S Smith of Dunedin asks :-

A recent newspaper article stated that a Czech coal-fired power station emitted more than 8 million tonnes of carbon dioxide per year. How can carbon emissions be weighed?

Katja Riedel, an atmospheric chemist with the National Institute of Water and Atmospheric Research (NIWA), responded.

You are right the carbon emissions are not weighed; weighing a gas is a quite complicated business. Instead the emissions are calculated. In the case of this Czech power station I assume that they know exactly how much coal they burn during the year. You then have to determine or guess how clean the coal is and how much of this coal is actually carbon. This is the crucial part of the calculation, since emission factors vary significantly by coal. Carbon dioxide (CO2) emission factors have been developed based on a database of coal sample analysis. The typical carbon content for coal ranges from 60 per cent to more than 80 per cent.

Every carbon atom that is burned will produce one CO2 molecule. The molecular weight of CO2 is 44 g/mol (you can look this up in the Periodic Table of Elements, 12g/mol for carbon plus 2 times 16 g/mol for the oxygen). If you then take the total weight of the carbon in the coal and multiply it with molecular weight of the CO2 divided by the molecular weight of the carbon (44/12 = 3.6) you get the weight of the CO2 that is produced.

The burning of fossil fuels, not only coal, but also oil and gas, produces CO2, currently world wide more than 8 gigatons of carbon are burned per year (1 gigaton is 1 billion tons). This makes with our calculation nearly 30 giga tons CO2 per year, a huge amount.