I have old tractor with Bridgestone tires. When I press a clean white tissue against the tire, a black spot show on the tissue. Is that substance carbon black and it’s harmful to soil?
Compressed air intended to clean analytical devices in the lab / medical use (e.g., scuba diving) has to pass filters to remove small droplets of oil (an aeorsol) which may originate from the lubrication of the compressor, and water.
I speculate the compressors at the gas / petrol station need not to fulfill quality standards this high and over time (years) these droplets accumulate in the tires with every refill of compressed air. By smaller diameter of the corresponding molecules and lesser surface tension, it is easier for air to pass through small defects of the tire mantle, bypass the neck of the valve, than it is for air. (I back this up with the observation that I have to refill the tires of my bicycle at least monthly, despite the presta valves itself as well as their inner screw and the cap appear to be tightly closed.) As for the composition, beside water (by humidity of the air compressed) and lubricant, it were plausible if this blend equally contains some soot/carbon black and rubber from the vulcanization, as well as debris because of the mechanical stress the polymer has to sustain (youtube hosts some videos of gopro cameras mounted inside a tire in used, example).