1 ppm – how much is that in mg/l?

That depends on the medium. In water, 1 ppm ≈ 1 mg/l for dissolved substances, because a litre of water weighs about one kilogram. In air, ppm is a volume fraction: 1 ppm CO₂ corresponds to about 1.8 mg/m³ at 25 °C, i.e. 0.0018 mg/l. For other gases the factor changes with the molar mass.