Troposphere aqueous-phase chemistry

The aqueous-phase and gas-phase chemistries of HO, are sufficiently closely coupled that the chemistry shown in Tables 8.11 and 8.12 can affect gas-phase concentrations as well. For example, including the aqueous-phase chemistry in models of tropospheric ozone formation alters predicted 03 concentrations, although whether the perturbation is significant is subject to some controversy (e.g., see Lelieveld and Crutzen, 1990 Jonson and Isaksen, 1993 Walcek et al., 1997 Liang and Jacob, 1997). 

Caltech unified GCM (Global) GISS GCM IF Harvard tropospheric Os-NO -hydrocarbon chemistry (305-346 reactions, 110-225 species) bulk aqueous-phase chemistry of S(IV) (5 equilibria and 3 kinetic reactions) prognostic aerosol/ cloud treatments with prescribed size distribution Global chemistry-aerosol interactions aerosol direct radiative forcing the role of heterogeneous chemistry impact of future climate change on O3 and aerosols Liao et al. (2003), Liao and Seinfeld (2005)... 

Let us first consider briefly the sources of nitrate in the troposphere aqueous phase. In modelling tropospheric multiphase chemistry, a number of more or less recognized gas phase and aqueous phase reactions are included which produce nitric species. We focus on the following main modes leading to nitrate within a droplet ... 

Table 5.1 classifies how chemical regimes meet in the climate system. We see that almost normal conditions occur and extreme low and high temperatures border the climate system. The chemistry described in the following chapters concerns almost these normal conditions of the climate system. We focus on the troposphere and the interfaces. For example, aqueous phase chemistry in cloud droplets does not differ principally from surface water chemistry (aquatic chemistry) and much soil chemistry does not differ from aerosol chemistry (colloidal chemistry). Plant chemistry, however, is different and only by using the generic terms (Chapter 2.2.2.S) of inorganic interfacial chemistry can we link it. The chemistry of the atmosphere is widely described (Seinfeld and Pandis 1998, Wameck 1999, Finlay-son-Pitts and Pitts 2000, Wayne 2000, Brasseur et al. 2003) and the branches in atmospheric chemistry are well defined (Fig. 5.2). 

H2O2 is rather stable (Finlayson-Pitts and Pitts 1986) in the gas phase, i.e. it does not undergo fast photochemical and gas phase reactions. The only important sinks in the boundary layer are dry deposition and scavenging by clouds (with subsequent aqueous phase chemistry) and precipitation (wet deposition). In the free troposphere (FT), H2O2 photolysis is regarded as an important radical feedback (Crutzen 1999). The photodissociation of H2O2 into two OH radicals was first shown by Urey et al. (1929). 

Herrmann, H., A. Tilgner, P. Barzaghi, Z. Majdik, S. Gligorovski, L. Poulain and A. Monod (2005) Towards a more detailed description of tropospheric aqueous phase organic chemistry CAPRAM 3.0. Atmospheric Environment 39, 4351-4363... 

Herrmann H, Hoffinann D, Schaefer T, Brauer P, Tilgner A (2010) Tropospheric aqueous-phase free-radical chemistry radical sources, spectra. Reaction kinetics and prediction tools. Chemphyschem ll(18) 3796-3822... 

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. 

Liang, J., and D. J. Jacob, Effect of Aqueous Phase Cloud Chemistry on Tropospheric Ozone, J. Geophys. Res., 102, 5993-6001 (1997). 

The net effect of these two links between sulfur and halogen chemistry is to decrease the gas phase concentration of SO2 via a reduced yield of SO2 from the oxidation of DMS and the stronger aqueous phase sink for S(IV) which results in enhanced uptake of SO2 by droplets and aerosols. A critical prerequisite for new particle formation in the marine troposphere is the reaction chain ... 

Herrmann H., B. Ervens, H.-W. Jacobi, R. Wolke, P. Nowacki and R. Zellner CAPRAM2.3 A chemical aqueous phase radical mechanism for tropospheric chemistry, J. Atmos. Chem. 36 (2000) 231 -284. 

Cloud processes have been predicted to have a significant effect on the chemistry of the clean troposphere (Lelieveld and Crutzen, 1990, 1991 Warneck, 1991, 1992). For example, the uptake of HCHO, HOz radicals, and N2Os into cloud droplets can lead to a decrease in the production of ozone. Removal of HCHO reduces the rate of gas-phase production of HOz radicals, and N205 into cloud droplets can lead to a decrease in the production of ozone. Removal of HCHO reduces the rate of gas-phase production of H02 radicals [reactions (33)—(36)1, and consequent conversion of NO to N02. Also, aqueous-phase reactions of H2C(OH)2, the hydrated form of HCHO, lead to the formation of 02, which can react with dissolved 03 to enhance the rate of transfer of 03 to the liquid phase over that based solely on physical solubility. Absorption of N2Os into... 

Iron complex photolysis is mie of the processes that produce reduced iron (Fe(n)) in a highly oxidizing enviromnent like the atmospheric aqueous phase. There are numerous other processes such as reactions with HO species or Cu(I)/ Cu(n) which can reduce or oxidize iron in the troposphere. These reactions can take place simultaneously and cause iron to undergo a so-caUed redox-cycling [167]. Because of the large number of complex interactions in the atmospheric chemistry of the transition metal iron, it is useful to utilize models to assess the impact of the complex iron photochemistry. 

Sander, R. Cmtzen, P.J., 2000 Comment on A Chemical Aqueous Phase Radical Mechanism for Tropospheric Chemistry by Herrmann et al. , in Chemosphere, 41 631—632. 

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