Measurements of nonmethane hydrocarbons (NMHCs) from urban plumes have previously been used to deduce average OH concentrations by monitoring the decay of the NMHCs from initial values and comparing their time rates of decay to the decay rate of a relatively inert tracer emitted from the same source. Under the assumption of no interaction between photochemistry and transport, these observationally derived indirect OH concentrations are lower for NMHCs with successively higher reactivities toward OH. In this study, analytical solutions to simple flow situations and a three-dimensional mesoscale model are used to test this assumption. When turbulent transport is parameterized in terms of diffusion coefficients, the models yield results that are consistent with the observations and suggest that the assumption is not generally valid. Daytime vertical mixing within the planetary boundary layer (PBL) and horizontal diffusion are two transport processes that cannot be assumed to be separable from species reactivity. The net effect is that under most daytime conditions the OH concentrations can be underpredicted by more than a factor of 2 when highly reactive NMHCs are used to derive OH concentrations in the usual manner.

However, these results only apply when the species used to derive OH concentrations are emitted from a continuous source. The assumption of separability beween photochemistry and transport is valid when only instantaneous or puff emissions are considered. An explanation of these effects is presented within the contex of the analytical results to some simply flow systems. Results from the three-dimensional model are used to illustrate the interaction of species reactivity and transport for more realistic parameterizations of atmospheric transport. The results of this study have import implications for experiments designed to indirectly determine OH concentrations and also for the treatment of turbulent transport in Lagrangian and Eulerian photochemical models. ¿ American Geophysical Union 1990