def drop_GHGs(df):
    """
    GHGs are included in some NEI datasets. If these data are not compiled together 
    with GHGRP, need to remove them as they will be tracked from a different source
    """
    # Flow names reflect source data prior to FEDEFL mapping
    flowlist = [
                'Carbon Dioxide',
                'Methane',
                'Nitrous Oxide',
                'Sulfur Hexafluoride',
                ]
    
    df = df.loc[~df['FlowName'].isin(flowlist)]
    
    return df