The role of long - range transport and domestic emissions in 1 determining atmospheric secondary inorganic particle 2 concentrations across the UK

13 Surface concentrations of secondary inorganic particle components over the UK have been 14 analysed for 2001-2010 using the EMEP4UK regional atmospheric chemistry transport model 15 and evaluated against measurements. In early 2003 an episode of substantially elevated 16 surface concentrations of ammonium nitrate was measured across the UK by the AGANet 17 network. The EMEP4UK model was able accurately to represent both the long-term decadal 18 surface concentrations and the episode in 2003. The latter was identified as consisting of three 19 separate episodes, each of less than 1 month duration, in February, March and April. The 20 primary cause of the elevated nitrate levels across the UK was meteorological, a persistent 21 high pressure system, but whose varying location impacted the relative importance of 22 transboundary versus domestic emissions. Whilst long-range transport dominated the elevated 23 nitrate in February, in contrast it was domestic emissions that mainly contributed to the March 24 episode, and for the April episode both domestic emissions and long-range transport 25 contributed. A prolonged episode such as the one in early 2003 can have substantial impact on 26 annual average concentrations. The episode led to annual concentration differences at the 27 regional scale of similar magnitude to those driven by long-term changes in precursor 28