For sprays of practical interest, the region where vigorous vaporization occurs is confined to a thin layer on their outer edges. The cubical array of droplets occupying a semi-infinite space provides a microscopic prototype to investigate this vaporization layer structure, upon which interactive droplet vaporization/combustion characteristics are examined to derive rational boundary conditions to be imposed at the spray edge when the spray is regarded as a continuum medium on large scales. It is found that not only the local droplet number density but also the gas-to-liquid density ratio plays an important role in excitation of prevailing vaporization/combustion modes, leading to their criteria in terms of the droplet separation distance as a function of the gas-to-liquid density ratio.