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.
<p><strong>Abstract.</strong> To better understand the formation and the oxidation pathways leading to gypsum-forming &#8220;black crusts&#8221; and investigate their bearing on the whole atmospheric SO<sub>2</sub> cycle, we measured the oxygen (&#948;<sup>17</sup>O, &#948;<sup>18</sup>O and &#8710;<sup>17</sup>O) and sulfur (&#948;<sup>33</sup>S, &#948;<sup>34</sup>S, &#948;<sup>36</sup>S, &#8710;<sup>33</sup>S and &#8710;<sup>36</sup>S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW-SE cross-section in the Parisian basin. The &#948;<sup>18</sup>O and &#948;<sup>34</sup>S, ranging between 7.5 and 16.7&#8201;&#177;&#8201;0.5&#8201;&#8240; (n&#8201;=&#8201;27, 2&#963;) and between &#8722;2.6 and 13.9&#8201;&#177;&#8201;0.2&#8201;&#8240; respectively, show anthropogenic SO<sub>2</sub> as the main sulfur source (from 2 to 81&#8201;%, in average ~30&#8201;%) with host-rock sulfates making the complement. This is supported by &#8710;<sup>17</sup>O-values (up to 2.6&#8201;&#8240;, in average ~0.86&#8201;&#8240;), requiring >&#8201;60&#8201;% of atmospheric sulfates in black crusts. Both negative &#8710;<sup>33</sup>S-&#8710;<sup>36</sup>S-values between &#8722;0.34 and 0.00&#8201;&#177;&#8201;0.01&#8201;&#8240; and between &#8722;0.7 and &#8722;0.2&#8201;&#177;&#8201;0.2&#8201;&#8240; respectively were measured in black crusts sulfates, that is typical of a magnetic isotope effect that would occur during the SO<sub>2</sub> oxidation on the building stone, leading to <sup>33</sup>S-depletion in black crust sulfates and subsequent <sup>33</sup>S-enrichment in residual SO<sub>2</sub>. Given that sulfate aerosols have mostly &#8710;<sup>33</sup>S&#8201;>&#8201;0&#8201;&#8240; and no processes can yet explain this enrichment, resulting in a non-consistent S-budget, black crust sulfates could well represent the complementary negative &#8710;<sup>33</sup>S-reservoir of the sulfate aerosols solving the atmospheric SO<sub>2</sub> budget.</p>
First of all, there was no rebuttal to our interpretations and the comments raised by the second referee were important to improved our manuscript. We almost followed all of them.
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