Toward the Origins of Quadruple Sulfur Isotope Anomalies in Modern Sulfate: A Multitracer Approach and Implications for Paleo- and Planetary Atmospheres

Abstract

Sulfur isotope mass-independent fractionation signatures (S-MIF) have been widely found in modern sulfate aerosols. Their mechanistic origins and potential implications for atmospheric sulfate formation chemistry are however poorly understood. Of particular importance is that the mystery of modern sulfate S-MIF hinders precise interpretations of S-MIF in geological sediment and meteorite samples. Here, we examine several proposed origins of modern sulfate S-MIF using a multiple-tracer approach. Quadruple sulfur isotopes of sulfate aerosols collected from Guangzhou, a megacity in South China, were measured along with various chemical and isotopic tracers for high-altitude air masses, biomass burning, and mineral dusts in the same samples. Similar to previous studies, we found that negative Δ36S values in modern sulfate aerosols are linked to combustions, and to a larger extent, to high-temperature processes that may involve symmetry-dependent recombination reactions of sulfur. The origins of Δ33S anomalies widely observed in modern sulfate aerosols remain elusive and require further investigation before we can unambiguously use this proxy to quantify sulfur emission and transformation pathways in the modern atmosphere. From a comparative planetology perspective, we highlight that a complete understanding of S-MIF in modern aerosols is crucial for tracking paleo- and planetary atmospheres by multiple sulfur isotope measurements in Phanerozoic sediments and Martian meteorites because their S-MIF magnitudes are similar.

Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution

“…In addition to stratospheric intrusions, a recent study argues that the heterogeneous sulfate formation on mineral dust surface may lead to positive Δ 33 S values of sulfate aerosols, based on the positive correlation of Δ 33 S with the Fe/Al . However, no correlation is observed between Δ 33 S and Ca 2+ (a mineral dust tracer) in this study (Figure ), which is consistent with results from Yin et al (2023) . The correlation between Δ 33 S values and mineral dust may depend on the parameters for mineral dust tracers.…”

“…28 However, no correlation is observed between Δ 33 S and Ca 2+ (a mineral dust tracer) in this study (Figure 3), which is consistent with results from Yin et al (2023). 39 The correlation between Δ 33 S values and mineral dust may depend on the parameters for mineral dust tracers. The Fe and Al concentrations of aerosols are not measured in this study but can be analyzed in the future.…”

“…Hence, besides stratospheric 2019), 20 Guo et al (2010), 25 Han et al (2017), 17 Romero and Thiemens (2003), 24 Dasari et al (2022), 28 sulfate, other processes related to sulfate formation in the troposphere may also lead to the positive Δ 33 S values. 39 As for the negative S-MIF, an experimental study shows that the primary sulfate from combustion processes has a negative Δ 33 S value, ranging from −0.227 to 0.031‰. 40 Previous studies suggest that the K + can be regarded as a tracer of biomass burning.…”

Tracing the origin of elevated springtime atmospheric sulfate on the southern Himalayan-Tibetan plateau

“…49 Given the lack of any existing solid physico-chemical evidence for S-MIF from biomass-based combustion processes, the magnitude and even ‘sign’ of isotope anomalies associated with such processes remain currently unclear. 26,35,45,46,49 Coupling isotope anomalies with appropriate chemical marker(s) can be attempted to better link source(s) to the origin of S-MIF. 35,50…”

“…Furthermore, other S-containing reservoirs have been suggested as a source of such negative D 33 S anomalies, thereby refuting the claim for them to be associated with biomass/biofuel combustion. 49 Given the lack of any existing solid physicochemical evidence for S-MIF from biomass-based combustion processes, the magnitude and even 'sign' of isotope anomalies associated with such processes remain currently unclear. 26,35,45,46,49 Coupling isotope anomalies with appropriate chemical marker(s) can be attempted to better link source(s) to the origin of S-MIF.…”