Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 3 2004 10.5194/acpd-4-3089-2004 http://www.atmos-chem-phys-discuss.net/4/3089/2004/ http://www.atmos-chem-phys-discuss.net/4/3089/2004/acpd-4-3089-2004.html http://www.atmos-chem-phys-discuss.net/4/3089/2004/acpd-4-3089-2004.pdf 3089 3126 2004-06-14 Implementing growth and sedimentation of NAT particles in a global Eulerian model M. M. P. van den Broek J. E. Williams williams@knmi.nl A. Bregman Space Research Organization of the Netherlands (SRON), Utrecht, The Netherlands Eindhoven University, Eindhoven, The Netherlands Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands Here we present a concise and efficient algorithm to mimic the growth and sedimentation of Nitric Acid Trihydate (NAT) particles in the polar vortex in a state-of-the-art 3D chemistry transport model. The particle growth and sedimentation are calculated using the microphysical formulation of Carslaw et al. (2002). Once formed, NAT particles are transported in the model as tracers in the form of size-segregated quantities. Two different approaches were adopted for this purpose: one assuming a fixed particle number density ("FixedDens") and the other assuming a discrete set of particle diameter values ("FixedRad"). Simulations were performed for three separate 10-day periods during the 1999–2000 Arctic winter and compared to the results of an existing Lagrangian model study, which uses similar microphysics in a computationally more expensive method for the simulation of NAT particle growth. The resulting maximum particle sizes for both our approaches compare favourably at 96 hPa with those obtained from this previous model study, and also in-situ observations related to the size of large NAT particles. Comparisons were made with a standard equilibrium approach and the differences in the redistribution of HNO₃ were found to be substantial. For both approaches the performance of the algorithm is rather insensitive to both the number of size bins and the shape of the size distribution. However, the percentage of HNO₃ sequestered into NAT is critically dependent on the total number density of particles prescribed for each size bin.