Enhanced heterogeneous uptake of sulfur dioxide on mineral particles through modification of iron speciation during simulated cloud processing

Abstract

Abstract. Iron-containing mineral aerosols play a key role in the oxidation\nof sulfur species in the atmosphere. Simulated cloud processing (CP) of\ntypical mineral particles, such as illite (IMt-2), nontronite (NAu-2),\nsmectite (SWy-2) and Arizona Test Dust (ATD) is shown here to modify sulfur\ndioxide ( SO2 ) uptake onto mineral surfaces. Heterogeneous oxidation of\n SO2 on particle surfaces was firstly investigated using an in situ DRIFTS\napparatus (diffuse reflectance infrared Fourier transform spectroscopy). Our results showed that the Brunauer–Emmett–Teller (BET) surface area normalized uptake\ncoefficients ( γBET ) of SO2 on the IMt-2, NAu-2, SWy-2 and\nATD samples after CP were 2.2, 4.1, 1.5 and 1.4 times higher than the\ncorresponding ones before CP, respectively. The DRIFTS results suggested\nthat CP increased the amounts of reactive sites (e.g., surface OH groups) on\nthe particle surfaces and thus enhanced the uptake of SO2 . Transmission\nelectron microscopy (TEM) showed\nthat the particles broke up into smaller pieces after CP, and thus produced\nmore active sites. The “free-Fe” measurements confirmed that more reactive\nFe species were present after CP, which could enhance the SO2 uptake\nmore effectively. Mossbauer spectroscopy further revealed\nthat the formed Fe phases were amorphous Fe(III) and nanosized ferrihydrite\nhybridized with Al\u2009 ∕ \u2009Si, which were possibly transformed from the Fe in the\naluminosilicate lattice. The modification of Fe speciation was driven by the\npH-dependent fluctuation coupling with Fe dissolution–precipitation\ncycles repeatedly during the experiment. Considering both the enhanced SO2 \nuptake and subsequent promotion of iron dissolution along with more active\nFe formation, which in turn led to more SO2 uptake, it was proposed\nthat there may be a positive feedback between SO2 uptake and iron\nmobilized on particle surfaces during CP, thereby affecting climate and\nbiogeochemical cycles. This self-amplifying mechanism generated on the\nparticle surfaces may also serve as the basis of high sulfate loading in\nsevere fog–haze events observed recently in China.