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[1] Signatures of sulfur mass-independent fractionation (S-MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO 2 is thought to be the most likely source for these S-MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S-MIF production. Laboratory SO 2 photolysis experiments are carried out with a flow-through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO 2 in 0.25 to 1 bar N 2 bath gas, in order to test the effect of SO 2 pressure on the production of S-MIF. Elemental sulfur products yield high d 34 S values up to 140 %, with d 33 S/d 34 S of 0.59 AE 0.04 and Δ 36 S/Δ 33 S ratios of À4.6 AE 1.3 with respect to initial SO 2 . The magnitude of the isotope effect strongly depends on SO 2 partial pressure, with larger fractionations at higher SO 2 pressures, but saturates at an SO 2 column density of 10 18 molecules cm À2 . The observed pressure dependence and d 33 S/d 34 S and Δ 36 S/Δ 33 S ratios are consistent with model calculations based on synthesized SO 2 isotopologue cross sections, suggesting a significant contribution of isotopologue self-shielding to S-MIF for high SO 2 pressure (>0.1 mbar) experiments. Results of dual-cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ 36 S/Δ 33 S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.Citation: Ono, S., A. R. Whitehill, and J. R. Lyons (2013), Contribution of isotopologue self-shielding to sulfur massindependent fractionation during sulfur dioxide photolysis,

23A series of experiments were carried out to determine the clumped ( 13 CH 3 D) 24 methane kinetic isotope effects during oxidation of methane by OH and Cl radicals, the 25 major sink reactions for atmospheric methane. Experiments were performed in a 100 L 26 quartz photochemical reactor, in which OH was produced from the reaction of O( 1 D) 27 (from O 3 photolysis) with H 2 O, and Cl was from photolysis of Cl 2 . Samples were taken 28 from the reaction cell and analyzed for methane ( 12 CH 4 , 12 CH 3 D, 13 CH 4 , 13 CH 3 D) 29 isotopologue ratios using tunable infrared differential laser absorption spectroscopy. 30Measured kinetic isotope effects for singly substituted species were consistent with 31 previous experimental studies. For doubly substituted methane, 13 CH 3 D, the observed 32 kinetic isotope effects closely follow the product of the kinetic isotope effects for the 13 C 33 and deuterium substituted species (i.e., 13,2 KIE = 13 KIE × 2 KIE). The deviation from this 34 relationship is 0.3‰ ± 1.2‰ and 3.5‰ ± 0.7‰ for OH and Cl oxidation, respectively. 35This is consistent with model calculations performed using quantum chemistry and 36 transition state theory. The OH and Cl reactions enrich the residual methane in the 37clumped isotopologue in open system reactions. In a closed system, however, this 38 effect is overtaken by the large D/H isotope effect, which causes the residual methane 39 to become anti-clumped relative to the initial methane. Based on these results, we 40 demonstrate that oxidation of methane by OH, the predominant oxidant for tropospheric 41 methane, will only have a minor (~0.3 ‰) impact on the clumped isotope signature 42 (Δ 13 CH 3 D, measured as a deviation from a stochastic distribution of isotopes) of 43 tropospheric methane. This paper shows that Δ 13 CH 3 D will provide constraints on 44 3 methane source strengths, and predicts that Δ 12 CH 2 D 2 can provide information on 45 methane sink strengths. 46 4

Signatures of mass-independent isotope fractionation (MIF) † Significant deviations from these mass-dependent scaling laws are referred to as mass-independent fractionation (MIF), and serve as important tracers in the earth and planetary sciences (see refs. 3-5).Early studies suggested that MIF could result only from nucleosynthetic processes (6), and the earliest measurements of oxygen MIF in calcium-aluminum inclusions of meteorites originally were interpreted to be nucleosynthetic in origin (7). It eventually was suggested (8) that chemical processes, such as tunneling or processes associated with predissociation, also might produce MIF. The first experimental evidence for a chemical origin of MIF came from ozone generated by an electric discharge or UV radiation (9, 10). The discovery of oxygen MIF in stratospheric ozone (11) soon triggered intense research into the physiochemical origin of MIF in the ozone system (see refs. 12-14). The possible chemical origins of MIF signatures still are poorly understood.For the sulfur isotope system ( 32 S, 33 S, 34 S, and 36 S), Farquhar et al. (15) made the remarkable discovery that mass-independent sulfur isotope fractionation (S-MIF) is prevalent in sedimentary rocks older than ca. 2.4 Ga but absent in rocks from subsequent periods. The disappearance of S-MIF at about 2.4 Ga (16, 17) signifies a fundamental change in the earth's surface sulfur cycles, and generally is linked to the suppression of both SO 2 photolysis and the formation of elemental sulfur aerosols by the rise of atmospheric oxygen levels (15,18,19). The Archean S-MIF is considered the most compelling evidence for an anoxic early atmosphere and constrains Archean oxygen levels to be less than 10−5 of present levels (19). This model of oxygen evolution, however, depends critically on the assumption that UV photolysis of SO 2 by ∼200 nm radiation is the ultimate source of the anomalous sulfur isotope signature (18). Constraining the source of the S-MIF requires a thorough understanding of the physiochemical origins of S-MIF during the photochemistry of SO 2 .SO 2 exhibits two strong absorption band systems in the UV region: one between 185 nm and 235 nm (C 1 B 2 ←X

Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the United States, it is acute in western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake Valley, Utah, demonstrate that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, Ox,total. We show that the traditional nitrogen oxide and volatile organic compound (NOx‐VOC) framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake Valley is responsive to VOCs control and, counterintuitively, not initially responsive to NOx control. We demonstrate simultaneous nitrate limitation and NOx saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution.