The approach described above applies to a regular rectangular grid that is contiguous in space. Similar thresholding can be applied to a collection of arbitrary points S that are not contiguous in space with two small modifications. First, the bounding box approach in Appendix B cannot be used since it is grid-based. Since the application is to simulate concentrations for real-world sensor networks with a small number of sensors, it is computationally feasible to skip this step and only apply the condition in (7) to determine which points will add substantially to the concentration. Second, the computational domain Ω must be modified to compute the exit time t e . Choosing Ω as the smallest rectangular box containing all points in S allows the same procedure described in Appendix C to be used to determine when a puff will leave the area of interest.