def get_nominal_conc(df, species):
    """
    Get the nominal hector concentration for a given species
    
    Parameters
    ----------
    df : Pandas DataFrame
        DataFrame containing output from a nominal hector run
    species : str
        Species to retrieve output for
    
    Return
    ------
    species_df : Pandas DataFrame
        Pandas DataFrame containing hector concentration output for the given
        species
    """
    species = species.upper()   # Ensure string is all uppercase
    
    # ensure correct string for atmospheric co2 concentration
    if (species == 'CO2'):
        species = 'Ca'
        
    species_df = df.loc[df['variable'] == species]
    return species_df