Heterogeneous Chemistry and the O3 Budget in the Lower Mid-Latitude Stratosphere

A photochemical box model including a detailed heterogeneous chemistrymodule has been used to analyze in detail the effects of temperature andaerosol surface area on odd oxygen production/depletion in the lowerstratosphere at 30° S. Results show that for background aerosolloading, the hydrolysis of BrONO₂ and N₂O₅are most important atall temperatures studied except when the temperature falls below about205 K, when ClONO₂ hydrolysis becomes most important. Thisprocessing leads to removal of active nitrogen to form nitric acid andenhancement of HO_x, BrO_x, ClO_x levels. Detailed O₃ budgets asa function of temperature are presented showing how ozone loss andproduction terms vary with changes in stratospheric sulfate aerosol loadingfor the individual families. For (most) aerosol loading levels, thelargest ozone losses occurred at warmer temperatures due to the strongtemperature dependence of the NO_x ozone-destroying reactions. Theexception to this occurred for the conditions representative of volcanicloading, which showed a strong increase in ozone destruction due toincreases in destruction from the ClO_x and HO_x families.The ozoneproduction term k[NO][HO₂] did not show a strong dependence oneithertemperature or aerosol loading, due to the offsetting effect of reducedNO_xand increased HO_x concentrations.