There are existing instruments used on SABRE and on balloons, and existing development efforts. For deployment (and even experimentation) one would likely want much more sampling to understand and validate physics; the issue is thus having more assets, rather than any technical difficulty in the ability to observe the required features of the aerosols (including size, spatial distribution, and composition).
Metric
Time to develop, build, and have balloon and aircraft infrastructure in place is more than 5 years from large-scale funding.
Uncertainty
For the instruments themselves, it is nearly certain that this is just a question of funding and isn't a limiting step. Note, though, that even deployment with existing aircraft at 13-15km will likely require some in situ sampling of aerosols at higher altitudes, so a possible limitation arises from that. The largest uncertainties are related to whether it is sufficient to only use balloons to sample, vs aircraft, and if so, how many aircraft would be needed for sampling, along with the potential for existing high-altitude assets to be retired without replacement in the near-term, leading to challenges to adequately sample aerosols above the ~15-16km ceiling of readily available aircraft.
Decision relevance
Without adequate in situ sampling, aerosol properties and chemistry will be difficult to validate.