Prabha R. Nair

Size Distribution of Coastal Aerosols: Effects of Local Sources and Sinks

Abstract Using aerosol optical depth as a function of wavelength obtained from ground-based multiwavelength radiometer observations, columnar size-distribution functions of aerosols have been derived. It has been found that the nature of the derived size-distribution function is strongly dependent on season. The derived size-distribution functions are discussed in term of seasonally dependent natural aerosol sources and sinks.

Wintertime spatial characteristics of boundary layer aerosols over peninsular India

During an intense field campaign for generating a spatial composite of aerosol characteristics over peninsular India, collocated measurements of the mass concentration and size distribution of near-surface aerosols were made onboard instrumented vehicles along the road network during the dry, winter season (February-March) of 2004. The study regions covered coastal, industrial, urban, village, remote, semiarid, and vegetated forestlands. The results showed (1) comparatively high aerosol (mass) concentrations (exceeding 50 μ g m(-3)), in general, along the coastal regions (east and west) and adjacent to urban locations, and (2) reduced mass concentration ( 50% of the total) of coarse-mode aerosols (>1 μ m). The spatial composite of accumulation-mode share to the total aerosol mass concentration agreed very well with the monthly mean spatial composite of aerosol fine-mode fraction for February 2004, deduced from Moderate-Resolution Imaging Spectroradiometer data for the study region, while a point by point comparison yielded a linear association with a slope of 1.09 and correlation coefficient of 0.79 for 76 independent data pairs. Pockets of enhanced aerosol concentration were observed around the industrialized and urban centers along the coast as well as inland. Aerosol size distributions were parameterized using a power law. Spatial variation of the retrieved aerosol size index shows relatively high values (>4) along the coast compared to interior continental regions except at a few locations. Urban locations showed steeper size spectra than the remote locations.

Sea-Breeze Front Effects on Boundary-layer Aerosols at a Tropical Coastal Station

The effects of sea breeze on optical depth, size distribution, and columnar loading of aerosols at the tropical coastal station of Trivandrum are studied. It has been observed that sea-breeze front activity results in a significant and short-lived enhancement in aerosol optical depth and columnar loading in contrast to the effects seen on normal sea-breeze days. Examination of the changes in columnar aerosol size distribution associated with sea-breeze activity revealed an enhancement of small-particle (size less than 0.28 [mu]m) concentration. The aerosol size distribution deduced from optical depth measurements generally show a pronounced bimodal structure associated with the frontal activity. 22 refs., 12 figs., 1 tab.

Time evolution of the optical effects and aerosol characteristics of Mt. Pinatubo origin from ground-based observations

Abstract The time evolution of the perturbations in aerosol spectral optical depths associated with the Mt. Pinatubo volcanic eruption of June 1991 is investigated using a ground-based multiwavelength solar radiometer at the tropical station, Trivandrum. The spectral optical depths of the volcanic aerosols deduced from the radiometer data are inverted to retrieve the size distributions of the particles, from which the effective radius and columnar mass loading are estimated. Observations showed that during the initial phase (within one year after the eruption) the optical depths showed fluctuations with two peaks, one in November 1991 and another in February 1992 superposed over a very weak decreasing trend. During this period, the size distributions have been generally bimodal, with a secondary large particle mode occurring at ∼0.75 μm, and the columnar mass loading varied between about 170 and 110 mg m−2. The optical depths and mass loading decreased with an e-folding time of about 15 months. The effective radius is found to increase from ∼0.35 μm in September 1991 to ∼0.60 μm in October 1992 and then remain rather steady, while the mass loading decreased to reach near-background levels (within error limits) by early 1993.

Surprising observation of large anthropogenic aerosol fraction over the "near-pristine" southern Bay of Bengal: Climate implications

The Bay of Bengal (BoB), a small oceanic region surrounded by landmasses with distinct natural and anthropogenic activities and under the influence of seasonally changing airmass types, is characterized by a rather complex and highly heterogeneous aerosol environment. Concurrent measurements of the physical, optical, and chemical (offline analysis) properties of BoB aerosols, made onboard extensive ship-cruises and aircraft sorties during Integrated Campaign for Aerosols, gases and Radiation Budget of March-April 2006, and satellite-retrieved aerosol optical depths and derived parameters, were synthesized following a synergistic approach to delineate the anthropogenic fraction to the composite aerosol parameters and its spatial variation. Quite interestingly and contrary to the general belief, our studies revealed that, despite of the very high aerosol loading (in the marine atmospheric boundary layer as well as in the vertical column) over the northern BoB and a steep decreasing gradient toward the southern latitudes, the anthropogenic fraction showed a steady increase from North to South (where no obvious anthropogenic source regions exist). Consequently, the direct radiative forcing at the top of the atmosphere due to anthropogenic aerosols remained nearly constant over the entire BoB with values in the range from -3.3 to -3.6 Wm(-2). This interesting finding, beyond doubts calls for a better understanding of the complex aerosol system over the BoB through more focused field campaigns.

Effects of changes in atmospheric water vapor content on physical properties of atmospheric aerosols at a coastal station

Abstract Observational results are presented on the effects of changes in the atmospheric water vapor content ( W g cm −2 ) on spectral optical depth ( τ pλ ) and the retrieved columnar size distributions of atmospheric aerosols over a natural, coastal environment obtained from simultaneous estimates of these parameters ( τ pλ and W ) using a ground based multiwavelength solar radiometer. It is found that, during meteorologically calm periods (i.e. in the absence of any significant air mass types), the aerosol optical depths increase with increase in W , with the shorter wavelengths being more sensitive to the changes in W , compared to the near infrared (IR) wavelengths. The aerosol (columnar) size distributions, retrieved from τ pλ as a function of W , are found to change from bimodal for very low values of W to unimodal for higher values of W . The mean radius and mode radii of the retrieved size distributions also increase with increase in W .

A study of equatorial wave characteristics using rockets, balloons, lidar and radar

Abstract A co-ordmated experimental campaign was conducted for 40 consecutive days from 21 February to 01 April 2000 using RH-200 rockets, balloons, Rayleigh lidar and MST radar, with the objective of delineating the equatorial waves and estimating momentum fluxes associated with them. Winds and temperatures in the troposphere, stratosphere and mesosphere over two low latitude stations Gadanki (13.5°N, 79.2°E) and SHAR (13.7°N, 80.2°E) were measured and were used for the study of equatorial waves and their interactions with the background mean flow in various atmospheric regions. The study shows the occurrence of a strong stratospheric cooling (∼25 K) anomaly along with a zonal wind anomaly and this low-latitude event appears to be linked to high-latitude stratospheric warming event and followed by subsequent generation of short period (∼5 days) oscillations lasting for a few cycles in the stratosphere. Slow and fast Kelvin waves and RG wave (∼-17-day and ∼7.2-day and ∼4.2-day periods respectively) have been identified. The mean flow acceleration produced by the divergence of the momentum flux due to the observed Kelvin waves in the 35–60 km height region were compared with the zonal flow accelerations computed from the observed zonal winds. Contribution by the slow and fast Kelvin waves was found to be only ∼25 % of the observed acceleration during the evolution of the westerly phase of the semi-annual oscillation.

Winter time chemical characteristics of aerosols over the Bay of Bengal: continental influence.

As part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) conducted under the Geosphere Biosphere Programme of Indian Space Research Organisation, ship-based aerosol sampling was carried out over the marine environment of Bay of Bengal (BoB) during the northern winter months of December 2008 to January 2009. About 101 aerosol samples were collected, covering the region from 3.4° to 21° N latitude and 76° to 98° E longitude—the largest area covered—including the south east (SE) BoB for the first time. These samples were subjected to gravimetric and chemical analysis and the total aerosol loading as well the mass concentration of the ionic species namely F−, Cl−, Br−, NO2−, NO3−, PO42−, SO42−, NH4+, etc. and the metallic species, Na, Mg, Ca, K, Al, Fe, Mn, Zn, and Pb were estimated for each sample. Based on the spatial distribution of individual chemical species, the air flow pattern, and airmass back trajectory analysis, the source characteristics of aerosols for different regions of BoB were identified. Significant level of continental pollution was noticed over BoB during winter. While transport of pollution from Indo-Gangetic Plain (IGP) contributed to aerosols over north BoB, those over SE BoB were influenced by SE Asia. A quantitative study on the wind-induced production of sea salt aerosols and a case study on the species dependent effect of rainfall are also presented in this paper.

Effect of physical properties of atmospheric aerosols on path radiance

Using a set of simplified radiative transfer equations, the effects of the changes in the physical properties (such as columnar size distribution, optical depth and refractive index) of atmospheric aerosols on the upwelling atmospheric path radiance are investigated for a nadir viewing remote sensor at various solar illumination angles in the context of developing operational methods for correcting radiances received by remote sensing satellites for atmospheric effects. It is seen that at low solar illumination angles and high haze conditions (aerosol optical depths > 0.5) the path radiance depends strongly on the nature of the aerosol size distribution, whereas for low to moderate values of aerosol optical depths ( < 0.5), the path radiance increases nearly linearly with optical depths. Spectral variations indicate a large increase in path radiance with decrease in wavelength.

An analysis of the effects of Mount Pinatubo aerosols on atmospheric radiances

Using a single layer radiative transfer approach, this paper studies the effects of Mount Pinatubos perturbations to atmospheric aerosol characteristics, as deduced from long-term ground based observations at a tropical station, on atmospheric radiances, as would be seen by a nadir-looking remote sensor in the visible and near infrared spectral range. We reveal that during the first one-year period after the eruption, direct radiance has been depleted by 30 to 40% while the path radiance increased up to 400%, (more at the near infrared wavelengths). The total radiance is found to decrease, significantly at the longer wavelengths, whereas at the shorter wavelengths the effect is much smaller.