Long term monitoring of reactive gases and water soluble aerosol components at a remote field site in South East Scotland

Abstract

Long term deposition of inorganic reactive gases, such as NH3, HNO3, HCl, SO2 and their aerosol counter parts (NH4 +, NO3-, Cl- and SO42-) have been demonstrated to contribute to the eutrophication and acidification of sensitive ecosystems. In addition,inorganic aerosols represent a significant proportion of tropospheric aerosols that have an important impact on human health and the climate system. Currently, PM2.5 and PM10 aerosols are mainly monitored by bulk mass methods. This however gives very little information on the contribution of different aerosol compounds to the total mass and on aerosol sources. Long term real time measurements of chemically speciated inorganic aerosols and their precursor gases are needed to understand the sources of emissions, and aerosol processes in the atmosphere in order to implement effective emission reduction measures. The MARGA (Monitoring instrument for AeRosols and reactive Gases, Applikon Analytical BV, Netherlands) is an instrument that provides long term real time (hourly) measurements of water soluble inorganic aerosols (NH4 +, NO3-, SO42-, Cl-) and their gas precursor (NH3, HNO3, HNO2, SO2, HCl). The MARGA is based on a wet chemistry method that utilises rotating annular wet denuders for the capture of reactive trace gases and steam jet aerosol collectors (SJACs) for the collection of water soluble aerosols. Analysis is performed by online ion chromatography, providing hourly concentration values. Since June 2006 a MARGA has been operated continuously at Auchencorth Moss in South East Scotland. The site is a European ‘Supersite’ within the Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe (EMEP). Annual average concentrations ranged from 1.77 μg m-3 to 1.06 μg m-3, 0.10 μg m-3 to 0.12 μg m-3, 0.27 μg m-3to 0.71 μg m-3 for NH3, HNO3 and SO2 respectively, from June 2006 to December 2009 at the site. High concentrations of NH3 and SO2 were frequently linked to wind direction, with the peaks in NH3 concentrations associated to local poultry farms, where as peaks in SO2 concentrations could be linked to coal fuelled power stations. Long range transport, on the other hand, could be used frequently to explain high concentration events of aerosols. High concentrations of NH4+, NO3- and SO42-, could be related to air masses originating over the UK, Western Europe or Eastern Europe and Russia, whereas high Na+ and Cl- concentrations could be linked to air masses originating either over the Atlantic Ocean or the Arctic. The result of long term monitoring using the MARGA system has led to a greater understanding of atmospheric composition and the ability to attribute sources following large pollution events at Auchencorth Moss