REACTION OF METHYLBUTENOL WITH HYDROXYL RADICAL - MECHANISM AND ATMOSPHERIC IMPLICATIONS

The tropospheric fate of 2-methyl-3-buten-2-ol (methylbutenol, MBO), a recently identified emission by vegetation, was investigated by measuring its UV absorption cross sections (210-300 nm) and the rate coefficient for its reaction with hydroxyl free radicals. UV absorption cross sections were found to be too small for photolysis to be an important removal pathway for MBO in the troposphere. The rate constant applicable under tropospheric conditions for the reaction of OH with MBO was determined to be k = (8.2 +/- 1.2) x 10(-12) e(((610+/-50/T)) cm(3) molecule(-1) s(-1). The OH reaction proceeds mainly via addition of the OH to the double bond in MBO. In the absence of O-2, about 15-20% of the adducts eliminate the alcohol-OH group. However, O-2 can scavenge the adduct before it decomposes at T < 300 K. This mechanism was confirmed by measuring the rate coefficients for the reactions of OD and (OH)-O-18 and determining the rate coefficient for the OK reaction in the presence of 7-13 Torr of O-2 and in SF6 buffer gas. The elimination of the alcohol-OH group was substantiated by observing OH production in the reactions of (OH)-O-18 and OD. The obtained OH reaction rate coefficient suggests that the primary daytime loss of MBO in the troposphere is via its reaction with OH.