Particulate matter at a rural location in southern England during 2006: Model sensitivities to precursor emissions

A moving air parcel trajectory model has been used to estimate the mid-afternoon mass concentrations of a number of suspended particulate matter (PM) components for each day of 2006 for a rural location, Harwell, Oxfordshire, in the southern UK. A large number of equally probable and randomly selected 96-h 3-dimensional air mass trajectories were used to describe the variability of the atmospheric transport paths during each day. A chemical kinetic description was given for the major PM formation processes. The linearity of the chemical production pathways forming the secondary PM components was examined by sensitivity studies to 30% reductions in SO2, NOx, NH3, VOC and CO emissions. The chemical environment revealed by these sensitivity studies appeared to be ‘ammonia-limited’. Consequently, PM mass concentrations appeared to be markedly non-linear with PM precursor emissions. Policy strategies for PM2.5 therefore need to take into account emission reductions for a wide range of primary PM components and secondary PM precursors and to focus primarily on the abatement of NH3. This complex interlinking may help to explain why PM levels have remained constant despite falling primary PM emissions.