Chemistry Open Book Paper
Nitrous Oxides (NO2 and NO or NOx), Carbon Monoxide (CO) and various Hydrocarbons (HC) are all pollutants that are produced by car engines. NOx is formed whenever fuel is burnt by the oxidation of Nitrogen in air that is over 800°C. This temperature is reached in lightning and in most combustion engines.
At these temperatures, oxygen molecules split into oxygen radicals.
O2 O + O
Each oxygen radical enters a catalytic cycle with nitrogen molecules to form nitrogen monoxide molecules and more oxygen radicals.
O + N2 NO + N
N + O2 NO + O
As NO cools, it reacts with more oxygen radicals and becomes NO2.
2NO + O2 2NO2
Hydrocarbons found in engine emissions are usually unburnt fuel molecules, while Carbon Monoxide is formed when processes usually giving CO2 are burnt with low levels of oxygen.
Combined, these pollutants are very bad for the atmosphere. NOx itself absorbs Infra-red radiation and can cause acid rain by reacting with OH radicals in water, but can cause more problems reacting with other things. For example, in sunlight, NOx reacts with hydrocarbons to produce harmful photochemical oxidants such as PAN and Ozone. Ozone is a pollutant in the lower atmosphere because it is toxic, irritant and can cause health problems.
Ozone is formed when NOx reacts with oxygen in sunlight.
NO2 NO + O
O + O2 O3
However, Ozone can also decompose with Nitrogen Monoxide.
NO + O3 NO2 + O2
This means that, if no extra NO2 is added to the atmosphere, the amount of ozone should stay at a steady state with no build-up of ozone. Extra NO2 causes the formation of ozone to be faster than the decomposition, causing a build-up of it.
Hydrocarbons found in a polluted atmosphere can react with NO instead of ozone to further help levels of ozone build up.
Different types of combustion engine produce different pollutants in different amounts. Here is a table showing what different amounts of...