An investigation of the high spectral resolution signatures and radiative forcing of tropospheric aerosol in the thermal infrared was conducted. To do so and to support advanced modeling of optical properties, a high spectral resolution library of atmospheric aerosol optical constants was developed. This library includes new optical constants of sulfate-nitrate-ammonium aqueous solutions and the collection of a broad range of existing optical constants for aerosol components, particularly mineral optical constants. The mineral optical constants were used to model and study infrared dust optical signatures as a function of composition, size, shape and mixing state. In particular, spherical and non-spherical optical models of dust particles were examined and compared to high spectral resolution laboratory extinction measurements. Then the performance of some of the most common effective medium approximations for internal mixtures was examined by modeling the optical constants of the newly determined sulfate-nitrate-ammonium mixtures. The optical signature analysis was applied to airborne and satellite high spectral resolution thermal infrared radiance data impacted by Saharan dust events. A new technique to retrieve dust microphysical properties from the dust spectral signature was developed and compared to a standard technique. The microphysics retrieved from this new technique and from a standard technique were then used to investigate the effects of dust on radiative forcing and cooling rates in the thermal IR.

Aerosols affect the global radiation budget which plays an important role in determining the state of the Earth's climate. The heterogeneous distribution of aerosols and the variety in their properties results in high uncertainty in the understanding of aerosols. Aerosols affect the radiation by scattering and absorption (direct effect) or by modifying the cloud properties which in turn affects the radiation (indirect effect). The current work focuses only on the direct radiative effect of aerosols. The change in the top-of-atmosphere (TOA) reflected flux due to the perturbation of aerosols and their properties is called direct aerosol radiative forcing (ARFTOA). Estimation of ARFTOA using aerosol properties is done by solving the radiative transfer equation using a radiative transfer model. However, before using the radiative transfer model, it has to be validated with observations for consistency. This is done to check if the model is able to replicate values close to actual observations. The current work uses the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The output radiative fluxes from SBDART are validated by comparing with the Clouds and the Earth's Radiant Energy System (CERES) satellite data. Under clear-skies SBDART agreed with observed fluxes at TOA well within the error limits of satellite observations. In the shortwave solar spectrum (0.25-4 µm) radiation is affected by change in various aerosol properties and also by water vapour and other gas molecules. To study the effect of each of these molecules separately on the aerosol forcing at TOA, SBDART is used. ARFTOA is found to depend on the aerosol loading (aerosol optical depth – AOD), aerosol type (SSA) and the angular distribution of scattered radiation (asymmetry parameter). The role of water vapour relative to the aerosol layer height was also investigated and for different aerosol types and aerosol layer heights, it was found that water vapour can induce a change of ~4 Wm-2 in TOA flux. The relative importance of aerosol scattering versus absorption is evaluated through a parameter called single scattering albedo (SSA) which can be estimated from satellites. SSA defined as the ratio of scattering efficiency to total extinction efficiency, depends on the aerosol composition and wavelength. Aerosols with SSA close to 1 (sea-salt, sulphates) scatter the radiation and cool the atmosphere. Aerosols with SSA < 0.9 (black carbon, dust) absorb radiation and warm the atmosphere. Over high reflective surfaces a small change in SSA can change forcing from negative (cooling) to positive (warming). This makes SSA one of the most important and uncertain aerosol parameters. Currently, the SSA retrievals from the Ozone Monitoring Instrument (OMI) are highly sensitive to sub-pixel cloud contamination and change in aerosol height. Using the sensitivity of OMI to aerosol absorption and the superior cloud masking technique and accurate AOD retrieval of Moderate Resolution Imaging Spectroradiometer (MODIS), an algorithm to retrieve SSA (OMI-MODIS) was developed. The algorithm was performed over global oceans (60S-60N) from 2008-2012. The difference in SSA estimated by OMI-MODIS and that of OMI depended on the aerosol type and aerosol layer height. Aerosol layer height plays an important role in the UV spectrum due to the dominance of Rayleigh scattering. This was verified using SBDART which otherwise would not have been possible using just satellite observations. Both the algorithms were validated with cruise measurements over Arabian Sea and Bay of Bengal. It was seen that when absorbing aerosols (low SSA values) were present closer to the surface, OMI overestimated the value of SSA. On the other hand OMI-MODIS algorithm, which made no assumption on the aerosol type or height, was better constrained than OMI and hence was closer to the cruise measurement The presence of clouds results in a more complex interaction between aerosols and radiation. Aerosols present above clouds are responsible to most of the direct radiative effect in cloudy regions. The ARFTOA depends not only on the aerosol properties but also on the relative position of aerosols with clouds. When absorbing aerosols are present above clouds, the ARFTOA is highly influenced by the albedo of the underlying surface. Recent studies, over regions influenced by biomass burning aerosol, have shown that it is possible to define a ‘critical cloud fraction’ (CCF) at which the aerosol direct radiative forcing switch from a cooling to a warming effect. Similar analysis was done over BoB (6.5-21.5N; 82.5-97.5E) for the years 2008-2011. Aerosol properties were taken from satellite observations. Satellites cannot provide for aerosols present at different heights and hence SBDART was used to calculate the forcing due to aerosols present only above clouds. Unlike previous studies which reported a single value of CCF, over BoB it was found that CCF varied from 0.28 to 0.13 from post-monsoon to winter as a result of shift from less absorbing to moderately absorbing aerosol. This implies that in winter, the absorbing aerosols present above clouds cause warming of the atmosphere even at low cloud fractions leading to lower CCF. The use of multiple satellites in improving the retrieval of SSA has been presented in this thesis. The effect of aerosols present above clouds on the radiative forcing at TOA is shown to be different between Bay of Bengal and Atlantic Ocean. This was due to the change in SSA of aerosols during different seasons. The effect of aerosol height, aerosol type and water vapour on the TOA flux estimation is also studied using a radiative transfer model.

This thesis describes laboratory measurements of the complex refractive index of volcanic ash particles. These measurements are needed to model the radiative impact of volcanic ash, vital for accurate satellite remote sensing. Three experimental methods have been developed, and the results for the complex refractive index and optical properties of a wide range of volcanic ash samples are presented. Measurements were made of the spectral transmission of radiation through suspended volcanic ash particles inside an aerosol cell, using a Fourier transform spectrometer at infrared wavelengths and two diffraction grating spectrometers covering ultraviolet, visible, and near-infrared wavelengths. In addition to the optical measurements, a suite of sampling and sizing instruments were connected downstream of the aerosol cell to measure the particle size distribution. The method was calibrated using two quartz samples. Mass extinction coefficients for nine volcanic ash samples, at 0.3-14 μm, are presented and show considerable variation. These variations are linked to the composition of the samples, measured using X-ray fluorescence (XRF) analysis. The complex refractive index, at 0.3-14 μm, of the two quartz samples and two samples of volcanic ash from the 2010 Eyjafjallajökull eruption were retrieved from the extinction measurements. The forward model used Mie theory and a classical damped harmonic oscillator (CDHO) model to represent the complex refractive index of the samples in terms of a finite set of band parameters, as well as the real refractive index of the sample in the small wavelength limit. Previous studies have shown that there is a redundancy in the retrievals between the band strength parameters and the real refractive index in the small wavelength limit, which can lead to spurious values for the retrieved complex refractive index. This problem was overcome by using an independent measurement of the real refractive index at a visible wavelength, to constrain the model parameter of the real refractive index in the short wavelength limit. Independent measurements of the complex refractive index at visible wavelengths are also important because the extinction produced at these wavelengths is highly sensitive to the particle size distribution, and any uncertainty in the measured size distribution will contribute to significant systematic error in the refractive index retrieved from extinction. The retrieved spectral complex refractive index of Eyjafjallajökull ash was applied using the ORAC retrieval scheme to measurements of the 2010 Eyjafjallajökull eruptionmade by theMODIS instrument aboard NASA's Terra satellite. Significant difference were found in the retrieved plume parameters of optical path, effective radius, and plume altitude, compared to assuming a literature measurement for the refractive index of pumice. For three discrete visible wavelengths (450, 546.7, and 650 nm) an optical microscope was used to make measurements of the complex refractive index of the volcanic ash samples. The long-established Becke line method was used to measure the real refractive index of the samples. For the imaginary refractive index, a new and novelmethod was developed involving measurements of the attenuation of light in individual particles. A strong linear correlation was found between the SiO₂ content of the samples and both their real and imaginary refractive indices at the visible wavelengths investigated. Furthermore, from the XRF compositional analysis of the samples values were calculated for the ratio of non-bridging oxygen atoms per tetrahedral cation (NBO/T), and it was found that NBO/T was an even stronger predictor of real refractive index at visible wavelengths. The optical microscope measurements could only be applied to particles with a radius larger than 10 μm. A new refractometer method was investigated for retrieving the real refractive index of submicron particles from colloidal reflectance measurements close to the critical angle in an internal reflection configuration. A coherent scattering model (CSM) was used to model the coherent reflection from a half-space of monodisperse or polydisperse particles, and a simple extension of the model is presented to properly account for the modified size distribution at the interface in an internal reflection set-up. A rigorous sensitivity analysis was performed to determine how experimental uncertainties propagate into uncertainty associated with the retrieved real refractive index, and the uncertainty due to non-spherical effects was estimated using T-matrix methods. Experimental reflectance data at a wavelength of 635 nm were obtained for spherical monodisperse polystyrene calibration particles, a polydisperse sand sample, and a polydisperse volcanic ash sample. The retrieved values for the real refractive index agreed, within propagated uncertainties, with values measured using other techniques. The method is shown to be a viable technique for measuring the real refractive index of small quantities of submicron particles, and can also retrieve the concentration and size of particles.