def get_ref_spec(spec='LIOx'):
    """
    Store of reference species for families

    Parameters
    ----------
    spec (str): species/tracer/variable name

    Returns
    -------
    ref_spec (str) reference species for a given family

    Notes
    -----
    This is for use in conbination  with functions that calculate relative values
    (e.g. in units of Ox, I, etc)
    """
    d = {
        'Cly': 'Cl',
        'Cl': 'Cl',
        'LOx': 'O3',
        'POx': 'O3',
        'LOX': 'O3',
        'POX': 'O3',
        'LIOx': 'I',
        'PIOx': 'I',
        'PClOx': 'Cl',
        'LClOx': 'Cl',
        'PClOxI': 'Cl',
        'LClOxI': 'Cl',
        'PClOxII': 'Cl',
        'LClOxII': 'Cl',
        'PClOxI': 'Cl',
        'LCI': 'Cl',
        'LCII': 'Cl',
        'PBrOx': 'Br',
        'LBrOx': 'Br',
        'Br': 'Br',
        'Bry': 'Br',
        'I': 'I',
        'Iy': 'I',
        'IxOy': 'I',
        # core species
        'SO4': 'S',
        'NIT': 'N',
        'NITs': 'N',
        'NH4': 'N',
        'ISOP': 'C',
        'NO': 'N',
        'NO2': 'N',
        'N2O5': 'N',
        'O3': 'O3',
        'SO2': 'S',
        # Other VOCs
        'ACET': 'C',
        'ALD2': 'C',
        'DMS': 'S',
        # include halogens
        'HOI': 'I',
        'I2': 'I',
        'CH3I': 'I',
        'CH3IT': 'I',  # Vestigle spec from v10 (is CH3I)...
        'CH2I2': 'I',
        'CH2IBr': 'I',
        'CH2ICl': 'I',
        'CHBr3': 'Br',
        'CH2Br2': 'Br',
        'CH3Br': 'Br',
        'CH2Cl2': 'Cl',
        'CHCl3': 'Cl',
        'CH3Cl': 'Cl',
        'HCl': 'Cl',
        'HBr': 'Br',
    }
    try:
        return d[spec]
    except KeyError:
        pstr = "WARNING: Just returning provided species ('{}') as ref_spec"
        print(pstr.format(spec))
        return spec