How to use the pvlib.atmosphere.relativeairmass function in pvlib

Dataframe with the following columns:
        ['solar_zenith', 'solar_azimuth', 'surface_tilt', 'surface_azimuth',
        'dhi', 'dni', 'vf_horizon', 'vf_circumsolar', 'vf_isotropic',
        'luminance_horizon', 'luminance_circumsolar', 'luminance_isotropic',
        'poa_isotropic', 'poa_circumsolar', 'poa_horizon', 'poa_total_diffuse']

    """
    # Create a dataframe to help filtering on all arrays
    df_inputs = pd.DataFrame(
        {'surface_tilt': surface_tilt, 'surface_azimuth': surface_azimuth,
         'solar_zenith': solar_zenith, 'solar_azimuth': solar_azimuth,
         'dni': dni, 'dhi': dhi},
        index=pd.DatetimeIndex(timestamps))

    dni_et = irradiance.extraradiation(df_inputs.index.dayofyear)
    am = atmosphere.relativeairmass(df_inputs.solar_zenith)

    # Need to treat the case when the sun is hitting the back surface of pvrow
    aoi_proj = aoi_projection(
        df_inputs.surface_tilt, df_inputs.surface_azimuth,
        df_inputs.solar_zenith, df_inputs.solar_azimuth)
    sun_hitting_back_surface = ((aoi_proj < 0) &
                                (df_inputs.solar_zenith <= 90))
    df_inputs_back_surface = df_inputs.loc[sun_hitting_back_surface].copy()
    # Reverse the surface normal to switch to back-surface circumsolar calc
    df_inputs_back_surface.loc[:, 'surface_azimuth'] = (
        df_inputs_back_surface.loc[:, 'surface_azimuth'] - 180.)
    df_inputs_back_surface.loc[:, 'surface_azimuth'] = np.mod(
        df_inputs_back_surface.loc[:, 'surface_azimuth'], 360.
    )
    df_inputs_back_surface.loc[:, 'surface_tilt'] = (
        180. - df_inputs_back_surface.surface_tilt)

----------
        .. [5] `pvlib globalinplane `_.
        .. [6] `pvlib atmosphere `_.

        See Also
        --------
        solarposition_hourly_mean, solarposition, angle_of_incidenc
        """
        # Determine the extraterrestrial radiation
        data['dni_extra'] = pvlib.irradiance.extraradiation(
            datetime_or_doy=data.index.dayofyear)

        # Determine the relative air mass
        data['airmass'] = pvlib.atmosphere.relativeairmass(data['zenith'])

        # Determine direct normal irradiation
        data['dni'] = (data['dirhi']) / np.sin(np.radians(90 - data['zenith']))

        # what for??
        data['dni'][data['zenith'] > 88] = data['dirhi']

        # Determine the sky diffuse irradiation in plane
        # with model of Perez (modell switch would be good)
        data['poa_sky_diffuse'] = pvlib.irradiance.perez(
            surface_tilt=self.powerplant.tilt,
            surface_azimuth=self.powerplant.azimuth,
            dhi=data['dhi'],
            dni=data['dni'],
            dni_extra=data['dni_extra'],
            solar_zenith=data['zenith'],

Function used to calculate the luminance and the view factor terms from the
    Perez diffuse light transposition model, as implemented in the
    ``pvlib-python`` library.

    :param df_inputs: class:`pandas.DataFrame` with following columns:
        ['solar_zenith', 'solar_azimuth', 'array_tilt', 'array_azimuth', 'dhi',
        'dni']. Units are: ['deg', 'deg', 'deg', 'deg', 'W/m2', 'W/m2']
    :return: class:`pandas.DataFrame` with the following columns:
        ['solar_zenith', 'solar_azimuth', 'array_tilt', 'array_azimuth', 'dhi',
        'dni', 'vf_horizon', 'vf_circumsolar', 'vf_isotropic',
        'luminance_horizon', 'luminance_circumsolar', 'luminance_isotropic',
        'poa_isotropic', 'poa_circumsolar', 'poa_horizon', 'poa_total_diffuse']
    """

    dni_et = irradiance.extraradiation(df_inputs.index.dayofyear)
    am = atmosphere.relativeairmass(df_inputs.solar_zenith)

    # Need to treat the case when the sun is hitting the back surface of pvrow
    aoi_proj = aoi_projection(df_inputs.array_tilt, df_inputs.array_azimuth,
                              df_inputs.solar_zenith, df_inputs.solar_azimuth)
    sun_hitting_back_surface = ((aoi_proj < 0) &
                                (df_inputs.solar_zenith <= 90))
    df_inputs_back_surface = df_inputs.loc[sun_hitting_back_surface]
    # Reverse the surface normal to switch to back-surface circumsolar calc
    df_inputs_back_surface.loc[:, 'array_azimuth'] -= 180.
    df_inputs_back_surface.loc[:, 'array_azimuth'] = np.mod(
        df_inputs_back_surface.loc[:, 'array_azimuth'], 360.
    )
    df_inputs_back_surface.loc[:, 'array_tilt'] = (
        180. - df_inputs_back_surface.array_tilt)

    if df_inputs_back_surface.shape[0] > 0: