The H₂SO₄‐HNO₃‐NH₃ system at high humidities and in fogs: 2. Comparison of field data with thermodynamic calculations

Abstract

Concentrations of HNO3(g ) and NH3(g ) determined in the field were compared to predictions from aerosol equilibrium models. The products of HNO3(g) and NH3(g) concentrations measured under cool and humid nonfoggy conditions agreed in magnitude with predictions from a comprehensive thermodynamic model for the atmospheric H2SO,•-HNO3-NH3-H20 system. Observed concentrations of NH3(g ) in fogs were generally consistent with those predicted at equilibrium with fog water, but important discrepancies were noted in some cases. These discrepancies may be due to fluctuations in fog water composition over the course of sample collection or to the sampling of nonfoggy pockets of air present within the fog. Detectable concentrations of HNO3(g ) (up to 23 neq m -3) were often found in fogs with pH < 5 and were attributed to the sampling of nonfoggy air or to the slow rate of HNO3(g) diffusion to fog droplets. Concentrations of HNO3(g ) in fogs with pH > 5 were below the detection limit of 4--8 neq [1983, 1984] JACOB ET AL.: H 2SO,•-HNO 3-NH 3 IN FOGS--THERMODYNAMIC CALCULATIONS

Ammonium Nitrate, Nitric Acid, and Ammonia Equilibrium in Wintertime Phoenix, Arizona

“…Doyle et al, 34 Grosjean, 35 Harrison arid Pio, 36 Hildemann et al, 37 Chang et al, 38 and Lewin et al 39 obtained reasonably good agreement with the theoretical values, while Stelson et al, 40 Cadle et al, 41 Tanner, 42 Anlauf et al, 43 Jacob et al, 44 and Allen et al 45 found significant deviations from theoretical estimates and, in some cases, a consistent bias. The amount of scatter in Figure 5 is consistent with that reported by several other studies.…”

Transformation of Individual Aerosol Particles in Acidic Fog Evolution

“…Wet acidic deposition can result not only from rain but also from fog and cloud. Measurements of the chemical composition of acid fog or cloud water by a number of investigators (Mrose, 1966;Okita, 1968;Hegg and Hobbs, 1982;Seigneur and Saxena 1984;Hegg et al, 1984;Daum et al, 1984;Jacob et al, 1986;Chang et al, 1990) have shown that sulfate and nitrate ions co-exist as major acidic anions. In some cases, the two ions, S02-4 and N0-3 , contributed over 90 % of the total anion content.…”

Acidic Sulfate Aerosols: Characterization and Exposure

“…Thermodynamic considerations suggest that aerosol sulfate/HsS04 mixtures should be in equilibrium with atmospheric ammonia (NH3) (35). NH3 is the principal neutralizing agent HS04, and the equilibrium level of INH3 for even slightly acidic sulfate is much below average ambient NHJ levels (36). Hence, acidic aerosol on filter membranes are subject to neutralization if NH3 ammonia-laden air is sampled later in the same interval.…”