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I’m doing research requiring daily total SO2 column data as inputs to a radiative transfer model (TUV) at one site 
(Billings, Oklahoma, Bounding Box:,"-97.495,36.595,-97.485,36.605"). I found the data/plot was published on the Giovanni 
website (http://giovanni.gsfc.nasa.gov). The data product I chose was “SO2 Column Amount (Planetary Boundary Layer) 
OMSO2e v003    (OMSO2e v003)”. The time range was from 2004-10-01 to 2016-12-15.

I have two questions about the SO2 data.

(1) Is there another daily total column SO2 data from OMI? Was I looking at the wrong data product?

(2) OMSO2e v003 data had a lot of negative values with mean value close to 0 (plot attached). Is it correct? 
Is there any procedure I should follow before using it?

SO2 time series question.png

 

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  1. Answer from a GES DISC internal subject matter expert:
     
    Please review the documentation for the OMSO2 data products:
    
    Level-3 gridded SO2 product
    https://disc.gsfc.nasa.gov/datacollection/OMSO2e_003.html
    
    Level-2 orbital SO2 data used as input to create Level-3
    https://disc.gsfc.nasa.gov/datacollection/OMSO2_003.html
    
    Negative values are a result of the retrieval algorithm (uses principal 
    component analysis PSA method). SO2 values are typically low, so the algorithm 
    gives slightly negative and positive SO2 values which will tend to average to 
    zero in these low SO2 regions. If there is a strong SO2 source from a power 
    plant, or a volcanic eruption, then you'll see a stronger signal and higher 
    SO2 value. The negative values from the retrieval are not removed so as not to 
    bias the data.
    
    At the moment the SO2 are available as a gridded daily Level-3 product OMSO2e 
    which contains SO2 at the planetary boundary layer (PBL) ~0.9 km.
    
    SO2 amounts at other altitudes are available in the Level-2 swath/orbital data 
    product OMSO2. They have SO2 at the PBL as well as at Lower Troposphere (TRL) 
    ~3km, Middle Troposphere (TRM) ~8km and Lower Stratosphere (STL) ~18km.