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. In addition, composition data is needed to refine the epidemiological understanding of the aerosol metrics most closely associated with human health impacts. 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. Furthermore, long-term data sets are necessary to validate national and international chemical transport models, which are used to inform policy. \ud \ud 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 soluble aerosols. Analysis is performed by online ion chromatography, providing hourly concentration values. Since June 2006 a MARGA has been operated continuously at the northern UK EMEP supersite ‘Auchencorth Moss’, monitoring the composition of both PM10 and PM2.5 aerosol. The work presented will focus on the MARGA method and the results of long term monitoring.\u
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