Deriving a speciated atmospheric nitrogen budget at Auchencorth Moss, a background site in South East Scotland

Since 2006, the EMEP supersite, Auchencorth Moss, has routinely measured HNO3, HONO, NO, NO2, NH3 and speciated aerosols including NH4+ and NO3- in PM2.5 and PM10. It is known that other reactive N species are important in the atmosphere at background sites including PANs, peroxy nitrates, alkyl nitrates, ClNO2 and N2O5 and routine measurements are not frequently assessed against these other species. The following study presents the highlights from an intensive study, in spring 2014, where non-routine measurements (TD-LIF and PAN GC) were carried out alongside routine measurements (MARGA, ANNOx, NOxThermoFisher Analyser). The objectives of the study were to understand further the role of non-routine measured species in the N budget at this site and to try to identify potential artefacts in current routine measurements. Initial comparison studies suggest that routine measurements capture well the temporal variations in NOx and HNO3, though questions remain on the accuracy of the measurements. During the study on average low concentrations of all species (NO2 = 1.58 ppb, NH3 = 2.3 ppb, HNO3 = 0.09 ppb, HONO= 0.07 ppb) were observed, though there were periods where polluted air masses arrived at the site resulting in an increase in both routine and non-routine measured species. As well as air masses transporting N species, there was evidence of atmospheric chemical transformations of N species at the site, including the photochemical production of PAN.