The sensitivities of emissions reductions for the mitigation of UK PM_2.5

Abstract. The reduction of ambient concentrations of fine particulate matter (PM_2.5) is a key objective for air pollution control policies in the UK and elsewhere. Long-term exposure to PM_2.5 has been identified as a major contributor to adverse human health effects in epidemiological studies and underpins ambient PM_2.5 legislation. As a range of emission sources and atmospheric chemistry transport processes contribute to PM_2.5 concentrations, atmospheric chemistry transport models are an essential tool to assess emissions control effectiveness. The EMEP4UK atmospheric chemistry transport model was used to investigate the impact of reductions in UK anthropogenic emissions of primary PM_2.5, NH₃, NO_x, SO_x or non-methane VOC on surface concentrations of PM_2.5 in the UK for a recent year (2010) and for a future current legislation emission (CLE) scenario (2030). In general, the sensitivity to UK mitigation is rather small. A 30 % reduction in UK emissions of any one of the above components yields (for the 2010 simulation) a maximum reduction in PM_2.5 in any given location of  ∼  0.6 µg m⁻³ (equivalent to  ∼  6 % of the modelled PM_2.5). On average across the UK, the sensitivity of PM_2.5 concentrations to a 30 % reduction in UK emissions of individual contributing components, for both the 2010 and 2030 CLE baselines, increases in the order NMVOC, NO_x, SO_x, NH₃ and primary PM_2.5; however there are strong spatial differences in the PM_2.5 sensitivities across the UK. Consequently, the sensitivity of PM_2.5 to individual component emissions reductions varies between area and population weighting. Reductions in NH₃ have the greatest effect on area-weighted PM_2.5. A full UK population weighting places greater emphasis on reductions of primary PM_2.5 emissions, which is simulated to be the most effective single-component control on PM_2.5 for the 2030 scenario. An important conclusion is that weighting corresponding to the average exposure indicator metric (using data from the 45 model grids containing a monitor whose measurements are used to calculate the UK AEI) further increases the emphasis on the effectiveness of primary PM_2.5 emissions reductions (and of NO_x emissions reductions) relative to the effectiveness of NH₃ emissions reductions. Reductions in primary PM_2.5 have the largest impact on the AEI in both 2010 and the 2030 CLE scenario. The summation of the modelled reductions to the UK PM_2.5 AEI from 30 % reductions in UK emissions of primary PM_2.5, NH₃, SO_x, NO_x and VOC totals 1.17 and 0.82 µg m⁻³ for the 2010 and 2030 CLE simulations, respectively (not accounting for non-linearity).