Gas/aerosol partitioning - 2. Global modeling results

[1] We present gas/aerosol partitioning calculations of multicomponent aerosols and aerosol associated water on a global scale. We have coupled a computationally efficient gas-aerosol scheme (EQSAM) to a global atmospheric chemistry-transport model (TM3). Our results show that gas/aerosol partitioning strongly affects the gas-phase concentrations at relatively low temperatures. During winter and at night during all seasons the calculated aerosol load, including water, is considerably higher than without accounting for gas/aerosol partitioning. The reason is that gaseous nitric acid near the surface is often neutralized by ammonia and therefore partitions almost completely into the aerosol phase to yield ammonium nitrate (NH4NO3). The aerosol NH4NO3 has a longer atmospheric residence time compared to the corresponding precursor gases (NH3 and HNO3) and can therefore be transported over larger distances, for instance from India to Africa and Europe. These modeling results are intriguing; however, verification requires in situ measurements. A comparison with a limited set of ground-based measurements indicates that our model yields realistic results for the ammonium-sulfate-nitrate-water aerosol system in relatively polluted locations where ammonium nitrate is important. For remote locations for which we underestimate the total aerosol load, however, it will be necessary to also account for other aerosol species such as sea salt, mineral dust and organic compounds. We further show that assumptions on turbulent mixing and model resolution have a much stronger effect on aerosol calculations than the uncertainties resulting from the simplifications made in EQSAM.