In the COVID-19 pandemic, many jurisdictions are now requiring that masks be worn in public, often including both indoor and outdoor settings. However, many sources (including widely cited preprints by Qian et al. and Nishiura et al.) also suggest that outdoor transmission of COVID-19 is much less likely than indoor transmission, which has led people to move some activities outside. In addition, some studies have apparently investigated how far droplets can spread due to the breathing/speaking/etc. of people with or without masks.
Has there been any study on the relationship between these factors? For instance, has anyone studied or modeled the concentration of droplets/aerosols that would be present for masked/unmasked people in indoor/outdoor areas? What is the relative effect of masks vs. outdoor environment on the likelihood of transmission (either of COVID-19 or other previously-studied diseases)?
(Please note that I'm not asking about the mechanical function of the mask, which clearly would be unchanged indoors or out. What I'm asking about is the effect of the mask on the actual risk of transmission, relative to or considered in the context of an indoor/outdoor environment.)