Anthropogenic activities, dominated by emissions of sulfur dioxide (SO 2 ), have perturbed the global sulfur (S) cycle. Uncertainties in timescales of S transport and chemistry in the atmosphere lead to uncertainties in the predicted impact of S emissions. Measurements of cosmogenic 35 S may potentially be used to resolve existing uncertainties in the photochemical and chemical transformation of S in the environment. The lack of a simple, effective, and highly sensitive technique to measure 35 S activity in samples with low activities may explain the scarcity of published measurements. We present a set of new sample handling and measurement procedures optimized for the measurement of 35 S in natural samples with activities as low as 0.20 dpm above background (2σ, integration time ¼ 2 hr). We also report simultaneous measurements of aerosol ( 35 SO 4 ) and gas phase ( 35 SO 2 ) collected at inland and coastal locations; the range of observed activities corresponds to SO 2 residence lifetimes of 0.2 AE 0.04 ðcoastalÞ − 22.3 dAE 0.04 ðinlandÞ. These optimized techniques offer the potential for resolving atmospheric processes that occur on 6-12-hour timescales as well as resolving transport phenomena such as stratospheric mixing into the troposphere.aerosol sulfate | sulfur cycle | dry deposition | sulfur dioxide residence times A ccording to the Intergovernmental Panel on Climate Change (1) anthropogenic emissions of sulfur dioxide have perturbed the sulfur cycle on local, regional, and global scales. These emissions and their oxidized end products lead to increases in acid rain (H 2 SO 4 ) and aerosol sulfate (SO 2− 4 ) concentrations that play a significant role in climate and the global sulfur cycle. Sulfurcontaining aerosol particles serve as cloud condensation nuclei affecting cloud formation and the hydrological cycle. Our knowledge of the chemical and photochemical processes that govern the chemical transformations and transport of sulfur compounds in the atmosphere is incomplete due to the complex, multivalent nature of sulfur and uncertainties in the understanding of aerosol chemistry. Sulfur in the atmosphere exists simultaneously as a solid and gas, further complicating matters. The development of new and/or improved analytical techniques to study the sulfur cycle on short timescales (hours to days) is, therefore, of considerable importance. Here we describe significant advances in the detection sensitivity of the cosmogenic isotope 35 S that can be used as a tracer of gas and aerosol phase lifetimes and turnover kinetics with high time and aerosol size resolutions.Stable isotopic measurements of atmospheric species such as nitrate (NO 1− 3 ) and sulfate (SO 2− 4 ) have recently been used to provide strong constraints on the oxidative processing of their precursors, NO x and SO x , in the atmosphere (2-6). The radionuclide 35 S (β-decay to 35 Cl, t 1∕2 ¼ 87.4 d) is continuously produced in the atmosphere by the interaction of cosmic rays with 40 Ar and provides an additional opportunity for tracing atmo...

The debate of life on Mars centers around the source of the globular, micrometer-sized mineral carbonates in the ALH84001 meteorite; consequently, the identification of Martian processes that form carbonates is critical. This paper reports a previously undescribed carbonate formation process that occurs on Earth and, likely, on Mars. We identified micrometer-sized carbonates in terrestrial aerosols that possess excess 17 O (0.4-3.9‰). The unique O-isotopic composition mechanistically describes the atmospheric heterogeneous chemical reaction on aerosol surfaces. Concomitant laboratory experiments define the transfer of ozone isotopic anomaly to carbonates via hydrogen peroxide formation when O 3 reacts with surface adsorbed water. This previously unidentified chemical reaction scenario provides an explanation for production of the isotopically anomalous carbonates found in the SNC (shergottites, nakhlaites, chassignites) Martian meteorites and terrestrial atmospheric carbonates. The anomalous hydrogen peroxide formed on the aerosol surfaces may transfer its O-isotopic signature to the water reservoir, thus producing mass independently fractionated secondary mineral evaporites. The formation of peroxide via heterogeneous chemistry on aerosol surfaces also reveals a previously undescribed oxidative process of utility in understanding ozone and oxygen chemistry, both on Mars and Earth.nanoparticles | mineral dust | heterogeneous chemical transformation | surface chemistry | mass-independent fractionation T he search for life beyond Earth is pursued based upon the requirement of liquid water both as a solvent and transport medium, and its biochemically unique role in providing support to cell structure (1). Central to the question of potential life on Mars is whether liquid water ever existed on the surface of Mars and, if so, under what climatic conditions (2). Apart from satellite observations of valleys and channels formed by aqueous activity, there are few quantitative measures (e.g., Thermal and Electrical Conductivity probe on Phoenix Lander; Compact Reconnaissance Imaging Spectrometer for Mars; High Energy Neutron Detector on board Mars Odyssey spacecraft) of significant amounts of liquid water at the surface of Mars today (3-5). Secondary minerals such as carbonates and sulfates record physicalchemical settings of the environment in which they are formed and geochemical relations between the atmosphere and the Martian surface (6). Unlike terrestrial carbonate sediments, the Martian surface lacks large amounts of carbonates despite a CO 2 -rich (95% vol∕vol) atmosphere, though, there is evidence of Mg-Fe rich carbonates (16-34 wt %) in the Columbia Hills and <5% CaCO 3 in Martian dust and soils (3,(7)(8)(9). In contrast to most Martian meteorites, ALH84001 contained substantial amounts of secondary carbonate minerals with an average age of 3.90 AE 0.04 billion years, contemporaneous with a wet period in Martian history (10). The formation of well-defined globular structures of a definite size range and their a...