Are there any other factors that affect soiling rates?

Yes. Again, location, location etc.. We find that there is a difference between soiling from dust, freeway exhaust and smog, salt water and agricultural areas.
In areas near freeways in urban areas where there is a lot of traffic can produce an oily film on the panels in addition to normal dirt. Airport area can also create a dirty deposit on solar panels and vehicles nearby from the jet exhaust. This should be cleaned as soon as possible from panels.
Another concern is areas near salt water. The wind blowing moisture off of the water tends to bring a salty deposit that can attack the aluminum frame and spot the panels. Again, it is good to remove this as often as possible to minimize salt water damage to panels.
Proximity to specific industrial businesses is another factor; for example, a solar array near a cement plant. Clearly this type of dust is going to adversely affect the solar panel more than in a normal residential area.
Yet another problem is bird droppings. Bird droppings can cause a major reduction in output as well as eventually destroy a solar panel if not removed.
The key to keeping your panels producing to the maximum potential is frequent cleaning so as to not allow buildup of dirt and debris.

Below are extreme examples