Ground-level ozone in eastern North America : its formation and transport

Abstract

Ozone (Os), a natural component of the troposphere, is augmented by photochemical processes involving manmade emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs). Sufficiently high concentrations of ozone are detrimental to the respiratory system. Ozone exposure also reduces crop yields and damages forests. This study attempts to explain the underlying factors which contribute to observed ozone levels.

Long range transport models of three species - NOx, VOCs and ozone - are developed for eastern North America. The seasonally averaged models include the essential physical and chemical processes in a relatively simple framework. NOx and VOCs are treated as primary species, i.e., they are modeled from their introduction into the atmosphere to their point of removal. Detailed emission inventories serve as input to the precursor models. Ozone is considered a secondary species because it is not directly emitted. Rather, its production is assumed to be a function of ambient NO, and VOCs levels.

Measured concentrations, available for NO 2 and ozone, are compared with model predictions and aid in determining adjustable model parameters. Predicted NOx concentrations are consistent with rural observations but underestimate sites influenced by nearby sources at which the long range assumptions break down. Local models which properly treat proximate sources account for the discrepancy. The VOCs model, having no measurements for verification, adopts parameters consistent with the NOx model and known chemical properties. Both biogenic and manmade emissions contribute to ambient VOCs levels. Biogenic emissions are found to be more important over most of ENA; anthropogenic sources of VOCs are dominant only in urban areas.

Consistent with empirical patterns, the ozone model predicts small regional gradients and hence a limited dependence on NOx and VOCs precursors. The natural background component is determined to be two-thirds of average ozone levels. Regional transport is significant; ozone lifetimes are estimated to be of the order of a day. The high background level and insensitivity to precursors suggests that significant reductions of average ozone concentrations will be difficult to achieve.