Stratospheric Aerosol Increases and Ozone Destruction: Implications from Mass Spectrometer Measurements

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Abstract
Stratospheric aerosol increases set the stage for increased halogen‐catalyzed ozone destruction. Most striking examples are polar stratospheric aerosols, so called polar stratospheric clouds (PSC's) and volcanic eruption clouds. Balloon‐ and rocket‐borne mass spectrometer measurements made by our reserach group have offered interesting new insights into these aerosol clouds. The present paper investigates, in the light of these measurements, the formation and role of the most common type of PSC's and volcanic eruption clouds. It is concluded that PSC's are mostly formed by nitric acid nucleation and/or ‐condensation leading to metastable HNO3/H2O‐condensates. In contrast, it seems that NAT (nitric acid trihydrate being the most stable HNO3/H2O‐condensate) does not form easily. The metastable condensates may be supercooled solution droplets or solid solutions, having lower existence temperatures than NAT. Volcanic H2SO4/H2O‐aerosols are formed by injection of sulfur‐bearing gases, mostly SO2, followed by photochemical conversion to gaseous H2SO4, which undergoes rapid condensation on preexisting H2SO4/H2O‐aerosols. The mass spectrometer measurements suggest that these H2SO4/H2O‐aerosols have an important role in setting the stage for halogen‐catalyzed ozone‐destruction.