Parts per million is defined as the mass of a solute in milligram per 1 liter of solvent. It is also a unit of concentration of a solution.
From the definition,
1 mg/l = 1 ppm
Converting μg/m3 to mg/l,
1 μg = 10^-6 g
Therfore, 1 mg = 10^3 μg
= 80 μg/m3 * 1 mg/10^3 μg
= 0.08 mg/m3
Molecular weight of SO2 = 32 + (16*2)
= 64 g/mol
Concentration (in ppm) = (concentration (mg/m3) * 24.45)/molecular weight of SO2
24.45 = volume (liters) of a mole (gram molecular weight) of a gas when the pressure is at 1 atmosphere (760 torr or 760 mm Hg) and at 25°C. where R is the ideal gas constant; T is the temperature in kelvins (273.16 + T°C); and P is the pressure in mm Hg.
