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How do I compare tropospheric_NO2_column_number_density(mol/m^2) of Sentinel-5p with EPA statewide emission data which unit is Ton or lb?

Goal: My goal is to compare remote sensing no2 emission to bottom-up no2 emission for electricity generation.

Current progress: I have calculated the total yearly emission of no2 for California. The data used for this is Sentinel-5p OFFL NO2 stored in Google Earth Engine. I first take the yearly sum of no2 (tropospheric_NO2_column_number_density) and use gee reduceRegion function to get a sum of yearly no2 of California.

I want to compare that value with EPA emission data which contain yearly statewide emission data. But the problem is tropospheric_NO2_column_number_density unit is mol/m^2 and EPA emission data in ton or lb.

Question: So how to compare these two value of the different units?

After searching I found this post in sentinel-5p forum which discusses converting mol/m^2 to ppb. But this doesn't solve my problem.

EDIT: After seeing @xunilk detailed answer, I came to know we can convert mol/m^2 to lb/m^2 and my question needs some clarification. If we convert and sum entire year no2 of a state then our final value's unit will be lb/m^2. For example, California yearly total no2 emission from s5p satellite is 104 lb/m^2. On the other hand, our bottom-up emission data describes that California yearly emission is 30000 ton. So how can we convert this lb/m^2 value to only ton ( not ton/m^2 ) and also how to convert lb/m^2 to lb (not lb/m^2)? Could we just multiply the entire California area in m^2 or is there any way to do that in satellite imagery?

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  • This is less of a GIS question than a scientific units one, for which Earth Science might be more appropriate.
    – Vince
    Mar 8 '20 at 12:00
  • @Vince It is a question of how converting mol/m2 to lb/m2 mapping over 'COPERNICUS/S5P/OFFL/L3_NO2' collection in GEE. It can be considered here.
    – xunilk
    Mar 8 '20 at 14:15
  • Based in your edit, I modified my script for calculating emissions in ton/m2 instead of lbs/m2 and total emissions in ton for California. I used a mean reducer. You can change it for a sum reducer and verify if values are about 30000 ton.
    – xunilk
    Mar 8 '20 at 22:02
  • Check out a similar question on Earth Science Stack Exhange.
    – Yaroslav
    Mar 10 '20 at 10:23
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One mol of NO2 is 46.0055 g (reference here) and 1 pound is 453.592 g so, conversion factor from mol to lbs is 0.1014 lbs/mol.

For converting mol/m2 to lbs/m2 in snippet code of this link Sentinel-5p OFFL NO2, you only have to map over collection by using this conversion factor (0.1014 lbs/mol); as in modified following script:

var collection = ee.ImageCollection('COPERNICUS/S5P/OFFL/L3_NO2')
  .select('tropospheric_NO2_column_number_density')
  .filterDate('2019-06-01', '2019-06-06');

var mol_to_lb = collection.map(function (image) {
  return image.multiply(0.1014);
});

var band_viz1 = {
  min: 0,
  max: 0.0002,
  palette: ['black', 'blue', 'purple', 'cyan', 'green', 'yellow', 'red']
};

var band_viz2 = {
  min: 0,
  max: 0.00002,
  palette: ['black', 'blue', 'purple', 'cyan', 'green', 'yellow', 'red']
};

Map.addLayer(collection.mean(), band_viz1, 'S5P N02_mol');
Map.addLayer(mol_to_lb.mean(), band_viz2, 'S5P N02_lbs');
Map.setCenter(65.27, 24.11, 4);

After running above script in GEE code editor, result was as follows. It can be observed in Inspector Tab, after clicking in an arbitrary point on map area, that this conversion was realized as expected.

enter image description here

Editing Note:

Based in your commentary and editing note, I load in my assets a Feature Collection for California state and modified my previous script for calculating emissions in Tm/m2 instead of lbs/m2. So, following script can calculate total emissions for California area in Tm (435.6976265829383 Tm) with a mean reducer.

var area = ee.FeatureCollection("users/joseguerreroa/california");

var collection = ee.ImageCollection('COPERNICUS/S5P/OFFL/L3_NO2')
  .select('tropospheric_NO2_column_number_density')
  .filterDate('2019-06-01', '2019-06-06');

var scale = collection.first()
    .projection().nominalScale();

print("pixel area", scale);

var mol_to_ton = collection.map(function (image) {
  return image.multiply(0.000046);
});

var calif_clip = mol_to_ton.mean().clip(area);

var band_viz2 = {
  min: 0,
  max: 0.000000002,
  palette: ['black', 'blue', 'purple', 'cyan', 'green', 'yellow', 'red']
};

Map.addLayer(calif_clip, band_viz2, 'S5P N02_lbs');
Map.centerObject(area, 5);

//calculating whole emission area in California m2 
var emission_area = ee.Image.pixelArea().reduceRegion({
  reducer: ee.Reducer.sum(),
  geometry: area,
  scale: 1113,
  maxPixels: 1e13
});

var california_area = emission_area.values().getInfo()[0];

print("California area", california_area);

// Reduce the region. The region parameter is the Feature geometry.
var mean_NO2_emission = calif_clip.reduceRegion({
  reducer: ee.Reducer.mean(),
  geometry: area,
  scale: 1113,
  maxPixels: 1e9
});

// The result is an emision in ton/m2.
var tot_emission = mean_NO2_emission.values().getInfo()[0];

print ("ton/m2", tot_emission);

var tot = ee.Number(tot_emission).multiply(california_area);

print("total emission(ton) of NO2 for California", tot);

After running above script, result can be visualized as follows:

enter image description here

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These are units measuring different things, so to compare them you need to make some assumptions. Mol is a unit that is proportional to mass, so it is comparable to the tons. When it is divided by m2, the number describes the distribution, not the release.

What you could.do is e.g. to divide the releases in each state with the area of the state. If all the atmospheric NO2 is from the power production releases in the same state, you should get a constant number.

(btw 1 mol of NO2 is 46grams)

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