Assessing the importance of heterogeneous reactions of polycyclic aromatic hydrocarbons in the urban atmosphere using the Multimedia Urban Model

The Multimedia Urban Model (MUM-Fate) of Diamond, M.L., Priemer, D.A., Law, N.L., 2001. Developing a multimedia model of chemical dynamics in an urban area. Chemosphere 44, 1655-1667 was used to evaluate the contribution of heterogeneous reactions of polycyclic aromatic hydrocarbons (PAHs) with gas-phase ozone to the total loss of these compounds in an illustrative urban environment. Recent laboratory studies by Kahan, T.F., Kwamena, N.-O.A., Donaldson, D.J., 2006. Heterogeneous ozonation kinetics of polycyclic aromatic hydrocarbons on organic films. Atmospheric Environment 40, 3448-3459 and Kwamena, N.-O.A., Thornton, J.A., Abbatt, J.P.D., 2004. Kinetics of surface-bound benzo[a] pyrene and ozone on solid organic and salt aerosols. Journal of Physical Chemistry A 108, 11626-11634 provided half-lives for these heterogeneous reactions on surface films and atmospheric particulate matter, respectively. Two model scenarios were studied, representing urban environments with varying amounts of impervious surface coverage. We investigated six PAHs spanning a wide range of physical and chemical properties; results for anthracene, pyrene and benzo[a]pyrene are presented here. Advection from air was the dominant loss process for all PAHs investigated. Heterogeneous reactions primarily on surface films but also on atmospheric particulate matter accounted for up to 75% of the reactive losses of lower volatility PAHs, which accumulate in condensed phases. The results suggest that surface films can be efficient reactive sinks for lower volatility PAHs.