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In the Arctic polar vortex of the 2009/10 winter temperatures were low enoughto allow widespread formation of polar stratospheric clouds (PSCs). Theseclouds occurred during the initial chlorine activation phase which providedthe opportunity to investigate the impact of PSCs on chlorine activation.Satellite observations of gas-phase species and PSCs are used in combinationwith trajectory modeling to assess this initial activation. The initialactivation occurred in association with the formation of PSCs over the eastcoast of Greenland at the beginning of January 2010. Although this area ofPSCs covered only a small portion of the vortex, it was responsible foralmost the entire initial activation of chlorine vortex wide. Observationsshow HCl (hydrochloric acid) mixing ratios decreased rapidly in anddownstream of this region. Trajectory calculations and simplifiedheterogeneous chemistry modeling confirmed that the initial chlorineactivation continued until ClONO<sub>2</sub> (chlorine nitrate) was completelydepleted and the activated air masses were advected throughout the polarvortex. For the calculation of heterogeneous reaction rates, surface areadensity is estimated from backscatter observations. Modeled heterogeneousreaction rates along trajectories intersecting with the PSCs indicate thatthe initial phase of chlorine activation occurred in just a few hours. Thesecalculations also indicate that chlorine activation on the binary backgroundaerosol is significantly slower than on the PSC particles and the observedchlorine activation can only be explained by an increase in surface areadensity due to PSC formation. Furthermore, there is a strong correlationbetween the magnitude of the observed HCl depletion and PSC surface areadensity

Vortex-wide chlorine activation by a mesoscale PSC event in the Arctic winter of 2009/10