Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) Model


 

Theory Investigation:Modelling of Cloud Microphysics, Heterogeneous Chemistry, and Photochemistry for SOLVE
Principal Investigator:Katja Drdla
Co-Investigators:Eric Jensen and Anthony Strawa
Organization:Mail Stop 245-4
Atmospheric Physics Branch
Ames Research Center
National Aeronautics and Space Administration
Moffett Field, CA 94035-1000
Investigation Description: The Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) Model is a trajectory model designed to simulate Polar Stratospheric Clouds (PSCs) and their impact on stratospheric chemistry. Many different scenarios for PSC formation can be evaluated within the model to determine which microphysical theories are most consistent with various measurements.

PSC and stratospheric aerosol size distributions are calculated using a full microphysics code, incorporating freezing, melting, nucleation, condensation, evaporation, and sedimentation. Simulated PSCs may be composed of liquid ternary solutions, nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), amorphous nitric acid solutions, or water ice; the specific combination present in any given model run is determined by the assumed PSC microphysics.

The microphysics is fully integrated with a complete photochemical model (57 species grouped in 28 families, with 177 reactions, of which 38 are photolysis and 12 are heterogeneous). Heterogeneous reaction rates are calculated using the modelled size distribution, with explicit bin-by-bin calculation of diffusion rates and available reactants.

During the SOLVE campaign, model results will be compared to ER-2 and DC-8 measurements of PSCs and chemistry. For ER-2 simulations, the model is initialized using ER-2 measurements of conditions (temperature, pressure, ozone column, reflectivity) and long-lived tracers (sulphate volume, H2O, NOy*, Cly, Bry, O3, CH4). Model results can then be compared to PSC measurements (MASP size distribution, gas- and condensed-phase HNO3, enhanced NOy) and chemical species (ClO, ClONO2, Cl2O2, HCl, BrO, NO, NO2, OH, HO2). On the DC-8, the model can be compared with lidar backscatter and depolarization measurements.

Such comparisons will be used to test our understanding of PSC formation processes, in particular the composition of given PSCs and the freezing rates of the aerosol particles. The integrated chemistry will be used to test chlorine activation and deactivation rates, and evaluate resulting ozone loss rates.

Reference:

K. Drdla,Applications of a Model of Polar Stratospheric Clouds and Heterogeneous Chemistry, Ph.D. Thesis, University of California at Los Angeles, 1996.