G. S. MEENA and D. B. JADHAV
Indian Institute of Tropical Meteorology, Pashan, Pune–411008, India
Corresponding author: G. S. Meena; e–mail: gsm@tropmet.res.in

Received March 21, 2006; accepted January 24, 2007

RESUMEN

El estudio de la variación diurna y estacional de los gases traza atmosféricos es esencial para entender nuestra atmósfera. Para esto, en el período 2000–2003 se realizaron observaciones diarias de luz zenital dispersa utilizando un espectrómetro UV–visible. Para observar las variaciones diurnas de NO₂, O₃, H₂O y O₄, se obtuvieron las densidades de columna inclinada (SCD) desde las mañanas (ángulo zenital solar (SZA) = 90º) hasta las tardes (SZA = 90º). Para el estudio de las variaciones estacionales de NO₂ y O₃, se obtuvieron las densidades de columna vertical (VCD) durante el período mencionado. Para los datos de VCD de NO2 y O3, se encontró una correlación positiva de r = 0.72 para las mañanas y de r = 0.79 para las tardes. Se compararon las observaciones de satélite con las espectroscópicas y se observó una buena correlación. Los valores más altos de VCD para NO₂ y O3 aparecen en los meses de verano (mayo y junio) y los más bajos en los meses de invierno (diciembre y enero). Las VCD vespertinas del NO2 son más altas que las matutinas. Para la ciudad de Puna (18º32' N, 73º51' E) se encontró un cambio estacional interesante: las tasas vespertina/matutina (PM/AM) de NO2 y de las temperaturas máximas/mínimas son mayores en los meses de invierno que en los de verano para todo el período de estudio. En los meses de invierno la tasa PM/AM del NO2 llega hasta 3.8 y la menor para los meses de verano es de 1.25. Durante el día, el N₂O₅ puede fotolizarse para regenerar NO2, lo que se refleja en las tardes. En el invierno las noches son más largas, por lo que durante ellas la conversión de NO₂ en N₂O₅ es mayor y los valores de NO2 son menores en las mañanas, lo que produce una tasa PM/AM elevada. La tasa PM/AM del O₃ es ligeramente más elevada en los meses de invierno que en los de verano.

ABSTRACT

Study of diurnal and seasonal variation of atmospheric trace gases is essential to understand our atmosphere. For this, daily zenith–sky scattered light observations have been made by UV–visible spectrometer during the period 2000–2003. Slant column densities (SCD) from morning (solar zenith angle SZA = 90º) to evening (SZA = 90º) were retrieved to see the diurnal variation of NO₂, O₃, H₂O and O₄. For the study of seasonal behavior of NO₂ and O₃, vertical column densities (VCD) were retrieved during the above period. For the whole period, NO₂ and O₃ VCD are found in a positive correlation of r = 0.72 for the morning data and r = 0.79 for the evening data. Satellites borne observations are compared with the spectroscopic observations, which are found in good correlation. It is seen that highest NO₂ and O₃ VCD are found in summer months (May and June) and lowest in winter months (December and January). Evening NO2 VCD are found higher compared to morning. There is found an interesting seasonal change that at Pune (18º32' N, 73º51' E), the evening–to–morning (PM/AM) ratios of NO₂ as well as temperature maximum/minimum ratios are higher in winter months and lower in summer months during the above period. In winter months NO₂ PM/AM ratio goes up to 3.8 and in summer months lowest ratio is 1.25. During the day, N₂O₅ can be photolyzed to regenerate NO₂, which reflects in the evening hours. In the winter, nights are longest; therefore, during night NO₂ to N₂O₅ conversion is more, hence in the morning NO₂ value will be less that leads to high PM/AM ratio. O₃ PM/AM ratio is slightly higher in winter months compared to summer months.

Keywords: Ozone, nitrogen dioxide, vertical column density, solar zenith angle, diurnal and seasonal variation.

1. Introduction

Tropospheric NO₂ plays a key role in both stratospheric and tropospheric chemistry. In the troposphere the photolysis of NO₂ results in the formation of O₃ (Bradshaw et al., 2000). NO₂ can react with O₃ to form the nitrate radical (NO₃), which is a strong oxidant and plays an important role in NO_x polluted areas at night (Wayne, 1991).

Ozone is one of the most extensively measured trace gases in the atmosphere. Column amounts of O₃ vary with season and altitude. WMO (1990a) has reported that the highest ozone abundances are found at high latitudes in winter and early spring, while the lowest values are located in the tropics. Most ozone is produced at low latitudes, where the solar irradiance is highest. Large day–to–day variations were found to be more at higher latitudes. Latitudinal and seasonal distributions of NO2 have been conducted by Kreher et al. (1995) and by Senne et al. (1996). Daily/seasonal variations in vertical column densities (VCD) of NO₂ and O₃ have been studied using spectroscopic observations (Syed et al., 1981; McKenzie and Johnston, 1982; Bhonde et al., 1992; Solomon et al., 1992; Kondo et al., 1994; Gil et al., 1996; Nichol et al., 1996; Koike et al., 1999; Jadhav et al., 1999; Meena et al., 2004).

Water vapour in the lower stratosphere is a very effective greenhouse gas. The atmospheric water vapour content responds to changes in temperature, microphysical processes and the atmospheric circulation. Water vapour concentration varies substantially in both the vertical and horizontal (Jayaraman and Subbaraya, 1988). Clouds are intimately connected to the water vapour pattern, as clouds occur in connection with high relative humidity, and cloud processes in turn affect the moisture distribution. In the atmospheric physics, O₄ is also an absorber of solar radiation (e.g., Pfeilstickeret al., 1997; Solomon et al., 1998) and toll to infer atmospheric photon path lengths, and cloud heights (e.g., Erle et al., 1995). Pfeilsticker et al. (1997) have reported that O₄ band does not saturate for cloudy skies. Therefore, O₄ can be considered to identify the clouds, which affect the measurements.

In this paper, for the study of diurnal variation differential slant column densities (SCD) of NO₂, O₃, H₂O and O₄ are presented for few days from morning to evening. For the study of diurnal and seasonal variation of NO₂ and O₃, UV–visible spectrometer is operated during the period 2000–2003. Study of diurnal and seasonal variations of NO₂ and O₃ have been made and daily morning/evening variations in NO₂ and O₃ are discussed. The evening to morning (PM/AM) ratios of NO₂ and O₃ are discussed with maximum/minimum temperature ratios during this period. Spectroscopic observations are compared with satellite borne Nimbus–7 Total Ozone Mapping Spectrometer (TOMS) observations.

2. Methodology

Zenith scattered light spectra recorded with the spectrometer are analyzed using differential optical absorption spectroscopy (DOAS) technique (Noxon, 1975; Solomon et al., 1987; Pommereau and Goutail, 1988; Bhonde et al., 1992; Platt, 1994; Londhe et al., 1999; Jadhav, 2002; Meena et al., 2003; Bhosale et al., 2004) to determine vertical column amounts of atmospheric trace gases such as O₃ and NO₂. The DOAS analysis algorithm uses the log ratio of a twilight spectrum to midday one, i.e. in determination of the slant column amount; Lambert–Beer's law is used. In the log ratio spectrum of wavelength range 462–498 nm; NO₂, O₃, H₂O and O₄ have their absorption signatures. SCD of these gases are derived simultaneously by matrix inversion method. To retrieve vertical column density (VCD), it is necessary to divide the SCD by a factor. For large solar zenith angle (SZA) sunlight passes nearly tangentially through the atmosphere before being scattered down to ground and the absorption path enhances by a factor called air mass factor (AMF).

AMF for different SZA are calculated using single scattering radiative transfer (RT) model (Solomon et al., 1987; Meena et al., 2003). In order to retrieve VCD, there are two approaches. The first approach is to retrieve ozone vertical columns that plots the observed differential slant column densities (SCD_diff) against the corresponding AMF (called a Langley plot). Then the slope of the line is equal to the VCD, as shown in equation (1). The Y–intercept should be equal to the negative of the column density of an absorber in the reference spectrum (RCD):

Equation (1) can be rearranged to solve for VCD at particular SZA, θ:                  

In equation (1) and (2), AMF(θ) is the air mass factor at SZA θ.

The second approach is based on the assumption that the vertical column in the background measurement is the same as in the actual measurement, and the differences in vertical columns are only due to changes in light path. With this assumption, equation (1) can be written as:

appropriate for NO₂ because NO₂ concentrations change during twilight. Thus for NO₂, equation (3) can be applied at 90º SZA.

3. Observations

Zenith–sky spectra (462–498 nm) are recorded daily by UV–visible spectrometer. Full day observations, from morning (SZA = 90º) to evening (SZA = 90º) on 18, 19, 21, 22 and 23 October, 2000, have been carried out to see the diurnal variation of trace gases such as NO₂, O₃, H₂O and 0₄.  Daily morning/evening observations have been made to see the seasonal variations of NO₂ and O₃ at Pune (18°32' N, 73°51' E) during the period 2000–2003. To retrieve the RCD, plots of SCD_diff against a set of the corresponding AMF (Langley plot) are needed. Hence, the set of AMF (SZA) for respective SZA is acquired by RT model. For the sake of comparison, data of O₃ VCD from Dobson instrument of Pune station are collected from the web site http://www.msc–smc.ec.gc.ca/woudc/index_e.html of World Ozone Data Centre (WODC), and TOMS data for the same station are collected from http://jwocky.gsfc.nasa.gov/eptoms/ep.html. Daily maximum/ minimum temperature is collected from Indian Daily Weather Report (IDWR) reported by Indian Meteorological Department to understand the variation in NO₂ PM/AM ratio.

4. Results and discussion

 4.1 Diurnal variation

As an example, full day observations on 18, 19, 21, 22 and 23 October 2000 have been made for the study of diurnal variations of NO₂, O₃, H₂O and O₄. Figures 1a and b shows the SCD_diff of NO₂ and O₃, respectively, with SZA for the above mentioned days. Similarly, H₂O and O₄ SCD_diff are shown in Figure 1c and d. Here, SCD_diff is the difference of the SCD in observed spectrum of particular SZA and reference spectrum of 30° SZA. Therefore, SCD_diff at 30º SZA will be zero for all the gases. In both the figures, SCD_diff of above gases are smoothly increasing with increasing SZA due to increasing atmospheric light paths approaching higher SZA.

From Figure 1a, it is seen that evening NO2 SCD_diff are higher than morning SCD_diff at same SZA. At 70º SZA, percentage difference between evening and morning NO₂ SCD_diffvaries from 50 to 57%; similarly, at 80º SZA percentage difference varies from 59 to 66% between evening and morning NO₂ SCD_diff. From Figure 1b, it is seen that O₃ SCD_diff at morning and evening hours are almost similar. O₃ SCD_diff are increasing with SZA due to increasing absorption path at higher SZA. From Figure 1c and d, it has been seen that H₂O and O₄ SCD_diff variations are more between the selected days compared to NO₂ and O₃. Water vapour has large variability in the troposphere and stratosphere. The major absorption is in the lower layer of the atmosphere. Hence, the zenith dependence of wather vapour variability is different compared to other three gases. The atmospheric content of this compound responds to changes in temperature, microphysical processes and the atmospheric circulation. Water vapour concentration varies substantially in both the vertical and horizontal. Clouds are intimately connected to its pattern, as clouds occur in connection with high relative humidity, and cloud processes in turn affect the moisture distribution. Table I shows SCD_diff of NO₂, O₃, H₂O and O₄ at morning (AM) and evening (PM) 90º SZA for selected days. From Table I, it is seen that the evening NO₂ values are almost double of morning NO₂ values at 90º SZA. The percentage difference between evening and morning NO₂ SCD_diff values varies from 78 to 107% at 90º SZA during those days. These higher values of NO₂ in the evening particularly at SZA greater than 85º compared to morning NO₂ may have occurred due to increased rate of NO–NO₂ conversion and decreases in the rate of photolysis of NO₂ during evening twilight period. From Table I, it has been seen that evening densities of O₃ are 1 –5 % higher than morning densities at 90º SZA during those days. H₂O SCD_diff at evening 90º SZA are found to be 2–11% higher than morning SCD_diff of same SZA. Similarly, O₄ SCD_diff are 0–26% higher than morning SCD_diff at evening 90º SZA. On October 19, O₄ SCD_diff at evening is 26% higher than morning. It is also seen that on this day H₂O SCD_diff at evening 90º SZA is 11% higher than morning compared to other days. These higher values of O₄ and H₂O on October 19, at evening 90º SZA may be due to cloud layer occurred at evening hours. Meena et al. (2004) have been noticed that more effects of clouds are seen in increased absorption of tropospheric species like H₂O and O₄, which are good indicative of the increased absorption path lengths due to multiple Mie–scattering inside clouds.

Figure 2 shows the plots of SCD_diff vs AMF (Langley plot) of O₃ at evening on October 18, 19, 21 and 22. The correlation coefficients (r) between SCD_diff and AMF are 0.99 for 18, 19 and 21 October and 0.98 for 22 October. The correlation coefficients are significant at 1% level. In Figure 2, straight lines are showing the linear fit to the data. From the Langley plots, it is seen that data follow the straight line. According to equation (1), Y intercept gives the slant column amount in reference spectrum and slope of Langley plots gives the VCD, which are listed in Table II. This table, it is seen that the VCD derived by the slope of Langley plots and VCD90 using equation (2) have not much difference. They appear to be in good agreement. VCD derived from our spectrometer at evening 90° SZA are observed to be less by 2 to 10 DU compared to Dobson instrument values (V_Dob) and 6 to 22 DU less from TOMS observed values. The percentage difference in VCD derived by Dobson and our spectrometer varies from 0.8 to 4%. Similarly, the percentage difference between VCD obtained from our spectrometer and TOMS varies from 1 to 8% on these days. There are a number of factors contributing to the total error in the VCD. The basic uncertainty of O₃ and NO₂ is given by the random error from the fit. The random error is 1% for O₃ and 2% for NO₂. Uncertainty in cross sections is 1.2% for O₃ and 2.3% for NO₂. Pseudo–random error causing structure in the "filling–in" of absorption features by Raman scattered light is 1% for O₃ and 5% for NO₂. Another factor is that associated with the calculated AMF due to approximations in the RT model, which is 2.7% for O₃ and 5% for NO₂. The root–sum–square error calculated from these various error sources is approximately 3% for O₃ and 8% for NO₂.

Figures 3a and b show the daily VCD of NO₂ and O₃, respectively, obtained in the morning and evening hours during the period May 4, 2000 to May 24, 2003. VCD were derived from SCD at 90° SZA using the AMF of 18.5 for NO₂ and 18.3 for O₃, which were calculated by RT model for 90° SZA. Also, daily ozone values obtained by Dobson spectrophotometer are plotted in Figure 3b for the above period. From Figure 3a, it is seen that maximum NO₂ VCD are found in summer months (May and June) and minimum in winter months (December and January) for all the years.

Maximum NO₂ VCD (average of May and June) and minimum NO₂ VCD (average of December and January) of morning and evening hours are listed in Table IIIa. From this table, it is seen that at morning and evening hours, maxima of NO₂ of VCD have an increasing trend during the period 2000–2003. The maximum NO₂ VCD in the morning hours are increasing per year in the order of 0.4 x 10¹⁵ – 0.5 x 10¹⁵ molecules cm–² (i.e., 7–8%) and in evening hours it is increasing 5–6% per year. The percentage difference between maximum NO₂ VCD at morning and evening hours varies from 29 to 42%; similarly, percentage difference between minimum NO₂ VCD at morning and evening hours varies from 150 to 172% during the period 2000–2003. Regarding the NO₂ concentration, it was observed at Pune that two wheelers contribute 74 and cars 20% to the total pollution. The air quality of Pune's surroundings is deteriorating rapidly. Every day hundreds of tons of polluted air are released. Vehicular contribution of Pune's air pollution is approximately 40%. The average air pollution levels in the city are given as: PM₁₀ 83 ppm, NO₂ 62 ppm and SO₂ 37 ppm. The observation station is in upwind and hence the effect of Pune pollution depends upon wind direction. However, the tropospheric contribution will only affect the observations and stratospheric contributions are representative of large circulation patterns.

From Figure 3b, it is seen that maximum O₃ VCD are observed in summer months (May and June) and minimum in winter months (December and January). From the figure it is also seen that morning and evening VCD are in good agreement with Dobson spectrophotometer values throughout the observational period. Maximum O₃ VCD (average of May and June) and minimum O₃ VCD (average of December and January) at morning and evening hours are listed in Table IIIb. From table, it is seen that evening O₃ VCD are higher than morning O₃ VCD. The percentage difference between maximum O₃ VCD at morning and evening hours varies from 3 to 10% and the percentage difference between minimum O₃ VCD at morning and evening hours varies from 2 to 18% during the period 2000–2003. Figure 3c shows correlation between NO₂ and O₃ for morning (AM) and evening (PM) data. For the whole period, a positive correlation of r = 0.72 in the morning data and r = 0.79 in the evening have been found between NO₂ and O₃ total column amounts.

It has been discussed above that maximum NO₂ and O₃ VCD are observed in May and June during all the years, therefore it can be say that NO₂ and O₃ peaks are one month ahead from maximum temperature peaks. A good agreement is seen between O₃ and minimum temperature variations. It is found from Figure 4b that TOMS values are higher than spectrometer values. The TOMS instrument measures UV radiances backscattered by the underlying atmosphere and earth's surface or clouds. The measured radiances are a function of total column ozone, the vertical distribution of that column, solar zenith angle, satellite zenith angle (azimuthal angle and scan angle), and the pressure level and reflectivity of the lower boundary. In clear sky conditions the contribution of backscattered radiation from the atmosphere dominates over the reflected radiation from earth's surface, the effective scattering surface for the backscattered UV radiation to the TOMS instrument is at the middle to upper troposphere (Hudson et al., 1995; Klenk et al., 1982).

For ozone above the middle to upper troposphere, the TOMS algorithm can retrieve the actual amount of ozone above the effective scattering surface with little error. However, if the assumed ozone profile below the effective scattering surface is different from the actual ozone profile, the retrieval will sense less than the actual deviation because some of the backscattered radiation will not have passed through the middle and lower troposphere. Then, the retrieved total ozone is either overestimated or underestimated depending on whether the assumed ozone amount below the effective scattering surface is less than or greater than the actual ozone amount. In the cloudy sky, incorrect cloud height lead to three errors like errors in the added ozone below clouds and errors in the retrieved ozone above clouds.

The knowledge of ozone and temperature profiles is critical for the calculation of ozone values at higher solar zenith angles. The effect of different profiles on the accuracy of ozone retrievals is an important part. Therefore, the reason of the observed discrepancies may be in the TOMS algorithm, which uses climatological profiles rather than the actual profile at the time of the measurement. High summer ozone concentrations are confined to increased duration of UV–radiation during summer months. Ozone produced in the tropical region gets transported to higher latitudes by the stratospheric mean diabatic circulation. Ozone rich–air during winter moves poleward and downward, which explains the seasonal variations in ozone. Also, day–to–day variability in total ozone is assumed to be caused by changing weather patterns in the troposphere and its effects may extent to the lower stratospheric levels. Ozone has a strong seasonal variation but weak diurnal variation at twilight, with maxima in the summer and minima in the winter. However, the abundance of lower stratospheric ozone is strongly dependent on transport, so that interannual variations in dynamic behavior can also be a source of ozone variability (Hood et al., 1997).

Figures 5a and b show the NO₂ and O₃ evening–to–morning (PM/AM) ratio, respectively, during 2000–2003. Also, Figure 5a shows temperature maximum/minimum (max/min) ratio. Figure 5c shows the correlation between NO₂ PM/AM ratio and temperature max/min ratio during 2000–2003. From Figure 5a, it is found that at Pune the PM/AM ratio of NO₂ density varies from 1.25 to 3.8 during the above period. There is an interesting seasonal change in the PM/AM ratio. NO₂ PM/AM ratios are higher in winter months and lower in summer months for all the years. In winter months NO₂ PM/AM ratio goes up to 3.8 and in summer months lowest ratio is 1.25. From Figure 5a, it is seen that max/min temperature ratios are higher in winter months and lower in summer months during 2000–2003. Decreasing trend is observed in the maxima of both NO₂ PM/AM ratio and temperature max/min ratio in winter seasons during the above period. When the max/min temperature differences are more, NO₂ PM/AM differences are also observed more. The NO₂ PM/AM ratio has been plotted against the temperature max/min ratio with a correlation coefficient of 0.55, which is shown in Figure 5c. Deviation from linearity indicates other processes than pure photochemical and homogeneous chemistry. The NO₂ concentration and its PM/AM ratio is considered to change with the atmospheric temperature in a day/night period.

Temperature of the atmosphere controls the rate of thermal decomposition of N₂O₅ concentrations. N₂O₅ forms during the dark hours and is destroyed by photolysis and thermal decomposition. N₂O₅ concentration is a function of temperature and wavelength. In the winter nights N₂O₅ forms more due to more dark hours and in day time it is photolyzed to produce NO₂, which is reflecting in the evening (PM) hours. Hence, in winter PM/AM ratios are higher than in summer.

Koike et al. (1999) have analyzed the series of PM/AM ratio values of the VCD of NO₂ and found a maximum ratio of 2.32 in winter and a minimum of 1.25 in summer. They obtained residual series of ratio values after eliminating seasonal trends and by considering PM value of the day and AM value of the next day for the two stations: Moshiri (44.4° N, 142.3° E) and Rikubetsu (43.5° N, 143.8° E). A positive correlation between these two stations before and after eliminating seasonal trend is suggestive of day–to–day variation of PM/AM ratios in the VCD of NO₂ in association with the real change in the atmosphere. Otten et al. (1998) observed a PM/AM ratio of the VCD of NO₂ of highest value 4.0 during the mid–winter of 1994/1995 at Kiruna (67.9° N, 21.1° E). The analysis at Pune also shows large day–to–day variability in the VCD of NO₂ with maximum PM/AM ratio of about 3.8 in winter compared to small day–to–day variations in spring and summer with PM/AM ratio of about 1.25. However, tropospheric NO₂ pollution probably by local traffic during the day could also enhance the PM/AM ratio in total atmospheric NO₂, in particular during winter when the stratospheric NO₂ is low, and the near surface inversion prevents an efficient dispersion of polluted air (Fiedler et al., 1993). During the night NO₂ can be oxidized by O₃ to form NO₃, a strong atmospheric oxidant and a precursor to the formation of N₂O₅. During the day N₂O₅ can be photolyzed to regenerate NO₂ (Nichol et al., 1996). In contrast, N₂O₅ can have morning concentrations in the lower stratosphere comparable to NO_x, even though its production requires the formation of NO₃.

NO₂ + O₃ NO₃ + O₂

NO₃ + hv NO + O₂

NO₂ + O

NO₃ + NO₂ + M N₂ O₅ + M

In the winter, nights are longest; therefore, during night NO₂ to N₂O₅ conversion is more and hence in the morning NO₂ value will be less, what leads to high PM/AM ratio. During daytime, photochemical reactions take place in the presence of sunlight and N₂O5 that, formed due to oxidation of NO₂ during night, gets photolyzed to reproduce NO₂ (Kostadinov et al., 1999). The formation of NO₂ during the day leads to an increase in the VCD of NO₂:

N₂O₅ + hv NO₃ + NO₂

The increase in NO₂ VCD is also possible due to increased pollution during day, which reflects in the evening hours as it is seen from figures. At twilight, NO₂ density depends on NO and controlled by the following reactions:

NO + O₃ NO₂ + O₂

NO₂ + hv NO + O

NO₂ + O NO + O₂

During evening twilight hours, the rapid decreases in the rate of photolysis of NO₂ and the abundances of atomic oxygen lead to conversion of NO into NO₂. Thus, the total number of NO₂ molecules can increase substantially at evening hours and hence the AM/PM differences (Solomon et al., 1987; Kerr et al., 1977; Boughner et al., 1980). These AM/PM differences in VCD of NO₂ can also be seen in Figure 3a. The VCD data at Pune show strong seasonal and diurnal variations of NO₂, with maxima in summer at sunset and minima in winter at sunrise. This agrees with the results of earlier authors (Noxon et al., 1979; Syed et al., 1981; Johnston et al., 1989; Solomon et al., 1992; Bhonde et al., 1992; Lal et al., 1993; Kondo et al., 1994). From Figure 5b, O₃ PM/AM ratio is slightly higher in winter months compared to summer months. The average PM/AM ratio of about 1.03 is observed. Various nitrogen oxides like NO, NO₂ and NO₃ are involved in reactions, which affect O₃ abundance, N₂O₅ acts as a temporary reservoir for these nitrogen oxides species. Therefore, the concentration of N₂O₅ in the atmosphere contributes to the AM/PM O₃ abundance. However, there is not much difference seen in O₃ values. Hence, no much variation is seen in PM/AM ratio.

5. Conclusions

• Full day observations have been made from morning (SZA 90°) to evening (SZA 90°) to see the diurnal variations of trace gases such as O₃, NO₂, H₂O and O₄. SCD_diff of above gases increase smoothly as the AMF increases with SZA. It is seen that evening NO₂ SCD_diff are higher compared to morning SCD_diff. It is also noticed that evening NO₂ values are almost double of morning NO₂ values in the month of October 2000. These high values of NO₂ in the evening appear to be related to NO–NO₂ conversion that occurs in twilight period. In O₃, H₂O and O₄, no much differences are observed in morning and evening SCD_diff.

• A time series of daily VCD of NO₂ and O₃ at sunrise and sunset has been obtained during the period from May 2000 to May 2003. There have been found a good correlation between NO₂ and O₃ VCD during the period. The VCD data at Pune show strong seasonal and diurnal variations of NO₂ with maxima in summer at sunset and minima in winter at sunrise. Ozone has strong seasonal variations but weak diurnal variations at sunrise and sunset hours with maxima in summer and minima in winter. A good agreement is found between O₃ VCD and minimum temperature variations during the above period. High summer ozone values are confined to increased duration of UV–radiation during summer months.

• The percentage difference between VCD obtained from Dobson and our spectrometer varies from 0.8 to 4%. Similarly, in VCD obtained from our spectrometer and TOMS, the percentage difference varies from 1 to 8% during the above period.

• The NO₂ PM/AM ratios are found to be higher in winter months and lower in summer months. It has also been found that temperature max/min ratios are higher in winter and lower in summer months during 2000–2003. When the max/min temperature differences are more, NO₂ PM/AM differences are also observed more. In winter months NO₂ PM/AM ratio goes up to 3.8 and in summer months lowest ratio is 1.25. A positive correlation is found between NO₂ PM/AM ratio and temperature max/min ratio during the period 2000–2003.

• Temperature of the atmosphere controls the rate of the thermal decomposition of N₂O₅ concentrations. N₂O₅ forms during dark hours. In the winter, nights are longest; therefore, during night NO₂ to N₂O₅ conversion is more and hence in the morning NO₂ value will be less that leads to high PM/AM ratio. Further, during evening twilight hours, the rapid decreases in the rate of photolysis of NO₂ and the abundances of atomic oxygen lead to conversion of NO into NO₂. Thus, the total number of NO₂ molecules can increase substantially at evening hours, and hence the AM/PM differences. O₃ PM/AM ratio is found slightly higher in winter months compared to summer months.

Acknowledgements

The authors are thankful to Prof. B. N. Goswami Director IITM and Dr. P. C. S. Devara Head PM & A Division for his encouragement. The authors are thankful to the Department of Science and Technology (DST), New Delhi, Govt. of India for the financial support during the tenure of this work.

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