Nitrogen and Sulphur Deposition on Regional and Global Scales: a Multi-model Evaluation

We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NOy and NHx) and sulfate (SOx) to land and ocean surfaces. For the year 2030, the models are driven by 3 different emission scenarios: the first reflects current air quality legislation (CLE) around the world; the second represents an optimistic case of the maximum emissions reductions currently technologically feasible (MFR); the contrasting third is the pessimistic IPCC SRES A2 scenario. We perform an extensive evaluation of the present-day deposition using nearly all information on wet deposition available worldwide. Most models show a good agreement with observations in Europe and North America, whereas the model skills are poorer in India, South East Asia, Africa and South America. Nevertheless, in regions with quality controlled measurements 60-70% of the models calculate wet deposition rates that agree with measurements to within ±50%. Models systematically overestimate NHx deposition in South Asia, and underestimate NOy deposition in East Asia. We show that there are substantial differences between models for the removal mechanisms of NOy, NHx, and SOx, leading to ±1 σ variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of two outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Our models indicate that currently, 43%, 36% and 51% of all NOy, NHx, and SOx is deposited over the ocean, and 50-80% of the fraction of deposition on land falls on natural (non-agricultural) vegetation. We use a threshold of 1000 mg(N)m-2yr-1 (“critical load”) to indicate potential risk for ecosystems. Currently 11% of the world’s natural vegetation receives nitrogen deposition in excess of this threshold. The regions most concerned are the United States (20% of vegetation), Western Europe (30%), Eastern Europe (80%), South Asia (60%), East Asia (40%), South East Asia (30%), and Japan (50%). Deposition fluxes in the future are mainly driven by changes in emissions, and less importantly by changes in atmospheric chemistry and climate. Under CLE, NOy deposition remains roughly constant in 2030 in most parts of the world, except for Asia where NOy deposition further increases by 50% to 100%. In contrast, NOy deposition for MFR could decrease worldwide by 50%. A2 would imply further increases of NOy deposition in the polluted parts of the world by a factor of two. Following CLE, NHx deposition goes down by 20% in Europe, and further increases by 40-100% in parts of Central and South America, Africa, and parts of Asia. NHx deposition for A2 is rather similar to CLE. Deposition of SOx varies strongly among the scenarios. For CLE SOx deposition remains constant or decreases everywhere, except in Asia. Assuming MFR large reductions in deposition can be realized throughout the world; whereas A2 implies large increases in deposition everywhere except for North America and Europe. These results have important implications for ecosystem nitrogen loads; the global fraction of vegetation exposed to nitrogen loads in excess of 1 g(N)m-2yr-1 increases globally to 17% for CLE and 25% for A2. In MFR, the reductions in NOy are off-set by further increases for NHx deposition. The regions most concerned by exceedingly high nitrogen loads for CLE and A2 are Europe and Asia, but also parts of Africa.JRC.H.2-Climate chang