Thermodesorption of aerosol matter on multiple filters of different materials for a more detailed evaluation of sampling artifacts [An article from: Atmospheric Environment]

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Description:
Multiple, essentially identical samples of PM2.5, PM10 and TSP aerosol matter were collected on filters of cellulose acetate-nitrate membrane (CA), quartz fiber (QF) and glass fiber (GF) material. The samples were analyzed in terms of the gravimetric mass and the mass of nine inorganic ions. These parameters were also measured after step-wise thermodesorption of aerosol matter by 1-h heating in ambient air up to 350^oC. The observed thermograms of the analyzed ions were compared with results obtained using pure and mixed salts on filter. In summer the apparent mass concentration of aerosol matter collected on CA was always larger than on QF and GF filter. The excess mass on CA was found to be highly volatile, i.e. completely removable at 120^oC, and composed of both ionic and non-ionic matter. The apparent nitrate concentration sampled on QF and GF was almost an order of magnitude lower than on CA. The very pronounced nitrate losses from the fiber filters are attributed to volatilization of ammonium nitrate. In contrast, nitrate losses from CA were small or even negligible for two reasons, pile-up of aerosol matter predominantly on (rather than in) the filter (''cake'' formation) and, more importantly, re-adsorption of volatilized ammonia and nitric acid in the filter. Sampling on GF filters was found to suffer from severe problems due to chemical reactions between Na^+ of the glass and SO"4^2^- of the aerosol matter. A novel type of artifact was observed in sampling campaigns during fall. Presumably as a results of a high water content, the collected aerosol matter became liquefied and a large fraction of the water soluble components was driven through the filter into the support pad underneath. The negative ''wetting artifact'' was much more pronounced for the thin CA than for the relatively thick QF filters. The total amount of aerosol matter in the CA/pad and QF/pad combinations was the same, indicating that this kind of artifact can be corrected for. Ammonium nitrate-to-sulfate concentration ratios were low in summer (~ 0.5 for PM2.5) but high in fall (>= 2). Furthermore, the ion fractions were low in summer (~ 30%), but quite high in fall (>= 60%), and even higher in winter.