Non-linearities in chemical processes are recognised as being important in a number of areas of atmospheric science. In this paper we show simulations using an idealised plume model which describes the relaxation of an urban plume into the background atmosphere. As might be anticipated, the initial conditions of NOx, O3 and VOCs within the plume and background are important in determining the chemistry downstream of the source, but crucially for this study, the rate of mixing (on timescales appropriate to the real atmosphere) is found to alter the composition of the atmosphere significantly. The model shows that NO3 chemistry can play a major role in the oxidation of biogenic VOCs present in the background atmosphere. In addition, the reaction of hydrocarbons with NO3 potentially has important implications for NOy speciation because a significant fraction of organic nitrates thus formed are sufficiently long-lived to leave the planetary boundary layer. A particularly critical result of the model is that under certain NOx conditions, O3 surface deposition can be significantly inhibited, with consequent effects on the O3 budget.