Abstract. Atmospheric pollution has many profound effects on human
health, ecosystems, and the climate. Of concern are high concentrations and
deposition of reactive nitrogen (Nr) species, especially of reduced N
(gaseous NH3, particulate NH4+). Atmospheric chemistry and
transport models (ACTMs) are crucial to understanding sources and impacts of
Nr chemistry and its potential mitigation. Here we undertake the first
evaluation of the global version of the EMEP MSC-W ACTM driven by WRF
meteorology (1∘×1∘ resolution),
with a focus on surface concentrations and wet deposition of N and S species
relevant to investigation of atmospheric Nr and secondary inorganic
aerosol (SIA). The model–measurement comparison is conducted both spatially
and temporally, covering 10 monitoring networks worldwide. Model simulations
for 2010 compared use of both HTAP and ECLIPSEE (ECLIPSE annual total
with EDGAR monthly profile) emissions inventories; those for 2015 used
ECLIPSEE only. Simulations of primary pollutants are somewhat sensitive
to the choice of inventory in places where regional differences in primary
emissions between the two inventories are apparent (e.g. China) but are much
less sensitive for secondary components. For example, the difference in modelled
global annual mean surface NH3 concentration using the two 2010
inventories is 18 % (HTAP: 0.26 µg m−3; ECLIPSEE: 0.31 µg m−3) but is only 3.5 % for NH4+ (HTAP: 0.316 µg m−3; ECLIPSEE: 0.305 µg m−3). Comparisons of 2010 and
2015 surface concentrations between the model and measurements demonstrate that
the model captures the overall spatial and seasonal variations well for the
major inorganic pollutants NH3, NO2, SO2, HNO3,
NH4+, NO3-, and SO42- and their wet
deposition in East Asia, Southeast Asia, Europe, and North America. The
model shows better correlations with annual average measurements for
networks in Southeast Asia (mean R for seven species: R7‾=0.73),
Europe (R7‾=0.67), and North America (R7‾=0.63) than in East Asia (R5‾=0.35) (data for 2015),
which suggests potential issues with the measurements in the latter network.
Temporally, both model and measurements agree on higher NH3
concentrations in spring and summer and lower concentrations in winter. The
model slightly underestimates annual total precipitation measurements (by
13 %–45 %) but agrees well with the spatial variations in precipitation in
all four world regions (0.65–0.94 R range). High correlations between
measured and modelled NH4+ precipitation concentrations are also
observed in all regions except East Asia. For annual total wet deposition of
reduced N, the greatest consistency is in North America (0.75–0.82 R range),
followed by Southeast Asia (R=0.68) and Europe (R=0.61).
Model–measurement bias varies between species in different networks; for
example, bias for NH4+ and NO3- is largest in Europe and
North America and smallest in East Asia and Southeast Asia. The greater
uniformity in spatial correlations than in biases suggests that the major
driver of model–measurement discrepancies (aside from differing spatial
representativeness and uncertainties and biases in measurements) are
shortcomings in absolute emissions rather than in modelling the atmospheric
processes. The comprehensive evaluations presented in this study support the
application of this model framework for global analysis of current and
potential future budgets and deposition of Nr and SIA.
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