Atmos. Chem. Phys., 11, 4505–4520, 2011 www.atmos-chem-phys.net/11/4505/2011/ Atmospheric doi:10.5194/acp-11-4505-2011 Chemistry © Author(s) 2011. CC Attribution 3.0 License. and Physics Detailed heterogeneous oxidation of soot surfaces in a particle-resolved aerosol model J. C. Kaiser1,*, N. Riemer2, and D. A. Knopf3 1Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY, USA 2Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, USA 3Institute for Terrestrial and Planetary Atmospheres, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA *currently at: Department of Physics and Astronomy, Universitat¨ Wurzburg,¨ Wurzburg,¨ Germany Received: 19 January 2011 – Published in Atmos. Chem. Phys. Discuss.: 10 February 2011 Revised: 27 April 2011 – Accepted: 30 April 2011 – Published: 12 May 2011 Abstract. Using the particle-resolved aerosol model influence of heterogeneous reactions on soot particles on the PartMC-MOSAIC, we simulate the heterogeneous oxidation gas phase composition. The derived half-lives of surface- of a monolayer of polycyclic aromatic hydrocarbons (PAHs) bound PAHs and the time and particle population averaged on soot particles in an urban atmosphere. We focus on the in- uptake coefficients for O3 and NO2 presented in this paper teraction of the major atmospheric oxidants (O3, NO2, OH, can be used as parameterisations for the treatment of het- and NO3) with PAHs and include competitive co-adsorption erogeneous chemistry in large-scale atmospheric chemistry of water vapour for a range of atmospheric conditions. For models. the first time detailed heterogeneous chemistry based on the Poschl-Rudich-Ammann¨ (PRA) framework is modelled on soot particles with a realistic size distribution and a contin- uous range of chemical ages. We find PAH half-lives, τ1/2, 1 Introduction on the order of seconds during the night, when the PAHs are rapidly oxidised by the gas-surface reaction with NO . Dur- 3 Organic aerosol particles are abundant in the atmosphere ing the day, τ is on the order of minutes and determined 1/2 (Maria et al., 2004; Jimenez et al., 2009) and can affect hu- mostly by the surface layer reaction of PAHs with adsorbed man health upon respiration (Finlayson-Pitts and Pitts, 1997; O . Such short half-lives of surface-bound PAHs may lead 3 Poschl¨ , 2002; Bernstein et al., 2004) as well as global and to efficient conversion of hydrophobic soot into more hygro- regional climate via absorption or scattering of light and the scopic particles, thus increasing the particles’ aerosol-cloud ability to act as cloud condensation or ice nuclei (e.g., Ra- interaction potential. Despite its high reactivity OH appears manathan et al., 2001; Kanakidou et al., 2005; Forster et al., to have a negligible effect on PAH degradation which can be 2007; Knopf et al., 2010). An ubiquitous type of such parti- explained by its very low concentration in the atmosphere. cles is soot, which is found in diesel engine exhaust, for ex- An increase of relative humidity (RH) from 30 % to 80 % in- ample, due to incomplete combustion of the fuel (Finlayson- creases PAH half-lives by up to 50 % for daytime degradation Pitts and Pitts, 2000). and by up to 100 % or more for nighttime degradation. Up- take coefficients, averaged over the particle population, are During atmospheric transport particles can undergo phys- −7 ical and chemical changes by interaction with gas phase found to be relatively constant over time for O3 (∼ 2×10 −6 −6 −5 species (Weingartner et al., 1997; Wang et al., 2010), pro- to ∼ 2×10 ) and NO2 (∼ 5×10 to ∼ 10 ) at the differ- cesses collectively known as aging. For example, conden- ent levels of NOx emissions and RH considered in this study. sation of additional organic material may lead to the forma- In contrast, those for OH and NO3 depend strongly on the surface concentration of PAHs. We do not find a significant tion of secondary organic aerosol (e.g., Seinfeld and Pandis, 2006; Rudich et al., 2007; Hallquist et al., 2009; Jimenez et al., 2009) while chemical reactions can also modify the Correspondence to: N. Riemer particles’ composition (e.g., Rudich, 2003; Maria et al., ([email protected]) 2004; Rudich et al., 2007). Published by Copernicus Publications on behalf of the European Geosciences Union. 4506 J. C. Kaiser et al.: Model study of heterogeneous oxidation of soot Among the least understood processes affecting particle and particle bulk. This allows for a complete physical de- composition are heterogeneous oxidation reactions involv- scription of heterogeneous kinetics, e.g. of the uptake co- ing trace gases such as ozone (O3), nitrogen dioxide (NO2) efficients’ temporal evolution, by knowledge of the basic and the hydroxyl (OH) and nitrate (NO3) radicals (Rudich physico-chemical parameters such as accommodation coef- et al., 2007; Kolb et al., 2010). Reaction mechanisms and ficients, desorption lifetimes, and reaction rate coefficients. products that determine this so-called chemical aging are Previous studies have successfully employed the PRA still poorly known, so that modelling of the processes re- framework to reproduce experimental results (Ammann and mains difficult. However, laboratory studies show that het- Poschl¨ , 2007) and to address scenarios with a complexity be- erogeneous reactions do modify a particle’s surface compo- yond current laboratory measurement limitations (Shiraiwa sition and can thereby change its physico-chemical proper- et al., 2009). Springmann et al. (2009) improved upon this ties (Rudich, 2003; Knopf et al., 2006; Rudich et al., 2007; by coupling the PRA mechanism to a gas phase chemical Finlayson-Pitts, 2009; Kolb et al., 2010). For example, cer- mechanism to account for changes in atmospheric gas phase tain substances found on particle surfaces become mutagenic composition due to the diurnal photochemical cycle and het- upon nitration, i.e. reaction with NOx (Finlayson-Pitts and erogeneous chemistry. Pitts, 2000; Reisen and Arey, 2005). Also, oxidation of All the studies using the PRA framework found that uptake carbonaceous and organic aerosol particle surfaces can lead coefficients of O3 and NO2 on individual PAH-coated soot to increased hygroscopicity, enhancing the particles’ ability particles can vary over several orders of magnitude within to act as cloud condensation nuclei (Kotzick et al., 1997; simulation times on the order of one day. This is in sharp Chughtai et al., 1999; Broekhuizen et al., 2004; Huff Hartz contrast to some large-scale models that use reaction rate et al., 2005; Petters et al., 2006; Shilling et al., 2007; Liu coefficients which depend only on the amount of available et al., 2010). particle surface area (e.g., Bey et al., 2001; Tie et al., 2001; An important class of surface-bound organics found in Matthias et al., 2009, note that the terminology concerning the analysis of soot particles are polycyclic aromatic hydro- uptake or rate coefficients is different in these three papers). carbons (PAHs) (Rogge et al., 1993; Pakbin et al., 2009; However, in existing aerosol models it is usually not possible El Haddad et al., 2009; Kashiwakura and Sakamoto, 2010), to represent aerosol particles of different processing stages. some of which are toxic or carcinogenic (Finlayson-Pitts Thus, the effect of chemical aging on heterogeneous kinetics and Pitts, 2000). PAHs, such as benzo[a]pyrene or pyrene, is difficult to incorporate and commonly neglected. are generated along with the soot particles during combus- In the present paper we address this problem by inves- tion and some of them subsequently condense on the par- tigating detailed heterogeneous chemistry using a particle- ticles (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, resolved aerosol model framework. Our study makes the 2006). These chemicals can be regarded as similar to soot in following contributions: (1) We developed a model frame- terms of their molecular structure on soot surfaces where they work that employs the PRA mechanism within the particle- form graphene-like layers (Poschl¨ , 2005; Cain et al., 2010). resolved model PartMC-MOSAIC (Particle Monte Carlo Numerous laboratory and modelling studies have been con- model, coupled to the MOdel for Simulating Aerosol Interac- ducted to determine the mechanisms and parameters of het- tions and Chemistry, Riemer et al., 2009; Zaveri et al., 2008). erogeneous reactions between soot or soot-like substances This includes the heterogeneous interactions of the PAH- and atmospheric oxidants (Rogaski et al., 1997; Gerecke coated soot particles with four major atmospheric oxidants et al., 1998; Bertram et al., 2001; Poschl¨ et al., 2001; Arens (O3, NO2, OH, NO3) and the competitive co-adsorption of et al., 2002; Kwamena et al., 2004; Molina et al., 2004; Gross water vapour in an urban polluted environment; (2) Using and Bertram, 2008, and references therein). These studies this model we explicitly derived uptake coefficients for the found that OH and NO3 react very efficiently with PAH sur- oxidant species both on a per-particle level and on a parti- faces according to a first-order reaction rate (Bertram et al., cle population level, i.e. averaged over all particles in the 2001; Gross and Bertram, 2008). In contrast, O3 and NO2 modelled population. (3) Furthermore, we investigated the show a Langmuir-Hinshelwood type uptake behaviour whose process-resolved lifetimes of condensed phase PAHs on soot pseudo-first-order reaction rate is characterised by a depen- at different levels of NOx emissions. dence on gas phase concentration and available reaction sites This paper is organised as follows. In Sect. 2 we review the (Poschl¨ et al., 2001; Arens et al., 2002). underlying theory for the treatment of heterogeneous chem- Poschl¨ and coworkers developed a theoretical framework istry in our model and present the governing equations for for aerosol and cloud surface chemistry and gas-particle in- the coupled model.