Can Aerosols Catalyze the Reaction ofOH Radical with VOCs ?

Theodore S. Dibble,1 M. Srensen,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

IntroductionThe Work of Oh and Andino (Atmos. Env. 36 149-156)

Relative rate studies for reaction with OH radical with VOCsin 100-L Tedlar bag, 298 K

(NH4)2SO4 particles with area 1,400 or 3,400 m2 cm-3

D = 50 m (number) and 150 m (area)VOCs: C1-C6 alcohols, n-hexane, p-xylene

Analysis: Gas ChromatographyIf 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

100-L Pyrex bulb, 296 K(NH4)2SO4 or NH4NO3 or NaCl aerosol

Area: 4,000-50,000 m2 cm-3 (500 - 8,000 g/m3) much larger particle surface than Oh and Andino

VOCs: methanol, ethanol, phenol, p-xylene, ethylene, acetyleneAnalysis: FTIR for relative rates.

No Effect of Aerosol!Less than 10% effect

Absolute rate constants agree with literature

(Malene Srenson)

Aerosol Concentration ~13,000 m2/cm3 ~600 g/m3

Our Data

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 m2 cm-3

k’ = 0.2 or 0.5 s-1 ( = 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 0.0018 seconds

vs. k’ = 0.2 or 0.5 s-1 ( = 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 require200-540 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 requirePhenol: At least 60 molecules lost perEthanol: At least 6 OH radical collision Methanol: At least 1.5 with aerosol surface

…But not detectable in these experiments!

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Conclusion

Our experiments contradict results of Oh and AndinoResults of Oh and Andino physically implausibleNo solid evidence that aerosols can catalyze reaction of OH

with VOLATILE Organic Compounds

AcknowledgementsDanish Natural Science Research Council, NSF, C. Lohse