To better understand the formation and the oxidation pathways leading to gypsum-forming "black crusts" and investigate their bearing on the whole atmospheric SO 2 cycle , we measured the oxygen (δ 17 O, δ 18 O, and 17 O) and sulfur (δ 33 S, δ 34 S, δ 36 S, 33 S, and 36 S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW-SE cross section in the Parisian basin. The δ 18 O and δ 34 S 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 SO 2 as the main sulfur source (from ∼ 2 % to 81 %, average ∼ 30 %) with host-rock sulfates making the complement. This is supported by 17 O values (up to 2.6 ‰, on average ∼ 0.86 ‰), requiring > 60 % of atmospheric sulfates in black crusts. Negative 33 S and 36 S 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 SO 2 oxidation on the building stone, leading to 33 S depletion in black crust sulfates and subsequent 33 S enrichment in residual SO 2. Except for a few samples, sulfate aerosols mostly have 33 S 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 33 S reservoir of the sulfate aerosols, thus solving the atmospheric SO 2 budget.