Simulation of chemical processes occurring in an atmospheric boundary layer. Influence of light and biogenic hydrocarbons on the formation of oxidants

Abstract

A photochemical model has been developed to determine the impact of the emission of light and terpenic hydrocarbons (HCs) on the evolution of ozone, HNO₃ and PAN concentrations in the troposphere.

The present model, of Lagrangian type, describes the evolution of trace gases in an air mass moving over a large and homogeneous source area. The respective cycles of O₃, NO_x, methane and CO are considered. The vertical exchange is treated by introduction of a vertical diffusivity tensor K_z varying as a function of time and altitude.

The results of this study indicate that both the intensity of vertical exchanges of airborne elements and the ground deposition play an important role on the evolution of vertical concentration profiles. The influence of diffusivity on the integrated concentrations in the boundary layer remains low.

The introduction of light HCs (alkanes, alkenes) results in an increase in the O₃ concentration, correlated with the intensity of NO_x and HC emissions. For a given level of NO_x, both a decrease of the concentration in HNO₃ and an increase in the PAN concentration are observable when the production of light HCs increases. The sensitivity increases with the NO_x source.

The introduction of terpenic HCs causes a depletion of O₃ in the air masses characterized by low NO_x emission rates, and an enrichment in those with higher emission rates. In both cases, the amplitude of the variation increases with the importance of the source of terpenic HCs, and is found higher for α-pinene than for isoprene.

For a given NO_x emission rate, the HNO₃ concentrations are seen to decrease when the source of natural HCs increase, whereas the concentration in PAN increases. As observed for light HCs, the sensitivity shows a dependence on the magnitude of the NO_x source.