Reaction of Isoprene on Thin Sulfuric Acid Films: Kinetics, Uptake, and Product Analysis

A high vacuum Knudsen flow reactor was used to determine the reactive uptake coefficient, gamma, of isoprene on sulfuric acid films as a function of sulfuric acid weight percent, temperature, and relative humidity. No discernible dependence was observed for gamma over the range of temperatures (220 - 265 K) and pressures (10(-7) Torr -10(-4) Torr) studied. However, the uptake coefficient increased with increased sulfuric acid concentration between the range of 78 wt % (gamma(i), similar to 10(-4)) and 93 wt % (gamma(i) similar to 10(-3)). In addition to the Knudsen Cell, a bulk study was conducted between 60 and 85 wt % H2SO4 to quantify uptake at lower acid concentrations and to determine reaction products. After exposing sulfuric acid to gaseous isoprene the condensed phase products were extracted and analyzed using gas chromatography/mass spectrometry(GC/MS). Isoprene was observed to polymerize in the sulfuric acid and form yellow/red colored monoterpenes and cyclic sesquiterpenes. Finally, addition of water to the 85 wt % sulfuric acid/isoprene product mixture released these terpenes from the condensed phase into the gas phase. Together these experiments imply that direct isoprene uptake will not produce significant SOA; however, terpene production from the small uptake may be relevant for ultrafine particles and could affect growth and nucleation.