Exploring Atmospheric Aerosols by Twilight Photometry
Total Page：16
File Type：pdf, Size：1020Kb
1600 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 25 Exploring Atmospheric Aerosols by Twilight Photometry B. PADMA KUMARI,S.H.KULKARNI,D.B.JADHAV,A.L.LONDHE, AND H. K. TRIMBAKE Indian Institute of Tropical Meteorology, Pune, India (Manuscript received 6 November 2007, in final form 28 January 2008) ABSTRACT The instrument twilight photometer was designed, developed, and installed at the Indian Institute of Tropical Meteorology (IITM), Pune, India (18°43ЈN, 73°51ЈE), to monitor the vertical distribution of atmospheric aerosols. The instrument, based on passive remote sensing technique, is simple and inexpen- sive. It is operated only during twilights, and the method of retrieval of aerosol profile is based on a simple twilight technique. It functions at a single wavelength (660 nm), and a photomultiplier tube is used as a detector. The amplifier, an important component of the system, was designed and developed by connecting 10 single integrated-circuit (IC) amplifiers in parallel so that the noise at the output is drastically reduced and the sensitivity of the system has been increased. As a result, the vertical profiles are retrieved to a maximum of 120 km. A brief description of the basic principle of twilight technique, the experimental setup, and the method of retrieval of aerosol profiles using the above photometer are detailed in this paper. 1. Introduction The main factor controlling the course of the twilight phenomena is scattering of sunlight in the earth’s at- It is important to understand the role of aerosol mosphere and the accompanying attenuation of the di- physical and optical properties in altering the radiation rect solar rays. The most important circumstance, which budget of the earth’s atmosphere. Along with other gives an altitudinal sounding ability to the twilight constituents of the atmosphere, such as molecular gases event, is that only a comparatively thin layer of air and clouds, aerosols determine what fraction of the so- above the earth’s shadow contributes the maximum to lar radiation incident at the top of the atmosphere the sky brightness at every given moment. The loga- reaches the earth’s surface and what fraction of the rithmic gradient of twilight sky intensity at a fixed angle thermal radiation emitted from the earth escapes to above the horizon where the earth’s shadow traverses space. These two processes essentially determine the different layers of the atmosphere gives information earth’s climate. Retrieval of vertical profiles is impor- about the vertical distribution of aerosols in the atmo- tant to study the tropospheric and stratospheric aerosol sphere (Bigg 1956, 1964; Volz and Goody 1962; Shah properties separately. Long series of frequent observa- 1970; Jadhav and Londhe 1992; Nighut et al. 1999; tions averaged on a weekly or monthly basis are re- Mateshvili et al. 2000; Padma Kumari et al. 2003). This quired to study the long-term trends. In addition to in method is analogous to the method of rocket sounding. situ measurements carried out on balloons or rockets, In it the solar radiation scans the earth’s atmosphere ground-based remote sensing techniques appear well during twilight, and the scattered radiation received adapted for long-term monitoring. from any part of the sky is primarily due to the light The twilight photometry technique, involving scattered by illuminated molecules and aerosols. Any ground-based photometry of the twilight sky bright- rapid change in their concentration with height will lead ness, is used to derive the vertical distribution of dust to a corresponding change in illumination at the particles in the earth’s atmosphere. The term “twilight” ground. Thus the scattered intensity is assumed to be refers to the optical phenomena that take place in the proportional to the particle number density. earth’s atmosphere when the sun is near the horizon. The twilight photometric measurements have been carried out by a number of workers, and the results of Corresponding author address: B. Padma Kumari, Indian Insti- many of the earlier investigations have been reviewed tute of Tropical Meteorology, Pashan Rd., Pune 411 008, India. in detail by Rozenberg (1966). The twilight method has E-mail: [email protected] been used as a useful tool for the study of stratospheric DOI: 10.1175/2008JTECHA1090.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 09/24/21 11:54 PM UTC JTECHA1090 SEPTEMBER 2008 PADMA KUMARI ET AL. 1601 aerosol (Shaw 1980). Enhanced optical effects due to the eruption of Mount Agung on Bali (8°25ЈS, 115°30ЈE) on 17 March 1963 have been reported by many observers using photometric observations (Meinel and Meinel 1963, 1964; Volz 1964). Measure- ments of twilight-scattered light were made with a pho- tometer during the International Geophysical Year– International Geophysical Campaign period (1957–59), at Mount Abu, India (Shah 1970). The twilight method was used to detect volcanic dust from the eruption of FIG. 1. Schematic diagram of the twilight phenomenon. Mount St. Augustine, Alaska, in 1976 (Meinel et al. is assumed that the bulk of the scattered light comes to 1976). The occurrence of a novel feature associated an observer from the lowest, and therefore densest, with the stratospheric aerosol layer, due to the eruption layer in the sunlit atmosphere at the time of measure- of El Chichon volcano, in Mexico, has been reported at ment. The contribution of the rest of the atmosphere Ahmedabad, India (23°N, 72°30ЈE), using twilight sky above this layer can be neglected because of an expo- brightness measurements (Ashok et al. 1984). Twilight nential decrease of air density with increasing altitude. photometric observations carried out at location The height of this lowest layer, called the twilight layer, Pathardi, India (19°9Ј N, 75°10ЈE), during 1993 and increases with increasing earth’s shadow height. The 1994 showed the presence of a broad stratospheric lower atmospheric layers, now submerged in shadow, aerosol layer peaking at ϳ20 km, indicating the effect no longer contribute to the sky brightness, and the scat- of the Mount Pinatubo, Philippines, eruption (Nighut et tered light comes more and more from the higher alti- al. 1999). Recently the seasonal variability in the strato- tudes, which are still illuminated by direct sunlight. spheric aerosol layer in the current volcanically quies- cent period has also been studied using the twilight 3. Instrument design method (Padma Kumari et al. 2006). Twilight photometry has also been utilized to study The twilight photometer was designed and developed the influx of extraterrestrial dust in the upper atmo- indigenously at IITM. A block diagram of this photom- sphere and its subsequent descent to lower altitudes as eter is shown in Fig. 2. The various components of the it could probe the atmosphere up to mesospheric alti- photometer are described below. tudes (Link 1975; Mateshvili et al. 1997; Mateshvili et a. Telescopic lens al. 1999, 2000; Padma Kumari et al. 2003, 2005). The present paper contains a brief description of the basic The photometer consists of a convex lens of diameter principle of the twilight technique, and the experimen- 15 cm having a focal length of 22 cm. It is used as a tal setup and method of retrieval of aerosol profiles by using the twilight photometer, which is designed, devel- oped, and installed at the Indian Institute of Tropical Meteorology (IITM), Pune, India, are detailed. 2. Basic principle of twilight technique A schematic diagram of the twilight phenomena is shown in Fig. 1. When the sun is within 0°–12° below the horizon, the lower part of the atmosphere comes under the earth’s shadow while the upper part is sunlit. The boundary between the illuminated and shadowed parts is monotonously shifting up during the evening twilight and down during the morning twilight. The twi- light technique is based on the fact that the luminosity of the twilight sky at a given moment depends on the momentary height of earth’s shadow. The twilight sky brightness at any given moment is caused by the sum of all light that is scattered toward an observer from all air molecules and aerosol particles above this boundary. It FIG. 2. Block diagram of the twilight photometer. Unauthenticated | Downloaded 09/24/21 11:54 PM UTC 1602 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 25 telescopic lens for gathering the scattered zenith sky light intensity from the ground. b. Filter A red glass filter peaking at 660 nm with a half- bandwidth of about 50 nm is used. The longest wave- length has been selected in order to reduce the Ray- leigh scattering contribution. The filter used has a wavelength cutoff at about 620 nm, and therefore most of the Chappius band with maximum ozone absorption at 610 nm would remain in the cutoff region of the filter. At this wavelength no other gas with maximum absorption is present. Therefore, the information that is obtained at this wavelength is predominantly of aerosol scattered light. c. Detector A photomultiplier tube (PMT) is used as a detector. Photomultipliers are extremely sensitive light detec- tors, providing a current output proportional to inci- FIG. 3. The response function of the photometer. dent light intensity. The PMT consists of a photoemis- sive cathode called a photocathode, focusing elec- trodes, series of dynodes for electron multiplication, QE curve for the PMT is given in Fig. 3. The photom- and an electron collector (anode) in a vacuum tube. eter response function is derived from the transmission When light (photons) falls on the photocathode, it function of the filter and the QE curve of the PMT. The emits electrons by photoelectric effect. These photo- overlapping of the two functions is the response func- electrons are electrostatically accelerated and focused tion of the photometer, seen in Fig.