Currently, more deposition of reduced nitrogen species to the UK is measure
d than can be accounted for from the calculated emissions. A newly develope
d longrange transport model was used to study this budget. Changes in horiz
ontal resolution of model and an improved parameterization of dry depositio
n of ammonia gave an improved agreement between measured and modelled depos
ition. However, the model still estimated only 65% of the measured depositi
on with a UK emissions term of 283 ktonnes N yr(-1). Scaling the emissions
by factors of x 1.2 and x 1.3 still resulted in the model underestimating o
f reduced nitrogen deposition. Modelled aerosol ammonium and ammonia gas co
ncentrations for the base-case were compared with observations. The ammonia
gas concentrations appear to be modelled reasonably well but the aerosol a
mmonium concentrations were underestimated by a factor of at least 2. This
is an important finding as this indicates that other parameters related to
wet deposition simulation are unlikely to be in gross error, implying that
the UK emissions term is too small. It was concluded that an emission sourc
e term of the order 400 ktonnes N yr(-1) is required to reconcile the model
with the measurements. Crown Copyright (C) 2001 Published by Elsevier Scie
nce Ltd. All rights reserved.