Abstract
Abstract. To better understand the formation and the oxidation pathways leading to gypsum-forming “black crusts” and investigate their bearing on the whole atmospheric SO2 cycle, we measured the oxygen (δ17O, δ18O, and Δ17O) and sulfur (δ33S, δ34S, δ36S, Δ33S, and Δ36S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW–SE cross section in the Parisian basin. The δ18O and δ34S values, ranging between 7.5 ‰ and 16.7±0.5 ‰ (n=27, 2σ) and between −2.66 ‰ and 13.99±0.20 ‰, respectively, show anthropogenic SO2 as the main sulfur source (from ∼2 % to 81 %, average ∼30 %) with host-rock sulfates making the complement. This is supported by Δ17O values (up to 2.6 ‰, on average ∼0.86 ‰), requiring > 60 % of atmospheric sulfates in black crusts. Negative Δ33S and Δ36S values between −0.34 ‰ and 0.00±0.01 ‰ and between −0.76 ‰ and -0.22±0.20 ‰, respectively, were measured in black crust sulfates, which is typical of a magnetic isotope effect that would occur during the SO2 oxidation on the building stone, leading to 33S depletion in black crust sulfates and subsequent 33S enrichment in residual SO2. Except for a few samples, sulfate aerosols mostly have Δ33S values > 0 ‰, and no processes can yet explain this enrichment, resulting in an inconsistent S budget: black crust sulfates could well represent the complementary negative Δ33S reservoir of the sulfate aerosols, thus solving the atmospheric SO2 budget.
Highlights
The oxidation of sulfur dioxide emitted into the atmosphere (between 100 and 110 Tg(SO2) yr−1; Klimont et al, 2013) can result in the formation of H2SO4 that forms sulfate aerosols; having light-scattering properties that alter the radiative balance of the planet
We present new oxygen and sulfur isotopic composition measurements of sulfate extracted from black crusts and report significant 17O, 33S, and 36S anomalies that help to discuss oxygen and sulfur isotopic variations both in term of source effects to elucidate their origin and in terms of fractionation processes leading to black crust formation in the Paris area
Our study shows that black crusts do preserve an atmospheric signal of SO2 oxidation, inferred from the nonzero
Summary
The oxidation of sulfur dioxide emitted into the atmosphere (between 100 and 110 Tg(SO2) yr−1; Klimont et al, 2013) can result in the formation of H2SO4 that forms sulfate aerosols; having light-scattering properties that alter the radiative balance of the planet. They modify the microphysical properties of clouds through the number and size of cloud condensation nuclei (CCN; e.g., Weber et al, 2001). Uncertainties regarding the formation of sulfate aerosols relate to the large variety of oxidants and conditions (e.g., pH), but, in view of their major impact on climate, a more accurate understanding of the formation of these particles is necessary
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