M.C. Subba Ramu

An experimental study of the role of radon and its daughter products in the conversion of sulphur dioxide into aerosol particles in the atmosphere

Abstract A set of experiments carried out to study the possible role of radon (Rn) and its daughters in the conversion of gaseous sulphur dioxide into particulate sulphate in the atmosphere is described. The experiments were performed under simulated atmospheric conditions in a reaction vessel containing known quantities of SO2, O2 and C2H4 in varying concentrations. The introduction of Rn gas of concentrations in the ranges prevalent in the atmosphere resulted in an enhancement of particle generation in the vessel. An increase of Rn concentration from 100 to 500 pCi m−3 resulted in an increase in the aerosol concentration in the reaction vessel from 6 × 104 to 1.4 × 10 5 cm −3 . Since the concentrations of Rn used for these experiments correspond to the average levels of Rn at ground level it can be expected that a significant fraction of the gas to particle formation could be influenced by the background concentrations of Rn. This observation has significant implications in the study of atmospheric aerosol formation mechanisms.

Calibration of a solid state nuclear track detector for the measurement of indoor levels of radon and its daughters.

Time-integrated measurements of environmental radiation levels are commonly carried out using solid state nuclear track detectors (SSNTD). These detectors are particularly suitable for monitoring indoor radiation, however they should be calibrated for the measurement of the levels of radon (Rn) and its daughters likely to be found in dwellings. This paper reports the results of experiments conducted to calibrate cellulose nitrate film. LR-115 type II, which is used for the measurement of Rn levels in indoor environments of dwellings in India. The detector was exposed to varying concentrations of Rn inside an exposure chamber both in Bare and in Cup with membrane modes. We obtained calibration factors of 1.05 x 10(-3) tracks cm2 day-1 per pCi m-3 (2.84 x 10(-2) tracks cm-2 day-1 per Bq m-3) and 4.8 x 10(2) tracks cm-2 day-1 per WL of Rn.

Calibration of LR-115 Plastic Track Detectors for Environmental Radon Measurements

Solid-state nuclear track detectors are widely used for integrated measure ments of radon and its daughter products in the environment. However, these detectors have to be calibrated before use in field measurements. In the present work, LR-115 type II plastic track detectors have been calibrated in bare mode in a chamber containing a known radon concentration. The cali bration factor measured for this mode was 125 tracks.cm-2.d-1 = 1 WL (3.12 x 10-2 tracks.cm -2.d-1 = 1 Bq.m-3).

Interlaboratory calibration of track-etch detectors for the measurement of radon and radon daughter levels

Abstract This paper reports the results of the second interlaboratory calibration experiments, which were carried out in two parts. In the first, five types of commercially available and economically cheap membranes were tested as the barrier against thoron for “radon-only” measurements. It was found that all the membranes tested are good barriers against thoron and can be used in the “radon-only” device. In the second, actual calibration experiments were performed with LR-115 type-II and CR-39 track-etch detectors for different radon measuring configurations in a radon exposure chamber.

Measurements of the equilibrium factor for 222Rn daughters in dwellings in India

As part of an ongoing project to assess the population dose from indoor radon and its daughters in India, a set of parallel measurements was made in more than 150 houses of different types to estimate indoor radon and progeny levels. Time-integrated solid state nuclear track detectors were employed for the measurement of radon concentration and potential alpha energy exposure level. From these values the equilibrium factor (F) between radon and its progeny was estimated for each house. An average F value of 0.39 was obtained.

Estimation of ventilation rates in dwellings

Abstract Several methods using gaseous tracers such as nitrous oxide, or the radioactive tracer 85 Kr have been used for estimating ventilation rates in rooms. This paper presents a method to evaluate the ventilation rate using radon daughter aerosol normally present in a room as a tracer. By using solid-state nuclear track detectors (SSNTD), radon and its progeny levels are determined. For a steady-state level of radon and its daughter concentrations, the ratio of working level (WL) to radon is estimated. This ratio is related to the pseudo -ventilation rate which is the sum of actual ventilation rate and plate-out rate as follows: WL = f a ×10 -5 (2.8+14 f b · f c ) C Rn The actual ventilation rate is obtained by subtracting the plate -out rates of attached and unattached fractions of radon daughters from the pseudo-ventilation rate. The results of the measurements carried out in 18 different types of houses in Bombay are presented.

Formation of aerosols in irradiated atmospheric air

Abstract Studies of the formation of aerosols in irradiated atmospheric air by gas-to-particle conversion are of fundamental importance, not only in understanding the physics and chemistry of the atmosphere, but also the environmental aerosol characterization which is needed for the evaluation of biologically significant doses to the human lung. Aerosols formed by ionizing radiation in atmospheric air are termed radiolytic aerosols. This paper describes a simple experimental method used for the generation of radiolytic aerosols, the measurements of concentration and size of these aerosols, and the effect of varying relative humidity of the atmospheric air. The growth of aerosols has been studied from the point of view of understanding the mechanism of formation of the radiolytic aerosols in uranium mines and in the free atmosphere.

A study of the formation of aerosols in filtered atmospheric air at Trombay, Bombay

Abstract A set of atmospheric measurements was carried out at Trombay, Bombay to study the correlation between the concentration variation of freshly formed atmospheric aerosols and the variations in the ambient concentrations of some of the atmospheric trace gases. Daily values of the aerosol concentration both in filtered and unfiltered air together with the concentrations of SO2, NO, O3 and α-activity were measured during a period of 11 months. Statistical analysis of the data shows that variation of particle concentration in the filtered air is a function of not only SO2 variation but also O3 and α-activity variations. The various parameters that are likely to be involved in the formation and the variations of small particle concentration in the atmosphere have been identified.

Behaviour of aerosols formed by clustering of molecules around gaseous ions

Abstract Behaviour of aerosols formed by the clustering of vapour molecules around negative gaseous ions has been studied using thorium-B atoms as tracer. Small negative ions of high mobility are first converted to large negative ions of low mobility by passing these through a flask containing the liquid whose vapour is to be studied, using nitrogen as carrier gas. The large ions so formed consist of clusters of molecules of the vapour. These clusters are found to be stable even after the charge is lost and behave as submicron aerosol. This aerosol, along with vapour and carrier gas, fills a neoprene reservoir. In the reservoir, the aerosol grows in size by coagulation and also probably by the diffusion of vapour molecules to the surface of the particles. Mean residence time of the aerosol particles in the neoprene reservoir at different times of growth is measured by introducing thoron gas in the reservoir and studying the change in activity due to thorium-B with time. The mean aerosol particle size at different times is estimated from the residence time in the reservoir. The aerosols investigated are the products of very early stages of nucleation by ions. The possible role of this mechanism in the formation of natural condensation nuclei is discussed.

Calibration of a diffusion sampler used for the measurement of unattached radon daughter products

Abstract A diffusion sampler for the collection of unattached radon daughter products has been calibrated and found to have an efficiency of about 90% at a flow rate of 11 min −1 . Previous lower efficiencies appear to have been caused by insufficient precautions in excluding aerosols from the calibration systems. Studies show that diffusion samplers can conveniently be used for investigation of the formation of radiolytic aerosols in filtered gases and air, and of the attachment of radon daughter atoms or ions to aerosols.