Everything from the 3 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Nitrogen is unreactive because the N≡N triple bond is very strong (944 kJ mol−1) and the molecule is non-polar, so it does not attract electrophiles or nucleophiles. Ammonia is a Brønsted–Lowry base: the lone pair on nitrogen accepts a proton, forming the ammonium ion, NH4+, by a dative bond. NH3 is pyramidal (107°) and NH4+ tetrahedral (109.5°). Ammonium salts react with warm alkali to release ammonia, which turns damp red litmus blue. Ammonium salts are used as fertilisers; their value depends on the percentage of nitrogen by mass.
Nitrogen is unreactive because of its strong triple bond and lack of polarity, not because it is small or a gas. When calculating percentage of nitrogen, count every nitrogen atom: NH4NO3 has two, (NH4)2SO4 has two, urea has two.
At the very high temperatures inside an engine (or in lightning), nitrogen and oxygen react: N2 + O2 → 2NO. In air NO is oxidised to NO2. Catalytic converters remove NO and CO together: 2CO + 2NO → 2CO2 + N2. In the atmosphere NO2 acts as a catalyst for the oxidation of SO2 to SO3, and NO2 also forms nitric acid, so both add to acid rain. NO and unburnt hydrocarbons in sunlight form photochemical smog containing peroxyacetyl nitrate (PAN). Nitrate fertilisers washed into water cause eutrophication.
In the oxidation of SO2, NO2 is a catalyst: it is used in the first step and regenerated in the second, so it is not used up overall. A catalytic converter removes CO, NO and hydrocarbons; it produces CO2, it does not remove it.
Fossil fuels contain sulfur, which burns to sulfur dioxide (S +4). In the air SO2 is oxidised to SO3 (S +6), helped by the NO2 catalyst, and dissolves in rain to form sulfuric acid, a major cause of acid rain. Acid rain damages limestone buildings, lakes, fish and trees. SO2 can be removed from power-station flue gases by reacting it with calcium carbonate or calcium oxide. In small amounts SO2 is a food preservative, because it is an antioxidant and kills bacteria. Industrially, SO2 is converted to SO3 in the Contact process to make sulfuric acid.
Acid rain is caused by sulfur and nitrogen oxides, not by carbon dioxide; rain is naturally slightly acidic (about pH 5.6) from dissolved CO2, but SO2 and NO2 push it much lower. Remember the oxidation number of sulfur: +4 in SO2 and H2SO3, +6 in SO3 and H2SO4.