Probably wrong units of SO2 concentration



  • Hi! Here's a screenshot from Android Russian version of Windy. I guess it should be micrograms not milligrams. Screenshot_20200823-021949_Windy.jpg


  • Sailor Moderator

    @AirPollutant
    No the unit is well mg/m2 not μg/m2. It’s not per cubic meter, but in a column from ground to the top of the atmosphere with a 1-square meter base. This has been changed recently:
    https://community.windy.com/topic/12819/what-happened-to-geos-5-source-for-so2?_=1598164817464
    Click About these data:
    DE04DD6E-6E76-4F36-A582-17C0DEE41E4A.jpeg



  • @idefix37 oh, I'm sorry, my bad... Is it possible to convert this to micrograms per m^3 somehow? I tried to google this with no success. I tried to use the formula (Ideal gas law):

    pV=(m/M) RT like this:
    For example in Moscow was 10 milligrams per m2 today
    Temperature = 23 °C = 296 K
    V = 1 m3
    So p = 10e-6 * g = 10e-6 * 9,81 = 9,81e-5 Pa (partial pressure of SO2)
    Molar mass M of SO2 is 64e-3 kg per mole

    m = pM/(RT) = 9,81e-5 * 64e-3 / (8,31 * 296) = 2,6 micrograms per m^3

    But I'm not quite sure if this correct convertion. Russian state threshold limit value of SO2 is 50 micrograms per m3 - it is about 195 milligrams per m2 (by the formula above) in Shanghai is now more than 230 mg/m2
    Is this convertion correct? (seems legit, but...)

    Also I've found another 'bug' : when I flip my phone the value can change up to 10 times in the same area
    Screenshot_2020-08-23-11-45-23-770_com.windyty.android.jpg Screenshot_2020-08-23-11-45-01-360_com.windyty.android.jpg


  • Sailor Moderator

    @AirPollutant
    I find strange the way how you convert mg/m2 to μg/m3 using the ideal gas law. I think you are interested by the SO2 concentration near the ground. But the values shown by Windy are the total SO2 mass in a column of 1 square meter section from surface up to the top of atmosphere (according to ECMWF up to 0.1 hPa level). In this column the mass of SO2 varies with altitude, first because the air and all gas density decreases (same as pressure) exponentially with altitude. In addition the concentration of SO2 is probably not uniformly distributed in this column. So it is about impossible to deduct from these SO2 values the concentration near the ground. Netherless this satellite data is interesting to show a SO2 plume from a volcano or from big polluting industrial complex for instance.
    However it is possible to get a better information by levels:
    http://rammb.cira.colostate.edu/training/visit/quick_guides/Quick_Guide_SO2_RGB.pdf



  • @idefix37 thank you! Your answers were helpful! I've googled more and found that this is not a trivial task to get ground concentrations from the satellite integral data. There are bunch of complex models and ways to do so. I wonder why the other pollutants are presented in micrograms per m3 and only the SO2 data is in 'unpractical' units. Wish you good luck!


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