Constructing the European aerosol nitrate concentration field from quality analysed data

We report on the construction of the aerosol nitrate concentration field in Europe from an interpolation of data. The first step was an active search for and a subsequent quality assessment of the (long term) data for this semi-volatile component, which is subject to a range of artefacts during sampling. Therefore first an overview of (filter) sampling methods in use in Europe and the associated sampling artefacts are presented. Denuder filter pack combinations have very minor artefacts and data obtained with this technique are therefore used as the starting point. Because of the paucity of data from these devices, values obtained with simple filter sampling were also considered after a critical analysis. We found that the largest artefact associated with inert filters (IF) (Teflon and Quartz) is evaporation of ammonium nitrate. Hence, concentrations obtained with the use of IF are lower values. Next, data from “total nitrate” filters were evaluated. Such filters collect both aerosol nitrate and nitric acid gas and the contribution of the gaseous compound was corrected for. The distances over which nitrate concentrations correlate was then analysed and it was found that in the winter half-year the correlation length was sufficiently long to warrant interpolation of the point data. The nitrate concentration field as obtained from interpolation of nitrate data in the period 1994–1997 was validated with intermediate point data from shorter lasting campaigns. The field has a large area of high nitrate concentrations ranging from southern England over continental western Europe into Poland, with concentrations of nitrate exceeding 5 μg m−3. Strong arguments are provided that the nitrate is predominantly present in the fine aerosol fraction (PM2.5). To the north nitrate concentrations range from 2.5 μg m−3 in southern Sweden to <0.5 μg m−3 in mid-Scandinavia. To the east and west of the central European maximum the nitrate concentration trails off in a more gradual way. It was inferred that the concentration of nitrate in winter in Europe north of the Alps, with the exception of France from which country no data could be found, is 60% or more of that of sulphate. It should further be mentioned that in the Po-Valley ammonium nitrate levels are as high as those in western Europe.