Modelling long-term sulphur and nitrogen deposition using Lagrangian chemical transport models

Lagrangian chemical transport models (LCTMs) have been used extensively in the past to predict seasonal and annual acid deposition. Because these models strike a different balance between level of process detail and computational cost than do Eulerian chemical transport models, LCTMs have been the preferred means for generating the reduced-form representations of atmospheric dispersion and removal called source-receptor matrices (SRMs). Such reduced-form acid deposition models are needed by integrated assessment models, life-cycle analysis models, and other multi-discipline model systems to link changes in emissions with changes in atmospheric concentrations and deposition.

This paper explores the adaptation of two regional LCTMs to simulate long-term sulphur and nitrogen ambient concentrations and dry and wet deposition in North America. First, the AES Lagrangian Sulphur Model (ALSM), a simple sulphur LCTM, has been applied to generate three types of sulphur-species SRMs for the 1990 meteorological year for 40 SO₂ emission regions and 200 receptors covering most of North America. Annual and quarterly results for the ALSM using full 1990 emissions show good agreement with 1990 observed air and precipitation chemistry station data. SRM predictions have also been evaluated against observed historical deposition trends. As well, spatial plots of unit SRM and actual-value SRM rows and columns provide useful insights into potential and actual source and receptor “footprints” for different regions of the continent.

Second, a more complex LCTM called LIAM (Lagrangian Inorganic Aerosol Model) is being developed to model sulphur, oxidized nitrogen, and reduced nitrogen concentrations and deposition with the objective of generating SRMs for nitrogen species. LIAM includes 28 transported gas-phase species plus particle sulphate, nitrate, and ammonium, a detailed gas-phase chemistry mechanism, simple NH₄NO₃ heterogeneous chemistry and SO₂ aqueous-phase chemistry, and dry and wet removal processes. Some preliminary LIAM results will be presented.