Sulfur Isotope Anomalies (Δ³³S) in Urban Air Pollution Linked to Mineral-Dust-Associated Sulfate

Abstract

Sulfate aerosols exert a net cooling effect on the earth-atmosphere system, yet their radiative forcing remains associated with largest of uncertainties in the assessment of climate change. One of the contributing factors is the poor understanding of the sulfate formation pathways, which are thought to be following mostly the mass-dependent fractionation model (i.e., Δ 33 S ~ 0). However, globally, urban sulfate aerosols exhibit significant non-zero Δ 33 S compositions (from -0.6‰ to +0.6‰), resulting in sulfur mass-independent fractionation (S-MIF) processes. The origin(s) of these S-MIF anomalies remain(s) unclear. Here, we conducted dual-isotope (Δ 33 S, δ 34 S) probing of sulfate aerosols from summertime megacity Delhi in South Asia. A shift towards concomitantly high Δ 33 S (from +0.2‰ to +0.5‰) and low δ 34 S (from +5‰ to +1‰) values was observed with the influx of mineral dust. The Fe-to-Al tracer showed significant correlations with sulfate loadings (R 2 =0.84) and Δ 33 S signatures (R 2 =0.77). As such, we postulate that the SO2 photo-oxidation on mineral dust generates S-MIF anomaly ~ +0.35±0.10‰, thereby also explaining the previously observed Δ 33 S values worldwide. Together, the findings help deconvolute S-isotope dynamics in urban regions wherein, contrary to prevailing paradigm, non-anthropogenic factor (i.e., mineral dust) is found to influence the aerosol sulfate-induced pollution affecting air quality/human health. SYNOPSISSulfate-related pollution in urban regions-affecting air quality/human health-could be linked to non-anthropogenic factor i.e., mineral dust.

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

“…Overall, uncertainties of δ 34 S, Δ 33 S, and Δ 36 S measurements attributed to the full procedure (including reduction, fluorination, purification, and IRMS measurements) are estimated to be 0.40, 0.064, and 0.20‰, respectively. Given Δ 33 S anomalies in aerosols are usually larger than 0.1‰, the error in Δ 33 S (0.064‰) due to the Thode solution reduction step in this study using the IRMS method, which remains lower than the multicollector-inductively coupled plasma mass spectrometer (MC-ICPMS) method (∼0.1‰), , would neither affect our data interpretation nor change our conclusion.…”

“…The prediction that heterogeneous sulfate formation leads to negative Δ 33 S values appears supported by subsequent findings of negative Δ 33 S values in black crust sulfate (Figure ), which was formed via SO 2 oxidation on carbonate building stones. , A magnetic isotope effect was further speculated given the decoupled Δ 33 S/Δ 36 S relationship, but the deep mechanism is not explicitly discussed. , Magnetic isotope effects that arise from spin-selective reactions are usually linked to radical pair reactions. , It remains unclear where the radical pair reaction is involved in the formation of black crust sulfate. Most recently, new studies of aerosol collected at India and open oceans argue that heterogeneous sulfate formation on mineral surface results in positive Δ 33 S values, , in contrast to previous arguments (negative Δ 33 S values). − In addition, the correlation between Δ 33 S and mineral dusts highly depends on the selection of mineral dust tracers. For example, although Δ 33 S values of India aerosol samples correlate with Fe/Al, such a relationship is not observed between Δ 33 S and Ca 2+ , similar to our work and other studies (Figure S4).…”

“…Based on the possible production of S-MIF in sulfur recombination reactions that may occur in gaseous thermochemical reactions such as combustion and volcanism, it is further argued that similar processes may occur in Archean, providing additional non-photochemical S-MIF processes that may need to be considered in interpreting Archean data. , This idea was preliminarily tested by physical chemistry experiments and atmospheric chemistry models . Subsequent multiple sulfur isotope measurements of modern sulfate aerosols however found that Δ 33 S anomalies are also correlated with mineral dusts. − It was preliminarily proposed that heterogeneous sulfur reactions account for such Δ 33 S anomalies, but Δ 33 S signs among different follow-up studies are different. − A deep mechanistic understanding on the possible chemical reaction that may lead to Δ 33 S anomalies is useful for using Δ 33 S to identify and quantify any missing sulfate formation pathway in the modern atmosphere with profound implications for air pollution and climate change. − …”

“…Figure 3. Quadruple sulfur isotope compositions of sulfate aerosols in this study and in the literature10,11,15,18,19,[36][37][38][39]43 (symbols are the same as Figure1). (A) δ 34 S/Δ 33 S; (B) δ 34 S/Δ 36 S; and (C) Δ 33 S/ Δ 36 S. Quadruple sulfur isotope compositions of coals collected from China are also shown as solid squares.…”

Sulfur-isotope anomalies recorded in Antarctic ice cores as a potential proxy for tracing past ozone layer depletion events

“…Evidence of S-MIF has been found in aerosol samples in present-day polluted urban and semi-urban environments ( 47–50 ). In such locations, mineral-dust-associated sulfate has been proposed to exhibit S-MIF ( 47 , 50 ), yet inherent mechanism(s) remain unclear and highly speculative. However, no such dust events have ever been reported for the period studied here ( 54 ), as also evidenced from the record of calcium concentrations (a marker for dust) in our dataset ( Figure S1a ; see the “Methods” section as well).…”