Chapter 8

The current concentration of carbon dioxide in the atmosphere is 365 ppmv. It was indicated in the text that annual anthropogenic additions to the atmosphere are about $7\mathrm{Ct}$ (as C) of which about $4\mathrm{Gt}$ are removed into oceans and the terrestrial environment. Use these numbers to estimate the yearly net increase in atmospheric carbon dioxide mixing ratio in ppmv.

There has been a steady decrease in the ratio of ${}^{14}\mathrm{C}$ to ${}^{12}\mathrm{C}$ in the atmosphere over the past decade. Explain how this is consistent with the view that the well documented increase in atmospheric carbon dioxide concentrations is primarily due to emissions from the combustion of fossil fuels.

Estimates (ref. 1) for emissions of methane to the atmosphere are given in the table below and the current atmospheric concentration is $1.77\mathrm{ppmv}$. Calculate its residence time. $$\begin{array}{|l|}\hline \text{ Sources of atmospheric methane in million tonnes per year }\\ \text{ Wetlands and other natural sources }& 160\\ \text{ Fossil-fuel-related sources }& 100\\ \text{ Other anthropogenic sources of biological origin }& 275\\ \hline\end{array}$$ There may be ${10}^{14}\mathrm{t}$ of methane hydrate $\left({\mathrm{CH}}_{4}6{\mathrm{H}}_2\mathrm{O}\right)$ in the permafrost below the ocean floors. If $1\mathrm{\%}$ of this were to melt per year, what would be the increased concentration of methane (ppmv $y^{-1}$ ) in the atmosphere neglecting any removal processes? What sinks for methane would play a role in reducing this concentration?

Recent work has shown that the flux of methane released from fens in the boreal forest area of Saskatchewan, Canada range from 176 to $2250\mathrm{mmol}{\mathrm{m}}^{-2}{\mathrm{y}}^{-1}$. Daily fluxes range from $1.08$ to $13.8\mathrm{mmol}{\mathrm{m}}^2{\mathrm{d}}^{-1}$. The data indicate that there are correlations between methane release and water depth (negative), water flow (negative), temperature (positive), and inorganic phosphorus in the sedimentary interstitial water (positive). Suggest reasons for these correlations. (Rask, H., D. W. Anderson, and I. Schoenau, Methane fluxes from boreal forest wetlands in Saskatchewan, Canada, Con. 1. Soil Sci., 76 (1996), 230 .

The Arrhenius parameters for the reaction $${\mathrm{N}}_2\mathrm{O}-{\mathrm{N}}_2+\mathrm{O}$$ are $A=7.94\times {10}^{11}{\mathrm{s}}^{-1}$ and $E_a=250\mathrm{kj}{\mathrm{mol}}^{-1}$. The reaction is first order. Calculate the rate constant and half-life of nitrous oxide assuming a tropospheric mixing ratio of $310\mathrm{ppbv}{\mathrm{N}}_2\mathrm{O}$ at ${20}^{\circ }\mathrm{C}$ and comment on the environmental significance of these results.

Discuss possible effects of the following on greenhouse gas chemistry. (a) The southern Pacific Ocean is seeded with the algal micronutrients zinc and iron. (b) CFCs cause further thinning of the ozone layer in the stratosphere. (c) Urban air pollution leads to increased tropospheric ozone concentrations. (d) Rice (paddy) is grown, under submerged conditions, on coarse sandy soils, rather than fine clay-rich soils.

One 'climate engineering' proposal for reducing the possibilities of global warming is to inject a sulfate aerosol into the stratosphere. Discuss the climatic and other atmospheric implications of this possible human intervention.