INTERCOMP2000: ionic constitution and comparison of filter and impactor

The field campaign INTERCOMP2000 was organised within the EUROTRAC-2 subproject AEROSOL for characterisation of aerosol at a rural site. The groups involved used a wide range of measurement methods for aerosol particles. Although the focus was on critical aerosol properties like mass, nitrate and carbon, in this paper particular attention is given to the role of inorganic soluble material being main part of the cloud condensation nuclei. Here, we compare methods used in Europe also for inorganic ion mass concentrations: three high-volume samplers (2 Digitel and 1 Sierra Andersen, equipped with quartz fibre filters), four low-volume samplers (1 Rupprecht Patashnik with Teflon filter; 3 stacked filter units with Teflon, cellulose ester or Whatman 41 filter), and 2 low-pressure impactors (Berner type with Tedlar foils). Ten parallel 24 h samples were compared. The data for the main ions nitrate, sulphate and ammonium agree well for the PM10 as well for PM2.5 aerosol fraction; relative standard deviation of about 20–40% were found. The single values for calcium, sodium and chloride which contribute only minor to the soluble inorganic mass scatter very strongly around the calculated averages: about 50% in PM10 mode, and even 100% in PM2.5 mode. While laboratory calibrations typically indicate performance close to design specifications, methods during field operation are subject to a number of sampling and handling artefacts. We know that the different sampling principles used in this study, and the analytical procedures done by each group with their own methodology will cause a main part of the observed uncertainties. In reality, due to different reasons (availability, costs, manpower, different analysis from the same sample, size and time resolution, etc) in many networks and field studies a high variability of methods for aerosol characterisation is used and often those experimental figures will be used for statistical interpretations. Thus, our paper will emphasise that harmonisation among different PM measurements is the “order of the day”.