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Abstract: Aerosol acidity is one of the most important parameters that can influence atmospheric visibility, climate change and human health. Based on continuous field measurements of inorganic aerosol species and their thermodynamic modeling on a time resolution of 1h, this study has investigated the acidic properties of PM2.5 and their relation with the formation of secondary inorganic aerosols (SIA). The study was conducted by taking into account the prevailing ambient temperature (T) and relative humidity (RH) in a tropical urban atmosphere. The in-situ aerosol pH (pHIS) on a 12h basis ranged from −0.20 to 1.46 during daytime with an average value of 0.48 and 0.23 to 1.53 during nighttime with an average value of 0.72. These diurnal variations suggest that the daytime aerosol was more acidic than that caused by the nighttime aerosol. The hourly values of pHIS showed a reverse trend as compared to that of in-situ aerosol acidity ([H+]Ins). The pHIS had its maximum values at 3:00 and at 20:00 and its minimum during 11:00 to 12:00. Correlation analyses revealed that the molar concentration ratio of ammonium to sulfate (RN/S), equivalent concentration ratio of cations to anions (RC/A), T and RH can be used as independent variables for prediction of pHIS. A multi-linear regression model consisting of RN/S, RC/A, T and RH was developed to estimate aerosol pHIS. [Copyright &y& Elsevier]