Last Sunday, March 28, I went up on the Magdalena Mountains (in central New Mexico) to pick up two instruments from the laboratory there, and saw what would qualify as Bishop's ring around the sun for most part of the day. The mountain ridge lies at an altitude of about 10,500 feet above sea level. The sky was very clear and dry, and there was light wind at the time.
One of the photos I took is shown to the right. It was taken with a Nikon D700 with a 24-70/2.8 AF-S lens set at 24mm focal length and f/13 aperture.
To the eye the ring had a pronounced blue aureole with brown outer ring. I think the altitude of the observation rules out low-altitude aerosols being responsible. The central blueish aureole was relatively small; it was only a couple degrees in diameter. I estimate the radius of the outer brown ring to have been about 10 to 15 degrees.
Although the atmosphere was stable and quiet at the time I saw the ring, the southwest of the USA including New Mexico has had very strong winds and dust storms over the past week. In general, the spring months see many strong windstorms in this area. I believe the ring was caused by fine dust in the upper atmosphere and not from volcanic activity somewhere.