occur around midday, but also the lower variability in this time, which can be explained by Figure 02, where they have a mean daily value higher global solar radiation and NO photolysis, which reaches maximum values and lower zenith angle.This causes a greater penetration of the solar rays (<400 nm), responsible for the photolysis of the NO2, since, due to the strong UV radiation, has high photochemical activity, in addition, there is the later oxidation of NO2 by part of the hydroxyl radical (OH) (NO2 + OH → HNO3), which is the main mechanism of NOx loss 18 .
The levels of NO2 observed in the atmosphere showed a general tendency in which they tend to decrease in the period of the day of greatest insolation and to grow at the end of the day (Figure 1).This fact suggests that the photochemical processes are the main mechanisms to remove NO2 from the atmosphere with formation of byproducts.The model that best explains this fact is the consumption of NO2 by direct photochemical reactions generating ozone.The formed O3 reacts with water vapor and generates HO radicals, this radical is the main chemical species involved in the NO2 consumption, with the production of HNO3, the reaction of formation of nitric acid is significant in the region under study, mainly due to the conditions Favorable conditions of insolation during a large part of the year, the presence of oxidants in the atmosphere is determinant for the formation of NO2 in the atmosphere, from the NO emitted by the combustion processes.The production of NO2 occurs mainly by the reaction of NO with O3 [19][20] .286 Such a variation is due mainly to the photochemical formation and meteorological conditions.The effect of the increasing solar radiation that occurs between 08:00 and 14:00 / 15:00, along with the height of the mixture layer entails the reduction of NOx concentration and a raising in the concentration of O3.The simultaneous measurement of O3 and UV during the daylight period (from 07:00 to 19:00, see Figure 2) shows that the concentration of O3 is strongly correlated to the UV irradiance (W/m²).The daily cycles of O3 and UV flux are similar, with the maximum O3 occurring at 14:00, i.e., approximately 1 or 2 hours after the maximum UV flux.The statistical analysis reveals that the correlation between O3 concentration and UV is significant with a correlation coefficient (R) of 0.79.
While O3 and a large percentage of NO2 are secondary contaminants, NO is a primary contaminant, formed through a complex set of chemical reactions.At 07:00, the sun light begins to induce a series of photochemical reactions.NO is converted in NO2 through a reaction with O3.During the shining hours, NO2 has converted again into NO because of photolysis, which induces the regeneration of O3.
Another factor influencing the atmospheric pollutant concentrations is the height of the mixture layer over the city.In a shiny day, pollutants are diluted when the mixture layer increases during the day and stays limited to the inside of NPBL during the night.Emitted pollutants, like NO, are kept underneath such an inversion, and it can cause an increase of the hourly average concentration of NOx overnight.

Chemistry of O3, NO and NO2
The basic chemistry that leads to the production and destruction of ozone has been detailed elsewhere. 26These equations form a cycle free of liquid chemistry, i.e., the global effect of reaction ( 2) is the opposite of reaction (1).Such reactions represent, therefore, a closed system for which the components NOx (NO and NO2) and Ox (O3 and O2) are referred separately.Along the daytime, concentrations of NO, NO2 and O3 remain typically balanced during a period of a few minutes.Such a situation is known as a photocatalytic state.The concentrations of NO, NO2 and O3 are related through the following equation: where t2 is the NO2 photolysis rate, and k1 is the coefficient of reaction between NO and O3.
The variation of t2/k1 average value along the time was obtained using the observed measurements of NO, NO2 and O3 is exhibited in Figure 3.The average value of t2/k1 is 16.9, with a minimum of 11.9 and maximum of 22.4 g m -³.The maximum value occurs typically at 08:00 hours.
The k1 coefficient varies as a function of temperature (T).Sienfeld and Pandis, 27 proposed the following equation for k1: As expected, the variation of k1 is similar to the variation of the average air temperature.

Diurnal variation of Ox
The difference between the diurnal and nocturnal Ox behavior shall be expected if the photochemical processes have any influence on the Ox levels in polluted areas.Figure 6 shows the daily variation of the average concentration value of Ox taken at intervals of 1 hour.The concentration of Ox, likewise the variation of O3 concentration, exhibits a peak at noon and lower concentrations during the night.The concentration of Ox slowly increases after the sun rises, attaining a maximum value during the day and, in the sequence, decreases until the next morning.This is due to the photochemical formation of O3. Figure 7 shows the daily variation of NO2/Ox.The differences on the partition of NO2 and O3 can be related to the rate of chemical processes or with the available time for them to occur.12.The total value of Ox raised with NOx, and the data have been linearly adjusted.Due to the influence of photochemical reactions on the O3 formation, differences between values and linearly adjusted equations of day and night have been found.It is noticeable that the total local Ox has a contribution independent of NOx and another one that is dependent.The first part is a regional contribution comparable to the regional level of O3, while the last term is a local contribution effectively correlated to the level of primary pollution.It has been verified that the dependent local contribution of NOx to Ox during the night is 37% lower than during the day.

Local and regional contributions to the oxidant
However, the approximately 75 g m -3 regional contribution is almost equivalent during the day and night.The regional contribution to O3 is consistent with the values observed by. 5,7 his result implies that the problem of air quality in Campo Grande is not just a local question, especially the street pollution, but also a regional issue of Campo Grande County.The territorial aspect observed for the analysis of O3 pollution is consistent with the results reported by. 5,24

CONCLUSION
The present results indicate that the diurnal cycle of ozone concentration has a peak around noon and smaller nocturnal concentrations.The ozone concentration slowly increases after the sun rising, reaching a maximum value during the daytime and, in the sequence, decreases until the next morning.This is due to the photochemical formation of O3.The shape and amplitude of ozone cycles are strongly influenced by meteorological conditions (solar radiation) and by the prevailing levels of precursors (NOx).At the studied region, the daily cycle of NO concentration stem from vehicle emissions and its conversion to NO2 has a huge impact on the daily cycle of ozone levels.A linear correlation between NO2 and NOx has also been determined, as well as between NO and NOx, while a polynomial correlation between O3 and NO2/NO has been found.These forms can be useful for the strategies of O3 provision and efforts of air pollution control.The Ox level is influenced by independent and dependent contributions of NO2.The first one is due to the regional concentration of O3, and the last one is correlated with the local level of primary pollution.

Figure 1 .
Figure 1.Average of measured values for a daily period of NO, NO2, NOx, O3 and Ox concentrations.The interval between measurements equals 1 hour.

Figure 2 .
Figure 2. Average concentrations of O3 and UV irradiance for the studied period.

NO2
molecule absorbing the excess of vibrational energy and thus stabilizing the O3 molecule that has been formed, normally it is N2 or O2; h represents the photon energy, with a 424 nm wavelength; and O is an active monoatomic molecule of oxygen.

Figure 4 5 )Figure 3 .Figure 4 .Figure 5 .
Figure 4 presents the variation of daily concentration of O3 as a function of the ratio [NO2]/[NO], sampled hourly.The level of O3 increases with the raise of [NO2]/[NO].According to Figure 4, the concentration of O3 increases rapidly for small values of [NO2]/[NO].For levels that are more elevated, the concentration of O3 gets close to reach a photocatalytic state, when it remains relatively stable.In our study, we adjust these data to a polynomial function of ln([NO2]/[NO]) that can be used to predict the concentration of O3 during the day:

Figure 6 .
Figure 6.Daily variation of Ox average values.

Figure 8 .Figure 9 .
Figure 8. Daily variation of NO2/Ox average values as a function of NOx level.When our data were adjusted to a linear function, the obtained correlation was weak.(a)

Figure 10 .
Figure 10.The average values of NOx plotted as a function of (a) NO2 and (b) NO for the daytime period.

Figure 11 .
Figure 11.The average values of NOx plotted as a function of (a) NO2 and (b) NO for the nighttime period.

Figure 12 .
Figure 12.Variation of daily average NOx concentration plotted as a function of Ox.