R.R. Reddy

Temporal and spectral characteristics of aerosol optical depths in a semi-arid region of southern India.

The spectral and temporal variations of aerosol optical depths (AOD) observed over Anantapur (a semi-arid region) located in the Southern part of India are investigated by analyzing the data obtained from a Multiwavelength Solar Radiometer (MWR) during January 2005-December 2006 (a total of 404 clear-sky observations) using the Langley technique. In this paper, we highlighted the studies on monthly, seasonal and spectral variations of aerosol optical depth and their implications. The results showed seasonal variation with higher values during pre-monsoon (March-May) and lower in the monsoon (June-November) season at all wavelengths. The pre-monsoon increase is found to be due to the high wind speed producing larger amounts of wind-driven dust particles. The post-monsoon (December-February) AOD values decrease more at higher wavelengths, indicating a general reduction in the number of bigger particles. Also during the post-monsoon, direction of winds in association with high or low pressure weather systems and the air brings more aerosol content to the region which is surrounded by a number of cement plants, lime kilns, slab polishing and brick making units. The quantity of AOD values in pre-monsoon is higher (low during post-monsoon) for wavelength, such as shortwave infrared (SWIR) or near infrared (NIR), which shows that coarse particles contribute more compare with the sub-micron particles. The composite aerosols near the surface follow suit with the share of the accumulation mode to the total mass concentration decreasing from approximately 70% to 30% from post-monsoon to pre-monsoon. Coarse mode particle loading observed to be high during pre-monsoon and accumulation mode particles observed to be high during post-monsoon. The backward trajectories at three representative altitudes with source point at the observing site indicate a possible transport from the outflow regions into Bay of Bengal, southern peninsular India and Arabian Sea. The temporal variations of AOD, Angstrom wavelength exponent and precipitable water content over Anantapur have also been compared with those reported from selected locations in India.

Inferring wavelength dependence of AOD and Ångström exponent over a sub-tropical station in South Africa using AERONET data: influence of meteorology, long-range transport and curvature effect.

Aerosol optical properties over a southern sub-tropical site Skukuza, South Africa were studied to determine the variability of the aerosol characteristics using CIMEL Sunphotometer data as part of the AErosol RObotic NETwork (AERONET) from December 2005 to November 2006. Aerosol optical depth (AOD), Ångström exponent (α), and columnar water vapor (CWV) data were collected, analyzed, and compiled. Participating in this network provided a unique opportunity for understanding the sources of aerosols affecting the atmosphere of South Africa (SA) and the regional radiation budget. The meteorological patterns significantly (p<0.05) influenced the amount and size distribution of the aerosols. Results showed that seasonal variation of AOD at 500 nm (AOD500) over the observation site were characterized by low values (0.10-0.13) in autumn, moderate values (0.14-0.16) in summer and winter seasons, and high to very high values (0.18-0.40) during the spring, with an overall mean value of 0.18±0.12. Ångström exponent α(440-870), varied from 0.5 to 2.89, with significant (p<0.0001) seasonal variability. CWV showed a strong annual cycle with maximum values in the summer and autumn seasons. The relationship between AOD, Ångström exponent (α), and CWV showed a strong dependence (p<0.0001) of α on AOD and CWV, while there was no significant correlation between AOD and CWV. Investigation of the adequacy of the simple use of the spectral AOD and Ångström exponent data was used in deriving the curvature (a2) showed to obtain information for determining the aerosol-particle size. The negative a2 values are characterized by aerosol-size dominated by fine-mode (0.1-1 μm), while the positive curvatures indicate abundance of coarse particles (>1 μm). Trajectory cluster analyses revealed that the air masses during the autumn and winter seasons have longer advection pathways, passing over the ocean and continent. This is reflected in the aerosol properties that are derived from the ocean, desert, and anthropogenic activities that include biomass burning and industrial pollution.

Size segregated mass concentration and size distribution of near surface aerosols over a tropical Indian semi-arid station, Anantapur: Impact of long range transport.

Regular measurements of size segregated as well as total mass concentration and size distribution of near surface composite aerosols, made using a ten-channel Quartz Crystal Microbalance (QCM) cascade impactor during the period of September 2007-May 2008 are used to study the aerosol characteristics in association with the synoptic meteorology. The total mass concentration varied from 59.70+/-1.48 to 41.40+/-1.72 microg m(-3), out of which accumulation mode dominated by approximately 50%. On a synoptic scale, aerosol mass concentration in the accumulation (submicron) mode gradually increased from an average low value of approximately 26.92+/-1.53 microg m(-3) during the post monsoon season (September-November) to approximately 34.95+/-1.32 microg m(-3) during winter (December-February) and reaching a peak value of approximately 43.56+/-1.42 microg m(-3) during the summer season (March-May). On the contrary, mass concentration of aerosols in the coarse (supermicron) mode increased from approximately 9.23+/-1.25 microg m(-3)during post monsoon season to reach a comparatively high value of approximately 25.89+/-1.95 microg m(-3) during dry winter months and a low value of approximately 8.07+/-0.76 microg m(-3) during the summer season. Effective radius, a parameter important in determining optical (scattering) properties of aerosol size distribution, varied between 0.104+/-0.08 microm and 0.167+/-0.06 microm with a mean value of 0.143+/-0.01 microm. The fine mode is highly reduced during the post monsoon period and the large and coarse modes continue to remain high (replenished) so that their relative dominance increases. It can be seen that among the two parameters measured, correlation of total mass concentration with air temperature is positive (R(2)=0.82) compared with relative humidity (RH) (R(2)=0.75).

Temperature and concentration dependence af energy gap and refrective index in certain mixed crystals and semiconductors

Abstract Variations of energy gap (Eg) and refractive index (n) with the concentration have been studied through a set of simple empirical equations proposed in the case of certain mixed crystals of technological importance. Similarly, another set of equations has been proposed to explain the temperature dependence of the energy gap in semiconductors such as GaS, GaSe, GaTe, SnS2 and SnSe2. The results obtained in both cases are found to be in excellent agreement with the experimental values. The proposed equations are proved to be simple and advantageous over others in the sense that less computational work is involved in the calculations of Eg and n.

Studies on the opto-electronic properties of alkali halides from optical electronegativities

Abstract A simple method of evaluating refractive indices from Duffys optical electronegativity values is proposed. In the proposed empirical relations the optical electronegativity for the anion as well as the cation are taken into consideration in order to obtain refractive indices in alkali halides. The refractive indices so obtained are used in the evaluation of electronic polarizabilities in these compounds and the results are found to be in good agreement with the standard values.

Studies on the opto-electronic properties of III-V and II-VI group semiconductors from optical electronegativities

Abstract New empirical relations have been proposed to evaluate bond energies ( E s ) in compound semiconductors, first from the knowledge of optical electronegativities of the constituent ions and secondly from the energy gap values. The validity of the two relations has been tested in the case of certain III–V and II–VI group semiconductors by comparing the calculated values of E s , with those in the literature. From the computed values of E s , refractive indices have been calculated. The Penn gap ( E p ), Fermi energy ( E F ) and S o -parameter for these semiconductors have also been determined. The estimated values of these parameters are utilized to evaluate the electronic polarizabilities (α). The computed values of a compare excellently with the standard data.

Ground-based in situ measurements of near-surface aerosol mass concentration over Anantapur: Heterogeneity in source impacts

Surface measurements of aerosol physical properties were made at Anantapur (14.62°N, 77.65°E, 331 m a.s.l), a semiarid rural site in India, during August 2008–July 2009. Measurements included the segregated sizes of aerosolsas as well as total mass concentration and size distributions of aerosols measured at low relative humidity (RH<75%) using a Quartz Crystal Microbalance (QCM) in the 25–0.05 μm aerodynamic diameter range. The hourly average total surface aerosol mass concentration in a day varied from 15 to 70 μg m−3, with a mean value of 34.02±9.05 μg m−3 for the entire study period. A clear diurnal pattern appeared in coarse, accumulation and nucleation-mode particle concentrations, with two local maxima occurring in early morning and late evening hours. The concentration of coarse-mode particles was high during the summer season, with a maximum concentration of 11.81±0.98 μg m−3 in the month of April, whereas accumulationmode concentration was observed to be high in the winter period contributed >68% to the total aerosol mass concentration. Accumulation aerosol mass fraction, Af (= Ma/Mt) was highest during winter (mean value of Af ∼ 0.80) and lowest (Af ∼ 0.64) during the monsoon season. The regression analysis shows that both Reff and Rm are dependent on coarse-mode aerosols. The relationship between the simultaneous measurements of daily mean aerosol optical depth at 500 nm (AOD500) and PM2.5 mass concentration ([PM2.5]) shows that surface-level aerosol mass concentration increases with the increase in columnar aerosol optical depth over the observation period.

Latitudinal variations of aerosols in the MABL over Bay of Bengal

Advection of aerosols from adjoint continental regions into the Bay of Bengal (BoB) is considered important in view of its potential aerosol radiative impacts on Indian summer monsoon. WICARB was conducted to study the spatio-temporal heterogeneity in AODs and aerosol mass concentration during the winter time. Measurements of AOD over BoB in W-ICARB campaign coupled with the MODIS level 3 data over the region are investigated to bring out the mean picture of AOD and the deviations in daily AODs from monthly means. The AODs are found to show a steep latitudinal gradient as the ship moves away from the coast towards the pristine oceanic region. Increased AOD at 500 nm and aerosol mass loading noticed with increase in latitude. The total mass concentration is found to vary between 15 and 45 μg m-3, with higher loadings near the east coast and northern parts of BoB. Latitudinal gradients in AOD and water vapor indicated higher aerosol loading with low water vapor situations over north BoB region suggesting predominance of coarse mode aerosols on certain occasions.