Yields of β -hydroxynitrates, dihydroxynitrates, and trihydroxynitrates formed from OH radical-initiated reactions of 2-methyl-1-alkenes

Yields ofβ -hydroxynitrates, dihydroxynitrates, and trihydroxynitrates, in particles formed from OH radical-initiated reactions of C9 -C15 2-methyl-1-alkenes in the presence of NOx were measured by using a thermal desorption particle beam mass spectrometer coupled to a high-performance liquid chromatograph with a UV-visible (UV-vis) detector. Yields ofβ -hydroxynitrates and dihydroxynitrates increased with carbon number primarily due to enhanced gas-to-particle partitioning before reaching plateaus at ≈C14 -C15 , where the compounds were essentially entirely in the particle phase. Plateau yields ofβ -hydroxynitrates, dihydroxynitrates, and trihydroxynitrates were 0.183 ± 0.005, 0.045 ± 0.005, and 0.034 ± 0.005, and, after normalization for OH radical addition to the C = C double bond, were 0.225 ± 0.007, 0.055 ± 0.006, and 0.042 ± 0.006. The fractions of 1-hydroxy and 2-hydroxyβ -hydroxynitrate isomers were 0.90/0.10. Yields measured here and in our previous study of reactions of linear internal alkenes and linear 1-alkenes indicate that, for these alkene classes, the relative branching ratios for forming tertiary, secondary, and primaryβ -hydroxyalkyl radicals by OH radical addition to the C = C double bond are 4.3/1.9/1.0, and the branching ratios for formingβ -hydroxynitrates from reactions of tertiary, secondary, and primaryβ -hydroxyperoxy radicals with NO are 0.25, 0.15, and 0.12. The effects of H2 O vapor and NH3 on yields were also explored.