To a large degree, oxidizing or Los Angeles-type smogs have supplanted reducing smog in importance in most industrialized countries. Oxidizing smogs are common in sunny places where there are large emissions to the atmosphere of nitric oxide and hydrocarbons, and where the atmospheric conditions are frequently stable. Oxidizing smogs form when those emitted (or primary) pollutants are transformed through photochemical reactions into secondary pollutants, the most important of which are the strong oxidant gases, ozone and peroxyacetyl nitrate. These secondary gases are the major components of oxidizing smog that are harmful to people and vegetation.
Typically, the concentrations of these various chemicals vary predictably during the day, depending on their rates of emission, the intensity of sunlight, and atmospheric stability. In the vicinity of Los Angeles, for example, ozone concentrations are largest in the early-tomid afternoon, after which these gases are diluted by fresh air blowing inland from the Pacific Ocean. These winds blow the polluted smog further inland, where pine forests are affected on the windward slopes of nearby mountains. The photochemical reactions also cease at night, because sunlight is not available then. This sort of daily cycle is typical of places that experience oxidizing smog.
Humans are sensitive to ozone, which causes irritation and damage to membranes of the respiratory system and eyes, and induces asthma. People vary greatly in their sensitivity to ozone, but hypersensitive individuals can suffer considerable discomfort from exposure to oxidizing smog. However, in contrast to some of the events of reducing smog, ozone and oxidizing smog more generally do not appear to cause the death of many large people. Ozone is also by far the most important gaseous pollutant in North America, in terms of causing damage to agricultural and wild plants.
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