| Abstrakt: |
New particle formation (NPF) is known to be an important source of atmospheric particles that impacts air quality, hydrological cycle, and climate. Although laboratory measurements indicate that ammonia enhances NPF, the physio-chemical processes underlying the observed effect of ammonia on NPF are yet to be understood. Here we present the first comprehensive kinetically-based H2SO4-H2O-NH3 ternary ion-mediated nucleation (TIMN) model that is based on the thermodynamic data derived from both quantum-chemical calculations and laboratory measurements. NH3 was found to reduce nucleation barriers for neutral, positively charged, and negatively charged clusters differently, due to large differences in the binding strength of NH3, H2O, and H2SO4 to small clusters of different charging states. The model reveals the general favor of nucleation of negative ions, followed by nucleation on positive ions and neutral nucleation, for which higher NH3 concentrations are needed, in excellent agreement with CLOUD measurements. The TIMN model explicitly resolves dependences of nucleation rates on all the key controlling parameters, and captures well the absolute values of nucleation rates as well as the dependence of TIMN rates on concentrations of NH3 and H2SO4, ionization rates, temperature, and relative humidity observed in the well-controlled CLOUD measurements. The kinetic model offers physio-chemical insights into the ternary nucleation process and provides an accurate approach to calculate TIMN rates under a wide range of atmospheric conditions. [ABSTRACT FROM AUTHOR] |
