The Ozone Monitoring Instrument (OMI) measures formaldehyde (HCHO) and nitrogen dioxide (NO2), and the ratio of HCHO to NO2 for August 2005 shows how these data can be used to develop effective strategies to improve air quality. Harmful levels of surface ozone can form through a complex series of reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. If the ratio HCHO/NO2 is less than 1 (e.g., Los Angeles, Chicago), ozone will decrease when emissions of anthropogenic VOCs are reduced. If the ratio is greater than 2, ozone will decrease when NOx emissions are reduced. Natural VOCs from trees are so high in the eastern United States that reducing NOx emissions is the effective strategy to improve air quality.