Ambient atmosphere have equal concentration of CO2, SO2, O3 and NO₂ in the units of PPM. Which of the gas will have highest concentration in µg/m³ units at same temperature and pressure conditions?

Correct option is B

• PPM= Parts per million (given as equal for all gases).

• Molecular Weight= Molar mass of the gas (g/mol).

• 24.45= Molar volume (L/mol) at standard conditions (25°C and 1 atm).

• 1000= Conversion factor from grams to micrograms (µg).

Since PPM, temperature, and pressure are the same for all gases, the only variable affecting the µg/m³ concentration is the molecular weight of each gas.

The molecular weights of the given gases are:

1. CO₂ (Carbon Dioxide)= 44 g/mol

2. SO₂ (Sulfur Dioxide)= 64 g/mol

3. O₃ (Ozone)= 48 g/mol

4. NO₂ (Nitrogen Dioxide)= 46 g/mol

Since SO₂ has the highest molecular weight (64 g/mol), it will yield the highest µg/m³ concentration when converted from PPM.

​SO₂ (Sulfur Dioxide) will have the highest concentration in µg/m³ when all gases are present at equal PPM levels.

Information booster 

• PPM (Parts Per Million): A volume-based unit representing the ratio of gas molecules to total air molecules.
     Example: 1 PPM CO₂ = 1 CO₂ molecule per 1 million air molecules.

  • Independent of molecular weight.

  • Used for regulatory standards (e.g., air quality limits).

• µg/m³ (Micrograms per Cubic Meter): Amass-based unit representing the actual mass of a gas per unit volume of air.

  • Depends on themolecular weightof the gas.

  • Critical for health/toxicity assessments (e.g., SO₂ is harmful even at low µg/m³).