B. H. Subbaraya

Global stratospheric distribution of halocarbons

A 16 yr database of stratospheric halocarbon profiles obtained by cryogenic whole air sampling and subsequent trace gas analyses is presented.Twenty-eight balloon ascents of cryogenic samplers were carried out at 44°N, supplemented by 2 balloon flights at 17.5°N and 3 flights at 69°N, between 1977 and 1993. The resultant vertical profiles of CCl4, CCl3F, CCl2F2, CClF3, CF4, CCl2FCClF2, CClF2.CClF2, CClF2CF3, CF3CF3, CH3Cl, CHClF2, CH3CCl3, CBrClF2 and CBrF3 were averaged using relative values with respect to tropospheric mixing ratios, yielding reference profiles for middle, low and high northern latitudes. In addition, latitude-altitude cross-sections of CCl3F, CCl2F2, and CH3CCl3 derived from the balloon data and additional aircraft data are shown for the northern hemisphere.

Compact light-emitting-diode sun photometer for atmospheric optical depth measurements

A new compact light-emitting diode (LED) sun photometer, in which a LED is used as a spectrally selective photodetector as well as a nonlinear feedback element in the operational amplifier, has been developed. The output voltage that is proportional to the logarithm of the incident solar intensity permits the direct measurement of atmospheric optical depths in selected spectral bands. Measurements made over Ahmedabad, India, show good agreement, within a few percent, of optical depths derived with a LED as a photodetector in a linear mode and with a LED as both a photodetector and a feedback element in an operational amplifier in log mode. The optical depths are also found to compare well with those obtained simultaneously with a conventional filter photometer.

Balloon-borne photometric studies of the stratospheric aerosol layer after Mt. Pinatubo eruption

Using Sun-tracking photometers on board balloons, the Pinatubo volcanic aerosol layer has been studied over Hyderabad (17.5°N) during October 1991 and April 1992. From the angular distribution of the scattered radiation intensity measurements the aerosol size parameter is derived. Over a decade of aerosol measurements at Hyderabad, aerosol extinction and number density obtained during October 1991 in the stratosphere are found to be the highest ever obtained with a distinct aerosol layer between 16 and 30 km. The derived aerosol size parameter shows layered structures. Analysis of the size parameter obtained during April 1992 indicates formation of aerosols at higher altitudes by coagulation with a subsequent reduction in the aerosol number density. The obtained results are found to agree well with that of an independent lidar measurement made over Ahmedabad (23°N) and with the stratospheric aerosol and gas experiment II results.

Methane flux measurements from paddy fields in the tropical Indian region

Abstract Methane flux measurements have been made from paddy fields in the Thanjavur region in southern India from September 1991 to January 1992. This is a major rice growing area having plenty of river and rain water. A perspex chamber was used to cover the plants. Samples from this chamber were collected using a metal bellows pump in small sample bottles. The methane flux values estimated from the analysis of these samples are mostly in the range of 15–25 mg m −2 h −1 . These preliminary results, from natural fields, give higher flux values than estimates from some other regions in India, as well as average values for the mid-latitude region, but values lower than from China.

ND:YAG BACKSCATTER LIDAR AT AHMEDABAD (23 N, 72.5 E) FOR TROPICAL MIDDLE ATMOSPHERIC STUDIES

A backscatter Nd:YAG laser radar, the first of this type in India to our knowledge, was developed at the Physical Research Laboratory, Ahmedabad (23° N, 72.5° E), for middle atmospheric studies. The system has been operational since April 1992. The system details and sample results on the Mt. Pinatubo volcanic aerosol layer and its decay as observed over this tropical site are presented and discussed. The future augmentation of the system and planned activities are also outlined.

The vertical distribution of aerosol concentration and size distribution function over the tropics and their role in radiation transfer

By measuring the atmospheric attenuation of the direct solar radiation and the angular distribution of the scattered radiation at different altitudes, the altitude profile of the aerosol concentration and size distribution function can be obtained. This principle has been employed to study the vertical distribution of aerosols over the tropical site, Thumba (8.5°N, 76.9°E), using rocket-borne photometers upto an altitude of 22km in February 1980. Later, a suntracking multichannel photometer was developed and flown on a balloon platform from Hyderabad (17.5°N, 78.6°E) on April 18, 1984 and aerosol measurements were made upto an altitude of 32km. Comparison of the two sets of data shows that the aerosols measured over Thumba are influenced by the near equatorial eruption of Sierra Negra volcano which occurred three months earlier. Mie scattering coefficients are computed for different wavelengths for the aerosol concentrations and size distribution functions measured over Thumba and Hyderabad and the spectral variations are compared.

ELECTRIC FIELDS IN THE E REGION DURING THE COUNTER ELECTROJET

The electron density distribution in the E region during counter electrojets is investigated. Rocket-borne Langmuir probes were flown from Thumba, India (dip 0.47 arcmin S) on August 17, 1972, a magnetically quiet day, and on April 25, a magnetically disturbed day, when ground-based magnetometers indicated a daytime decrease of the difference between equatorial and nonequatorial values of the horizontal component of the earths magnetic field from its nighttime base level. Electron density irregularities with scale sizes from 1 to 15 m were observed in bursts in the 87 to 95 km region, and continuously with varying amplitude in the 96 to 110 km region, while 30- to 300-m irregularities with sawtooth waveforms similar to normal daytime electrojets were found in the region of downward density gradients. Results indicate that the Hall polarization fields are downwards up to 110 km, and that during counter electrojet events, electric fields reverse not only below the electrojet region, but within the main electrojet region as well.

Mode radius and asymmetry factor of Mt. Pinatubo volcanic aerosols from balloon‐borne optical measurements over Hyderabad during October 1991

Using Sun-tracking photometers onboard balloon, the Pinatubo volcanic aerosol layer has been studied over Hyderabad (17.5°N) in October 1991, about 4 months after the eruption. From the aerosol extinction coefficients the mode radius and from the angular distribution of the scattered radiation intensity measurements the asymmetry factor g are determined. Mode radii are found to be in the range of 0.22±0.05 µm within the aerosol layer while g values are found to be 0.83±0.04, indicating the presence of larger particles. The mass determination reveals that the mass of the Pinatubo layer is about 0.053 gm−2 which is 3.75 times higher than the earlier reported value for El Chichon layer, about 4 months after the eruptions.

The vertical distribution of ozone in the equatorial zone

An attempt has been made to construct a mean vertical distribution of ozone over the equatorial site, Thumba (8.5°N, 76.9°E) using 19 ozone profiles obtained during 1980 to 1984 using rocket-borne in situ measurements in the altitude region of 20–60 km. For the 0–20 km region the mean of the several ozone profiles obtained from balloon ozonesonde is used. This mean ozone profile (0–60 km) which can be used as a reference profile shows the ozone maximum at 27 km with a ozone concentration of 3.45×1012 molecules/cc. The profile is also compared with various available satellite results for the equatorial region and mid-latitude K&M ozone model and discussed.

The ozonesonde intercomparison experiment at Thumba

An Indo-Soviet collaborative experiment on Ozonesonde Intercomparison was conducted at TERLS in March 1983. Thirteen rocket ozonesondes, eleven balloon ozonesondes and seven meteorological rockets were launched from Thumba. The rocket and balloon soundings were supported by on site Dobson spectrophotometric observations, surface ozone measurements as well as measurements with a Volz type filter photometer. The programme has yielded data on ozone vertical profiles from eleven rocketsondes, seven balloon-sondes and four sets of Umkehr observations. The data is studied with a view to intercompare the various sensors.