KINETICS OF SURFACE-BOUND BENZO[A]PYRENE AND OZONE ON ORGANIC AND INORGANIC AEROSOLS

N.-O. A. Kwamena, J. A. Thornton, J. P. D. AbbattDepartment of Chemistry, University of Toronto, Toronto, ON M5S 3H6 Canada

nkwamena@chem.utoronto.ca

Ozone Generating andDetecting Region

N2Ozone denuder

Aerosol Flow Tube withMoveable Injector

DMA CPC

FilterHolder

Homogeneous Aerosol Nucleation Sourceand Benzo[a]pyrene Coating Region

O2/N2

O3/N2

N2

Add-in flow

Atmospheric Chemistry of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are emitted to the atmosphere from combustion sources and many are known carcinogens. Larger PAHs, such as benzo[a]pyrene (BAP), may partition to fine particulate matter because of their low vapour pressures. This particulate matter can penetrate deep into the lungs resulting in allergenic, mutagenic or carcinogenic responses. Because the oxidation products of PAHs may be even more mutagenic or carcinogenic than the parent PAHs it is necessary to characterize the chemical and physical mechanisms that control the fate and transport of atmospheric PAHs. Therefore, we have undertaken a kinetic study of the oxidation of BaP bound to the surface of organic and inorganic aerosols to better understand aerosol-related health risks and improve urban air quality. This study is the first examination of the oxidation of a PAH on organic and inorganic aerosols in the sub-monolayer regime.

Aerosol Flow Tube for Surface Kinetics Studies

An analytical technique was developed so studies in the sub-monolayer regime could be performed under both dry and high relative humidity conditions.

Azelaic acid (C9 di-carboxylic acid) was used as the organic aerosol substrate.

Solid NaCl aerosols, generated by atomizing a NaCl solution and passing the output through a diffusion dryer, were used as the inorganic aerosol substrate.

Followng filter collection, the samples were extracted ultrasonically and analyzed using HPLC with fluorescence detection to monitor the BaP concentration.

Figure 1: Experimental set-up for kinetics experiments of BaP on organic aerosols

Figure 2: BaP surface concentration as a function of the inverse BaP coating region temperature

Generating Fractional Surface Coverages ofBenzo[a]pyrene on Organic Aerosols

Azelaic acid aerosols were introduced to various BaP coating region temperatures.

A BaP sublimation enthalpy of (124±3) kJ/mol was obtained for BaP on azelaic acid aerosols based on the Clasius-Clapeyron equation.

Pöschl et al. (2001) reported a sublimation enthalpy of (118±5) kJ/mol for BaP on spark discharge soot, which is in agreement with our measurement and the extrapolated literature value (118±2)kJ/mol.

Kinetics of Surface-Bound BaP on Azelaic Acid Aerosolsin the Presence of Ozone Under Dry and Wet Conditions

Kinetic experiments were performed at two different BaP fractional surface coverages (0.02 and 0.2 monolayers). The reaction between surface BaP and ozone is first order within experimental precision.

Preliminary results indicate that the reaction between surface-BaP and ozone is 2.5 times faster at 72% RH compared to dry conditions. This is in direct contrast to other measurements (Pöschl et al. 2001) where a suppression of this reaction under high relative humidity conditions was observed. Studies are ongoing to elucidate the reaction mechanism at high relative humidities.

Kinetics of Surface-Bound BaP on Azelaic Acid Aerosols:Implications for Reaction Mechanisms

Kinetics of Surface-Bound BaP on Sodium Chloride Aerosols

Kinetic experiments were performed on solid sodium chloride aerosols at high ozone concentrations (up to 31 ppm). No reaction between the surface-adsorbed BaP (submonolayer coverage) and ozone was observed.

Figure 5 suggests a Langmuir-Hinshelwood reaction mechanism, where ozone adsorbs to the surface and reacts with the surface-bound BaP.

By assuming a Langmuir isotherm, the pseudo- first order rate coefficient may be given by:

][OK1

][OKk

3O

3OImax1

3

3

obsk

Figure 5: The pseudo-first order rate coefficient as a functionof gas-phase ozone concentration

The lower KO3 value for azelaic acid suggests decreased partitioning of ozone to azelaic acid than to soot.

The similarity in the pseudo-first order rate coefficients (kImax) for this work and that done by

Pöschl et al. (2001) on soot aerosols indicates that the product of kII and Nsurf are similar.

Anthracene oxidation on an organic film displayed behaviour consistent with a Langmuir- Hinshelwood mechanism (Mmereki and Donaldson, 2003); however the kI

max is a factor of 10 lower.

The reaction between ozone and surface-bound PAHs appears to progress by the Langmuir Hinshelwood mechanism irrespective of the substrate. However, the adsorption of ozone differs from surface to surface (Pöschl et al. 2001; Mmereki and Donaldson, 2003).

Our results suggest a lifetime as long as 4 hours ([O3] = 30ppb, dry conditions) with respect to ozone oxidation for BaP bound to the surface of organic aerosols under atmospherically relevant ozone concentrations. This could be a significant loss process for BaP in the atmosphere.

Soot

Azelaic Acid

Substrate

aerosol

0.0153.2Pöschl et al (2001)

0.0230.042This work

kI max

(s-1)

KO3

(10-13 cm3)

Table 1: Comparison of adsorption equilibrium constants and the maximum pseudo-first order rate coefficient

AcknowledgementsWe would like to thank Dan Mathers and the Analest Facility for use of the HPLC withfluorescence detection. We also thank NSERC, TSRI and ASC-PRF for funding this project.

Figure 3: Kinetics at 0.2 monolayers of BaP on azelaic acid aerosols under dry conditions

Figure 4: Kinetics at 0.2 monolayers of BaP on azelaicacid aerosols at 72% relative humidity

ReferencesPöschl, U. et al., J. Phys. Chem. A 2001, 105, 4029 – 4041Mmereki, B. T., Donaldson, D. J., J Phys. Chem. A 2003, 107, 11038 – 11042

surfIINkk I max

The kImax is a product of the second order

rate constant (kII) and the number of surface sites for the adsorbed species (Nsurf):

0.02 monolayers

0.2 monolayers

1 monolayer