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

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 HNO 3 /H 2 O‐condensates. In contrast, it seems that NAT (nitric acid trihydrate being the most stable HNO 3 /H 2 O‐condensate) does not form easily. The metastable condensates may be supercooled solution droplets or solid solutions, having lower existence temperatures than NAT. Volcanic H 2 SO 4 /H 2 O‐aerosols are formed by injection of sulfur‐bearing gases, mostly SO 2 , followed by photochemical conversion to gaseous H 2 SO 4 , which undergoes rapid condensation on preexisting H 2 SO 4 /H 2 O‐aerosols. The mass spectrometer measurements suggest that these H 2 SO 4 /H 2 O‐aerosols have an important role in setting the stage for halogen‐catalyzed ozone‐destruction.