If the website is reporting ambient air pollution levels and labels them as mg/m², that is most likely a unit labeling error, and the intended unit is mg/m³.

No need to make assumptions 5 years after the referenced question.

I LOVE THIS SITE! But I have a question that I hope someone can help with. I was looking at the concentration of NO₂, SO₂ and PM2.5 and wanted to convert the reading into Parts Per Million (PPM). All the formulas I've found were for converting concentrations expressed in mass (in milligrams) per cubic meter (mg/m³); however the site is showing concentrations of Sulphur Dioxide per square meter (mg/m²), rather than per cubic meter. This doesn't make sense to me (meters squared vs meters cubed), and I believe is most likely an error, but this is far from my area of expertise, so I wanted to check and see if it is supposed to be mass per meter cubed (g/m³), or if mass per square meter is correct, what the formula would be to convert this reading to PPM? I found a conversion formula of Concentration (mg/m³) = 0.0409 X PPM X Molecular Weight.

If anyone can help clarify I would appreciate it.

Concentrations of gaseous pollutants such as NO₂ and SO₂ are physically meaningful only as mass per unit volume (e.g., mg/m³), because ppm is a volumetric (mole-fraction) unit. A value reported in mg/m² does not represent an air concentration; it usually refers to deposition or surface loading (how much mass settles on a surface), not the amount present in the air. Therefore, mg/m² cannot be directly converted to ppm without additional information such as the height of the air column or mixing layer thickness.

If the website is reporting ambient air pollution levels and labels them as mg/m², that is most likely a unit labeling error, and the intended unit is mg/m³. Only in that case does the standard conversion apply.

For gases, the correct conversion at standard conditions is:

ppm

mg/m
3
×
24.45
molecular weight
ppm=
molecular weight
mg/m
3
×24.45
​

(or rearranged forms of the same relationship, depending on temperature and pressure assumptions).

This distinction is important for accurate Parts Per Million Conversion, especially when comparing regulatory limits or health guidelines. If you want a reliable way to perform ppm ↔ mg/m³ calculations under standard conditions, tools like https://ppmcalculator.net/
are useful, provided the input units are volumetric (mg/m³), not areal (mg/m²).

In short:

mg/m³ → ppm ✔ valid (with assumptions)

mg/m² → ppm ✘ not valid without additional spatial data

But on website it is mg/m2 from Copernicus data:

It should be better to have the same data used both on website and in the app.

Anyway the mg/m2 are probably an error as the Copernicus website shows SO2 amounts expressed in μg/m3.