Rapid temperature fluctuations are shown to cause liquid H2SO4/HNO3/H2O stratospheric aerosols to depart considerably from thermodynamic equilibrium. While HNO3 uptake by larger droplets is diffusively hindered, small droplets can approach the composition of a pure binary HNO3/H2O solution with up to 52 wt% HNO3, 48 wt% H2O and very small amounts of H2SO4. The stoichiometry of these droplets is close to that of nitric acid trihydrate (NAT) and freezing experiments suggest that this could be a suitable pathway for the formation of frozen polar stratospheric clouds (PSCs) of type‐Ia.

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In 2011, the German Federal Ministry of Transport, Building and Urban Development laid the foundation of the Hans-Ertel Centre for Weather Research [Hans-Ertel-Zentrum für Wetterforschung (HErZ)] in order to better connect fundamental meteorological research and teaching at German universities and atmospheric research centers with the needs of the German national weather service Deutscher Wetterdienst (DWD). The concept for HErZ was developed by DWD and its scientific advisory board with input from the entire German meteorological community. It foresees core research funding of about €2,000,000 yr−1 over a 12-yr period, during which time permanent research groups must be established and DWD subjects strengthened in the university curriculum. Five priority research areas were identified: atmospheric dynamics and predictability, data assimilation, model development, climate monitoring and diagnostics, and the optimal use of information from weather forecasting and climate monitoring for the benefit of society. Following an open call, five groups were selected for funding for the first 4-yr phase by an international review panel. A dual project leadership with one leader employed by the academic institute and the other by DWD ensures that research and teaching in HErZ is attuned to DWD needs and priorities, fosters a close collaboration with DWD, and facilitates the transfer of fundamental research into operations. In this article, we describe the rationale behind HErZ and the road to its establishment, present some scientific highlights from the initial five research groups, and discuss the merits and future development of this new concept to better link academic research with the needs and challenges of a national weather service.

Abstract. We examine the impact of heterogeneous chemistry involving liquid aerosol and ice particles on net ozone (O3) 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 (NOx) and by the location of the air masses relative to the tropopause (TP). The net effect of heterogeneous chemistry is to decrease net O3 production below the TP (via heterogeneous HOe loss) and to cause O3 destruction above the TP (via heterogeneous chlorine (C1) activation). In the UT, gas phase chemistry due to non-methane hydrocarbons (NMHCs) can become as important for O3 chemistry as heterogeneous reactions, and removal of HOe by particles can become more important than changes of hydrogen oxides (HOx) through heterogeneous bromine (Br) chemistry. In the humid LS, C1 activation can become sufficiently large, so that O3 depletion occurs at all conceivable values of NO•. Such cold and humid conditions occur frequently enough to reduce the average ozone production rates in the midlatitude LS by more than 10%.