We examine the impact of heterogeneous chemistry involving liquid aerosol and ice particles on net ozone (O3) production rates under conditions representative of the midlatitude upper troposphere (UT) and lowermost stratosphere (LS). We demonstrate that heterogeneous effects are controlled by nitrogen oxides (NOx) and by the location of the air masses relative to the tropopause (TP). The net effect of heterogeneous chemistry is to decrease net O3 production below the TP (via heterogeneous HO2 loss) and to cause O3 destruction above the TP (via heterogeneous chlorine (CI) activation). In the UT, gas phase chemistry due to non-methane hydrocarbons (NMHCs) can become as important for O3 chemistry as heterogeneous reactions, and removal of HO2 by particles can become more important than changes of hydrogen oxides (HOx) through heterogeneous bromine (Br) chemistry. In the humid LS, Cl activation can become sufficiently large, so that O3 depletion occurs at all conceivable values of NOx. Such cold and humid conditions occur frequently enough to reduce the average ozone production rates in the midlatitude LS by more than 10%. ¿ 2001 American Geophysical Union