The answer is simple and yet hard to solve! The “gunk” growing on your house is probably a combination of mold and mildew. The problem is that mold and mildew both need moisture to survive which is why they develop on the North facing side of your home and the reason they continue to come back every few years…..if you have good paint!
The fact that the mold and mildew hold moisture will also explain why thr house may attract dirt and dust….especially if you live near new construction. The moisture helps the mold and mildew live…which holds moisture and helps attract more dirt and dust – it is like a snowball effect!