I see this term being used everywhere, but I have not been able to find a formal definition for "ambient aerosols".
In the field, in which I work, the term _ambient aerosol` is related to the water content of aerosol particles.
Commonly in the atmosphere, water is attached to particles. The amount of water attached to particles depends on the relative humidity (in ambient air). Increasing relative humidity means more water and, hence, larger particle/droplet diameter. When we provide a measured/modeled particle diameter, we need to note whether the diameter is given as dry, as wet at XX% relative humidity or as wet at ambient relative humidity.
Maybe it is also used in other contexts.