Aerosol can provide a sink for gas-phase species though heterogeneous reactions. This thesis examines these heterogeneous reactions in the troposphere, with a focus on reactive uptake of N20S and H02. Model sensitivity to the rate of uptake of N20 (as manifest by γN20S ) is found to be highly regional and seasonal. Due to the impact of photolysis and the location of emissions (of NO x and aerosol), heterogeneous chemistry is generally a greater driver of chemistry for the northern extra-tropics. The northern extra-tropics show significant and enhanced sensitivity to γN20S at lower values of γN20S than other regions, as NOx loss through the heterogeneous reaction is limited by production of N03. While the northern extra-tropics are more sensitive to heterogeneous chemistry of N20s, the extended impact of 03 means uptake to aerosol in the tropics is also important. Available laboratory data on the uptake of H02 is used to create a new parameterisation for γH02, with composition, temperature and humidity influencing the value of γH02,, The calculated value is an order of magnitude lower than previously found. Though the global impact of H02 uptake is small, there are regionally significant changes. The model shows sensitivity to the product of this reaction. The impact of heterogeneous chemistry under different emission scenarios (1985 to 2005) is in- vestigated. The impact of heterogeneous uptake (of N20S and H02) on H02 and oxidants is remarkably similar between the years, whereas the impact on components of inorganic aerosol (SO2-4-, N+4, NO-3)) is very different, due to the impact of SO2-4- on aerosol thermodynamics .