Because you're working on TROPOMI/ NO2 data which is in the molecules/cm2 unit and need to convert the molecules/cm2 into microgram/m3, you need to consider the following options:

• First, I assumed you were referring to the NO2 vertical column in the stratosphere based on this link (https://developers.google.com/earth-engine/datasets/catalog/COPERNICUS_S5P_NRTI_L3_NO2#bands). The value of this column changes depending on the latitude. Due to gas compression, Colder regions and seasons lead to a thinner stratosphere.
• Let assume that its thickness is 30 km (3x10⁶ cm) based in this link (https://sciencing.com/thick-thin-earths-atmosphere-19740.html). So, molecules/cm² are 3x10⁶ molecules/cm³ for a vertical column of 30 km.
• Now, you need to consider Avogadro's number (6.02x10²³ molecules/mol), molecular weight of NO2 (46 g/mol), conversion of grams in µg (1 g = 10⁶ µg) and conversion of cm³ in m³ (1 m³ = 10⁶ cm³). Combining all of them (I employed factor method), you got a conversion factor of 2.29x10⁻⁴ for converting values from molecules/cm² to µg/m³ based in these assumptions.
• By applying this conversion factor, you can acquire conversions from your original maps using map algebra.
• However, you should go over the calculations again to be sure there are no errors in the arithmetic.

for NO2- : 1 mmol/l = 14 mg/l

Thank you Belaroui Abdelhakim for the clarification that 1 mmol/l = 14 mg/l for NO2. Can this be applied to the SO2, CO, CH4, HCHO, and O3?

I can help you, please contact in private Valentine Mukanyandwi