Simulated Electronic Absorption Spectra of Sulfur-Containing Molecules Present in Earth's Atmosphere

We have calculated, ab initio, the electronic absorption spectrum of sulfuric acid (H₂SO₄) under atmospherically relevant conditions using a nuclear ensemble approach. The experimental electronic spectrum of H₂SO₄ is unknown so we benchmark our theoretical results by also considering other related sulfur-containing molecules, namely, sulfur dioxide (SO₂), sulfur trioxide (SO₃), hydrogen sulfide (H₂S), carbonyl sulfide (OCS), and carbon disulfide (CS₂), where experimental spectra are available. In general, we find very good agreement between our calculated spectra, which are based on underlying EOM-CCSD electronic structure calculations, and the available experimental spectra. We show that the computational cost of these calculated spectra can be substantively reduced with negligible loss of accuracy by using a combination of results obtained with the aug-cc-pV(D+d)Z+3 and aug-cc-pV(T+d)Z+3 basis sets. Our calculated cross-section for H₂SO₄ in the UV/VUV region is larger than previous theoretical estimates and greater than the experimentally measured upper limits. We suggest that further experimental attempts to measure the electronic absorption spectrum of H₂SO₄ in the actinic region (4.0-7.5 eV, 313-167 nm) region are warranted.