Enhancement of ammonium nitrate aerosol in the Northern Hemisphere lower stratosphere linked to Asian summer monsoon outflow

Abstract. This study examines how Asian Summer Monsoon (ASM) outflow perturbs the chemical composition of background aerosol in the extratropical lower stratosphere (ExLS). We analyze the summer-to-autumn transition in aerosol chemical composition using in-situ measurements from the ERICA instrument acquired during the PHILEAS aircraft campaign in August–September 2023 over the North Pacific, Alaska, northern Canada, and northern Europe. We observe an enrichment of ammonium and nitrate aerosol in the ExLS background air masses from summer to autumn, particularly at potential temperatures above 370 K (~13 km). Concurrently, the fraction of NO⁺-rich particles in the ExLS increases from August to September 2023. The corresponding mass spectra indicate internally mixed particles containing nitrate, sulfate, ammonium, and organic matter. Simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) show this seasonal transition is associated with the intrusion of relatively young air masses (<3.5 months old) originating from South Asia and the western Pacific into the ExLS, especially in autumn. These particles persist in the lower stratosphere for weeks up to months and undergo chemical aging. This aging is reflected by an observed increasing oxidative degree of organic matter, a decreasing nitrate-to-sulfate ratio, and an increasing ammonium-to-nitrate ratio, suggesting progressive sulfate incorporation and particle nitrate depletion. Overall, our results demonstrate that the ASM outflow can substantially shape ExLS background aerosol composition through the convective uplift, subsequent transport, and aging of ammonium- and nitrate-rich air masses from polluted surface regions, with important implications for stratospheric heterogeneous chemistry and aerosol-climate interactions.