A 3-DIMENSIONAL MODEL OF THE GLOBAL AMMONIA CYCLE

Using a three-dimensional (3-D) transport model of the troposphere, we calculated the global distributions of ammonia (NH3) and ammonium (NH4+), taking into account removal of NH3 on acidic aerosols, in liquid water clouds and by reaction with OH. Our estimated global 10 degrees x 10 degrees NH3 emission inventory of 45 Tg N-NH3 yr(-1) provides a reasonable agreement between calculated wet NH4+ deposition and measurements and of measured and modeled NH4+ in aerosols, although in Africa and Asia especially discrepancies exist. NH3 emissions from natural continental ecosystems were calculated applying a canopy compensation point and oceanic NH3 emissions were related to those of DMS (dimethylsulfide). In many regions of the earth, the pH found in rain and cloud water can be attributed to acidity derived from NO, SO2 and DMS emissions and alkalinity from NH3. In the remote lower troposphere, sulfate aerosols are calculated to be almost neutralized to ammonium sulfate (NH4)(2)SO4, whereas in the middle and upper troposphere, according to our calculations, the aerosol should be more acidic, as a result of the oxidation of DMS and SO2 throughout the troposphere and removal of NH3 on acidic aerosols at lower heights. Although the removal of NH3 by reaction with the OH radical is relatively slow, the intermediate NH2 radical can provide a substantial annual N2O source of 0.9(-0.4)(+0.9) Tg, thus contributing -0.4 by ca. 5% to estimated global N2O production. The oxidation by OH of NH3 from anthropogenic sources accounts for 10% of the estimated total anthropogenic sources of N2O. This source was not accounted for in previous studies, and is mainly located in the tropics, which have high NH3 and OH concentrations. Biomass burning plumes, containing high NOx and NH3 concentrations provide favourable conditions for gas phase N2O production. This source is probably underestimated in this model study, due to the coarse resolution of the 3-D model, and the rather low biomass burning NH3 and NOx emissions adopted. The estimate depends heavily on poorly known concentrations of NH3 (and NOx) in the tropics, and uncertainties in the rate constants of the reactions NH2 + NO2 --> N2O + H2O (R4), and NH2 + O-3 --> NH2O + O-2 (R7).