Figure 1.

Yields of secondary organic aerosol (SOA) from isoprene oxidation as reported by chamber studies in the literature and plotted as a function of the initial NO concentration and relative humidity (RH). Yields are defined as the mass of SOA produced per unit mass of isoprene oxidized. For studies with no detectable NO we plot the NO concentration as half the reported instrument detection limit, and stagger points as needed for clarity. Data are colored by relative humidity (RH). The thick grey line divides the low-NO_x and high-NO_x pathways as determined by the fate of the ISOPO₂ radical (HO₂ dominant for the low-NO_x pathway, NO dominant for the high-NO_x pathway). The transition between the two pathways occurs at a higher NO concentration than in the atmosphere because HO₂ concentrations in the chambers are usually much higher. Also shown as dashed line is the mean atmospheric yield of 3.3% for the Southeast US determined in our study.