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In January 2016 the United Kingdom’s southern European Monitoring and Evaluation Programme (EMEP) level-2 air pollution monitoring ‘supersite’ was relocated from Harwell, Oxfordshire to Chilbolton Observatory, Hampshire. As no co-location study was undertaken, this work retrospectively investigates whether the supersite relocation has led to discontinuities in the time series of concentrations of commonly studied gaseous pollutants (NOx, NH3, SO2 and O3) and particulate matter (PM2.5 and PM10). Two years of measurements pre- and post-relocation (2014–15 and 2016–17 respectively) were analysed in conjunction with meteorological variables and local emission data. The deweather package was applied to the concatenated time series to minimise the influence of meteorology. Similar average concentrations of PM2.5, PM10, SO2 and O3 were observed, but there were substantial differences in that of NOx and NH3 (increase by factors of ∼1.6 and ∼3, respectively). The considerably higher NH3 concentrations at Chilbolton are attributed to the close proximity of mixed farmland, in particular to a strong south-westerly source contributing to ∼50% of the annual average. NOx and PM concentrations in easterly winds arriving at Chilbolton are ∼2.7 and ∼1.5 times larger than at Harwell, from sources including the M3 motorway and Greater London. Westerly concentrations of NOx remain similar, therefore despite a higher frequency of westerly wind, annual mean concentrations are larger. Lower concentrations of PM arriving from the west result in similar annual averages. The secondary inorganic and black carbon components of PM were broadly similar between the sites. The differences in average NOx and NH3 at Chilbolton must be taken into account when considering long-term regional trends based on the southern UK supersite data.

A period of elevated surface concentrations of airborne particulate matter (PM) in the UK in spring 2014 was widely associated in the UK media with a Saharan dust plume. This might have led to over-emphasis on a natural phenomenon and consequently to a missed opportunity to inform the public and provide robust evidence for policy-makers about the observed characteristics and causes of this pollution event. In this work, the EMEP4UK regional atmospheric chemistry transport model (ACTM) was used in conjunction with speciated PM measurements to investigate the sources and long-range transport (including vertical) processes contributing to the chemical components of the elevated surface PM. It is shown that the elevated PM during this period was mainly driven by ammonium nitrate, much of which was derived from emissions outside the UK. In the early part of the episode, Saharan dust remained aloft above the UK; we show that a significant contribution of Saharan dust at surface level was restricted only to the latter part of the elevated PM period and to a relatively small geographic area in the southern part of the UK. The analyses presented in this paper illustrate the capability of advanced ACTMs, corroborated with chemically-speciated measurements, to identify the underlying causes of complex PM air pollution episodes. Specifically, the analyses highlight the substantial contribution of secondary inorganic ammonium nitrate PM, with agricultural ammonia emissions in continental Europe presenting a major driver. The findings suggest that more emphasis on reducing emissions in Europe would have marked benefits in reducing episodic PM2.5 concentrations in the UK.