Can Aerosols Catalyze the Reaction of OH Radical with VOCs ?

Can Aerosols Catalyze the Reaction of OH Radical with VOCs ? Theodore S. Dibble,1 M. Srensen,2 T. J. Wallington,3 M. D. Hurley,3 and O. J. Nielsen4 (1) SUNY-Environmental Science and Forestry (2) University of Southern Denmark (3) Ford Motor Company Scientific Research Lab. (4) University of Copenhagen Atmos. Env.36 (2002)5947-5952

Introduction The Work of Oh and Andino (Atmos. Env.36 149-156) Relative rate studies for reaction with OH radical with VOCs in 100-L Tedlar bag, 298 K (NH4)2SO4 particles with area 1,400 or 3,400 mm2 cm-3 D = 50 mm (number) and 150 mm (area) VOCs: C1-C6 alcohols, n-hexane, p-xylene Analysis: Gas Chromatography If relative rates same with and w/out aerosols, assume aerosols do not affect rates (n-hexane, p-xylene) Result: For methanol, ethanol, n-propanol, rate relative to p-xylene is enhanced ~20% by presence of aerosol. Loss of n-propanol also enhanced by NH4NO3 and NH4Cl

Why it Could Matter  Rate of removal of OH / computed local [OH]  Rates of VOC loss / ozone production  Change products of VOC degradation?  ozone production  hazardous air pollutants?  Affects on bulk/surface composition of aerosol?  effectiveness as CCN  radiative properties

Our Experiments (Malene Srenson) 100-L Pyrex bulb, 296 K (NH4)2SO4 or NH4NO3 or NaCl aerosol Area: 4,000-50,000 mm2 cm-3 (500 - 8,000 mg/m3) much larger particle surface than Oh and Andino VOCs: methanol, ethanol, phenol, p-xylene, ethylene, acetylene Analysis: FTIR for relative rates. No Effect of Aerosol! Less than 10% effect Absolute rate constants agree with literature

Our Data Aerosol Concentration ~13,000 mm2/cm3~600 mg/m3

Physical Plausibility Aerosol enhancement of rate of OH + VOC assumed due to surface catalysis: Compare rate of OH loss in gas phase versus surface Pseudo-first order rate constant for OH collision with surface k’ = (vOH/4) x Aerosol surface area/Volume vOH = mean velocity of OH in gas phase = 609 m s-1 at 298 K Aerosol surface area/Volume = 1400 or 3400 mm2 cm-3 k’ = 0.2 or 0.5 s-1 ( t = 5 or 2 seconds )

Rate of OH reaction in gas phase Pseudo-first order rate constant for reaction with n-propanol(g) k’ = k(OH+ n-propanol) x [n-propanol] k’  5.5 x 10-12 cm3 s-1 [1.0 x 1014 molecules cm-3] k’  550 s-1 or t  0.0018 seconds vs. k’ = 0.2 or 0.5 s-1 ( t = 5 or 2 seconds ) for surface collision OH reacts in gas phase at least 1000-2500 times faster than it collides with surface. 20% effect seen by Oh and Andino would require 200-540 n-propanol molecules lost with each collision!

Surface Reactions Could Occur Large [VOC]  only very efficient surface reaction detectable A 10% effect on VOC loss would require Phenol: At least 60 molecules lost per Ethanol: At least 6 OH radical collision Methanol: At least 1.5 with aerosol surface …But not detectable in these experiments! {

Conclusion Our experiments contradict results of Oh and Andino Results of Oh and Andino physically implausible No solid evidence that aerosols can catalyze reaction of OH with VOLATILE Organic Compounds Acknowledgements Danish Natural Science Research Council, NSF, C. Lohse

Can Aerosols Catalyze the Reaction of OH Radical with VOCs ?