Nitrogen Depositions to the OSPAR Maritime Area in the period 1990-2021
Executive Summary
Airborne nitrogen depositions to the OSPAR Maritime Area for the 32-year period 1990-2021 have been calculated with the EMEP MSC-W Chemistry Transport Model at a horizontal resolution of 0.1°longitude x 0.1°latitude, based on emission data updated in 2023 by the EMEP Centre on Emission Inventories and Projections. The 1990-2021 period is the longest period for which EMEP MSC-W has done this type of calculation for OSPAR until now.
Results for actual and normalized nitrogen depositions, as well as source-receptor relationships, are presented in this report and in the accompanying data sheets for the 5 OSPAR Regions, the 24 Exclusive Economic Zones (EEZs) within the OSPAR Maritime Area, the 25 so-called ‘partial EEZs’ (EEZs divided up into different OSPAR regions), and the 64 COMP4 (Fourth Common Procedure) Assessment Units. Normalization (“weather-averaging”) was done using meteorological data from the 2016-2021 period.
According to our model results, actual (i.e. non-normalized) depositions of oxidized nitrogen were clearly lower in 2021 than in 1990 in all OSPAR Regions, EEZs, partial EEZs, and COMP4 Assessment Units. In the case of reduced nitrogen depositions, there are both increases and decreases, depending on the receptor area under consideration. This is mainly due to the much less significant (or even absent) downward trends seen in the emissions of reduced nitrogen in many countries. However, total (oxidized+reduced) nitrogen depositions were lower in 2021 than in 1990 in all OSPAR Regions, EEZs, partial EEZs, and COMP4 Assessment Units.
A trend analysis for depositions over the 32-year period from 1990 to 2021, using the Mann-Kendall test, shows statistically significant downward trends in all receptor areas for oxidized nitrogen, while for reduced nitrogen there are far less areas with significant downward trends; indeed some of them show increases. In general, trends in normalized depositions are more significant than those in actual depositions because the interannual variability in meteorology is filtered out.
In general, receptor areas are most influenced by the countries adjacent to them, but large emitters can make important contributions even if they are far away, mainly as oxidized nitrogen deposition. The largest contribution to nitrogen deposition in OSPAR Regions II and III is made by the United Kingdom, while OSPAR Region IV receives the single-largest contribution from Spain; the more remote Regions I and V are strongly influenced by the boundary condition (i.e. sources outside the EMEP model domain).