Particle pH can affect many aerosol processes, including gas-particle partitioning, SOA formation, and mobilization of toxic redox metals. pH is challenging to directly measure and often improperly characterized by proxies like ion balances or molar ratios of measured aerosol ionic species. We present a detailed analysis predicting pH with a thermodynamic model, verify the prediction, and test pH sensitivity to model inputs based on data from the SOAS field campaign.