Changing supersites: assessing the impact of the southern UK EMEP supersite relocation on measured atmospheric composition

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, SO(2)and O-3) and particulate matter (PM(2.5)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. Thedeweatherpackage was applied to the concatenated time series to minimise the influence of meteorology. Similar average concentrations of PM2.5, PM10, SO(2)and O(3)were observed, but there were substantial differences in that of NO(x)and NH3(increase by factors of similar to 1.6 and similar to 3, respectively). The considerably higher NH(3)concentrations at Chilbolton are attributed to the close proximity of mixed farmland, in particular to a strong south-westerly source contributing to similar to 50% of the annual average. NO(x)and PM concentrations in easterly winds arriving at Chilbolton are similar to 2.7 and similar to 1.5 times larger than at Harwell, from sources including the M3 motorway and Greater London. Westerly concentrations of NO(x)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 NO(x)and NH(3)at Chilbolton must be taken into account when considering long-term regional trends based on the southern UK supersite data.