Chemical Weathering Models

For a variety of practical reasons, great effort has been expended in recent years to understand controls on and to predict rates of chemical weathering in soils. A principle reason has been a need to assess the effects of acid precipitation on the chemistry of soils and thus on the health of affected plants and trees. Soil acidification from acid precipitation has been a serious problem in industrialized areas where soils are thin or absent and bedrock and resultant soils lack carbonate or reactive silicate minerals (cf. Likens et al. 1977 Berner and Berner 1996). Soil acidification in such areas has caused the acidification of adjacent streams and even underlying groundwaters (cf. Bottcher et al. 1985 Hansen and Postma 1995). 

In the mass-balance approach an attempt is made to determine field weathering rates from element flux calculations, usually focusing on the plant-nutrient important base cations (Ca- Mg , K, and Na+) or on silica (cf. Velbel 1985). In some studies balances are also computed for Al (Swoboda-Colberg and Drever 1992) and/or the the major anions and nitrogen species (Likens et al. 1977 Mast et al. 1990). A general mass-balance equation for the base cations (BC) might be 

TABLE 7.7 Weathering rates, / ,v (keq/ha/y), calculated from compositional differences between the top and bottom of a soil near Gardsjon, Sweden 

Mineral K Bottom of profile composition (wt %) Ions released 

Source After H. Sverdrup and P. Warfvinge. Weathering of primary silicate minerals in the natural soil environment in relation to a chemical weathering model. Water, Air and Soil Pollution 38 387-408. 1988 by Kluwer Academic Publ. Used by permission. 

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Kaminski JW, Neary L, Struzewska J, McConnell JC, Lupu A, Jarosz J, Toyota K, Gong SL, Cote J, Liu X, Chance K, Richter A (2008) GEM-AQ, an on-line global multiscale chemical weather modelling system model description and evaluation of gas phase chemistry processes. Atmos Chem Phys 8 3255-3281. doi 10.5194/acp-8-3255-2008 Kuo HL (1974) Eurther studies on the parametrization of the influence of cumulus convection on largescale flow. J Atmos Sci 31 1232-1240... 

Inclusion of the sea breeze process which is crucial for chemical weather modelling in coastal regions. The meteorological driver needs to describe properly the processes relevant to sea breeze circulations. 

Release modeling system. Contains database of chemicals and characteristics which may be modified by user. User selects chemical, weather conditions and type of release for simple or heavy gas modeling. Output is numeric for times and distances with graphic capabilities. 

Fig. 9-3 Conceptual model to describe the interaction between chemical weathering of bedrock and down-slope transport of solid erosion products. It is assumed that chemical weathering is required to generate loose solid erosion products of the bedrock. Solid curve portrays a hypothetical relationship between soil thickness and rate of chemical weathering of bedrock. Dotted lines correspond to different potential transport capacities. Low potential transport capacity is expected on a flat terrain, whereas high transport is expected on steep terrain. For moderate capacity, C and F are equilibrium points. (Modified with permission from R. F. Stallard, River chemistry, geology, geomorphology, and soils in the Amazon and Orinoco basins. In J. I. Drever, ed. (1985), "The Chemistry of Weathering," D. Reidel Publishing Co., Dordrecht, The Netherlands.)...

Figure 9-3 portrays a hypothetical model of how chemical weathering and transport processes interact to control soil thicknesses. The relationship between soil thickness and rate at which chemical weathering can generate loose solid material is indicated by the solid curve. The rate at which transport processes can potentially remove loose solid weathering products is indicated by horizontal dotted lines. The rate of generation by chemical weathering initially increases as more water has the opporhmity to interact with bedrock in the soil. As soil thick-... 

In this chapter, we build on applications in the previous chapter (Chapter 26), where we considered the kinetics of mineral dissolution and precipitation. Here, we construct simple reactive transport models of the chemical weathering of minerals, as it might occur in shallow aquifers and soils. 

Two different approaches have been taken by researchers to determine the secondary mineralogy of CCBs (1) direct observation, which is accomplished via analysis of weathered ash materials, and (2) prediction, based on chemical equilibrium solubility calculations for ash pore-waters and/or experimental ash leachate or extractant solutions. Because the secondary phases are typically present in very low abundance, their characterization by direct analysis is difficult. On the other hand, predictions based on chemical equilibrium modelling or laboratory leaching experiments may not be reliable indicators of element leachability or accurately indicate the secondary phases that will form under field conditions (Eighmy et al. 1994 Janssen-Jurkovicova et al. 1994). 

In perspective, integrated NWP-ACTM modelling may be a promising way for future atmospheric simulation systems leading to a new generation of models for improved meteorological, environmental and chemical weather forecasting. 

The on-line implementation of environmental processes in the GEM model allows running in global uniform, global variable, and limited area configurations, allowing for multiscale chemical weather forecasting (CWF) modelling. This approach provides access to all required dynamics and physics fields for chemistry at every time step. The on-line implementation of chemistry and aerosol processes... 

Air quality modelling at met.no consists of three different systems, all coupled offline to our numerical weather prediction (NWP) models. These are (1) a nuclear emergency system, (2) an urban air quality (AQ) forecasting system and (3) a longterm air quality chemical transport model routinely used in Europe to determine transboundary pollution fluxes. 

The Enviro-HIRLAM is an online coupled NWP and ACT model for research and forecasting of both meteorological and chemical weather (Fig. 16.4). The integrated modelling system is developed by DMI and other collaborators (Chenevez et al. 2004 Baklanov et al. 2004, 2008a Korsholm et al. 2008a, Korsholm 2009) and included by the European HIRLAM consortium as the baseline system in the HIR-LAM Chemical Branch (https //hirlam.org/trac/wiki), it is used in several countries. 

The Atmospheric Chemistry Transport modelling system used is based on the off-line coupled CAMx and HIRLAM models has been developed to simulate particulate and gas-phase air pollution on different scales. It has been used to simulate short and longterm releases of different chemical species and air pollution episodes. At present it is run in a pre-operational mode 4 times per day based on 3D meteorological fields produced by the HIRLAM NWP model. Currently this modelling system is setup to perform chemical weather forecasts for a series of chemical species (such as O3, NO, NO2, CO and SO2) and forecasted 2D fields at surface are available for each model as well as an ensemble of models (based on 12 European regional air quality models). The simulated output is publicly available and it is placed at the ECMWF website (http //gems.ecmwf.int/d/products/raq/forecasts/) of the EC FP6 GEMS project. 

There are several shortcomings of the current HIRLAM and other NWP models for use in air pollution modelling and Chemical Weather Forecasting (CWF), including the following ... 

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