Mean profiles of trace reactive species in the unpolluted marine surface layer

Abstract

We have investigated several aspects of trace gas photochemistry in the marine boundary layer using a time-dependent transport-kinetics model with one-dimensional eddy diffusion. The photochemical scheme in the model (Thompson and Cicerone, 1982) is represented by a conventional complement of reactions involving O, H, N, and methane-derived organic species; boundary conditions are assigned which give low surface mixing ratios of O₃ and NOₓ (Routhier et al., 1980; McFarland et al., 1979) characteristic of the remote marine environment. Altitude dependent eddy diffusion coefficients in the surface layer (1 mm to 100 m) are based on the formulation of Businger et al. (1971) for temperature diffusivity in an unstably stratified surface layer. Diffusion coefficients in the rest of the convective boundary layer (the mixed layer) are taken from Lamb and Durran (1978). The surface is assumed to be the tropical ocean with a steady state mixed layer. In the simulations described here, particular attention has been given to the distribution of odd nitrogen and the NO₂-NO-O₃ photostationary state in the surface boundary layer. Calculated profiles of NO, NO₂, O₃, and HNO₃ show definite gradients in the surface layer. When the ocean is assumed to be a source of NO, mixing ratios on the order of a few parts per trillion can be supported by an upflux of ~10⁸ cm⁻² s⁻¹. If a surface input of NO is not assumed, NOₓ levels are much lower. This is consistent with measurements in the Equatorial Pacific (McFarland et al., 1979; Zafiriou et al., 1980; Liu et al., 1983) and suggests that NO upwelling may be significant in the local budget of NOₓ in certain remote marine environments. Calculated values of the O₃-NO-NO₂ photostationary state ratio, Rₚₛ, and NO/HNO₃ show appreciable variation within the surface layer. The latter ratio is sensitive to the NO upflux and to heterogeneous removal of HNO₃. Rₚₛ decreases away from the surface and is unity only at one point. Both chemical and micrometeorological factors (Lenschow, 1982) contribute to nonunity values of Rₚₛ. Significant departures from a profile characteristic of a nonreactive species are shown to occur in the NO profile as low as 0.2 m above the surface.