The effect of varying primary emissions on the concentrations of inorganic aerosols predicted by the enhanced UK Photochemical Trajectory Model

An enhanced Photochemical Trajectory Model (PTM) has been used to simulate concentrations of secondary inorganic aerosol (for the purposes of this work, sulphate, nitrate, chloride and ammonium) in PM10 over a two-month period at a rural site in central southern England (Harwell). Judged against a base year of 2007, emissions of precursor gases, SO2, NOx and NH3 have been varied over plausible ranges, occurring across the UK only, mainland Europe only, or the whole of Europe. The model is able to reproduce observed non-linearities and shows that abatement is less than proportional in all cases. Additionally, abatement of sulphur dioxide leads to increased nitrate concentrations. The combination of a weak response of nitrate to reductions in NOx emissions, and the effect of sulphur dioxide reductions in increasing nitrate is consistent with the very small recent observed trends in nitrate concentrations over the UK. A scenario for 2020 in which emissions of SO2, NOx and NH3 fall to 64%, 75% and 96% respectively of their 2007 baseline levels across the whole of Europe shows a reduction of 2 μg m−3 in secondary inorganic aerosol which is 13% below the baseline case for a two month period in 2007, due mostly to a fall in sulphate and ammonium. As this was a relatively high pollution period, it is estimated that over a full year, the reduction is more likely to be around 1 μg m−3.

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► Secondary inorganic aerosol has been simulated for a site in central southern England.
► Non-linearities in precursor–pollutant relationships are well reproduced.
► In line with some other model studies, abatement of sulphur dioxide leads to increased nitrate.
► Low response of nitrate aerosol to NOx reductions is predicted.
► Total reduction of secondary inorganic components of about 1 μg m−3 between 2007 and 2020 is predicted.