Marine boundary layer

Figure 5 Schematic illustration of the sources and sinks of DMS in the marine boundary layer of the atmosphere and the oceanic mixed layer (Taken from Bigg," with permission of Cambridge University Press)...

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

Three-dimensional representation of the latitudinal distribution of atmospheric carbon dioxide in the marine boundary layer. Data from the NOAA CMDL cooperative air sampling network were used. The surface represents data smoothed in time and latitude. The Norwegian and Swedish flask sampling effort at Zeppelin Station is shown in the inset as flask monthly means. (Figure kindly provided by Dr Pieter Tans and Dr Thomas Conway of NOAA (CMDL).)... 

Sievering, H., Boatman, J., Gorman, E., Kim, Y., Anderson, L., Ennis, G., Luria, M., and Pandis, S. (1992). Removal of sulphur from the marine boundary layer by ozone oxidation in sea-salt aerosols. Nature, 360, 571-573. 

Eledgecock IM, Pirrone N. 2004. Chasing quicksilver modeling the atmospheric lifetime of Hg( g) in the marine boundary layer at various latitudes. Environ Sci Technol 38 69-76. 

Pirrone N, Pacyna JM, Munthe J, Barth H. 2003a. Dynamic processes of mercury and other trace contaminants in the marine boundary layer of European seas — ELOISE 11. Atmos Environ 37(S1) 1-177. 

Sprovieri F, Pirrone N, Gardfeldt K, Sommar J. 2003. Atmospheric mercury speciation in the marine boundary layer along 6000 km cruise path over the Mediterranean Sea. Atmos Environ 37(S1) 63-72. 

Weiss-Penzias P, Jaffe DA, McClintick A, Prestbo EM, Landis MS. 2003. Gaseous elemental mercury in the marine boundary layer evidence for rapid removal in anthropogenic pollution. Environ Sci Technol 37 3755-3763. 

The model results were compared with the HOx concentrations measured by the FAGE (Fluorescence Assay by Gas Expansion) technique during four days of clean Southern Ocean marine boundary layer (MBL) air. The models overestimated OH concentrations by about 10% on two days and about 20% on the other two days. HO2 concentrations were measured during two of these days and the models overestimated the measured concentrations by about 40%. Better agreement with measured HO2 was observed by using data from several MBL aerosol measurements to estimate the aerosol surface area and by increasing the HO2 uptake coefficient to unity. This reduced the modelled HO2 overestimate by 40%, with little effect on OH, because of the poor HO2 to OH conversion at the low ambient NOx concentrations. 

In studies comparing measured and modelled HOx radical concentrations, the models usually overestimate [OH] by 20-50%. A detailed review of the comparisons of modelled and measured concentrations of OH and H02 can be found in Heard and Pilling (2003). In particular, several studies have been made in the marine boundary layer. 

This paper investigates the radical chemistry of the clean marine boundary layer in the Southern Ocean during the SOAPEX-2 (Southern Ocean Photochemistry Experiment 2) campaign using an observationally constrained box-model based on the Master Chemical Mechanism (Jenkin et al., 1997, 2003 Saunders et al., 2003). The primary aim of SOAPEX-2 was to study free radical chemistry in the remote marine boundary layer in the Southern Hemisphere. Sections 2 and 3 of this paper describe the SOAPEX-2 site and the measurements that were made during the campaign. Section 4 describes the models used and Sect. 5 presents the results. Finally, Sect. 6 contains the summary and the conclusions. 

Heterogeneous uptake on surfaces has also been documented for various free radicals (DeMore et al., 1994). Table 3 shows values of the gas/surface reaction probabilities (y) of the species assumed to undergo loss to aerosol surface in the model. Only the species where a reaction probability has been measured at a reasonable boundary layer temperature (i.e. >273 K) and on a suitable surface for the marine boundary layer (NaCl(s) or liquid water) have been included. Unless stated otherwise, values for uptake onto NaCl(s), the most likely aerosol surface in the MBL (Gras and Ayers, 1983), have been used. Where reaction probabilities are unavailable mass accommodation coefficients (a) have been used instead. The experimental values of the reaction probability are expected to be smaller than or equal to the mass accommodation coefficients because a is just the probability that a molecule is taken up on the particle surface, while y takes into account the uptake, the gas phase diffusion and the reaction with other species in the particle (Ravishankara, 1997). 

February) were selected as representative of the extremely clean conditions of the Southern Hemisphere Marine Boundary Layer. These very clean conditions (NO<3 ppt) correspond to the cleanest conditions under which radical measurements have been taken at ground level in the Southern Pacific Ocean. The two models agree to within 5-10% or less. 

Covert, D. S., Gras, J. L., Wiedensohler, A., and Stratmann, F. Comparison of directly measured CCN with CCN modeled from the number-size distribution in the marine boundary layer during ACE 1 at Cape Grim, Tasmania, J. Geophys. Res.-A., 103, 16597-16608,1998. 

Keene, W., Jacob, D. J., and Fan, S. M. Reactive chlorine A potential sink for dimethylsulfide and hydrocarbons in the marine boundary layer, Atmos. Environ., 30, R1-R3, 1996. 

Monks, P. S., Carpenter, L. J., Penkett, S. A., and Ayers, G. P. Night-time peroxy radical chemistry in the remote marine boundary layer over the Southern ocean, Geophys. Res. Lett., 23, 535-538, 1996. 

Raes, F., VanDingenen, R., Cuevas, E., VanVelthoven, P. F. J., and Prospero, J. Observations of aerosols in the free troposphere and marine boundary layer of the subtropical Northeast Atlantic Discussion of processes determining their size distribution, J. Geophys. Res.-A., 102, 21 315-21 328,1997. 

Marine mercury cycle. All fluxes are in units of Mmol/y. Preindustrial fluxes are in parentheses. The marine boundary layer is the iower part of the troposphere in which mercury transformations associated with the air-sea interface occur. Source After Mason, R. R, and G.-R. Sheu (2002). Global Blogeochemical Cycles 16, GB001440. 

Disselkamp, R.S., Chapman, E.G., Barchet, W.R., Colson, S.D., Howd, C.D. BrCl production in NaBr/NaCl/HNOs/Os solutions representative of sea-salt aerosols in the marine boundary layer, Geophys. Res. Lett., 26(13) 2183-2186, 1999. 

Vila-Guerau de Arellano, J., P. G. Duynkerke, and M. van Weele, Tethered-Balloon Measurements of Actinic Flux in a Cloud-Capped Marine Boundary Layer, . /. Geophys. Res., 99, 3699-3705 (1994). 

In short, while there is evidence that atomic chlorine is generated from sea salt reactions and contributes to organic oxidations in the marine boundary layer, the nature and strength of the sources remain to be elucidated. 

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