PNNL

Abstract

An earlier study evaluating the dust life cycle in EAMv1 has revealed that the simulated global mean dust lifetime is substantially shorter when higher vertical resolution is used, primarily due to significant strengthening of dust dry removal in source regions. This paper demonstrates that the sequential splitting of aerosol emissions, dry removal, and turbulent mixing in the model's time integration loop, especially the calculation of dry removal after surface emissions and before turbulent mixing, is the primary reason for the vertical resolution sensitivity reported in that earlier study. Based on this reasoning, we propose a simple revision to the numerical process coupling scheme, which moves the application of the surface emissions to after dry removal and before turbulent mixing. The revised scheme allows newly emitted particles to be transported aloft by turbulence before being removed from the atmosphere, and hence better resembles the dust life cycle in the real world. Sensitivity experiments show that the revised process coupling substantially weakens dry removal and strengthens vertical mixing in dust source regions. It also strengthens the large-scale transport from source to non-source regions, strengthens dry removal outside the source regions, and strengthens wet removal and activation globally. In wind-nudged simulations of the year 2010 at 1-degree horizontal resolution with 72 vertical layers, the revised process coupling leads to a 39% increase in the global annual mean dust burden and an increase of dust lifetime from 1.9 days to 2.6 days when tuning parameters are kept unchanged. The revised process coupling is implemented for all aerosol species in EAMv1. The same qualitative changes in process rates are seen in dust, sea salt, marine organic aerosols (MOA), black carbon (BC), and primary organic aerosols (POA), as these species have significant sources from surface emissions. Quantitatively, the changes are large for dust and sea salt but are considerably smaller for the predominantly sub-micron species (MOA, BC, and POA). The impacts on sulfate and secondary organic aerosols are very small, as these species have little or no surface emissions.