Implementing growth and sedimentation of NAT particles in a global Eulerian model

[摘要] Here we present a concise and efficient algorithm to mimic the growth andsedimentation of Nitric Acid Trihydate (NAT) particles in the polar vortexin a state-of-the-art 3D chemistry transport model. The particle growth andsedimentation are calculated using the microphysical formulation of Carslawet al. (2002). Once formed, NAT particles are transported in the model astracers in the form of size-segregated quantities or size bins. Two different approacheswere adopted for this purpose: one assuming a fixed particle number density("FixedDens") and the other assuming a discrete set of particle diametervalues ("FixedRad"). Simulations were performed for three separate 10-dayperiods during the 1999-2000 Arctic winter and compared to the results of anexisting Lagrangian model study, which uses similar microphysics in acomputationally more expensive method for the simulation of NAT particlegrowth. The resulting particle sizes for both our approaches compare favourably at 430K withthose obtained from this previous model study, and also in-situ observations relatedto the size of large NAT particles. The particle growth is faster for "FixedDens" resulting ina difference in (de)nitrification by a factor of ~2 for all three simulation periods.Comparisons were made with a standard equilibrium approachand the differences in the redistribution of HNO₃ were found to be substantial. For bothapproaches the performance of the algorithm is rather insensitive to boththe number of size bins and the shape of the size distribution, and show a weak dependence on theprescribed total particle number density during the coldest period. This results in an increase of 7% for the "FixedRad" approach and 17% for the "FixedDens" approach when increasing the total particle number density by a factor of 2.5.

[学科分类] 大气科学