def explode_pm_to_src(dfrel, dfpm, datecolname='Date', pm10colname='PM10'):
    """Return the proportion of each source for a given PM timeserie

    Parameters
    ----------

    dfrel : pd.DataFrame
        Monthly relative contribution of each sources.
    dfpm : pd.DataFrame
        PM concentration timeserie, with a date column
    datecolname : str, default 'Date'
        Name of the date column in dfpm
    pm10colname : str, default 'PM10'
        Name of the PM column in dfpm

    Returns
    -------

    dfpm_bysrc : pd.DataFrame with the absolute contribution of the sources as
        new columns
    """
    dfpm_bysrc = dfpm.copy()
    dfpm_bysrc['month'] = dfpm_bysrc[datecolname].apply(lambda x: x.month)
    for c in dfrel.columns:
        for m in range(1, 13):
            idx = dfpm_bysrc['month'] == m
            dfpm_bysrc.loc[idx, c] = dfpm_bysrc.loc[idx, pm10colname
                ] * dfrel.loc[m, c]
            if c == 'Unnamed: 0':
                print(c)
    return dfpm_bysrc
