The role of multimedia mass balance models for assessing the effects of volatile organic compound emissions on urban air quality
A multimedia mass balance model is described that has the potential to contribute to improved assessment of the levels and fate of volatile organic compounds (VOCs) in urban environments. It determines the fate and concentration of organic chemicals in the urban atmosphere under both steady-state and dynamic conditions. It can be used to estimate the fraction of the mass emitted that may participate in photochemical reactions leading to ozone formation, as distinct from the fraction which is dissipated by other processes. Five compartments are included: air (including aerosols), surface water, soil, vegetation and a non-polar organic urban film that coats impervious urban surfaces. Five test chemicals are assessed, viz. pentane, toluene, naphthalene, anthracene and pyrene, representing a range in volatility and atmospheric reactivity. The steady-state results suggest that the more volatile chemicals (pentane and toluene) remain almost entirely in the gaseous phase and advection and atmospheric reaction losses are the only important processes. For the less volatile chemicals, with liquid vapor pressures below a “cut-off” of approximately 10 Pa, partitioning to other media increases, notably to soil and water, thus a lower fraction of the emitted mass reacts in the atmosphere and is a potential ozone precursor. The dynamic results show how changes in the hydroxyl radical (OH) concentrations and advection rates affect chemical fate. It is suggested that the model can contribute to improved assessment of the fate of a variety of organic chemicals in urban areas, especially if certain inter-media partitioning and transport parameters can be better quantified.