During the CLU/ESCOMPTE experimental campaign (June-July 2001) two optical (0.93 m) large aperture scintillometers (LAS) were installed over the city center of Marseille (France) with path lengths of approximately 2 kms, with the goal of evaluating the use of the LAS to estimate spatially integrated sensible heat fluxes. Two problems are addressed. The first one deals with atmospheric absorption: given that in an ideal 'lossless' atmosphere, optical scintillation is a purely refractive process, absorption by the atmosphere can add to the variance observed in the scintillation measurements and thus can cause an over estimation of the sensible heat flux. The second question is related to the influence of the instrument aperture diameter on high frequency fluctuations which is still uncertain. We use high frequency (1kHz) raw scintillation measurements to investigate the influence of refractive/absorption fluctuations on the observed variance in relation to optical atmospheric scintillation. We present results obtained using spectral analyze in an urban environment under varying meteorological conditions, along with estimations of the impact of absorption on flux estimates. Initial results show the following: (i) we observe similarity with pastureland, as found in the literature, and (ii) we see differences due to the absorption contribution between the two paths. A detailed analysis is presented.