Degradation of SO2, NO2 and NH3 leading to formation of secondary inorganic aerosols: An environmental chamber study

We have examined the interactions of gaseous pollutants and primary aerosols that can produce secondary inorganic aerosols. The specific objective was to estimate degradation rates of precursor gases (NH3, NO2 and SO2) responsible for formation of secondary inorganic aerosols. A Teflon-based outdoor environmental chamber facility (volume 12.5 m3) was built and checked for wall losses, leaks, solar transparency and ability to simulate photochemical reactions. The chamber was equipped with state-of-the-art instrumentation to monitor concentration–time profiles of precursor gases, ozone, and aerosol. A total of 14 experimental runs were carried out for estimating the degradation of precursor gases. The following initial conditions were maintained in the chamber: NO2 = 246 ± 104 ppb(v), NH3 = 548 ± 83 ppb(v), SO2 = 238 ± 107 ppb(v), O3 = 50 ± 11 ppb(v), PM2.5 aerosol = 283438 ± 60524 No./litre. The concentration–time profile of gases followed first-order decay and were used for estimating degradation rates (NO2 = 0.26 ± 0.15 h−1, SO2 = 0.31 ± 0.17 h−1, NH3 = 0.35 ± 0.21 h−1). We observed that degradation rates showed a statistical significant positive correlation (at 5% level of significance) with the initial PM2.5 levels in the chamber (coefficient of correlation: 0.63 for NO2; 0.62 for NH3 and 0.51 for SO2), suggesting that the existing surface of the aerosol could play a significant role in degradation of precursor gases. One or more gaseous species can be adsorbed on to the existing particles and these may undergo heterogeneous or homogeneous chemical transformation to produce secondary inorganic aerosols. Through correlation analysis, we have observed that degradation rates of precursor gases were dependent on initial molar ratio of (NH3)/(NO2 + SO2), indicative of ammonia-rich and ammonia-poor situations for eventual production of ammonium salts.

► Degradation rates of precursor gases were estimated as: NO = 0.45 ± 0.11 h−1, NO2 = 0.26 ± 0.15 h−1, SO2 = 0.31 ± 0.17 h−1, NH3 = 0.35 ± 0.21 h−1.
► Existing surface of primary aerosol plays a significant role in degradation of precursor gases.
► A higher initial molar ratio of (NH3) to (NO2 + SO2) leads to greater degradation rates of the precursor gases.