A model of photochemical smog including gas-to-particle conversion for humid and dry climates

A model of photochemical smog is presented that includes the interaction of the gas phase molecules with aerosol particles. The model demonstrates how high relative humidity maintains the lower oxidant levels observed in humid climate cities such as Houston as compared to dry climate cities such as Los Angeles. A quasi-Langrangian approach is taken by modeling a homogeneous air parcel which follows an air trajectory. Four of the twenty species modeled are solved by the quasi steady state approximation (QSSA). The set of sixteen ordinary nonlinear stiff differential equations is solved by Michelsen's (1976) semi-implicit Runge-Kutta method. This numerical method directly handles the explicit time dependence of the emission rates, photodissociative reaction rates, and the empirically determined aerosol growth patterns. At high relative humidity there is an increase in the growth of aerosols resulting in a large available surface area for collision. The model assumes that certain molecular species that either directly or indirectly contribute to oxidant levels are partially removed through absorption by aerosols. Empirically determined aerosol growth patterns were presented for humid and dry climates. The humid versus dry climate aerosol growth patterns were compared under a variety of theoretical absorption conditions. Case studies included studying the effect of different combinations of molecules and radicals considered to be absorbed at various accommodation coefficients, the effect of initial concentrations of the species, and the effect of different emission rates. When only radicals were considered absorbed the model showed insignificant differences between the two types of aerosol growth patterns. When molecules as well as radicals were considered absorbed, oxidant levels were considerably lower for humid climate aerosol growth patterns under all conditions studied.