Seasonal and diurnal behavior of NO2 column density in Antarctica as observed by Mark IV, Brewer Ozone Spectrophotometer Number 153

Available online at www.sciencedirect.com

Advances in Space Research 47 (2011) 86–93 www.elsevier.com/locate/asr

Seasonal and diurnal behavior of NO2 column density in Antarctica as observed by Mark IV, Brewer Ozone Spectrophotometer Number 153 D.K. Chakrabarty a,⇑, S.K. Peshin b,1 a

b

Physical Research Laboratory, Ahmedabad 380009, India India Meteorology Department, Lodi Road, New Delhi 110003, India

Received 10 March 2010; received in revised form 20 August 2010; accepted 27 August 2010

Abstract The Meteorology Department of India has been measuring vertical column density of NO2 at Maitri (70.7°S, 11.7°E), Antarctica since July 1999 using a Mark IV, Brewer Ozone Spectrophotometer. Maitri is situated at the south of the Antarctic circle. An analysis of 6 years of data shows that NO2 column has seasonal variation with a maximum value during summer. It is also found that during the period when sun does not set, the NO2 column exhibits a diurnal variation, with a peak around noon and lower values in the morning and afternoon hours. Using a simple steady-state chemical reaction scheme, an attempt has been made to explain these features. Ó 2010 Published by Elsevier Ltd. on behalf of COSPAR. Keywords: NO2; NO; UV-B; Antarctica

1. Introduction Nitrogen dioxide is a member of the NOx family which catalytically destroys ozone in the stratosphere. For this reason, and since it is easy to measure, measurements of this species have been made by various groups at various locations on the earth by using ground-based, balloonborne and space craft techniques. One important feature of this species that has emerged from theses studies, especially in low and mid-latitude, is that it builds up during morning, remains almost constant during afternoon hours and then decays during nighttime. This makes its sunrise density lower than the sunset density. Accordingly, during the middle of November to the middle of January when sun

⇑ Corresponding author. Tel.: +91 79 26749841; fax: +91 79 26314900; mobile: 9327020993. E-mail addresses: dkchakrabarty@rediffmail.com (D.K. Chakrabarty), [email protected] (S.K. Peshin). 1 Fax: +91 7924699216; moblie: 9810240574.

0273-1177/$36.00 Ó 2010 Published by Elsevier Ltd. on behalf of COSPAR. doi:10.1016/j.asr.2010.08.034

does not set (no-night period) in Antarctica, a diurnal variation of NO2 is not expected. The nature of NO2 and O3 variations during an ozone-hole event was studied using short-term data series obtained by visible absorption spectroscopy technique earlier by Shibasaki et al. (1986), Keys and Johnston (1986), Sanders et al. (1989) and Gil and Cacho (1999). Recently Yela et al. (2000) studied the seasonal evolution of NO2 and O3 outside, at the edge and inside the Antarctic polar vortex using the same technique. These studies were based on observations taken during the twilight period near 90° solar zenith angle. These measurements are at twilight because they use the visible absorption band of ozone, which is too weak for useful measurements at small solar zenith angle near noon. India Meteorology Department has been measuring the column densities of NO2, SO2 and O3 and UV-B flux at the ground using a Brewer spectrophotometer at Maitri (70.7°S, 11.7°E) since July 1999. The relationship between SO2, O3 and UV-B flux at the ground during the ozone-hole period has been reported earlier (Chakrabarty and Peshin,

D.K. Chakrabarty, S.K. Peshin / Advances in Space Research 47 (2011) 86–93

2007). In this paper the results of study on seasonal and diurnal behavior of NO2 during a period with no nighttime are presented. 2. Instrumentation and data

O3 (D.U.)

An automated Brewer spectrophotometer (Mark IV, No. 153) was installed at Maitri by the India Meteorological Department in July 1999. Regular measurements of the total column O3, NO2, SO2 and the maximum value of UVB flux were begun there in September 1999. The details of this instrument and the methods by which these parameters are measured are described by Vanicek et al. (2003), Kerr et al. (1980, 1985) and Chakrabarty and Peshin (2007). Three similar Brewer instruments are also operating at India Meteorological Department in New Delhi, Pune and Kodaikanal where Dobson instruments are taking routine O3 observations daily. Simultaneous running of Standard Dobson Instrument No. 112 and Brewer instruments at New Delhi has shown that ozone values obtained by these two instruments do not differ by more than 2%. The data obtained at Maitri for the years 1999– 2005 have been used in the present work.

87

3. Results Fig. 1 shows a plot of the vertical column density of O3, NO2, and the maximum value of UV-B flux for the period September 14, 1999 to December 30, 2003 measured at Maitri (70.7°S, 11.7°E) using the Brewer spectrophotometer by the India Meteorological Department. These observations were made under clear sky condition. Measurements were made in both ds (direct sun) and zs (zenith sky) modes. The zs values obtained by us under disturbed condition may not be correct because the sky chart supplied with the Brewer depends on implicit correlations between climatological change in ozone column and ozone distribution. However, in this study, we have not used any data obtained in the zs mode; we have used the data obtained in the ds mode only for clear sky condition. Fig. 1(a) shows a plot of daily ozone values obtained in the ds mode. The ozone-hole event is clearly seen in this figure during all the years. Starting from the middle of September, as the days pass, ozone column decreases until about the middle of November when it reaches a minimum. After this time, it increases rapidly and reaches a maximum

450 400 350 300 250 200 150 100 50 0

Figure 1a

28-Aug-99 27-Feb-00 28-Aug-00 27-Feb-01 29-Aug-01 28-Feb-02 30-Aug-02 1-M ar-03

31-Aug-03

1.4

NO2 (D.U.)

1.2 1 0.8

Figure 1b

0.6 0.4 0.2 0 28-Aug-99 27-Feb-00 28-Aug-00 27-Feb-01 29-Aug-01 28-Feb-02 30-Aug-02 1-M ar-03

31-Aug-03

UV-B Flux (mW/m 2)

300 250 200 150

Figure 1c

100 50 0 28-Aug-99 27-Feb-00 28-Aug-00 27-Feb-01 29-Aug-01 28-Feb-02 30-Aug-02 1-M ar-03

31-Aug-03

Date Fig. 1. Daily values obtained by Brewer spectrophotometer from September 14, 1999 to December 30, 2003. (a) Vertical column density of O3, (b) vertical column density of NO2, and (c) maximum UV-B flux values.

D.K. Chakrabarty, S.K. Peshin / Advances in Space Research 47 (2011) 86–93

3.1. Seasonal variation We have shown the raw data in Fig. 1 which exhibits a lot of scatter. We have, therefore, considered the monthly averaged values. The monthly averaged values of NO2 column for September, October, November, December, January, February and March for 1999–2000, 2000–2001, 2001– 2002, 2002–2003 and 2005 have been plotted in Fig. 2. Data are not available for 2004. Two things emerge from this figure. One, there is a seasonal variation with the NO2 column increasing from September until about the middle of January after which it decreases. This kind of

250

1.4

1.2 UV-B Flux NO2 column

200

1 150

0.8

0.6

100

NO2 column (D.U.)

in January. From January onwards, ozone slowly decreases until about April. After April data are not reliable due to cloudy and bad weather conditions. The zs ozone values were found to be within ±5% of the ds values. But the nature of variation of ozone in both the modes was seen to be the same. It is to be noted in Fig. 1(a) that we do not have regular data for about three months, May, June and July, during winter when there is no sunlight. Fig. 1(b) shows plot of the daily values of the NO2 column for the same period mentioned above. It can be seen from this figure that, starting from the middle of September, the NO2 column increases until about the middle of January and then decreases. The same pattern is seen in all the years analyzed. But the magnitude of increase of NO2 column from the middle of September to the middle of January is different in different years. In 1999 the increase is from 0.1 to 1 DU and in 2003 the increase is from 0.1 to  0.6 DU. Also there is no similarity in the nature of NO2 and O3 variations. The nature of NO2 and O3 variations in Antarctica was studied using the visible absorption spectroscopy technique earlier by Shibasaki et al. (1986), Keys and Johnston (1986), Sanders et al. (1989) and more recently by Yela et al. (2000). These studies are based on observations taken during the twilight period near 90° solar zenith angles, whereas our study is based on direct sun observations taken at noon time and at a time of continuous daylight. Fig. 1(c) shows plot of UV-B flux values measured at the ground for the same period as mentioned above. One can see from this figure that from the middle of September, UV-B flux increases, reaches its peak towards the middle of November and after November, UV-B values start decreasing, till up to December/January. The same trend is seen in all the years analyzed. A comparison of Fig. 1(a) with Fig. 1(c) shows that the variation of ozone column is neither in phase nor opposite in phase to that of UV-B flux. The nature of correlation between O3 column and UV-B flux at ground has been studied earlier (Chakrabarty and Peshin, 2007). A comparison of Fig. 1(b) with Fig. 1(c) shows that though the nature of variation of NO2 and UV-B flux is the same but there is a shift in time, with the UV-B flux reaching its peak before the NO2 column does.

UV-B Flux (mW/m 2)

88

0.4 50 0.2

0 Jul-98

Dec-99

Apr-01

Sep-02

Jan-04

May-05

Oct-06

0 Feb-08

Date

Fig. 2. Monthly averaged values of NO2 column and UV-B flux at ground for September, October, November, December, January, February and March for 1999–2000, 2000–2001, 2001–2002, 2002–2003 and 2005. Data are not available for 2004.

variation could also be due to an extra terrestrial calibration error as a result of seasonal changes in solar zenith angle and, therefore, air mass. Due to seasonal changes in the solar zenith angle and, therefore, air mass, if any correction has to be applied for extraterrestrial calibration error for obtaining values in different seasons, that might have been incorporated in the Brewer software. Even if it is not, the possibility of such an error to affect our results to a significant extent is low because the instrument has been running since 1999 and the standard lamp tests done during this period shows no drift in R5/R6 values, e.g. in 1999 the values were 3257/1674 and in 2004 the values were 3259/1675. This shows the stability condition of the instrument. Also our measurements were made for zenith angle less than 75° in the direct sun mode. We have not applied any correction in the Gate values. Two, the peak NO2 value increases from 1999–2000 to 2001–2002 and then decreases. The peak values during 2002–2003, 2003–2004 and 2005 are much lower than those during 1999–2000, 2000–2001 and 2001–2002. Such a behavior is not seen either in the O3 column or in the UV-B flux values. Our averaged peak value varies from 0.46 in 1999 to 1.17 in 2001 to 0.28 DU in 2006. Such a behavior was also observed by Cook and Roscoe (2009) and ascribed by them to changes in Brewer–Dobson circulation. Gil and Cacho (1999) measured at the Antarctica Peninsula Marambio (64°S, 56°W) during the spring of 1982 (8 September – 28 November) and found an NO2 vertical column 1.5– 2.5  1015 cm2 in the morning and 2–3  1015 cm2 in the evening. Yela et al (2000) have measured NO2 values inside the Antarctic polar vortex at Belgrano (78°S, 35°W) during 1994–1999 and have given the average summer values 4–6  1015 cm2. Our averaged peak value in summer of 1999 is 1.23  1016 cm2, higher than the Yela et al. (2000)’s values by a factor of 2. It may be noted here that both Gil and Cacho (1999) and Yela et al. (2000)’s val-

D.K. Chakrabarty, S.K. Peshin / Advances in Space Research 47 (2011) 86–93

ues are averaged slant column values for solar zenith angle 88°–92° twilight condition in all the seasons, converted to vertical column values. They measured scattered sun light intensity. Whereas our values are slant column values for solar zenith angles lying between 47° and 73° during noon time in different season, converted to vertical column values. We measured direct sun light intensity. The conversion of slant column to vertical column is dependent on vertical distribution of species which is assumed. Air mass factor calculated with this assumption will be different in different season because vertical distribution will be different in different seasons. Also error in calculating air mass factor for large zenith angle could be significant. Yela et al. (2000) have not considered any seasonal variation of air mass factor. Brewer instrument calculates air mass factor not just the secant of the solar zenith angle but by a formula that includes the height of the layer and curvature of the solar beam during their passing through the atmosphere. In our case since solar zenith angle is less than 75°, the error in calculating air mass factor will not be significant. The air mass values which come in the Gate output, agree very well with the secant law in 47°–73° solar zenith angles. We have not applied any diurnal or temporal correction in the Gate output values. The UV-B flux raw data shown in Fig. 1(c) exhibits a lot of scatter. We have, therefore, considered their monthly averaged values. The monthly averaged values of UV-B flux corresponding to the NO2 column are also shown in Fig. 2. An increase in UV-B flux with an increase in NO2 column and vice versa is also seen in this figure. The UV-B radiation emitted from the Sun is almost completely absorbed by the ozone layer in the stratosphere and only a very small fraction reaches the Earth’s surface. Consequently, a change in the O3 amount will change the UV-B flux reaching the surface of the Earth. The NO2 layer is situated in the altitude region of 20–25 km but the depletion of ozone takes place in the altitude below this altitude region, 14– 21 km. We have examined correlation between UV-B flux measured at ground and NO2 column density. It has been found that during September to December UV-B flux and NO2 column density have correlation coefficient 0.91, 0.87, 0.79, 0.98, 0.91 and 0.56 for 1999, 2000, 2001, 2002, 2003 and 2006, respectively. The poor correlation in 2006 is due to smaller number of observations made during that year. For January to April there is no correlation. The good correlation between UV-B flux measured at ground and NO2 column density during September to December is due to the fact that the two peaks (UV-B flux peak and NO2 column peak) are in the same phase and no correlation during January to April is due to the fact that the two peaks are not in the same phase, there is some time lag. During the period when there is depletion of ozone, the UV-B flux measured at ground will be the same as that in the region lying between 20–25 and 14–21 km altitude. This feature has been further discussed in Section 4.

89

3.2. Diurnal variation The diurnal variation of NO2 column for six days in November 2003 under continuous daylight condition is presented in Fig. 3(a–f). Normally observations are made once per day. But multiple observations were taken during November, 2003 to study a solar eclipse phenomenon in Antarctica which occurred on 23 November, 2003. Therefore, frequent observations are not available for other months and years. One can see from these figures that there is a diurnal variation in the NO2 column: it builds up in the morning, reaches a maximum at noon and decays in the afternoon. Several authors have shown that at mid- and low-latitudes the NO2 column is expected to increase slowly throughout the day due to the photolysis of N2O5 (reactions (R7) and (R6)) and decay at night by reaction (R5) (e.g. Roscoe et al., 1986). To study the direct control of solar radiation on NO2 column, corresponding solar zenith angle values have also been plotted in Fig. 3. NO2 column and zenith angle appears to be in anti-phase but with a time lag of about 2–3 h, zenith angle becoming minimum around 1100 h and NO2 column becoming maximum between 1400–1500 h. It appears from Fig. 3 that there is indeed a diurnal variation of NO2 column with morning and evening values lower than the noontime values. To examine the diurnal variation of NO2 column further all the data points of Fig. 3(a–f) have been plotted together in Fig. 4. A trend line (continuous line) drawn by the computer through these data points clearly shows the diurnal variation. An average of all these data points has been calculated. It is represented by ANO2. Since there is a lot of scatter in the data, we have calculated the average values in time interval of 1 h. All the data points between 8:00 and 9:00 GMT have been averaged. This is represented by TNO2. Then the difference between ANO2 and TNO2 has been calculated. This difference value has been shown in Fig. 4 at 8:30 GMT by a big solid square. Similarly all the data points lying between 9:00 and 10:00 GMT have been averaged and its difference from ANO2 has been shown at 9:30 GMT in Fig. 4 by a big solid square and so on. A trend line (broken line) drawn by the computer through these difference values also show a diurnal trend. 4. Discussion In winter during night when there is no sunlight, NO2 is converted in to N2O5, HNO3 and probably HNO3(H2O)3 solid crystals. After winter, when sunlight appears, these species are photo-dissociated and produce NO and NO2. We take a simple chemistry of NO2 used by us earlier (Chakrabarty et al., 2001). It contains the following chemical reactions: NO þ O3 ! NO2 þ O2 k 1 ¼ 3:0  1012  expð1500=T Þcm3 s1 ðR1Þ NO2 þ hm ! NO þ O

J 2 ¼ 6:52  103 s1

ðR2Þ

90

D.K. Chakrabarty, S.K. Peshin / Advances in Space Research 47 (2011) 86–93 0.9

90 80

1.2

(b) 25 Nov 2003

(a) 30 Nov 2003 75

1.1

70

0.8

70

0.7

0.7

60

0.6

50 0.6

50

0.5

NO 2 column (D.U.)

0.8 55

NO 2 column (D.U.)

0.9 60

Zenith angle (degree)

65

Zenith angle (degree)

80

1

45 0.5

40

0.4

40 Z-Angle NO2-COL

35 30 8:00

9:12

10:24

Z-Angle NO2-COL

0.4

11:36

12:48

14:00

15:12

16:24

17:36

18:48

30 8:00

0.3 20:00

9:12

10:24

11:36

12:48

14:00

15:12

16:24

17:36

18:48

0.3 20:00

Time (GMT)

Time (GMT)

90

0.6

90

0.75 (c) 24 Nov 2003

(d) 23 Nov 2003 80

0.55

70

0.5

60

0.45

50

0.4

0.6 0.55

60 0.5 0.45

50

Zenith angle (degree)

70

NO 2 column (D.U.)

0.4 40

0.35

40 Z-Angle NO2-COL

30 8:00

9:12

10:24

0.35

11:36

12:48

14:00

15:12

16:24

17:36

18:48

Z-Angle NO2-COL

0.3 20:00

30 8:00

9:12

10:24

11:36

12:48

Time (GMT)

15:12

16:24

17:36

18:48

0.60

0.65

90

(e) 19 Nov 2003

(f) 16 Nov 2003 0.55

0.6

80

70

0.55

60 55

0.45

50 0.40

Zenith angle (degree)

0.50

NO 2 column (D.U.)

65

Zenith angle (degree)

0.3 20:00

Time (GMT)

80 75

14:00

70 0.5 60 0.45

NO 2 column (D.U.)

Zenith angle (degree)

0.65

NO 2 column (D.U.), NO density (relative unit)

0.7 80

50

45

0.4

40 0.35 35 30 8:00

40

9:12

10:24

0.35

Z-Angle NO2-COL

NO2 COL Z-Angle 11:36

12:48

14:00

15:12

16:24

17:36

18:48

0.30 20:00

30 8:00

9:12

10:24

11:36

12:48

14:00

15:12

16:24

17:36

18:48

0.3 20:00

Time (GMT)

Time (GMT)

Fig. 3. (a–f) Diurnal variation of NO2 column density for 30, 25, 24, 23, 19 and 16 November 2003. Corresponding solar zenith angles are also shown.

NO2 þ O ! NO þ O2 k 3 ¼ 5:6  10

NO2 þ O3 ! NO3 þ O2 12

3

 expð180=T Þcm s

k 4 ¼ 1:2  1013  expð2450=T Þcm3 s1

1

ðR3Þ

ðR4Þ

D.K. Chakrabarty, S.K. Peshin / Advances in Space Research 47 (2011) 86–93 0.9

0.15

0.8

0.1

0.05 0.6 0.5

0

0.4

-0.05

0.3

TNO2-ANO2 (D.U.)

NO 2 column (D.U.)

0.7

-0.1 0.2 0.1 0 7:00

NO2 col (left Y axis) TNO2-ANO2 (right Y axis) Poly. (TNO2-ANO2 (right Y axis)) Poly. (NO2 col (left Y axis))

-0.15

-0.2 9:24

11:48

14:12

16:36

19:00

21:24

Time (GMT)

Fig. 4. Difference between the average of all the data points of Fig. 3(a–f) (ANO2) and the average of data points in 1 h interval (TNO2).

NO2 þ NO3 þ M ! N2 O5 þ M 4:4

k 5 ¼ 2:0  1030  ð300=T Þ cm6 s1 ðR5Þ NO3 þ hm ! NO2 þ O J 6 ¼ 2:01  102 s1 N2 O5 þ hm ! NO2 þ NO3

J 7 ¼ 3:0  105 s1

ðR6Þ ðR7Þ

N2 O5 þ M ! NO2 þ NO3 þ M k 8 ¼ 5:8  1027 cm3 s1

ðR8Þ

The rate coefficients mentioned above are from Sander et al. (2003). A panel of 11 scientists (Sander et al., 2003) has examined all the available reaction rates of atmospheric importance and has given the best values in NASA-JPL 2002 Publication 02-25 to be used for model studies. Using the above reactions, the continuity equation of NO2 with time at a particular height for daytime can be written as: d½NO2 =dt ¼ k 1 ½NO½O3  þ J 6 ½NO3  þ k 8 ½N2 O5 ½M þ J 7 ½N2 O5   J 2 ½NO2   k 3 ½O½NO2   k 4 ½O3 ½NO2   k 5 ½NO3 ½NO2 ½M

ð1Þ

The concentration of NO3 and N2O5 is negligible during daytime. Also k3[O] and k4[O3] are about two orders of magnitude smaller than J2. Then from Eq. (1), for steady-state condition, the concentration of NO2 at a particular height during daytime is given by: ½NO2  ¼ k 1 ½NO½O3 =J 2

ð2Þ

One can see from Eq. (2) that NO2 concentration depends on k1 (which is temperature dependent), [NO], [O3], and J2. NO2 layer is situated at an altitude above 20 km. Our balloon measurements show that in this altitude region, during September to December, there is almost no variation in [O3] but there is an increase in temperature by about 20 K. This increase in temperature will increase the value of k1 by a factor of 1.98. As a result, there will be an in-

91

crease in the density of NO2 by a factor of 1.98. There will be an increase in photo-dissociation rate J2 by a factor of 2.34 as the solar zenith angle decreases from September (the average lowest value of noon time zenith angle in September = 73°) to December (the average lowest value of noon time zenith angle in December = 47°) during noon time. The effect of that increase in J2 should decrease the density of NO2 column by a factor of 2.34. When the effects of both k1 and J2 are incorporated in Eq. (2), the density of NO2 column is not found to change significantly from spring to summer. In the polar night, during winter, NO does not exist. With the appearance of the sun, the density of NO starts increasing rapidly. As a result, a large increase in NO concentration from September to December takes place. This increase in NO concentration could explain the seasonal increase in [NO2] as seen in Fig. 2. Here there could be the effect of transport also to increase or decrease [NO2] which we have not taken into consideration. For nighttime, Eq. (1) can be written as: d½NO2 =dt ¼  k 3 ½O½NO2   k 4 ½O3 ½NO2   k 5 ½NO3 ½NO2 ½M

ð3Þ

In this equation, k3[O][NO2] is negligible compared to k4[O3][NO2]. Also the rate-limiting step governing the formation of N2O5 at night is R4 which is quickly followed by R5. Then the decay of NO2 at night is given by d½NO2 =dt ¼ 2k 4 ½O3 ½NO2 

ð4Þ

If sr and ss denote sunrise and sunset times and Dt = duration of night (=ss–sr times) then one can write ½NO2 sr ¼ ½NO2 ss  expð2k 4 ½O3 DtÞ

ð5Þ

It is clear from Eq. (5) that [NO2]sr depends on [NO2]ss, k4, [O3] and Dt. In the middle of September Dt = 13 h (sun sets around 1600 UT and rises around 0500 UT) and in the middle of December (since there is no sunset) Dt = 0 h. If we take T = 220 K and [O3] = 4  1012 cm3 at 20 km, then according to Eq (5), [NO2]sr in December will be about a factor of 2 greater than [NO2]sr in September. This increased sunrise time NO2 value from September to December will increase the noon time NO2 value from September to December. Yela et al. (2000) have studied the effect of sunlit hours on the ratio of evening to morning values of NO2. When Dt = 0 h, Eq. (5) gives [NO2]sr = [NO2]ss. That means when there is no loss of NO2 due to the absence of darkness; there should not be any diurnal variation, which is contrary to what has been observed by us (Fig. 3). During the day time, solar zenith angle starts decreasing from morning onwards, becomes the minimum during noon time and then starts increasing. A decrease in zenith angle increases the value of J2. This should decrease the value of NO2 with the decrease of zenith angle, opposite to what is observed. Thus an increase in photo-dissociation rate of NO2 cannot explain the diurnal variation of NO2. Also no significant change in temperature during a day is likely. As a result, k1 does not change during a

92

D.K. Chakrabarty, S.K. Peshin / Advances in Space Research 47 (2011) 86–93

day. Then, according to Eq. (2), the nature of diurnal variation in NO2, as shown in Fig. 3, can be explained by diurnal variation of NO. The above discussion shows that both seasonal and diurnal variation of NO2 can be explained by an increase in NO density with the decrease in solar zenith angle and vice versa. An increase in NO density with the decrease in solar zenith angle is possible through the following two reactions: O3 þ hm ! Oð1 DÞ þ O2 ð1 Dg Þ J 9 ¼ 1:3  105 s1

ðR9Þ

1

Oð DÞ þ N2 O ! NO þ NO k 10 ¼ 6:7  1011 cm3 s1

ðR10Þ

1

Oð DÞ þ N2 þ M ! N2 O þ M 0:6

k 11 ¼ 3:5  1037  ð300=T Þ cm6 s1 ðR11Þ With the decrease of zenith angle (during spring to summer), the value of J9 (in Hartley continuum, k < 295– 300 nm) will increase which will increase the concentration of O(1D) leading to an increase in NO density via reaction (R10). An increase in O(1D) concentration will also increase the concentration of N2O through the reaction (R11) which will further increase the concentration of NO through (R10). Reactions (R9)–(R11) also explain the feature shown in Fig. 2 that UV-B (k, 280–320 nm) flux and NO2 column density have good correlation. Due to the decrease of zenith angle from 73° to 43° during spring to summer, the value of J9 will increase by a factor of 2.34, thus leading to an increase in the value of NO2 column at least by the same factor. According to Chipperfield and his coworkers, seasonal cycle of NO2 is dominated by exchange with HNO3, whose reaction with OH produces NO2, and there is more OH when there is more sunlight hence the summer maximum in NO2. Their theory is as follows: HNO3 þ OH ! NO3 þ H2 O k 12 ¼ 7:2  1015  e785=T cm3 s1

ðR12Þ

NO3 produces NO2 by reaction (R6). O(1D) produced by R9 reacts with many species; two important among them produces OH as follows: Oð1 DÞ þ H2 O ! OH þ OH 10

1

ðR13Þ

k 14 ¼ 1:5  1010 cm3 s1

ðR14Þ

k 13 ¼ 2:2  10

3

cm s

OH þ NO2 þ M ! HNO3 þ M 3:2

k 15 ¼ 2:0  1030  ð300=T Þ cm6 s1 ðR15Þ 1

O( D) is also quickly deactivated by collision with major species and make transitions to ground state O which subsequently form O3. O(1D) also reacts with O3 to form ground state O or O2. We have not considered these complicated exchange processes in our study. The physical mechanism by which an increase in NO density and hence NO2 density could take place during summer is through reactions (R9)–(R11). We have not considered HNO3 exchange channel in our calculation because for that we have to consider several species whose concentration is not precisely known. Also there is less HNO3 in the poles. In fact, the principal source of all nitrogen compounds is N2O which after reaction with O(1D) will produce NO and NO2. Since there is more O(1D) during summer, there will be more NO and more NO2 during summer. 5. Conclusion The India Meteorology Department has measured simultaneously NO2, and O3, column amounts and UV-B flux at the ground for more than 5 years at Maitri by Brewer spectrophotometer. Analysis of data shows that NO2 has seasonal variation with peak in summer. The peak NO2 value increases from 1999–2000 to 2001–2002 and then decreases. The peak values during 2002–2003, 2003– 2004 and 2005 are much lower than those during 1999– 2000, 2000–2001 and 2001–2002. Such a behavior is not seen either in O3 column or in UV-B flux values. Our averaged peak value varies from 0.46 in 1999 to 1.17 in 2001 to 0.28 DU in 2006. Analysis also shows that during no-night period in summer, NO2 has diurnal variation with peak around noon time. We explain the seasonal and diurnal variations of NO2 column by an increase in NO density from winter to summer and an increase in NO density with decrease in solar zenith angle during the daytime. Both neutral chemistry model study that includes transport and measurements are needed to validate our results. This should be one of the topics of study for SPARC. Acknowledgement We thank one referee for taking pain to go through the manuscript critically and giving suggestions which have greatly improved the quality of the paper.

Oð1 DÞ þ CH4 ! OH þ CH3

Both (R13) and (R14) are much faster than (R9). Therefore more O(1D) is likely to go in (R13) and (R14) channels than in (R9). But OH is lost by reactions with many species including one which destroys NO2 through the following reaction:

References Chakrabarty, D.K., Peshin, S.K. Effect of stratospheric O3 depletion on tropospheric SO2 column in Antarctica. Journal of Atmospheric and Solar-Terrestrial Physics 69, 1377–1387, 2007. Chakrabarty, D.K., Peshin, S.K., Srivastav, S.K., Shah, N.C., Pandya, K.V. Further evidence of total ozone variation during solar eclipse of 1995. Journal of Geophysical Research 106, 3213–3218, 2001.

D.K. Chakrabarty, S.K. Peshin / Advances in Space Research 47 (2011) 86–93 Cook, P.A., Roscoe, H.K. Variability and trends in the stratospheric NO2 in Antarctic summer, and implications for stratospheric NOy. Atmospheric Chemistry and Physics 9, 3601–3612, 2009. Gil, M., Cacho, J. NO2 total column during the 1999 spring at Antarctica peninsula. Journal of Atmospheric Chemistry 15, 187–200, 1999. Kerr, J.B., McElroy, C.T., Olafson, R.A. Measurement of ozone with the Brewer ozone spectrophotometer, in: Paper Presented at the International Quadrennial Ozone Symposium, sponsored by the International Ozone Commission and the International Association for Meteorology and Atmospheric Physics. Boulder, Colo, 1980. Kerr, J.B., McElroy, C.T., Wardie, D.I., Olafson, R.A., Evans, W.F.J. The automated Brewer spectrophotometer, in: Zerefos, C.S., Ghazi, A. (Eds.), Atmospheric Ozone. D. Reidel, Norwell, pp. 396–401, 1985. Keys, J.G., Johnston, P.V. Stratospheric NO2 and O3 in Antarctica: dynamic and chemically controlled variations. Geophysical Research Letters 13, 1260–1263, 1986. Roscoe, H.K., Kerridge, B.J., Gray, L.J., Wells, R.J., Pyle, J.A. Simultaneous measurements of stratospheric NO and NO2 and their comparison with model predictions. Journal of Geophysical Research 91, 5405–5419, 1986.

93

Sander, S.P., Friedl, R.R., Golden, D.M., et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies. JPL Publication 02-05, 2003. Sanders, R.W., Solomon, S., Carroll, M.A., Schmeltekopf, A.L. Visible and near-ultraviolet spectroscopy at McMurdo station, Antaratica, 4. Overview and daily measurements of NO2, O3 and OClO during 1987. Journal of Geophysical Research 94, 11381–11391, 1989. Shibasaki, K., Iwagami, N., Ogawa, T. Stratospheric nitrogen dioxide observed by ground-based and balloon-borne techniques at Syowa station (69.0°S, 39.6°E). Geophysical Research Letters 13, 1268–1271, 1986. Vanicek, K., Stanek, M., Dubrovsky, M. Evaluation of Dobson and Brewer total ozone observations from Hardec Kralove, Czech Republic, 1961–2002. Report of the project CANDIDOZ, Working Group WG-1, 5th RTD Framework Programme, Project No. EVK2CT-2001-00133. Prague, Czech Republic, 2003 Yela, M., Rodriguez, S., Gil, M., Iglesias, J., Ozu, R., Diaz, S., Chipperfield, M. Seasonal evolution of NO2 and O3 outside, at the edge and inside the Antarctic Polar Vortex: Comparison with the SLIMCAT model. http://www.aero.jussieu.fr/~sparc, 2000.