On the impact of heterogeneous chemistry on ozone in the tropopause region

We examine the impact of heterogeneous chemistry involving liquid aerosol and ice particles on net ozone (O 3 ) 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 (NO x ) and by the location of the air masses relative to the tropopause (TP). The net effect of heterogeneous chemistry is to decrease net O 3 production below the TP (via heterogeneous HO 2 loss) and to cause O 3 destruction above the TP (via heterogeneous chlorine (Cl) activation). In the UT, gas phase chemistry due to non‐methane hydrocarbons (NMHCs) can become as important for O 3 chemistry as heterogeneous reactions, and removal of HO 2 by particles can become more important than changes of hydrogen oxides (HO x ) through heterogeneous bromine (Br) chemistry. In the humid LS, Cl activation can become sufficiently large, so that O 3 depletion occurs at all conceivable values of NO x . Such cold and humid conditions occur frequently enough to reduce the average ozone production rates in the midlatitude LS by more than 10%.