Measurement, analysis, and modeling of gas-to-particle conversion between ammonia, acid gases, and fine particles.

摘要

Since 1990, the population of hogs in eastern North Carolina has increased sharply resulting in increased emissions of ammonia. An Annular Denuder System (ADS) was used, which consisted of a cyclone separator, two diffusion denuders coated with sodium carbonate and citric acid, respectively, and a filter pack consisting of Teflon and nylon filters in series. The ADS measured ammonia, acid gases, and fine particles in ambient atmosphere at a commercial hog farm in Eastern North Carolina from April 1998 to March 1999. The sodium carbonate coated denuders yielded average acid gas concentrations of 0.23 μg/m 3 HCl (±0.20 μg/m3); 1.10 μg/m 3 HONO (±1.17 μg/m3); 1.14 μg/m 3 HNO₃ (±0.81 μg/m3), and 1.61 μg/m 3 SO₂ (±1.58 μg/m3). The citric acid coated denuders yielded an average concentration of 17.89 μg/m 3 NH₃ (±15.03 μg/m3). The filters yielded average fine aerosol (i.e., fine particular matter, Dp ≤ 2.5 μm) concentrations of 1.64 μg/m3 NH₄+ (±1.26 μg/m3); 0.26 μg/m3 Cl − (±0.69 μg/m3); 1.92 μg/m 3 NO₃− (±1.09 μg/m 3), and 3.18 μg/m3 SO₄2− (±3.12 μg/m3).; Using the data collected from the study sites, we evaluated the seasonal variations and the effects of relative humidity on fine particle species. Based on the measurements of ammonia, acid gases, and fine particles, the mean pseudo-first-order rate constant, k_S, between NH₃ and H₂SO₄ aerosol is estimated to be 3.70 (±2.99) × 10−3 sec−1. The rate constant was found to increase as temperature increases, and decrease with increasing relative humidity.; The equilibrium time constant was determined based on the estimated kinetic rate constants and the observed inorganic components of atmospheric aerosols. The average value of equilibrium time constant was determined to be 17.01 (±12.19) minutes for ambient equilibrium time between ammonia, nitric acid gas and ammonium nitrate aerosol; and 10.83 (±8.97) minutes for ammonia, hydrochloric acid, and ammonium chloride. The aerosol chemical compositions predicted by the use of a thermodynamic equilibrium model (EQUISOLV II) were in good agreement for ammonium and under-estimated nitrate aerosol when compared with the observed chemical composition at the experimental site.