Unraveling the Mysteries of Sulfate Aerosols in the US: Insights from Oxygen Isotopes

Sulfate aerosols (pSO₄) play a critical role in climate and air quality; however, current atmospheric models often struggle to simulate pSO₄ concentrations accurately due to uncertainties in chemical production mechanisms. The formation of pSO₄ involves complex gas-phase and aqueous-phase oxidation reactions, each with different implications for particle nucleation and condensation. Additionally, there are various primary sources of pSO₄, including anthropogenic and natural emissions. These diverse pathways and sources of pSO₄ complicate our understanding of its atmospheric burden.

Oxygen isotope mass-independent fractionation (Δ¹⁷O) has been suggested as a useful tool to evaluate sulfate oxidation chemistry and sources, thereby improving model simulations. In this work, we modeled Δ¹⁷O of pSO₄ for the first time using the Community Multiscale Air Quality Model (CMAQ) with the sulfate tracking mechanism (STM) script across the contiguous US.

Our results reveal significant spatiotemporal variability, with higher Δ¹⁷O values simulated in certain regions and during summertime due to increased pSO₄ formation via aqueous-phase O₃ reactions. Comparisons of the simulated Δ¹⁷O values with observations in the northeastern US indicate that the CMAQ model substantially overpredicts the importance of aqueous-phase O₃ oxidation reactions, particularly during the summer months. Our results reveal significant seasonal shift with a northeastern US Δ¹⁷O(pSO₄) average of 1.257±1.543‰ and 3.587±1.758‰ during winter and summer respectively, reflecting an increase of aqueous-phase O3 oxidation reactions during the summertime. Further, regions with elevated NH3 emissions tended to have higher Δ¹⁷O(pSO₄) values due to higher cloud water pH conditions favoring aqueous-phase O₃ oxidation. This period is also when CMAQ tends to underpredict sulfate concentrations, suggesting the omission of a key sulfate formation pathway with a low Δ¹⁷O value during the summer. This work underscores the importance of integrating novel isotopic observational data with advanced modeling techniques to address critical uncertainties in atmospheric chemistry. Such integration is crucial for informing strategies to mitigate air pollution and its associated impacts on climate and human health.