JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. D3, PAGES 4788-4796,JUNE 20, 1984 Mean Profiles of Trace Reactive Species in the Unpolluted Marine SurfaceLayer ANNE M. THOMPSON AND DONALD H. LENSCHOW National Center for Atmospheric Research We haveinvestigated several aspects of tracegas photochemistry in the marineboundary 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 organicspecies; boundaryconditions are assigned which give low surface mixing ratios of O 3 and NO x (Routhieret al., 1980; McFarland et al., 1979) characteristic of the remotemarine environment. Altitude dependent eddy diffusion coefficients in the surface layer (1 mm to 100 m) are based on the formulationof Businger et al. (1971)for temperature diffusivity in an unstably stratified surfacelayer. 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 tropicalocean with a steady state mixedlayer.In the simulations described here, particular attention has beengivento the distribution of odd nitrogen and the NO2-NO-O 3 photostationary statein the surface boundary layer.Calculated profiles of NO, NO2, 03, and HNO 3 show definite gradients in the surface layer. When the ocean is assumed to be a source of NO, mixingratioson the order of a few parts per trillioncanbe supported by an upflux of •- 10 a cm-2 s-•. If a surface inputof NO is not assumed, NOx levels are much lower.This is consistent with measurements in the EquatorialPacific (McFarland et al., 1979;Zafiriou et al., 1980;Liu et al., 1983) and suggests that NO upwelling may be significant in the localbudget of NO• in certain remote marine environments. Calculated values of the O3-NO-NO 2 photostationary state ratio, Rps, and NO/HNO 3 show appreciable variation within thesurface layer. The latter ratiois sensitive to theNO upflux andto heterogeneous removal of HNO3.Rps decreases away from the surface and is unity only at one point. Both chemical and micrometeorological factors (Lens- chow, 1982) contribute to nonunity values of Rvs. Significant departures from a profile characteristic of a nonreactive species are shownto occurin the NO profile as low as 0.2 m abovethe surface. 1. INTRODUCTION Trace gases in the boundary layer are subject to various micrometeorological and physicochemical influences. In the surface layer (the lowest few meters of the boundary layer where the fluxesof conserved species can be considered con- stant) turbulent mixing on a rapid time scale can upset the photochemical equilibrium relations among reactive trace gases[Lenschow,1982; Fitzjarrald and Lenschow, 1983]. In this sameregion,heterogeneous effects--the interaction of cer- tain gases with aerosols, fog, and falling rain and snow---can alsodisruptsimple photochemistry. The resultis a high degree of variability in boundarylayer tracegasdistribution on small time and space scales. We have reported previously on certain aspectsof this variability in modeling studies of the remote marine tropo- sphere [Thompsonand Cicerone, 1982; Thompson and Zafi- riou, 1983]. Using a one-dimensional photochemical-transport model with a moderately refined boundary layer (minimum vertical resolution = 6.25 m), we demonstrated that near- surface mixing ratios of NO,, (NO,, - NO + NO2) and soluble gases like HNO3, H202, and CH3OOH are very sensitive to surface fluxes,heterogeneous removal, and near-surface eddy diffusion rates. The purposeof the presentinvestigation is to focus in more detail on a few of these processes, specifically to examine with a model the chemical fine structure in the lowest 100 m of the atmosphere, where ground, tower, and ship- based measurements of chemical concentrations are obtained. In particular we simulateconditions typical of a suppressed marine trade wind regime.Although disturbedsynopticsitu- ations are potentially important for large-scale exchange be- tween the boundary layer and the free troposphere [Chatfield and Crutzen, 1984; Gidel, 1983], as well as for lightning- produced NO,, [Noxon, 1976; Chameides et al., 1977], they are not includedin this study because their effects on the bound- ary layer are more difficult to quantify. We also look at the chemical compositionin the rest of the convective boundary layer (i.e., the mixed layer above the surface layer) where mixing times and the photochemical reaction times considered here are comparableand a state closerto photochemical equi- librium is predicted theoretically [Lenschow, 1982]. Our model is described in section 2. Basic chemical and transport features are the same as in Thompson and Cicerone [1982], but in the presentset of simulations the altitude grid has been compressed and refined. We have also introduced a simplifiededdy-diffusion treatment that represents mixing in the tropical marine boundary layer. Section3 summarizes re- sultsof typical species diurnal cycles obtained from the modi- fied transport scheme. We also report findings from a series of model sensitivity runs designedto look closely at the odd nitrogen system in the surface layer. Here we have been moti- vated by several concerns:(1) the role of an oceanic NO source[Zafiriou and McFarland, 1981] in determiningbound- ary layer NOx; (2) the effects of heterogeneous removal and ocean-derived NO on the NO/HNO3 ratio; (3) the structure of the NO-O3-NO 2 photostationary state ratio in the surface layer, where considerable structureand deviation from unity have been predicted [Fitzjarrald and Lenschow, 1983; Calvert and Stockwell,1983]. 2. MODEL DESCRIPTION Copyright 1984by the American Geophysical Union. Paper number 4D0313. 0148-0227/84/004D-0313505.00 Chemical Kinetics Chemical distributions of trace gases are determined as a function of altitude from a system of one-dimensional 4788