The size of the bounding box in equation ( ) is determined entirely by σ t y , σ t z . Since both σ y and σ z monotonically increase downwind of the source, using the time-varying value of σ y , σ z enables dynamic resizing of the bounding box during advection. At some point, the bounding box will leave the computational domain and simulation of that puff can stop. The exit time t e when a puff will leave the computational domain can be computed by creating an initial bounding box centered on the source using σ m y,z = max σ y,z and marching forward in time to when the box exits the domain. Note that the value of t e can be efficiently calculated using an Axis Aligned Bounding Box (AABB) approach described in Appendix C. A summary of our fast method for simulating Gaussian puffs is described in Algorithm 1 and a pictorial representation is shown in Fig. .