def _find_aerosol(cad_score, confidence):
    """
    Calculate an aerosol mask based on the given CAD score array.

    :param int confidence: The confidence above which to count a retrieval as an aerosol or not (should be +ve)
    :param ndarray cad_score:
    :return bool ndarray : aerosol mask, True values are where there *are* aerosols with given confidence
    """
    return -cad_score > confidence