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The profiles of the broad emission lines of active galactic nuclei (AGNs) andthe time delays in their response to changes in the ionizing continuum ("lags")give information about the structure and kinematics of the inner regions ofAGNs. Line profiles are also our main way of estimating the masses of thesupermassive black holes (SMBHs). However, the profiles often showill-understood, asymmetric structure and velocity-dependent lags vary withtime. Here we show that partial obscuration of the broad-line region (BLR) byoutflowing, compact, dusty clumps produces asymmetries and velocity-dependentlags similar to those observed. Our model explains previously inexplicablechanges in the ratios of the hydrogen lines with time and velocity, the lack ofcorrelation of changes in line profiles with variability of the central engine,the velocity dependence of lags, and the change of lags with time. We proposethat changes on timescales longer than the light-crossing time do not come fromdynamical changes in the BLR, but are a natural result of the effect ofoutflowing dusty clumps driven by radiation pressure acting on the dust. Themotion of these clumps offers an explanation of long-term changes inpolarization. The effects of the dust complicate the study of the structure andkinematics of the BLR and the search for sub-parsec SMBH binaries. Partialobscuration of the accretion disc can also provide the local fluctuations inluminosity that can explain sizes deduced from microlensing.

Partial dust obscuration in active galactic nuclei as a cause of broad-line profile and lag variability, and apparent accretion disc inhomogeneities