What about the 'Warm Sector'? This seems to fit better with the description of persistent haze than mist/fog explanations which surely should vary during the day.
The reduced viz in the warm sector (between warm and cold fronts) can persist for a day or two and make cross country VFR flying impractical despite otherwise stable conditions.
It seems to involve clean maritime (SW) air so I'm not convinced about particulates. But if it is trapped humidity, why doesn't it burn off in the afternoon?
Can someone please explain what causes this reduced viz and where it goes after the passage of the cold front? The idea that the wind just blows it away doesn't quite work for me.
I realise I'm probably displaying cataclysmic ignorance here!