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More than a hundred million people in cities that fail to meet international for air quality. to improve conditions in these urban have usually focused on reducing of reactive hydrocarbons (such unburned gasoline vapors), nitrogen oxide radicals (NO and NO_2, together known as _x), and primary and secondary sources of matter (such as diesel smoke sulfur dioxide). Control strategies have over time in response to evolving about atmospheric photochemistry the impact of urban emissions on air downwind of cities (1, 2). The results by Li et al. on page 1657 of this issue (3) may require another rethinking of these strategies.
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Wennberg et al. (2008) studied this question.
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