def _find_aerosols(obs, is_falling, is_liquid):
    """Estimates aerosols from lidar backscattering.

    Aerosols are lidar signals that are: a) not falling, b) not liquid droplets.

    Args:
        obs (ClassData): The :class:`ClassData` instance.
        is_falling (ndarray): 2-D boolean array of falling hydrometeors.
        is_liquid (ndarray): 2-D boolean array of liquid droplets.

    Returns:
        ndarray: 2-D boolean array containing aerosols.

    """
    is_beta = ~obs.beta.mask
    return is_beta & ~is_falling & ~is_liquid