Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ³³S reservoir of sulfate aerosols?

• Main Authors: I. Genot, D. Au Yang, E. Martin, P. Cartigny, E. Legendre, M. De Rafelis
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-04-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/4255/2020/acp-20-4255-2020.pdf
• ISSN: 1680-7316; 1680-7324

<p>To better understand the formation and the oxidation pathways leading to gypsum-forming “black crusts” and investigate their bearing on the whole atmospheric <span class="inline-formula">SO₂</span> cycle, we measured the oxygen (<span class="inline-formula"><i>δ</i>¹⁷O</span>, <span class="inline-formula"><i>δ</i>¹⁸O</span>, and <span class="inline-formula">Δ¹⁷O</span>) and sulfur (<span class="inline-formula"><i>δ</i>³³S</span>, <span class="inline-formula"><i>δ</i>³⁴S</span>, <span class="inline-formula"><i>δ</i>³⁶S</span>, <span class="inline-formula">Δ³³S</span>, and <span class="inline-formula">Δ³⁶S</span>) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW–SE cross section in the Parisian basin. The <span class="inline-formula"><i>δ</i>¹⁸O</span> and <span class="inline-formula"><i>δ</i>³⁴S</span> values, ranging between 7.5&thinsp;‰ and <span class="inline-formula">16.7±0.5</span>&thinsp;‰ (<span class="inline-formula"><i>n</i>=27</span>, <span class="inline-formula">2<i>σ</i></span>) and between <span class="inline-formula">−2.66</span>&thinsp;‰ and <span class="inline-formula">13.99±0.20</span>&thinsp;‰, respectively, show anthropogenic <span class="inline-formula">SO₂</span> as the main sulfur source (from <span class="inline-formula">∼2</span>&thinsp;% to 81&thinsp;%, average <span class="inline-formula">∼30</span>&thinsp;%) with host-rock sulfates making the complement. This is supported by <span class="inline-formula">Δ¹⁷O</span> values (up to 2.6&thinsp;‰, on average <span class="inline-formula">∼0.86</span>&thinsp;‰), requiring &gt;&thinsp;60&thinsp;% of atmospheric sulfates in black crusts. Negative <span class="inline-formula">Δ³³S</span> and <span class="inline-formula">Δ³⁶S</span> values between <span class="inline-formula">−0.34</span>&thinsp;‰ and <span class="inline-formula">0.00±0.01</span>&thinsp;‰ and between <span class="inline-formula">−0.76</span>&thinsp;‰ and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.22</mn><mo>±</mo><mn mathvariant="normal">0.20</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="1b368e87126b5ee755521a605ed743d4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-4255-2020-ie00001.svg" width="64pt" height="10pt" src="acp-20-4255-2020-ie00001.png"/></svg:svg></span></span>&thinsp;‰, respectively, were measured in black crust sulfates, which is typical of a magnetic isotope effect that would occur during the <span class="inline-formula">SO₂</span> oxidation on the building stone, leading to <span class="inline-formula">³³S</span> depletion in black crust sulfates and subsequent <span class="inline-formula">³³S</span> enrichment in residual <span class="inline-formula">SO₂</span>. Except for a few samples, sulfate aerosols mostly have <span class="inline-formula">Δ³³S</span> values &gt;&thinsp;0&thinsp;‰, and no processes can yet explain this enrichment, resulting in an inconsistent S budget: black crust sulfates could well represent the complementary negative <span class="inline-formula">Δ³³S</span> reservoir of the sulfate aerosols, thus solving the atmospheric <span class="inline-formula">SO₂</span> budget.</p>