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The solar UV radiation has prominent impacts on human life, animals and plants with positive and negative effects. Atmospheric ozone, which is formed from the photo- dissociation of molecular oxygen mainly present in the stratosphere, absorbs a significant fraction of solar UV radiation .The ozone in the stratosphere acts as a protective layer to prevent UV radiation from reaching on the surface of the earth. Thus the intensity of solar UV radiation on the surface has a strong dependence on the Total Ozone Column (TOC). The UV irradiance on earth surface primarily depends on geometrical factors such as solar zenith angle, altitude, latitude and on other atmospheric parameters as well. This is an attempt to study the variation of solar UV flux at its four discrete wavelengths ranging from 305 -380 nm at Kannur (11.93 N, 75.36E) located in the northern part of Kerala. This is the first attempt to establish a correlation study of these two vital parameters at this location. Subsequently, a correlation of TOC and UV irradiance is pertinent to realize the radiation budget at this location using AURA OMI data. This paper pertains to the correlation of day to day, month to month temporal variation of total ozone column (DU) and UV irradiance (w/m2).The study reveals that a small change in ozone concentration can significantly increase the UV radiation which resulted sun burn occurred in this location.
Ozone being a trace gas in the earth's atmosphere plays a crucial role in the chemistry of the atmosphere (Satheesh et al 2010). A number of factors contribute to ozone's importance but perhaps the most imperative feature is the relationship between the UV absorption efficiency of ozone and the intensity of solar UV radiation received on the surface which is harmful to living beings (Chakrabarthy et al, 1998). The study of ultraviolet (UV) solar radiation reaching the Earth's surface has achieved a global concern after the report of ozone depletion (WMO, 2006). Thus, it is of a great importance to continue with high accuracy in the UV radiation measurements at different locations. Although the Sun emits a large spectrum of radiation, the UV radiation constitutes a small part of it. Regarding its biological effects, it can be divided in three main ranges UV-A (315-400nm), UV-B (280-315nm) and UV-C(100-280nm) (Chubarova et al., 2002). While the UV-C radiation is completely absorbed by atmospheric Oxygen (O2) and Ozone (O3) in the middle and upper atmosphere, the UVB radiation is absorbed efficiently though not completely by the stratospheric ozone (Purkait et al). Around 300-320nm wavelength region, the sole agent for absorption of this portion of the solar radiation is the atmospheric Ozone. Therefore, the intensity of solar UVB radiation falling on the surface depends on the ozone layer concentration and it is realized that more ozone should give less UV-B radiation at ground level (Niranjan et al 2004). The UVB radiation has been reported to cause skin cancer and several other effects on human health as well as detrimental effects on animal and plants (Buchard, 2008). Satellite UV data complement ground based measurements providing global daily maps with uniform geographical coverage from a single instrument (Anton et al, 2007). It has been classified that UVB radiation present in the solar energy is absorbed by ozone in the stratosphere and below; more ozone should give less UVB at ground level. As a general rule, the relationship between UV radiation and O3 is not easily found in the data, for many parameters besides O3 (as solar zenith angle, cloud cover, aerosol optical thickness, precipitable water column) have direct influence on the UVB irradiance (Blumthaler, 1993). The main objective of this study is to investigate the relation between the variation of total ozone column on clear sky days and its impact on UV irradiance and at this location over a span of one year.
The UV irradiance and TOC data sets consists of observations carried out by satellite -borne instrumentation, (TOMS, OMI). The Total Ozone Mapping spectrometer (TOMS) launched on board the Earth Probe satellite of NASA in July 1996 is still continuing by long term daily mapping of the global distribution of the distribution of ozone over a column of atmosphere. The Ozone Monitoring Instrument (OMI) has been operating since July 2004 on board EOS Aura which is a nadir- viewing imaging spectrograph that measures solar radiation back scattered by earth atmosphere and surface over the wavelength range from 300 to 500 nm with a spectral resolution of about 0.5nm.OMI combines the advantage of GOME and SCIAMACHY with the advantages of TOMS, measuring the complete spectrum in the ultraviolet /visible wavelength range with a very high special resolution (13 km x 24 km) and daily global coverage. In this study, data were retrieved only at cloudless, blue-sky conditions at similar solar zenith angles to eliminate much of the influences of non- ozone parameters described earlier. The UV irradiance and TOC were analyzed at pre-monsoon period (May), monsoon (June), post-monsoon (October) and winter (December) seasons at Kannur. The data correspond to 305 nm solar radiation and respective TOC were analysed in detail in a pre-monsoon season (May) in 2009 is depicted in fig.1.This pattern remains the same for all seasons and the variations in a specific season is shown in the figure.
Fig.1. Variation of clear sky irradiance for 305nm with TOC
Fig.2 shows the correlation between TOC and CS irradiance to explore the correlation between these two at Kannur during the pre-monsoon period in May 2009. The R2 value for the linear fit is found to be 0.944 and thus a good correlation is obtained.
Fig2. Correlation between TOC and CS irradiance on clear sky days during May 2009
Figure 3 shows a prominent illustration of the inverse relationship between UVB and O3 during post monsoon season (October) similar to that shown in Fig.1.
Fig.3.Correlation between TOC and CS irradiance on clear sky days during October 2009
Effective UVB and total ozone column are still exhibiting an anti correlation with each other and the correlation is statistically significant (r=-0.972), probably due to the same aerosol optical thickness. The squared correlation is also statistically important (R2=0.986).This analysis could further estimate that a decline in 100 DU ozone produces about 70 mW/m2 effective UV. The correlation between TOC and CS irradiance during the month of monsoon (June) 2009 is shown in the figure 4.
Fig.4.Correlation between TOC and CS irradiance on clear sky days during June 2009
During the month of June 2009, the effective UVB and TOC show still anti correlate (slope = 2.25) with each other and the correlation is statistically significant. The squared correlation is fairly not good (R2=0.896) compared to the previous analysis. This may be due to the aerosol loading and cloud cover. This indicates that a change in 100 DU ozone produces about 200 mw/m2 effective UV. The correlation between the total ozone column and the CS irradiance during winter month (December 2009) is shown in the figure 5.
Fig.5.Correlation between TOC and CS irradiance on clear sky days during December 2009
During the winter season the effective UVB and the total ozone column are still anti correlate (slope= -1.44) with each other and the correlation is statistically significant, possibly due to the aerosol loading. The squared correlation is also statistically important (R2=0.998). This means that a change in 100 DU ozone columns reduces about 110 mW/m2 effective UV. The Seasonal variation of squared correlation is shown in the table1.
Table1. Seasonal variation of squared correlation
UV index at Kannur and Palakkad during March 2010
In March 2010, wide spread sun burn cases were recorded in Kerala State for the first time and several cases were recorded over Kannur and Palakkad. Thus in this study an attempt was initiated to estimate the UV index at these two regions.
The variation of average value of TOC at Kannur and Palakkad during the month of March from 2005 to 2010 is shown in the following Table 2.
Table.2 Yearly variation of TOC at Kannur and Palakkad
The UV indices at Kannur and Palakkad were retrieved during March 2010 and the estimated values are 5.4 and 6 respectively with clear sky. When the UV index is over 9, UV-B is extremely strong and cause sun burn in less than 15 minutes of exposure in sunlight.
Results and Discussion
The solar UV irradiances on clear sky days during pre-monsoon, monsoon, post-monsoon and winter seasons have been successfully retrieved from OMI and this has been correlated with TOC over Kannur. As expected, a negative correlation has been obtained in all our observations. However, the correlation is much strong during post-monsoon and winter days due to relatively clean atmosphere after severe south -west monsoon activity in Kannur. During the pre-monsoon period, the atmosphere becomes quite turbid owing to heavy winds that transport aerosols and trace gases and these constituents make the degree of correlation relatively poor. The excess heat and sunburn reported in March 2010 may be due to a decline in TOC which enhances UVB over this region. Thus it is evident that the correlation of solar UV irradiance with changes in TOC becomes more precise during post monsoon and winter months over Kannur. It is proposed to extend this study over a long period of time to yield the exact correlation of these vital parameters by incorporating other dominant elements like Aerosol Optical Thickness (AOT) since marine aerosols are much abundant over this location since is confined in the coastal belt of the Arabian Sea.