What's the prospect of air quality forecasting based on machine learning?
While the prospects are more of an economic question, I can answer that there is at least one study using machine learning methods (https://etda.libraries.psu.edu/catalog/13561nvb5011).

There are problems such as the attribution of error to physical processes are not feasible, the spatial extent of the model is very limited, the accuracy of the model is dependent on the history and integrity of the data, etc. Perhaps a hybrid, some sort of Model Output Statistics could be formed, but this would be doubly expensive.