To Reveal the Aerosol Vertical Profiles in Terms of Aerosol Number Density

Aerosol measurements have been carried out at Kolhapur (16°42′N, 74°14′E) by using newly designed Semiautomatic Twilight Photometer. It is observed that, the AND over Kolhapur in volcanically quiescent period declines from 185 particles per cm3 (~6 Km) to about 1 particle per cm3 (~44 Km). The aerosol layer peak occurs at ~20 Km (5 particles per cm3). Mesospheric AND decreases from ~700 particles per dm3 (~50 Km) to ~ 225 particles per dm3 (~85 Km). Thermospheric AND vary from 225 particles per dm3 (~85 Km) to ~105 particles per dm3 (~250 Km). After ~ 250km, many deviations noticed up to 350km in between 100-120 particles per dm3. The main aim of this study is to demonstrate the measurement capabilities offered by semiautomatic twilight photometer and it is the first attempt in India made by the author to estimate mesospheric and thermospheric aerosol number density per cubic decimeter (AND/ dm3).


INTRODUCTION
Aerosols affect the progress of twilight, as more the number of aerosols in the atmosphere, more the sky brightness during the twilight period. Hence, the measurements for the twilight sky brightness carried out by using a twilight photometric technique to retrieve the aerosol vertical profiles. During the course of the study, the measurement of the atmospheric aerosols carried out by using semiautomatic twilight photometer during the period of 1January 2009 to 31 December 2011at Kolhapur (16°42′N, 74°14′E), Maharashtra state, India. The twilight scattering method yields a reasonable qualitative picture of the vertical distribution of aerosols from about 6 km to a maximum of 350 km. This being a passive technique, clear sky conditions are preferable for obtaining the vertical profile of aerosols.
Logarithmic gradient of the intensity cannot give the information about the aerosol number density (AND). Therefore, an imperial formula was derived with the help of actual Lidar observation and Photometric observations. The main aim of this study is to demonstrate the measurement capabilities offered by semiautomatic twilight photometer and it is the first attempt in India made by the authors to estimate mesospheric and thermospheric aerosol number density per cubic decimeter (AND/ dm3).

INSTRUMENTAL SETUP
The instrument, semiautomatic twilight photometer, has been newly designed, developed and tested at IITM, Pune, India. The semiautomatic twilight photometer consists of a simple experimental set up. It comprises of a telescopic lens of diameter 15 cm having a focal length of 35 cm.amd a red glass filter peaking at 670 nm with a half band width of about 50 nm. The red filter of 2cm diameter and an aperture of 0.6 cm diameter are placed at the focal length of convex lens, provides approximately 10 field of view [(Aperture diameter/Focal length of lens) X57= (0.6cm/35cm) X 7=0.9771 degree]. A photomultiplier tube (PMT)-9658B is used as a detector. The PMT requires a high voltage i.e. 700Volts and hence a high voltage power supply is used. The output signal (current) of the PMT is of the order of nano to microamperes. The amplitude or strength of this low signal is amplified by using newly designed fast preamplifier. The more details regarding the instrument and Fast pre-amplifier were given elsewhere, (Mane et al., 2012, a and b). The amplifier output recorded by the digital multimeter, Rishcom-100, having an adapter can store the data automatically for every 10secs in the form of date, time and intensity in Volts.

RESULTS AND DISCUSSION
The basic principle of the twilight sounding method is given elsewhere, (Mane et al., 2012, c). Thus according to Shah (1970), Here 'I' is the observed intensity; 'dI' and 'dh' are the differences in intensities and shadow heights, respectively observed at time 't' and '(t+dt)'.
The aerosol number density was calculated by using an imperial formula derived by actual LIDAR observations and Twilight Photometric observations as stated below:

RESEARCH PAPER
The stratosphere is the second major layer of Earth's atmosphere situated between about 17 km and 50 km altitude above the surface of the Earth. Figure-3 shows typical vertical profile for stratosphere, (Fig3: a); which is also divided into three parts. In lower stratosphere, the value of AND starts increasing slightly from 3 particles per cm 3 (~17 Km) due to the presence of stratospheric dust layer, also called as Junge layer (Junge et.al., 1961). Two aerosol layer peaks occur at ~20 Km and ~24Km having the aerosol number density of 5 particles per cm 3 for each (Fig3:b). After that, the value of AND decreases as the altitude increases. It becomes 1 particle per cm 3 at ~44 Km (Fig3: c).
After 45 Km the AND is less than 1 particle per cm 3 . It is inconvenient to say some fraction of particle per cm 3 (i.e. say 0.944 particle per cm 3 ). So the AND for the upper stratosphere is expressed in terms of the aerosol number density per dm 3 (AND/ dm 3 ). Thus, it is convenient to say 944 particles per dm 3 instead of 0.944 particles per cm 3 . The AND in the upper stratosphere vary from 944 particles per dm 3 (~45 Km) to 722 particles per dm 3 (~50 Km) (Fig3: d).
In short the aerosol number density in stratosphere vary from 3 particles per cm 3 (~17 Km) to 1 particles per cm 3 (~44 Km). The aerosol layer peak occurs at ~20 Km having the aerosol number density of 5 particles per cm 3 . Ramachandran et al. (1994) have reported that the particle density over Hyderabad in volcanically quiescent period (22 October 1985) vary from 80 particles per cm 3 (at 6 Km) to 9 particles per cm 3 (at 32 Km). Also Nighut et al. (1999) have reported that the particle density over location Pathardi in volcanically quiescent period (29 October 1993) vary from 100 particles per cm 3 (at 6 Km) to 1 particles per cm 3 (at 50 Km). The aerosol number density obtained in present work is supporting the results of earlier workers.

Vertical profiles of the Mesospheric aerosol number density per dm 3
The mesosphere is the third major layer of the Earth's atmosphere located about 50 to 85 kilometers above the Earth's surface. It is the most poorly understood part of the atmosphere. In the present work an attempt is made to calculate the mesospheric aerosol number density per dm 3 (AND/ dm 3 ) by TSM, for the first time in India. All of the earlier workers obtained AND up to stratospheric level only.

SUMMARY AND CONCLUSIONS
The measurements using the twilight sounding method presented in this paper suggest the following, I. The newly designed Semiautomatic twilight photometer yields a reasonable qualitative picture of the vertical distribution of aerosol number density (AND) from about 6 km to a maximum of 350 km. II. Tropospheric aerosol number density over Kolhapur in volcanically quiescent period (3 January 2010-Morning) vary from 185 particles per cm 3 at ~6 Km to about 2 particles per cm 3 at ~15 Km.
III. Stratospheric aerosol number density vary from 3 particles per cm 3 (~17 Km) to 1 particles per cm 3 (~44 Km). The aerosol layer peak occurs at ~20 Km having the aerosol number density of 5 particles per cm 3 .
IV. Mesospheric AND decreases with altitude from ~700 particles per dm 3 (~50 Km) to on an average of 225 particles per dm 3 (~85 Km).

Acknowledgements:
Author is grateful to the Shivaji University authorities, Kolhapur, for the encouragement during the course of this work and also the SERB, DST, India, for providing funds for the development of twilight photometer.