Here in Vermont, where sun angles tend to be pretty low much of the year, and where much of the available land is not tillable, there may be less need of selective panels. I'm seeing a number of solar farms in which the panels are mounted a bit higher than normal, providing sheep pasture below. Eat and mow.
anything that reduces exposed surface will reduce energy production. Depending on location, Snow or Sand are the major issues.
It's one of the issues here in Vermont, where I am often surprised at how little is done to clear the snow off them. Of course eventually it slides off or melts, but I know if I had my own solar farm I'd be out there. Maybe for commercial ones the loss of energy for a few days in the dead of winter isn't enough to warrant the cost of labor for sweeping them off.
I was thinking more of deposits from atmospheric pollution -y'know, from any nearby(?) nice clean coal power plants or heavy industry.
That's something we thankfully need not consider, though I suppose there are some places where woodstove ash might settle. Heavy industry is pretty much a thing of the past. Hydro,wind, and solar make up something like 83 percent of our electric generation, with another ~17 percent from "biomass" (think wood chips and the like), making 99.9 percent "renewable." Much of the year, we get enough rain to keep panels clean. But snow...we got us some snow!