That's an interesting article. The atmospheric conditions of the time could have well played a part in palette selection, but Monet also suffered from worsening vision as he got older, and wouldn't do anything about it. He is said to have had cataracts, and it's a known phenonomenon that their presence will scatter light in unusual ways. People today with cataracts are advised not to drive at night because of this occurrence. Monet likely used it to his advantage.
James Whistler was also mentioned in the article - he was a Tonalist, painting in deliberate subdued palettes like his Nocturnes. Even his famous portrait of his mother follows the typical palette of Tonalism, and is an interior scene.
Not a great stretch to imagine different skies from countryside to city, so in that regard the smoky trains, boats, etc., could affect atmospheric haze.
Good link, though, and an interesting read.