光化学烟雾Photochemical smog

NO₂ 吸收波长小于 420 nm 的光分解为 NO 和 O 原子，O 与 O₂ 结合生成 O₃。没有有机物时 O₃ 又被 NO 消耗，浓度维持低水平。挥发性有机物被 OH 自由基氧化生成过氧自由基（RO₂、HO₂），它们把 NO 氧化为 NO₂ 而不消耗 O₃，于是 O₃ 不断积累。过氧乙酰自由基与 NO₂ 结合生成 PAN，刺激眼睛并损害植物。

NO₂ absorbs light below 420 nm and splits into NO and an O atom, which combines with O₂ to give O₃. Without organics, NO consumes the O₃ again and levels stay low. Volatile organics oxidised by OH radicals form peroxy radicals (RO₂ and HO₂) that convert NO to NO₂ without using up O₃, so ozone accumulates. Peroxyacetyl radicals combine with NO₂ to form PAN, which irritates the eyes and damages plants.

1940 年代起美国洛杉矶频繁出现刺眼的烟雾，与伦敦以煤烟和 SO₂ 为主的烟雾截然不同。1952 年加州理工学院的哈根-斯密特证明它是汽车尾气中的烃类和氮氧化物在阳光下反应的产物，被称为“光化学烟雾之父”。

From the 1940s Los Angeles suffered frequent eye-stinging smog quite unlike London's coal-smoke and SO₂ smog. In 1952 Arie Haagen-Smit of Caltech showed it forms when hydrocarbons and nitrogen oxides from car exhaust react in sunlight, and he became known as the father of air-pollution control.

近地面臭氧刺激呼吸道、加重哮喘、降低农作物产量。治理需要同时控制 NOₓ 和 VOCs：机动车三效催化转化器、燃油蒸气回收、工业涂装溶剂替代和电厂脱硝。由于 O₃ 生成对两类前体物呈非线性响应，需要根据当地是“VOC 控制区”还是“NOₓ 控制区”制定减排比例。

Ground-level ozone irritates the airways, worsens asthma and lowers crop yields. Control requires cutting both NOₓ and VOCs: three-way catalytic converters, fuel-vapour recovery, low-solvent industrial coatings and power-plant denitrification. Because ozone responds non-linearly to its two precursor groups, the balance of cuts depends on whether an area is VOC-limited or NOₓ-limited.