Week 12: Ammonia in the Troposphere

Wednesday, 11^th November, 2008, lecture I (pdf)

Overhead sources are
Chemistry of the Natural Atmospheres, Peter Warneck, 2^nd Ed., Academic Press, 1999

Atmospheric Change, T.E. Graedel and P.J. Crutzen, Wiley-Interscience, 1994

Points/Topics to remember from this week’s classes:

• Ammonia is particularly relevant for aerosol science, particularly because it is the dominant (only relevant) base in the atmosphere. Ammonia essentially neutralizes the major atmospheric acidic species, sulfuric and nitric acid in the troposphere. Ammonia is very polar and has high water solubility. It partitions into the liquid phase forming ammonium (NH₄⁺) rapidly, especially when the pH is low
• The tropospheric lifetime of ammonia is less than 10 days, dominated by wet and dry deposition. Its reaction with the ·OH radical

NH₃ + ·OH → H₂O + NH₂·

… is slow, the equivalent of a lifetime exceeding 100 days, wherefore only a small amount of NH₃ is chemically lost (only a few studies exist on this)

• Ammonia mixing ratios are highly variable near the surface and fall off rapidly with elevation. NH₃/NH₄⁺ ratios are generally smaller than 1, reflecting ammonia’s rapid conversion into ammonium. However, high ammonia mixing ratios are not necessarily converted into ammonium immediately if the conditions for partitioning are not favorable, such as low sulfate concentrations
• Ternary nucleation theory suggests that ammonia is essential for new particle formation, interacting with H₂SO₄ and water in effectively nucleating gas-phase species into clusters into particles. Hence, the role of ammonia might be essential for earth’s climate through its role in particle formation

o recent discoveries suggest that simple amines, the organic analogs of ammonia, are much more efficient contributors to the ternary nucleation process. Like ammonia, amines have a principle source in excrement decay (see below)

• Sources of ammonia are the enzymatic decomposition of urea ((NH₂)₂CO) and amino acids, and soil (and plant) nitrogen dynamics

o urea is a basic component of all excrements, be they from animals or humans. Animal husbandry has approximately doubled the atmospheric ammonia budget due to large emissions from domestic animal feces, a field that has been intensely studied in the last 10 years. Other contributions are from human and wild animal excrements

o The Haber-Bosch process of ammonia (http://en.wikipedia.org/wiki/Haber-Bosch) production from N₂ and H₂ (an anthropogenic nitrogen fixation process!) was first patented before World War I, and is the major pathway of synthetic fertilizer production. The use of ammonium nitrate and other fertilizers has not only lead to the green revolution, but also to additional ammonia, as well as N₂O and NO emissions from fertilized soils

o Ammonium is the highly preferred form of assimilated nitrogen in microbial and plant biology; atmospheric ammonia can be assimilated by plants when internal ammonium concentrations are low, or ammonia is emitted (from plants, soils and the oceans) when atmospheric concentrations are low. On a global basis, there is an estimated net ammonia flux from the biosphere is into the atmosphere

• as pH is so important for ammonia fluxes, I have reviewed typical pH values for natural waters, both in the atmosphere and at the surface, alongside some typical ion concentrations in these waters