Abstract. The state of mixing of aerosols significantly influence their transformation, deposition, radiative forcing and health effects. We report the realistic (as in the atmosphere) state of mixing and morphology of aerosols at two nearby but contrasting environments; the urban region Bengaluru and a remote region Challakere. Ambient aerosols are collected on filter substrates using a High-Volume Sampler and analysed using Scanning Electron Microscope 15 (SEM) equipped with Energy Dispersive X-Ray spectroscopy (EDX). The results show that the prevailing state of mixing of aerosols is 'core-shell' with SiO2 as the core and Carbon-SiO2-others (Calcium-Magnesium-Aluminium and other dust origin elements) combinations as the shell. On an average, for Bengaluru (Challakere), 66% (51%) of the core surface is coated. The sample-wise mean Carbon content of the composite particles reaches as high as ~26% (~9%) at Bengaluru (Challakere). The ambient black carbon (BC) mass concentration and the amount of rainfall that 20 occurs just prior to the end of sampling are found to significantly influence the Carbon content of the particles. To the best of our knowledge, this is a first-of-its kind study over Indian region, coupling realistic aerosol observations and spectroscopy along with advanced image processing techniques to investigate the state of mixing and morphology of atmospheric aerosols at single particle resolution. This knowledge regarding the real-life state of mixing of aerosols would be useful in constraining the models studying aerosol-radiation interactions. 25 1 Introduction:

We report the effect of aerosol-induced local atmospheric heating and the resulting changes in the lower atmospheric optical turbulence on the performance of Free-Space Optical (FSO) communication links. A closed form mathematical expression is derived to estimate the influence of aerosol-induced warming on the Bit Error Rate (BER) of a Binary Phase Shift Keying FSO communication link through Gamma-Gamma modeled turbulence. Our results demonstrate a strong impact, with the aerosol-induced turbulence taking a toll on the signal-to-noise ratio of ~20 dB for a BER of 10 −9 . Aerosol-induced warming produces significant variations in BER compared to the clear atmospheric conditions and can subdue the benefits of improved beam alignment. IntroductionFree Space Optical (FSO) communication is a line of sight technology, where data laden optical signals propagate through the atmosphere characterized by fluctuations in the thermodynamical properties such as temperature, pressure and wind velocity and direction etc., superposed on the regular variations [1]. These random fluctuations cause wave-front distortion and signal degradation at the receiver. Continuously varying nature of the signalboth temporally and spatially -makes the retrieval of information more difficult. Wave front distortion of laser beam propagating through the atmosphere has been studied extensively and reported to have limiting effects on the performance of communication systems [2][3][4][5]. Such distortions arise due to the interaction of the wave front with (a) thermal eddies and (b) gas molecules and suspended particles (aerosols) in the atmosphere. Random fluctuations in the intensity of a beam propagating through the atmospheric turbulence are quantified by the refractive index structure parameter (C n 2 ). Several models are in use to estimate clear air optical turbulence [6]. Recent studies on the modulation of C n 2 due to variations in the atmospheric residence time and vertical distribution of aerosols [7,8] have clearly quantified the aerosol-induced optical scintillations through absorption, scattering and radiative effects, when they are present close to the surface or in the elevated layers [9] of the Earth's lower atmosphere (troposphere). Extinction effects of aerosols on optical turbulence [10][11][12] and FSO communication links [5,13,14] were reported earlier. Commonly used intensity fluctuation models [6] neglect the heating effects of absorbing atmospheric aerosols (such as black carbon and dust, which strongly absorb in the visible through near infra-red wavelengths of the incident solar energy). Hence, the modulation of atmospheric C n 2 by aerosol-induced warming and its consequence on FSO communication systems have not been investigated extensively. Under this backdrop, the present work focuses on the radiative

Effects of absorbing atmospheric aerosols in modulating the tropospheric refractive index structure parameter (Cn2) are estimated using high resolution radiosonde and multi-satellite data along with a radiative transfer model. We report the influence of variations in residence time and vertical distribution of aerosols in modulating Cn2 and why the aerosol induced atmospheric heating needs to be considered while estimating a free space optical communication link budget. The results show that performance of the link is seriously affected if large concentrations of absorbing aerosols reside for a long time in the atmospheric path.

Propagation through turbulent media produces complex amplitude fluctuations and temporal spreading of narrow optical pulses. Light-absorbing aerosols present in the atmospheric transmission path will perturb the refractive index structure parameter ( C n 2 ) through atmospheric heating. The consequent enhancement in broadening and attenuation of ultrashort (femtosecond) optical pulses has been calculated by combining multi-satellite observations, radiosonde profiles and computational radiative transfer. It is shown that narrower optical pulses are more vulnerable to aerosol-induced impairments while broader pulses are more resilient, notwithstanding three to four orders of enhanced optical scintillation.

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