def _get_coefficients(crossection, cn):
    """
    Calculates the extinction, scattering or absorbtion coefficient

    Parameters
    ----------
    crosssection:   float
                    Units are um^2
    cn:             float
                    Particle concentration in cc^-1

    Returns
    --------
    coefficient in m^-1.  This is the differential AOD.
    """
    crossection = crossection.copy()
    cn = cn.copy()
    crossection *= 1e-12  # conversion from um^2 to m^2
    cn *= 1e6  # conversion from cm^-3 to m^-3
    coefficient = cn * crossection

    # print('cn',cn)
    # print('crossection', crossection)
    # print('coeff',coefficient)
    # print('\n')

    return coefficient