Not really, no. The speed of light in air is very nearly identical to the speed of light in a vacuum. Wikipedia gives the refractive index of air as 1.0003, which is the ratio between the speed of light in a vacuum and the speed of light in air.
The use of air as a medium rather than a vacuum is unlikely to be a significant source of error in an amateur measurement of c.
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Not really, no. The speed of light in air is very nearly identical to the speed of light in a vacuum. Wikipedia gives the refractive index of air as 1.0003, which is the ratio between the speed of light in a vacuum and the speed of light in air.
The use of air as a medium rather than a vacuum is unlikely to be a significant source of error in an amateur measurement of c.