The footprint for flux measurements, from backward Lagrangian stochastic models

Backward Lagrangian stochastic models calculate particle trajectories in the atmosphere upstream of an observation point. A feature of these models is the ease with which the vertical flux contribution from surface area sources can be calculated. The flux contribution at an observation point P is found by summing the ratio of the particle vertical velocity at P to the ''touchdown'' velocity for particles which impact the ground within the source boundary.