To be accurate, ppm is simply a ratio and has no dimensions (units of measurement). So, 1 ppm can be 1 gram in a million grams or 1 litre in a million litres or one molecule in a million molecules. Your answer of 441.5ppm is correct. When dealing with aqueous solutions, ppm and mg/l are considered to be the same thing despite the fact that mg/l is a weight per volume. However in aqueous solutions this is ok (ish) since water has a density of 1 and 1 litre will weigh 1,000,000 mg.
When dealing with concentrations of volatile analytes in air, then ppm means something completely different! In this case, 1 ppm means one molecule of analyte in one million molecules of air. So to convert ppm to mg/l, you need to know the analyte's molecular weight and it's molar volume (22.4 litres ish).