Air Quality Modeling

7.4 Third Generation Modeling

7.4.1 Formation of Particulate Matter and Ozone
For ozone formation, reactive hydrocarbons play key roles in the generation and cycling of free radicals. One important problem is related to the suitability of present mechanisms in the evaluation of the effectiveness of emissions reduction strategies for ozone abatement. Present chemical mechanisms were developed primarily to predict ozone concentrations, and are based largely on smog chamber studies conducted under conditions of relatively high reactive hydrocarbon (RHC) to NO~ conditions, so it doesn’t work well under lower RHC/NOx ratios. Another important issue in regard to the treatment of reactive hydrocarbons in comprehensive models is related to the question of which species to include explicitly (such as biogenic hydrocarbons, oxygenated compounds).

For particulate formation, heterogeneous reactions between gas-phase species and aerosols may affect the atmospheric concentrations and life time of some species, and reactions that occur on the surfaces of aerosol particles may exert a significant influence on various biogeochemical cycles. It has not been rigorously determined how particulate compounds including hydrocarbons, elemental carbonaceous materials, and sulfate form in the atmosphere. Binary homogeneous nucleation and heterogeneous nucleation seem to be more likely pathways. In light of these evidences, the chemical transformation submodels should be structured to allow simultaneous treatment of aerosol physics and thermodynamics, aqueous phase chemistry on wetted particles, and homogeneous gas-phase chemical processes. A detailed description of aerosol dynamics for the aerosol module is in McMurry (2000).

Many models ignore the effects of clouds and/or aerosols. In some urban locations, clouds do not form frequently during the summer; thus, cloud formation can be neglected. The effects of aerosols in unpolluted air are often neglected, when insoluble gases such as O3, NO, and NO2 are simulated. When aerosols grow to cloud-sized drops, their effects on ozone are more significant and should be simulated. In the stratosphere, ice crystals and volcanic aerosols catalyze reactions that release chlorine, which reduce ozone concentrations. Thus, simulating particles in the stratosphere is important for determining ozone concentrations when polar stratospheric clouds or aerosols from recent volcanic eruptions are present.