PM1 Liquid Water and pH Determined with a Thermodynamic Model for the AEROMMA Field Campaign

MAGESH KUMARAN MOHAN, Alison Piasecki, Ann M. Middlebrook, Amy P. Sullivan, Emily Lill, Ilana Pollack, Stelios Kakavas, Spyros Ν. Pandis, Athanasios Nenes, Rodney J. Weber, Georgia Institute of Technology

     Abstract Number: 383
     Working Group: Coast to Coast Campaigns on Aerosols, Clouds, Chemistry, and Air Quality

Abstract
Particle acidity, or pH, governs numerous aerosol processes that impacts aerosol concentration, composition, and toxicity. Here we assess fine particle water and pH over several cities across the US using aircraft measurements from the AEROMMA study in summer 2023. Liquid water content and pH, based on both ionic and organic species, were predicted with a thermodynamic model (ISORROPIA-LITE) with gas and PM1 composition measurements, and meteorological parameters (RH >40% and temperature) as input variables. The model was evaluated by comparing predicted and observed NH3-NH4+ partitioning, and a good correlation was found (Slope=0.91, R2 = 0.81). Based on the analysis with preliminary data, the pH was consistent across all the cities at altitudes between 1 and 12 kft and ranged between 1.6 to 3.1 with an average pH of 2.3 ± 1.2 units for the study. This is slightly less acidic compared to previous ground based and aircraft studies. Water content associated with organic species on average made up 60% of the total LWC, while sulfate on average contributed 90% to the inorganic water fraction. However, the pH predicted without considering organic water was only about 0.4 units lower. Layers of high smoke levels were intercepted over urban areas in some flights. The average pH in those smoke layers (altitudes 4 to 10 kft) was about 1 unit higher than in areas with less smoke (average pH of 2.2). NO3- concentrations were generally low in all regions studied (average concentration 1 μg/m3) and 1 µg/m3 higher in the smoke plumes studied. Previous studies have reported pH of ~ 6 in fresh smoke, while these results are consistent with studies of more aged smoke. Further analysis with more accurate data will provide additional insights on pH and LWC over cities, in smoke layers and more remote continental regions.