Long Term Changes Of Total Ozone

Published Date: 02 Nov 2017

B. M. Vyasa [] and Vimal Saraswata, b

aDepartment of Physics, M. L. Sukhadia University, Udaipur – 313 001, India ; bPacific College of Engineering, Udaipur – 313 003, INDIA

Abstract

This paper focuses the long-term declining change in TOC derived from satellite measurements of twenty-five-year period over two close Asian tropical sites Karachi and Mt. Abu. It is observed that a strong declining trend in TOC, with the good significance level above 95%, is seen to be higher magnitude, i.e., 4 to 10DU/decade in September to December and a weak statistical significance level below 85% with the lower magnitude of 2 to 4DU/ decade in pre-monsoon months over both places. However, during the monsoon months, a small declining trend is observed about 1DU/decade, and this variation is statistically insignificant. Further, the long-term changes in TOC exhibit the seasonal dependence with more negative change of 10DU/decade in winter over Karachi and the less i.e., -7 DU/decade over Mt. Abu. The consequence of such a net long-term declining change in the TOC as high as 10 DU/decade is expected to be serious for environmental implication due to overall enhancement in 18% of ground level solar UV-radiation from their normal value over the tropic. In order to identify some of the plausible causes of the depletion trend in the ozone concentration with stratospheric temperature and solar activity, it is quite clear that there is a strong league between the seasonal dependence of long-term declining trend of TOC/decade with air temperature at 10mb or stratospheric cooling. At the same time, there is a less significant long-term variation in TOC due to changing the solar activity level.

Key words: Total ozone column content; Long-term trend; stratospheric cooling

PACS Nos.: 92.05.df; 92.60.hd; 92.70cp

Background

The monitoring of the Total Ozone Content (TOC) is of great scientific relevance in present scenario to the global Earth’s climate and environmental issue in view of their active roles in complete absorption of the ultraviolet (UV) solar radiation below 280 nm and partially attenuation between UV-B (ultraviolet–Biological Radiation) solar radiation 290 to 320 nm i.e., radiative balance of the upper troposphere and lower stratosphere (Munro et al. 1998, Kondratyev and Varotsos 2000).

The presence of ozone in the Earth’s atmosphere protects the human biological life system from harmful solar UV-B radiation and also plays a pivotal role in controlling the thermal structure of the stratosphere (Ghude et al. 2005). Therefore, its measurement is important in the assessment of the radiative forcing of climate and protecting our human health from the incident harmful solar UV-B radiation by absorption in stratospheric height from 15 to 40 km (McKinley and Differy 1987, Pyle et al. 1994, Katsambas et al. 1997, Palancar and Toseli 2002, Ghude et al. 2008). About 90% of the TOC concentration resides in the stratospheric region, and its daily creation is primarily governed by dissociation of oxygen gas with UV incident solar radiation (below 280 nm) by photochemical reaction to form the highly reactive atomic oxygen, which quickly combines with molecular oxygen to form ozone (Ghosh and Midya 1994). However, in the special context of the tropical region, about 40% of global ozone is produced due to availability of a large abundance of the incident solar radiation over the outskirts of tropical region, and it is transported towards the higher latitude by atmospheric transport mechanisms (e.g., Brewer-Dobson circulation). Thus, the large amount of ozone accumulates at higher latitudes than the lower latitude, where it is produced (Mohankumar 2008). Hence, net TOC over tropical latitude is not only governed by absorption of UV-B radiation but also by the clouds, aerosols, large scale atmospheric transportation or stratospheric dynamics phenomena, i.e., Brewer-Dobson circulation as well as by reactions with long lived chemicals as a catalyst, such as, chlorine, bromine, hydrogen and nitrogen (chlorofluorocarbon) in the atmosphere, especially in low temperatures (e.g., Varotsos 2002, Mohankumar 2008, Krizan et al. 2011). In the year 1985, the ozone hole over Antarctic region was first well detected along with the cause of depletion of TOC concentration due to the availability of the emitted long-lived Ozone Depletion Substance (ODS) (Farman et al. 1985). Subsequently, a declining trend of TOC was observed even in middle latitudes of both hemispheres (Rowland et al. 1988). In this context, similar feature was also reported by Bialek (2006) using TOC data over Northern Hemisphere mid-latitude stations, who displayed the negative trend of TOC to be strongest in winter/spring and weakest in autumn for the period 1980-2003.

Earlier researchers demonstrate the consequences of the ozone reduction in terms of enhancement in the incident solar UV radiation of shorter wavelengths ranging above 320 nm and allow more biologically damaging UV-B radiation to reach the Earth’s surface. They have established the crucial statement and important finding that about 1% decrease in TOC could cause about 2% increase in the harmful UV-B radiation (Briihl and Crutzen 1989, Ghude et al. 2008, Varotsos 1994, 2005, Udelhofen et al. 1999).

Keeping the above views and perspectives in mind, the largest number of TOC studies has been made in past decades targeting to elucidate the variability of TOC at distinct parts of the globe in several time scales ranging from solar cycle (11years) to planetary wave scale (few days) including seasonal, monthly and diurnal variations, etc. (Wirth 1993, Logan 1994, Fusco and Salby 1999, Chandra et al. 1999, Chakrabarty et al. 1998, WMO 1999, 2003, Dobson et al. 1929). But, after the great scientific discovery of the present century, "Ozone Hole" phenomena over Antarctic region, assessment of long-term changes in TOC over other parts of the world has been motivated from these works and now become frontier research topics (Dobson et al. 1929, Godson, 1960, Petzold et al. 1994, Chandra et al. 1996, Chakrabarty et al. 1998, Petzold 1999, Harris et al. 2001, Singh et al., 2002, Sahoo et al. 2005, Bialek 2006). Earlier works based on the ground and satellite measurements have shown an average annual declining trend in the higher TOC range from 30 DU/decade to 20 DU/decade over North America, Europe, Asia and much of Africa, Australia and South America (Varotsos and Cracknell 1994, Bojkov et al. 1995, Chandra and Varotsos 1995, Gernandt et al. 1995, Kondratyev et al. 1995, Chandra et al. 1996, McPeters et al. 1996, Harris et al. 2001, Efstathiou et al. 2003, Stolarski et al. 2006).

Over the Indian subcontinent or tropical region, which is a region of great relevance due to maximum ozone production site and observed the lowest TOC magnitude, the first ever resulted about the declining of the ozone per decade over the Indian locations was reported as high as about 10 to 5DU/decade by Chakrabarty et al. (1998). Later, Singh et al. (2002) and Sahoo et al. (2005) have sited similar features over Northern and Central Indian region. Recently, Tandon and Attri (2011) have created a lot of attention and discussed similar detailed investigation of long-term variability of TOC over a number of Indian sites covered from Northern to Southern and Eastern to Central Indian major cities. Their findings pointed out that the larger rate of annual decline of TOC is also high about 9DU/decade over Northern Indian zone compared to those observed low value of -5DU/decade over Central Indian region. However, a statistically insignificant in long-term TOC declining trend is seen over the Southern Indian region. Previous researchers interpreted their results of annual declination of TOC based on a number of hypotheses of stratospheric dynamic parameters, mainly of chemistry coupled processes or chemical depletion by long lived halogen species as catalysis (Petzold et al. 1994, Petzold, 1999, Singh et al. 2002, Patra and Santhanam 2002). In the direction to identify the possible causes for the depletion of TOC, a couple of Indian researchers like Jana and Nandi (2005a, 2005b, 2006), Jana et al. (2011), Midya and Saha (2011) & Saha et al. (2011) have reported several aspects assuming the interrelation of ozone depletion among corresponding decrease on night-airglow emission intensities per decade, thunderstorm activities and variable components of 10.7 cm of solar flux (SF) in different seasons over several parts of Indian regions.

Realizing the importance of the net increase in incident UV radiation intensities due to declining of TOC, an attempt has been made in the present work to extend similar studies about long-term changes in the TOC, specifically on a monthly basis over two close Asian tropical locations, i.e., Karachi (Pakistan) (24.87ï‚°N, 67.05ï‚°E, 6 meters) and Mount Abu (Mt. Abu, India) (24.60ï‚°N, 72.72ï‚°E, 1220 meters) using satellite measurements of TOC for the period 1979 to 2004. As the majority of earlier studies are based on long-term changes in TOC basis on annual TOC value, the very few studies have been available in the literature on monthly or even on a seasonal basis of the declining trend in the TOC. Therefore, in the present investigation, long-term changes in TOC on the monthly basis for the period of 1979 to 2004 have been described over two close tropical stations. Furthermore, in the present study, our main focus is also put on identifications of its most possible causes based on stratospheric Air Temperature at 10mb (AT) as well as on the solar activity level (Hadjinicolaou et al. 1997, 2002, Singh et al. 2002).

The main reason for choosing these particular sites is that these two tropical sites are quite near to each other, i.e., rather than similar latitude and longitude but the differences are in their altitudes. One of them (Karachi) is located at sea level, while the other (Mt. Abu) is nearby the top of the atmospheric boundary layer. Furthermore, Karachi is one of the major densely populated and highly industrialized polluted urban city concerned to the large varieties of anthropogenic activities, i.e., industries, transportations, vehicles, etc., whereas Mt. Abu is the clean and fewer populated hilly rural region.

Dataset

The longer period data sets of two and half decade used in the present study are (i) daily value of TOC raw data set over Karachi (Pakistan) and Mt. Abu (India) (ii) monthly mean value of stratospheric atmospheric parameters likes AT over the similar geographical regime. The daily TOC values are measured by Total Ozone Mapping Spectrometer (TOMS) instruments flown on the three polar satellites plate forms Nimbus –7 (N7) from January 1979 to June 1993, Meteor – 3 (M3) from July 1993 to November 1994 and Earth Probe (EP) during the period from July 1996 to December, 2005. Such daily raw TOC data are accessed from ftp://toms.gsfc.nasa.gov/pub/ for the present study. The average monthly values of stratospheric atmospheric parameters like AT are retrieved from http://www.ready.noaa.gov.

Data analysis and Methodology

The daily TOC values are used to compute the individual monthly average value along with their corresponding standard deviation values for the complete study period to eliminate its monthly and seasonal dependence. The year to year variation of each  average monthly value of Solar Fluxes (SF) at 10.7 cm, a parameter for solar activity level, were also used for the same specified period to understand the extent of variation in the TOC due to solar cycle variation. In order to delineate the long-term change in the TOC and identification of its possible causes, particular monthly mean values of the TOC of the whole specified period are plotted with function of years along with similar yearly average monthly values of stratospheric atmospheric temperature parameter, i.e., AT over the similar geographical site. Besides these, such monthly time series of TOC and AT are subjected to statistical linear regression analysis to obtain the trend analysis parameters like slopes and correlation coefficients (R) etc. of the linearly fitted line. Such statistically linearly regression fitted analysis variables are arranged in the tabular form (Table 1). The average reference monthly values of TOC are also calculated from the average of its all individual monthly mean values of the entire period as given in Table 2. It will give the inference about the base level of monthly TOC value, which will further give the indication about the understanding of statistical significance in terms of long-term variability of TOC for the selected study period. The slope and R values give the information about the changing of the relevant parameters per year. In order to evaluate the statistical relevance of long-term trend TOC in different months as well as dependence with stratospheric temperature at 10mb and SF 10.7cm, the probability (P) is evaluated from their respective correlation coefficient values and number of data point for each individual group of the month during the specified study period. The percentage of the confidence significance level (% level) is computed from the P values using the following expression as

= 100 × (1P).

Table 1

Table 2

In order to assess the contribution of the negative trend of TOC per decade due to AT and SF, the variation of the R and P for O3 v/s AT and O3 v/s SF for each month is illustrated in Figure 1. The results, obtained from the analysis of the tables and figures, have been discussed in the next section.

Fig. 1

Results and Discussion

It is found from the statistical analysis parameters as given in Table 1, that a strong statistically significant declining trend in TOC, with the good significance level above 95%, is seen to be higher magnitude about 4 to 10DU per decade in September to December. In pre-monsoon, the lower magnitude of TOC depletion about 2 to 4 DU per decade is observed with the weak statistical significance level of below 85%. An insignificant negative trend of TOC about 1 DU/decade is seen in monsoon months over both places. Thus, the magnitude of declining TOC value per decade gradually reduce from the winter months onward, and it further continuously decrease from pre-monsoon to monsoon months as also depicted in Figure 2. Furthermore, the highest change in its declining trend per decade value is found in December, which is more over Karachi (-10 DU per decade) in comparison to their observed corresponding fluctuation -7 DU per decade over Mt. Abu. Hence, the long-term changes in TOC exhibit the distinct seasonal dependence with maximum negative change 10 DU/decade and 7 DU/decade in winter, followed by pre monsoon and post monsoon (4 to 7DU/decade) and the minimum (1 to 2DU/decade) in monsoon months over Karachi and Mt. Abu, respectively. Although, Jain et al. (2008) have also reported the similar features from analysis of TOMS ozone, available data set for the period 1979-2004 over Indian station, i.e., New Delhi. They also observed that a greater declining trend in TOC was observed in the range of a 7-9DU / decade during post-monsoon to winter, followed by a 4-6DU/decade in pre-monsoon and insignificant statistical change in TOC magnitude as low as 2DU/decade in the remaining monsoon months. Likewise, higher-order fluctuation in ozone declining trend was reported during the winter season from the analysis of long-term trend in TOC over mid and high-latitude stations, which further supports the present findings (Bialek 2006). They suggested one of the possible causes of long-term change in the TOC based on changes in the atmospheric transport of ozone or circulation in both the troposphere and stratosphere (Godson 1960).

Fig. 2

To infer about the annual average monthly variations in the TOC based on the 25-year data, it is quite obvious from Table 2 that the mean monthly TOC values over Karachi systematically started to increase from a minimum value of 241 DU in December onward and attain its peak value as high as 279 DU in June, afterwards its magnitude gradually decreases from 277DU to 247DU during July to November. In case of Mt. Abu, its overall nature of mean monthly variation in the TOC is quite comparable and similar to the average monthly behavior of TOC over Karachi. The higher TOC value over Mt. Abu is found in the magnitude of 276 to 270 DU from April to August, intermediate range from 265 to 255 DU during September, October, February and March and lower values occur in the range of 238 to 244 DU in the remaining months, i.e., in winter months. Thus, deviation of TOC magnitude over Karachi to Mt. Abu is seen in order of +3 DU. Its difference may be attributed due to a difference of altitude of two sites or availability of the extra ozone as surface ozone level present between the ground levels to 1.3 km altitude due to extra anthropogenic activities. Since previous TOC studies over other tropical stations highlight the similar features such as the higher TOC magnitude (270-285 DU) in pre-monsoon month and its lower magnitude (240-245 DU) in winter. It is also expected from the increase of the average temperature from winter month to summer month and the correspondingly increase in TOC from winter to summer by photolysis, suggesting the dominance of photochemical production (London 1979). It further confirms and supports to earlier workers' findings with the present results of seasonal variation of TOC from analysis of more than the two-decade period of TOC measurements (Shimazaki 1985, Kondratyev and Varotsos 2000, Pandey and Vyas 2004, Mohankumar 2008). Such seasonal TOC features can also be well correlated with one of more plausible parameters based on corresponding average monthly variation in AT and solar activity index F10.7 cm. From the Table 2; it is quite imperative that average monthly variation in the TOC is seen more positively linked to the corresponding monthly variation of AT i.e., the amount of increase in TOC is in accordance with similar enhancement in its AT magnitude. At the same time, there are either inconsistent or very low correlation values (0.2 to 0.5) between the monthly behaviors of TOC with the corresponding monthly variation of solar activity level compared to those observed higher correlation value of TOC with the AT. In this context, Midya and Saha (2011) have also shown almost similar observations , which partially corroborate to the present work that the significant positive correlation value (0.5 to 0.6) is observed between the varying component of solar activity flux (F10.7 cm) with the rate of change in TOC during the post-monsoon and winter season, while in the pre-monsoon and monsoon period, an insignificant co-variation (value less than 0.15) is reported between rate of change of total column ozone with the increasing variable component of 10.7 cm SF over Dum-Dun (22.63ï‚°N, 88.43ï‚°E), India.

The decadal variations in AT and SF as the function of month are also illustrated in Figure 2. It is seen that decreasing trend in AT over lower stratospheric layer is found to be higher order of 1.2 ± 0.015K per decade with the best statistical significance level during the winter months, while its lesser values up to 0.2 ± 0.008K/decade with good statistical significance levels in April to August, month. Thus, the stratospheric cooling phenomenon is more prominent in winter month, which is here coinciding with the similar features as an observed highest change in a negative decadal change in TOC in winter months. The declining trend in TOC value and stratospheric temperature, i.e., cooling effect are found to be higher about 4 to 10 DU and 1.2K per decade, correspondingly, in winter months, lower values about 1 to 4 DU and 0.2K per decade relatively, in the monsoon period. As far as concerned with year to year change in monthly value of SF with the long-term trend of TOC, it is clearly stated that statistically significant negative trend in the lower range of SF magnitude (5 units) is found in monsoon months; while in another month except winter month their corresponding values vary from 5 to 10 units. The SF changes per decade in winter months are also statistically insignificant with the variation of TOC. At this juncture, it is an also noticeable fact that the yearly change in monthly TOC values' exhibit more closely association with a similar trend of year to year change of AT than observed corresponding variation in the SF values. This feature gives the support and indication about the interrelation between the declining of TOC with cooling of stratospheric phenomena. Another argument about the declining of TOC due to the presence of ODS can also lead to strong stratospheric cooling in winter. Ramaswamy et al. (1996) have also established the similar interrelation between depletion of ozone and stratospheric cooling.

In order to better understanding as well as established the statistical significance relevance of the present reported results, variation of the correlation coefficient values between average monthly variations of TOC with the AT and SF for each particular month during the entire study period is portrayed along with their corresponding probability values (Figure 1). It is very interesting to note that in case of O3 v/s AT, the highest correlation coefficient values in the range of 0.6 to 0.7 (with the best percentage of significance level up to 99.99%) are observed in most of the months except July to September over Karachi and June to September over Mt. Abu. However, in case of O3 v/s SF, the lower correlation coefficient values in the range of 0.4 to 0.5, with percentage of maximum significance level value 90%, are found during most of the month in case of O3 v/s SF. Such results give the clear indication that there is the strong significance of interrelation between the seasonal dependence of long-term change in the declining trend of TOC with the stratospheric cooling event. Nevertheless, there is less appreciable long-term variation in TOC due to SF or solar activity level.

Although, the observed features of highest TOC value in pre-monsoon can be well documented by assuming the argument suggested in the introduction section. Since the primary mechanism for production of stratospheric ozone is through solar UV-radiation, thus greater amount of TOC is generated due by linking the photochemical activity with the presence of the incident solar radiation UV-level or indirectly dependent with the solar activity level. Due to availability of the higher amount of solar UV radiation level or high value of SF at F10.7 cm or in high solar activity period and its consequences on larger rate of photochemical activity and reaction rates lead to production of more ozone in hot months. It further gives an indication of the higher ozone level and higher stratospheric temperature in pre-monsoon month, which are furthermore supported by the present findings. It may also argue with a similar analogy that a strong declining in TOC undergoes in winter season may be associated with more cooling in stratospheric height and corroborated to the present observations. Based on the aforesaid discussion, it is quite obvious that consequence of long-term negative trend in TOC during winter may also seen in reducing the heating rate or enhance the stratospheric cooling due to availability of lesser amount of TOC in long term basis. It may also be pointed at here that in addition to the earlier, other important possible cause for the long-term declining trend in TOC could also be due to presence of longer lived chemical species as a chemical catalyst of ozone-depleting substances (Farman et al. 1985).

Conclusion

This study has presented the long-term change and trend in TOC on the monthly basis over two close Asian tropical site based on TOMS satellite measurement period from 1979 to 2004 with aiming further at investigating the possible causes for such negative change of TOC in per decade. Based on the above discussions, the following main conclusion can be drawn, which are summarized as below:

Long-term trend analysis of TOC on a monthly basis at the above-mentioned sites exhibit distinct seasonal dependence. Its monthly trend reveals the more pronounced long term declining trend in TOC (9-7 DU/decade) with the best statistical significance level above 99.9% in the winter and post-moonsoon months. However, in pre-monsoon months, its lower negative trend in the TOC level (6 DU/decade) is seen in the same period with good statistical significance level. There is either absence or statistically insignificant long-term change in TOC in monsoon months. The consequence of such a net long term declining change in TOC order as high as 9 DU is in the form of the overall enhancement in 18% of ground level UV- radiation from their normal value over the tropic.

The seasonal change in long-term change in TOC has shown good similarities with more seasonal changes in long-term variation in AT in comparison to the less seasonal change in long-term changes in solar activity intensity. It is more worthwhile to be noticed on here that depletion of TOC may also be responsible to produce the stratospheric cooling phenomenon or vice versa.