Hi,
The Air Start Unit purpose is to start the engine by providing pressurized air. It is not intented to to provide "clean air" for the air conditioning system, same for Boeing aircraft btw. Like the common air compressor found in garage the air coming out smells bad because it is contamined by the oil which lubricates the compressor.
Air conditioning on ground can be provided by the APU (very noisy) if running but also by preconditioned air unit (less noisy but still noisy ^^), you can easily recognized them with their big yellow pipes coming out.
Hope it helps