Adsorption of ammonia on soot at low temperatures

Ammonia is ar,arguably the most important alkaline trace species in the atmosphere. In addition to major ammonia sources on the ground, ammonium has been observed in aircraft exhaust that injects ammonia or ammonium directly into the upper troposphere. If ammonia adsorbs to a soot surface, then reactions in the troposphere between ammonia and other trace gases may be facilitated by soot particles. We examined the uptake of ammonia on n-hexane soot using transmission Fourier transform infrared spectroscopy to quantify the uptake of the gas on soot particles between 115 and 173 K and up to 0.1 Pa (10(-3) Torr) NH3. At temperatures above 173 K, we see virtually no adsorption of ammonia on soot for NH3 pressures up to 0.1 Pa (10(-3) Torr). Between 115 and 173 K we find that, upon exposure to the soot, some ammonia sticks rapidly while additional ammonia adsorbs more slowly to the soot. For fast adsorption, the uptake coefficient ranges from similar to 0.02 +/- 0.01 at 115 K to greater than or equal to(1.5 +/- 0.8) x 10(-4) at 153 K. The amount of rapid uptake of NH3 on soot follows the shape of a Langmuir adsorption isotherm. On the heterogeneous soot surface, this simple uptake behavior implies that ammonia adsorbs to soot on to a group of sites with similar binding characteristics. Between 123 and 153 K the saturation coverage is roughly 0.1 monolayer with a binding energy of 24 +/- 1 kJ/mol. At 115 K up to a full monolayer adsorbs with roughly 21 kJ/mol binding energy. Ammonia adsorption on soot is not likely to be significant at temperatures and ammonia partial pressures found in the troposphere.