This study has investigated the chemical association among acidic gases, ammonia and secondary inorganic aerosols based on hourly measurements in a tropical urban atmosphere. The 24 hr average concentrations of SO2, NH3, HONO, HNO3 and HCl were 21.77, 2.47, 1.73, 3.00 and 0.08 μg/m3, respectively while those of SO42-, NO3-, Cl-, Na+, K+, NH4+, Ca2+ and Mg2+ in PM2.5 were 4.41, 1.29, 0.28, 0.30, 0.32, 1.76, 0.14 and 0.07 μg/m3, respectively. The results of this study for SO2, NH3, HONO, HCl, SO42- and Cl- showed significant diurnal variations, whereas there was a lack of significant diurnal variations for HNO3, NO3-, Na+, NH4+, Ca2+ and Mg2+. Analysis of the charge balance of ionic species indicated that sufficient NH3 was present most of the time to neutralize both H2SO4 and HNO3 to form (NH4)2SO4 and NH4NO3. The conversion of SO2 into SO42- and HNO3 into NO3- was observed to be sensitive to changes in temperature and relative humidity, respectively. The study area experienced ambient relative humidity, which was higher than the estimated deliquescence relative humidity of NH4NO3 most of the time during the measurement period. As a result, the NH4NO3 formation was thermodynamically favorable during both daytime and nighttime. However, NH4Cl formation was not favored under ammonia-poor conditions. It was observed that biomass burning could trigger nitrate and chloride formation in the ambient air. © Taiwan Association for Aerosol Research.