Magnetic accumulations in urban environment

Let me preface by saying that I have no extensive background in science, biology, medicine, geology. I consider myself more artist than scientist, so I'll try to refrain from making definitive claims about the things I'm seeing. Instead, I hope to be able to present findings and engage others who may be able to describe what I'm finding and determine if any of it warrants further investigation.

Magnetic granuals in my environment

I've recently moved into the heart of Seattle to a high-rise building where I have rooftop access. My location is only a few miles from SEATAC Airport and we have consistent air traffic taxiing above us. I've noted dark stains that coat most buildings and accumlate particularly after a good rain. Out of sheer curiosity I ran a magnet along one such accumulation and to my surprise a great deal of it transfered and stuck to my magnet. Keep in mind this sample was taken from an elevated position with minimal environmental disturbance, so I believe it makes an ideal location to test what kinds of things may be present in our environment, or raining down from out atmosphere.

What could it be?

Observing small amounts which I scraped off a magnet and viewed under a microscope, I see small granuals that appear to loosely bind to fibers and filaments.

What could be causing this and where is it coming from? I speculate that it is somehow falling and collecting on the rooftop via air traffic overhead. I've also collected magnetic samples from other locations around the building: garage, exterior, interior. But the fact that it is present so high up and isolated from the usual city disturbances seems to suggest it's arriving there from above.

Is it in your environment?

I've also scraped samples from my vacuum filter and I believe my test should be relatively easily recreated. All you need is a magnet, a vacuum or a location with minor environmental disturbance variables, and a surface/container to scrape magnetic granuals onto. I'm very interested to learn if this is present in other locations, or if perhaps my proximity to a major airport could be the biggest contributing factor.

I'll post more info if I find it. But that's what I've got to present for now.

Will