The words "dirty fog" evoke a sense of mystery and unease, but there are other synonymous phrases that paint a similar picture. "Smog" brings to mind a thick, yellowish haze that can obscure visibility. "Haze" is a less dense version of smog, but can still cause a similar effect. "Mist" is a more ethereal term that describes a light fog which can create a dreamy or mysterious atmosphere. "Murk" is another synonym, which invokes a sense of darkness and shadows. Overall, these phrases capture a similar feeling to "dirty fog," and can be used to describe a variety of atmospheric conditions.