Fine Particle Liquid Water and Acidity over North American Cities during the Summer of 2023

Abstract. Fine aerosol liquid water content (ALWC) and acidity (pH) are co-determined, pH primarily reflects the ratio of hydrogen-ion concentrations in air (H⁺_air) to ALWC. Inorganic ions dominate H⁺_air and often ALWC, whereas organic aerosol (OA) mainly adds water. Added OA water, however, shifts gas–particle partitioning of semi-volatile species, altering H⁺_air itself, and thus pH and the aqueous-phase processes it governs. We characterize PM₁ ALWC and pH over four North American cities using airborne AEROMMA measurements (June–August 2023), including periods influenced by aged wildfire smoke with high OA but little effect on inorganic species. ALWC and pH were predicted with ISORROPIA-Lite, which includes OA water, and evaluated against measured partitioning of NH₃–NH₄⁺ and HNO₃–NO₃^-. Predicted ammonia partitioning agreed with observations (R² > 0.75, within ~±10 %), whereas nitrate was systematically over-predicted by ~27 %. Outside smoke, inorganic ions dominated ALWC despite being a minor mass fraction; within smoke, organic water dominated (45–65 %). Across all cities, pH remained low and varied little (1.5–2.5, 10th–90th percentile). Particle-phase fractions were 0.17–0.56 for NH₄⁺ but only 0.1–0.22 for NO₃^-, giving nitrate less redistribution and pH-buffering capacity than NH₄⁺. Including OA water raised pH during smoke by at most 0.62 units and improved HNO₃–NO₃^- agreement. Summertime PM₁ acidity thus remains persistently low and thermodynamically stabilized across diverse composition regimes, with wide-ranging implications for regulatory, environmental, and human-health impacts.