Transformation of atmospheric ammonia and acid gases into components of PM2.5: an environmental chamber study

INTRODUCTION: The kinetics of the transformation of ammonia and acid gases into components of PM2.5 has been examined. The interactions of existing aerosols and meteorology with the transformation mechanism have also been investigated. The specific objective was to discern the kinetics for the gas-to-particle conversion processes where the reactions of NH3 with H2SO4, HNO3, and HCl take place to form (NH4)2SO4, NH4NO3, and NH4Cl, respectively, in PM2.5. MATERIALS AND METHODS: A Teflon-based outdoor environmental chamber facility (volume of 12.5 m3) with state-of-the-art instrumentation to monitor the concentration–time profiles of precursor gases, ozone, and aerosol and meteorological parameters was built to simulate photochemical reactions. RESULTS AND DISCUSSION: The reaction rate constants of NH3 with H2SO4, HNO3, and HCl (i.e., k S, k N, and k Cl) were estimated as (1) k S = 2.68 × 10−4 (±1.38 × 10−4) m3/μmol/s, (2) k N = 1.59 × 10−4 (±8.97 × 10−5) m3/μmol/s, and (3) k Cl = 5.16 × 10−5 (±3.50 × 10−5) m3/μmol/s. The rate constants k S and k N showed significant day–night variations, whereas k Cl did not show any significant variation. The D/N (i.e., daytime/nighttime values) ratio was 1.3 for k S and 0.33 for k N. The significant role of temperature, solar radiation, and O3 concentration in the formation of (NH4)2SO4 was recognized from the correlation analysis of k S with these factors. The negative correlations of temperature with k N and k Cl indicate that the reactions for the formation of NH4NO3 and NH4Cl seem to be reversible under higher temperature due to their semivolatile nature. It was observed that the rate constants (k S, k N, and k Cl) showed a positive correlation with the initial PM2.5 levels in the chamber, suggesting that the existing surface of the aerosol could play a significant role in the formation of (NH4)2SO4, NH4NO3, and NH4Cl. CONCLUSIONS: Therefore, this study recommends an intelligent control of primary aerosols and precursor gases (NO x , SO2, and NH3) for achieving reduction in PM2.5 levels.