Modeling the possible role of iodine oxides in atmospheric new particle formation

[摘要] We studied the possible role of iodine oxides in atmospheric new particleformation with the one-dimensional marine boundary layer model MISTRA,which includes chemistry in the gas and aerosol phase as well as aerosolmicrophysics. The chemical reaction set focuses on halogen(Cl-Br-I) chemistry. We included a two-step nucleation parameterization, wherein the first step, the "real" nucleation process is parameterized,i.e., the formation of cluster-sized nuclei via homogeneous condensation of gases.We considered both ternary sulfuric acid-ammonia-water nucleation andhomomolecular homogeneous OIO nucleation.For the latter, we derived a parameterization basedon combined laboratory-model studies. The second step of the nucleationparameterization treats the "apparent" nucleation rate, i.e., the growthof clusters into the model's lowest size bin by condensable vapors such as OIO.We compared different scenarios for a clean marine versus a polluted continentalbackground atmosphere. In every scenario, we assumed the air to move,independent of its origin, first over a coastal region (where it is exposedto surface fluxes of different reactive iodine precursors) and later over the open ocean.According to these sensitivity studies, in the clean marine background atmosphereOIO can be responsible for bothhomogeneous nuclei formation and the subsequent growth of the clustersto detectable sizes. In contrast to this, in the continental case with its higherlevels of pollutants, gas phase OIO mixing ratios, and hence related nucleation rates,are significantly lower. Compared to ternary H₂SO₄-NH₃-H₂Onucleation, homogeneous OIO nucleation can be neglectedfor new particle formation in this case, but OIO can contribute toearly particle growth, i.e., to apparent nucleation rates.In general, we found OIO to be more important for the growth of newly formedparticles than for the formation of new nuclei.According to our studies, observations of particle "bursts" can onlybe explained by hot spot-like, not by homogeneously distributed emissions.

[学科分类] 大气科学